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The word ‘ organic’ signifies life. Therefore, all
substances which were obtained directly or indirectly
from living organisms, plants and animals were called
organic compounds and the branch of chemistry which
deals with these compounds was called organic
chemistry.
Modern definition of organic chemistry :
Organic chemistry is a chemistry of hydrocarbons and
their derivatives in which covalently bonded carbon is
an essential constituent.
Berzelius put forward a theory in 1815 known as
vital force theory. According to this theory, " organic
compounds could be prepared only by living organism
under the influence of a mysterious force known as vital
force". Accidental synthesis of urea by Wohler and
synthesis of acetic acid by Kolbe led to the fall of this
theory.
O
NHCNO NH C NH
4 ^2 2
CH CHO CH COOH
O 3
[] 3
Berthelot prepared methane in laboratory and
the most abundant organic compound is cellulose
which is a polymer of glucose. Kekule and Couper
proposed the tetravalency of carbon and wrote the first
structural formula. In 1874, Van't Hoff and Le Bell
suggested a tetrahedron model of carbon.
Purification and Characterisation of organic
compounds
The study of organic compounds starts with the
characterisation of the compound and the
determination of its molecular structure. The procedure
generally employed for this purpose consists of the
following steps :
(1) Purification of organic compounds
(2) Qualitative analysis of organic compounds
(3) Quantitative analysis of organic compounds
(4) Determination of molecular mass of organic
compounds
(5) Calculation of Empirical formula and
Molecular formula of organic compounds
(6) Determination of structure of organic
compounds by spectroscopic and diffraction methods
(1) Purification of organic compounds : A large
number of methods are available for the purification of
substances. The choice of method, however, depends
upon the nature of substance (whether solid or liquid)
and the type of impurities present in it. Following
methods are commonly used for this purpose,
(i) Simple crystallisation
(ii) Fractional crystallisation,
(iii) Sublimation
(iv) Simple distillation
(v) Fractional distillation
(vi) Distillation under reduced pressure
(vii) Steam distillation
(viii) Azeotropic distillation
(ix) Chromatography
(x) Differential extraction
(xi) Chemical methods
(Ammonium
cyanate) (^) (First organic compound
synthesised in laboratory)
Urea
Acetic acid
(First organic compound
synthesised from its
elements)
Acetaldehyde
Chapter
Purification, Classification and Nomenclature of Organic
compounds
(i) Simple crystallisation : This is the most
common method used to purify organic solids. It is
based upon the fact that whenever a crystal is formed,
it tends to leave out the impurities. For crystallisation,
a suitable solvent is one (a) which dissolves more of
the substance at higher temperature than at room
temperature (b) in which impurities are either
insoluble or dissolve to an extent that they remain in
solution (in the mother liquor) upon crystallisation,
(c) which is not highly inflammable and (d) which
does not react chemically with the compound to be
crystallized. The most commonly used solvents for
crystallisation are : water, alcohol, ether, chloroform,
carbon- tetrachloride, acetone, benzene, petroleum
ether etc.
Examples : (a) Sugar having an impurity of
common salt can be crystallized from hot ethanol since
sugar dissolves in hot ethanol but common salt does
not.
(b) A mixture of benzoic acid and naphthalene can
be separated from hot water in which benzoic acid
dissolves but naphthalene does not.
(ii) Fractional crystallisation : The process of
separation of different components of a mixture by
repeated crystallisations is called fractional
crystallisation. The mixture is dissolved in a solvent in
which the two components have different solubilities.
When a hot saturated solution of this mixture is
allowed to cool, the less soluble component crystallises
out first while the more soluble substance remains in
solution. The mother liquor left after crystallisation of
the less soluble component is again concentrated and
then allowed to cool when the crystals of the more
soluble component are obtained. The two components
thus separated are recrystallized from the same or
different solvent to yield both the components of the
mixture in pure form.
Fractional crystallisation can be used to separate
a mixture of KClO 3 (less soluble) and KCl (more
soluble).
(iii) Sublimation : Certain organic solids on
heating directly change from solid to vapour state
without passing through a liquid state, such substances
are called sublimable and this process is called
sublimation.
The sublimation process is used for the separation
of sublimable volatile compounds from non sublimable
impurities. The process is generally used for the
purification of camphor , naphthalene, anthracene,
benzoic acid NH 4 Cl , HgCl 2 , solid SO 2 , Iodine and
salicylic acid etc containing non-volatile impurities.
(iv) Simple distillation : Distillation is the joint
process of vapourisation and condensation. This
method is used for the purification of liquids which boil
without decomposition and contain non-volatile
impurities. This method can also be used for separating
liquids having sufficient difference in their boiling
points. This method can be used to separate a mixture
of
(a) chloroform (b. p. 334 K ) and aniline (b. p. 457
K )
(b) ether (b. p. 308 K ) and toluene (b. p. 384 K )
(v) Fractional distillation : This process is used
to separate a mixture of two or more miscible liquids
which have boiling points close to each other. Since in
this process, the distillate is collected in fractions
under different temperatures, it is known as fractional
distillation. This process is carried out by using
fractionating columns. Fractionating column is a
special type of long glass tube provided with
obstructions to the passage of the vapour upwards and
that of liquid downwards. This method may be used to
separate a mixture of acetone (b. p. 330 K ) and methyl
alcohol (b. p. 338 K ) or a mixture of benzene and
toluene. One of the technological applications of
fractional distillation is to separate different fractions
of crude oil in petroleum industry into various useful
fractions such as gasoline, kerosene oil, diesel oil,
lubricating oil etc.
(vi) Distillation under reduced pressure : This
method is used for the purification of high boiling
liquids and liquids which decompose at or below their
boiling points.
The crude liquid is heated in distillation flask
fitted with a water condenser, receiver and vacuum
pump. As the pressure is reduced, the liquid begins to
boil at a much lower temperature than its normal
boiling point. The vapour is condensed by water
condenser and the pure liquid collects in the receiver.
Glycerol which decomposes at its boiling point
(563 K ) under atmospheric pressure can be distilled
without decomposition at 453 K under 12 mm of Hg.
Similarly, sugarcane juice is concentrated in sugar
industry by evaporation under reduced pressure which
saves a lot of fuel.
(vii) Steam distillation : This method is
applicable for the separation and purification of those
organic compounds (solids or liquids) which (a) are
insoluble in water (b) are volatile in steam (c) possess
a high vapour pressure (10- 15 mm Hg ) at 373 K and (d)
contain non-volatile impurities.
Aniline (b. p. 457 K ) can be purified by steam
distillation since it boils at a temperature of 371.5 K in
Solid Vapour
Heat
Cool
acetone (0.5%) and methanol (3%). Acetic acid can be
separated from this mixture by treating it with milk of
lime when acetic acid forms the calcium salt. The
reaction mixture on distillation gives a mixture of
acetone and methanol (which can be further separated
by fractional distillation into individual components as
mentioned above) while the calcium salt remains as
residue in the flask. The calcium salt is then
decomposed with dil HCl and distilled to afford acetic
acid.
(c) A mixture of 1
o , 2
o and 3
o amines can be
separated using either benzenesulphonyl chloride
( Hinsberg's reagent ) or diethyl oxalate ( Hoffmann's
method ).
(d) Purification of commercial benzene :
Commercial benzene obtained from coal-tar distillation
contains 3-5% thiophene as an impurity which can be
removed by extraction with conc. H 2 SO 4. This
purification is based upon the fact that thiophene
undergoes sulphonation much more easily than
benzene. Thus, when commercial benzene is shaken
with conc. H 2 SO 4 in a separating funnel, thiophene
undergoes sulphonation to form thiophene- 2 - sulphonic
acid which dissolves in conc. H 2 SO 4 while benzene
does not.
Roomtemp H 2 SO 4
H 2 O
After this treatment, the benzene layer is
removed, washed with water to remove unreacted
H 2 SO 4
, dried over anhyd. CaCl 2 and then distilled to
give pure benzene.
(e) Absolute alcohol from rectified spirit : The
rectified spirit (ethanol : H 2 O , 95. 87 : 4. 13 by weight) is
kept over a calculated amount of active quick lime
( CaO ) for few hours and then refluxed. During this
process, water present in rectified spirit combines with
CaO to form Ca ( OH ) 2. When the resulting mixture is
distilled, absolute alcohol distils over leaving behind,
Ca ( OH ) 2.
Drying of Organic Substances. (1) For solids :
Most solids are dried first by pressing them gently
between folds of filter papers. Compounds which
neither decompose on heating nor melt below 100
o C are
dried by keeping them in steam or oven maintained at
o C. Substances, which decompose on heating are
dried by keeping them in a vacuum desiccator
containing a suitable dehydrating agent like fused
CaCl 2 , conc. H 2 SO 4 , P 4 O 10 , solid KOH or NaOH, etc
(desiccant).
(2) For liquids : Organic liquids are generally
dried by keeping them over night in contact with a
dehydrating (desiccating) agent which does not react
chemically with the liquid to be dried. Commonly used
dehydrating agents are quick lime, anhydrous CaCl 2 ,
fused 4
CuSO or CaSO , KOH 4
, metallic sodium or
potassium, etc.
Criteria of purity of organic compounds : The
purity of an organic compound can be ascertained by
determining its some physical constants like m.p., b.p.,
specific gravity, refractive index and viscosity. In usual
practice, sharp m.p. (in case of solids) and boiling point
(in case of liquids) are used as criteria for purity
because their determination is feasible in the
laboratory. A pure organic solid has a definite and sharp
(sudden, rapid and complete) melting point, while an
impure substance has a lower and indefinite melting
point.
(1) Mixed melting point : The melting point of
two thoroughly mixed substances is called mixed
melting point. This can also be used for ascertaining
the purity of a compound.
The substance, whose purity is to be tested, is
mixed with a pure sample of the same compound. The
melting point of the mixture is determined. If the
melting point of the mixture is sharp and comes out to
be the same as that of pure compound, it is sure that
the compound under test is pure. On the other hand, if
the melting point of the mixture is less than the
melting point of the pure compound, the compound in
question is not pure.
(2) Qualitative analysis : (Detection of Elements
The qualitative analysis of an organic compound
involves the detection of all the elements present in it.
Carbon is an essential constituent of an organic
compound whereas hydrogen is nearly always present.
On heating the organic compound with dry cupric
oxide, carbon is oxidized to 2
CO and hydrogen to HO 2
CO 2 is detected by lime water which turns milky while
H 2 O is detected by anhydrous CuSO 4 (white) which
turns it blue. This method is known as copper oxide
test.
C 2 CuO CO 2 2 Cu
Heat ;
Ca OH CO CaCO H 2 O Milky
2 3 Limewater
H CuO H 2 O Cu
Heat 2^ ;
(Hydrated)
Blue
2 4 2
(Anhydrous)
Colourles
CuSO (^) 4 5 HO CuSO. 5 HO
s
Thiophen
e
(Conc. S )
SO 3 H
Thiophene- 2 - sulphonic
acid
(Dissolves in conc.
)
S
If the substance under investigation is a volatile
liquid or gas, the vapours are passed over heated
copper oxide kept in combustion tube and the gaseous
products are tested as above.
Lassaigne method
This is used to detect nitrogen, halogen and
sulphur. Organic compounds are fused with dry sodium
in a fusion-tube and fused mass after extraction with
HO
2
is boiled and filtered. Filtrate called sodium
extract (S.E.) is used to detect elements (other than C
and H ) and the tests are given in the table.
Organic compounds being covalents normally do
not have ionisable groups, hence direct test is not
possible.
Fusion with (^) Na forms soluble salt (like
NaCl , NaCN etc.) which can be easily detected.
This test fails in case of diazo compounds.
Sometimes when the amount of nitrogen
present is small, the prussian blue is present in
colloidal form and the solution looks green.
Table : 22.2 Lassaigne method (Detection of elements)
Elemen
t
Sodium Extract (S.E.) Confirmed Test Reaction
Nitroge
n
( S. E .)
Na C N NaCN
S.E.+ FeSO NaOH 4 , boil
and cool + FeCl conc. HCl 3
Blue or green colour
2 NaCN FeSO 4 Fe ( CN ) 2 Na 2 SO 4
Sodium ferrocyanide
Fe ( CN ) 2 4 NaCN Na 4 [ Fe ( CN ) 6 ]
Na FeCN FeCl Fe FeCN NaCl
HCl 3 [ ( )] 4 [ ( )] 12
(Prussian blue)
Ferric ferrocyanide
Sulphu
r (S.E.)
2
2 Na S NaS
(i) S.E. + sodium nitro
prusside
(ii)S.E+
CH (^) 3 CO 2 H ( CH 3 CO 2 ) 2 Pb
A black ppt.
(i)
(Purple)
4 5 Sodium nitroprusside
Na (^) 2 S Na 2 [ Fe ( CN ) 5 NO ] Na [ Fe ( CN ) NO. S ] or
( Violet)
Sodium thionitroprusside
Na 3 [ Fe ( ONSNa )( CN ) 5 ]
(ii) Na S CHCOO Pb PbS CHCOONa blackppt
CHCOOH 3 .
2 (^3 ) 2 2 3
Haloge
n
( S. E .)
Na X NaX
( X = Cl, Br, I )
S.E. HNO 3 (^) AgNO 3
(i) White ppt soluble in aq
NH 3 confirms Cl.
(ii) Pale yellow ppt
partially soluble in aq. NH 3
confirms Br.
(iii) Yellow ppt insoluble in
aq NH 3 confirms I.
ppt
HNO NaX AgNO AgX
3 3
soluble
3 32 Whiteppt
AgCl 2 NH ( aq )[ Ag ( NH ) ] Cl
Partially soluble
3 32 Yellowppt.
AgBr 2 NH ( aq )[ Ag ( NH ) ] Br
AgI NH 3 ( aq )Insoluble
Nitroge
n and
sulphur
togethe
r
(S.E.)
Na C N S NaCNS
with excess of Na the
thiocyanate formed
decomposes into
cyanide and sulphide.
NaCNS 2 Na NaCN
Na 2 S
As in test for nitrogen;
instead of green or blue
colour, blood red
colouration confirms
presence of N and S both.
3 NaCNS FeCl [ Fe ( SCN ) or[ Fe ( SCN )] Cl 3 NaCl
(Blood red colour)
Ferric sulphocyanide
Table : 22.3 Other methods for detection of elements
Element Test
Nitrogen Soda lime test : A pinch of an organic compound is heated strongly with soda lime ( NaOH CaO )in a
test tube. If ammonia gas evolves, it indicates nitrogen. 3 3 Acetamide
CH (^) 3 CONH 2 NaOH CHCOONa NH
CaO .
This test is, however, not reliable since certain compounds like nitro, azo etc do not evolve
NH 3 when heated with soda lime.
Table : 22.4 Quantitative estimation of elements in organic compounds
Element (^) Method and its principle Formula
Carbon and
Hydrogen
Liebig's combustion method : In this method, a
known weight of organic compound is heated
with pure and dry cupric oxide in a steam of pure
and dry oxygen, when carbon is oxidised to
carbon dioxide while hydrogen is oxidised to
water. From the weight of CO 2 and H (^) 2 O , the
percentage of C and H can be calculated.
HO
y O xCO
y C (^) xH y x 2 2 2 4 2
(i) 100 44
12
Weightoforg.compound
Weightof % of
2
CO C
(ii) 100 18
2
Weightoforg.compound
Weightof % of
2
HO H
Nitrogen (i) Duma's method : Elemental nitrogen is
converted into molecular nitrogen by a suitable
chemical method and its voiume is changed to
STP data.
C 2 H 3 CuO CO 2 H 2 O 3 Cu
2 N 2 CuO N 2 oxide of nitrogen
Oxidesofnitrogen Cu N 2 CuO
(ii) Kjeldahl's method : Nitrogen in organic
compound is converted into NH 3 by suitable
chemical method which, in turn, is absorbed by
V 1 mL of N 1 H 2 SO 4_._
N ( from organiccompound) conc. H 2 SO 4 ( NH 4 ) 2 SO 4
( NH (^) 4 ) 2 SO 4 2 NaOH Na 2 SO 4 2 H 2 O 2 NH 3
% of N = 100 22400
28 W
V
Where, V= volume of 2
N in nitrometer (in ml )
at NTP,
W= Weight of substance taken;
W
N V N
%of
Note : This method is, however, not applicable
to compounds containing nitrogen in the ring
(e.g. Pyridine, quinoline etc) and compounds
containing nitro and azo (– N = N – ) groups
since nitrogen in these compounds is not
completely converted into ( NH 4 ) 2 SO 4 during
digestion.
Halogens (^) (i) Carius method : The method is based on the
fact that when an organic compound containing
halogen ( Cl, Br, or I ) is heated in a sealed tube
with fuming nitric acid in presence of silver
nitrate, silver halide is formed. From the mass of
silver halide formed, the percentage of the
halogen can be calculated.
100 Massofsubstancetaken
Massof formed
% of
AgCl Cl
100 Massofsubstancetaken
Massof formed
188
80 % of
AgBr Br
100 Massofsubstancetaken
Massof formed
235
127 % of
Agl I
(ii) Schiff's and Piria method : In this method
the accurately weighed organic compound (0.15 –
0.25 g ) is taken in a small platinum crucible with
a mixture of lime and sodium carbonate,
( CaO Na 2 CO 3 ). It is now heated strongly and then
cooled and dissolved in dilute nitric acid in a
beaker. The solution is then filtered and the
halide is precipitated with silver nitrate solution.
Halogen is now calculated as in Carius method.
Sulphur Carius method : When an organic compound
containing sulphur is heated with fuming nitric
acid, sulphur is oxidised to sulphuric acid. This is
precipitated as barium sulphate by adding
barium chloride solution. From the amount of
barium sulphate, percentage of sulphur can be
calculated.
2 4
heat S HNO 3 (fuming) HSO
H SO BaCl BaSO 2 HCl
white ppt
2 4 2 4
100 Massofsubstancetaken
Massof formed
233
32 % of
BaSO 4 S
phosphoro
us
Carius method : The organic compound
containing phosphorus is heated with fuming
nitric acid. Phosphorus is oxidised to phosphoric
acid. It is precipitated as magnesium ammonium
100 Massofsubstancetaken
Massof formed
222
62 % ofP
Mg 2 P 2 O 7
phosphate, MgNH 4 PO 4 , by the addition of
magnesia mixture
( MgSO (^) 4 NH 4 OH NH 4 Cl ). The magnesium
ammonium phosphate is washed, dried and
ignited when it is converted to magnesium
pyrophosphate ( Mg 2 P 2 O 7 ).
MgNH PO MgPO NH HO
heat 2 4 4 227 2 3 2
From the mass of magnesium pyro-phosphate,
the percentage of phosphorus in the compound
can be calculated.
Oxygen (i) The usual method of determining the
percentage of oxygen in an organic compound is
by the method of difference. All the elements
except oxygen present in the organic compound
are estimated and the total of their percentages
subtracted from 100 to get the percentage of
oxygen.
(ii) Aluise's method :. Organic compound
containing oxygen is heated with graphite and
CO formed is quantitatively converted into CO 2
on reaction with I 2 O 5.
Oxygen
Pyrolysis Org. compound
O C CO
oC 2 2
1100 2 ^
5 CO I 2 O 5 I 2 5 CO 2
Percentage of oxygen = 100 – (Sum of the
percentages of all other elements)
g
CO
g
O CO
16 44
2
100 massoforg.compd.
massofCO
44
16 % of
2 O
(4) Determination of Molecular Mass : The
molecular mass of the organic compounds can be
determined by various methods.
(i) Physical methods for volatile compounds
(a) Victor Meyer's method : Molecular mass of
volatile liquids and solids can be easily determined
from the application of Avogadro hypothesis according
to which the mass of 22.4 litres or 22400 ml of the
vapour of any volatile substance at NTP is equal to the
molecular mass of the substance.
In Victor Meyer's method, a known mass of the
volatile substance is vaporised in a Victor Meyer's tube.
The vapours formed displace an equal volume of air
into a graduated tube. The volume of air collected in
graduated tube is measured under experimental
conditions. This volume is converted to NTP conditions.
Calculations : Mass of the organic substance
Wg
Let the volume of the air displaced be Vml 1
Temperature T 1 K
Pressure (after deducting aqueous tension)
p 1 mm
Let the volume at NTP be V 2 ml
Applying gas equation, 760
1
1 1 2
T
p V V
V 2 ml of vapours weight at NTP = Wg
22400 ml of vapour weight at NTP =
M
V
W
2
Alternatemethod : Vapour density of substance
Massof 1 mlofhydrogenatNTP
Massof 1 mlof vapoursatNTP
or V. D.
- 00009
W / V 2 ( Mass of 1 ml of H 2 at
NTP
0. 00009 g^ or^2 /^22400 )
or V. D.
2 ^0.^00009
V
W
Mol. Mass,
- 00009
V
W
M VD
(b) Hofmann's method : The method is applied to
those substances which are not stable at their boiling
points, but which may be volatilised without
decomposition under reduced pressure. A known mass
of the substance is vaporised above a mercury column
in a barometric tube and the volume of the vapour
formed is recorded. It is then reduced to NTP
conditions. The molecular mass of the organic
which will require 1000 ml of a normal alkali solution
for complete neutralisation can be calculated. This
mass of the acid will be its equivalent mass.
Onegramequivalentof alkali
1000 ml 1 N alkalisolu tion One gram equivalent of the
acid
Calculations : Suppose w g of the organic acid
requires V ml N 1 alkali solution for complete
neutralisation.
V ml N 1 alkali solution wgm acid
So 1000 ml N 1 alkali solution g V N
w 1000
1
acid
one gram equivalent acid
Equivalent mass of the acid 1000
1
V N
w
Thus, Molecular mass of the acid = Eq. mass
basicity
In the case of organic bases, the known mass of
the base is titrated against a standard solution of an
acid. Knowing the volume of the acid solution used, the
mass of the organic base which will require 1000 ml of
a normal acid solution for complete neutralisation can
be calculated. This mass will be the equivalent mass of
the base.
Onegramequivalentof the acid
1000 mlN acid solution One gram equivalent of the
base
Molecular mass of the base (^) Eq. mass acidity
(5) Calculation of Empirical and Molecular
formula
(i) Empirical formula : Empirical formula of a
substance gives the simplest whole number ratio
between the atoms of the various elements present in
one molecule of the substance. For example, empirical
formula of glucose is CH 2 O , i.e. for each carbon atom,
there are two H- atoms and one oxygen atom. Its
molecular formula is however, C 6 H 12 O 6.
Calculation of empirical formula : The steps
involved in the calculation are as follows,
(a) Divide the percentage of each element by its
atomic mass. This gives the relative number of atoms.
(b) Divide the figures obtained in step (i) by the
lowest one. This gives the simplest ratio of the various
elements present.
(c) If the simplest ratio obtained in step (ii) is not
a whole number ratio, then multiply all the figures with
a suitable integer i.e., 2, 3, etc. to make it simplest
whole number ratio.
(d) Write down the symbols of the various
elements side by side with the above numbers at the
lower right corner of each. This gives the empirical or
the simplest formula.
(ii) Molecular formula : Molecular formula of a
substance gives the actual number of atoms present in
one molecule of the substance.
Molecular formula = n Empirical formula
Where, n is a simple integer 1, 2, 3,...... etc. given
by the equation,
Empiricalformulamassof thecompound
Molecularmassof thecompound n
where the molecular mass of the compound is
determined experimentally by any one of the methods
discussed former, empirical formula mass is calculated
by adding the atomic masses of all the atoms present in
the empirical formula.
(iii) Molecular formula of gaseous hydrocarbons
(Eudiometry)
Eudiometry is a direct method for determination
of molecular formula of gaseous hydrocarbons without
determining the percentage composition of various
elements in it and without knowing the molecular
weight of the hydrocarbon. The actual method used
involves the following steps,
(a) A known volume of the gaseous hydrocarbon
is mixed with an excess (known or unknown volume) of
oxygen in the eudiometer tube kept in a trough of
mercury.
(b) The mixture is exploded by passing an electric
spark between the platinum electrodes. As a result,
carbon and hydrogen of the hydrocarbon are oxidised
to CO 2 and HO 2
vapours respectively.
(c) The tube is allowed to cool to room
temperature when water vapours condense to give
liquid water which has a negligible volume as
compared to the volume of water vapours, Thus, the
gaseous mixture left behind in the eudiometer tube
after explosion and cooling consists of only CO 2 and
unused O 2.
(d) Caustic potash or caustic soda solution is then
introduced into the eudiometer tube which absorbs
2
CO completely and only unused 2
O is left behind.
2 NaOH CO 2 Na 2 CO 3 H 2 O
Thus, the decrease in volume on introducing
NaOH or KOH solution gives the volume of 2
CO
formed. Sometimes, the volume of O 2 left unused is
found by introducing pyrogallol and noting the
decrease in volume.
Calculation : From the volume of CO 2 formed and
the total volume of O 2 used, it is possible to calculate
the molecular formula of gaseous hydrocarbon with the
help of the following equation.
1 vol ( / 4 )vol vol / 2 vol
( / 4 ) 2 2 / (^22)
x y x y
C (^) xHy x y O xCO y HO
(Negligible volume on
condensation)
From the above equation, it is evident that for one
volume of hydrocarbon,
(a) ( x y / 4 )volume of 2
O is used
(b) x volume of 2
CO is produced
(c) y/ 2 volume of H 2 O vapours is produced which
condense to give liquid H 2 O with negligible volume.
(d) Contraction on explosion and cooling
[( 1 x y / 4 ) x ] 1 y / 4
By equating the experimental values with the
theoretical values from the above combustion equation,
the values of x and y and hence the molecular formula
of the gaseous hydrocarbon can be easily determined.
(6) Determination of structure by spectroscopic
and diffraction methods : The structures of organic
substances are determined by spectroscopic and
diffraction methods.
Classification of organic compounds
Organic compounds have been classified on the
basis of carbon skeleton (structure) or functional
groups or the concept of homology.
(1) Classification based on structure
(i) Acyclic or open-chain compounds : Organic
compounds in which all the carbon atoms are linked to
one another to form open chains (straight or branched)
are called acyclic or open chain compounds. These may
be either saturated or unsaturated. For example,
Butane
CH 3 CH 2 CH 2 CH 3
Isobutane
3 |
3 3
CH
CH CH CH
1 - Butene
3 2 2
CH CHCH CH
3, 3 - Dimethy l-1-buty ne
3
3
|
|
3
CH
CH
CH C C CH
These compounds are also called as aliphatic
compounds.
(ii) Cyclic or closed-chain compounds : Cyclic
compounds contain at least one ring or closed chain of
atoms. The compounds with only one ring of atoms in
the molecule are known as monocyclic but those with
more than one ring of atoms are termed as polycyclic.
These are further divided into two subgroups.
(a) Homocyclic or carbocyclic : These are the
compounds having a ring or rings of carbon atoms only
in the molecule. The carbocyclic or homocyclic
compounds may again be divided into two types :
Alicyclic compounds : These are the compounds
which contain rings of three or more carbon atoms.
These resemble with aliphatic compounds than
aromatic compounds in many respects. That is why
these are named alicyclic, i.e. , aliphatic cyclic. These
are also termed as polymethylenes. Some of the
examples are,
Aromatic compounds : These compounds consist
of at least one benzene ring, i.e., a six-membered
carbocyclic ring having alternate single and double
bonds. Generally, these compounds have some fragrant
odour and hence, named as aromatic ( Greek word
aroma meaning sweet smell ).
These are also called benzenoid aromatics.
Non-benzenoid aromatics : There are aromatic
compounds, which have structural units different from
benzenoid type and are known as Non-benzenoid
aromatics e.g. Tropolone, azulene etc.
Cyclopropane (^) Cyclobutane Cyclohexane
Naphthalene
(Bicyclic)
Benzene
(Monocyclic)
HO
O
Tropolone
Azulene
Table : 22.
Class Functional group Class Functional group
Olefins/Alkenes (ene) Acid halides (Alkanoyl
halids)
O
C X
||
(Acylhalide)
Acetylenes/Alkynes
(yne)
C C ^ Amides (Alkanamides) O
C NH
||
2 (Amide)
Alkyl Halides F , Cl , Br , I (Halo) Acid anhydrides
(Alkanoic anhydrides)
O
C
O
C O
|| ||
(Anhydride)
Alcohols (Alkanols) – OH (Hydroxy) Esters (Alkylalkanoates) |
|
||
C
O
C O (Ester)
Ethers (Alkoxyalkanes) (Alkoxy)
|
|
|
|
C O C
Cyanides/Nitriles
(Alkanenitrile)
C N (Cyano)
Aldehydes (Alkanals)
O
C H ||
(Aldehydic) Isocyanides – N C (Isocyano)
Ketones (Alkanones) O
C
||
^ (Carbonyl)
Nitro compounds
(Nitroalkanes)
(Nitro)
Carboxylic acid
(Alkanoic acid)
O
C OH
||
^ (Carboxyl)
Amines
(Amino)
(3) Homologous series : A homologous series can
be defined as a group of compounds in which the various
members have similar structural features and similar
chemical properties and the successive members differ in
their molecular formula by 2
CH group.
Characteristics of homologous series
(i) All the members of a series can be represented
by the general formula. For example, the members of
the alcohol family are represented by the formula
C (^) n H 2 n 1 OH where n may have values 1, 2, 3..... etc.
(ii) Two successive members differ in their
formula by CH 2 group or by 14 atomic mass units
(iii) Different members in a family have common
functional group e.g., the members of the alcohol
family have OH group as the functional group.
(iv) The members in any particular family have
almost identical chemical properties and their physical
properties such as melting point, boiling point, density,
solubility etc., show a proper gradation with the
increase in the molecular mass.
(v) The members present in a particular series
can be prepared almost by similar methods known as
the general methods of preparation.
(4) Saturated and unsaturated compounds : If,
in an organic compound containing two or more carbon
atoms, there are only single bonds between carbon
atoms, then the compound is said to be saturated, e.g.
ethane, n- propyl alcohol, acetaldehyde etc.
Ethane
|
|
|
|
H
H
C H
H
H
H C
n-propy lalcohol
|
|
|
|
|
|
;
H
H
C O H
H
H
C
H
H
H C
Acetaldehyde
|
| H
H O
H
H C C
On the other hand, if the compound contains at
least one pair of adjacent carbon atoms linked by a
multiple bond, then that compound is said to be
unsaturated , e.g, ethylene, acetylene, vinyl alcohol,
acraldehyde etc.
Acety lene
H C C H
Viny lalcohol
| |
H
C OH
H
H C
Acraldehy de
| |
H
C C
H
H C
Nomenclature of organic compounds
H
H
C = C
H
H
Ethylen
e
O
H
C = C
N
O
O
N
H
H
Nomenclature means the assignment of names to
organic compounds. There are two main systems of
nomenclature of organic compounds.
(1) Trivial system : This is the oldest system of
naming organic compounds. The trivial name was
generally based on the source, some property or some
other reason. Quite frequently, the names chosen had
Latin or Greek roots. For example,
(i) Acetic acid derives its name from vinegar of
which it is the chief constituent (Latin : acetum =
vinegar).
(ii) Formic acid was named as it was obtained
from red ants. The Greek word for the red ants is
formicus.
(iii) The names oxalic acid ( oxalus ), malic acid
( pyrus malus ), citric acid ( citrus ) have been derived
from botanical sources given in parentheses.
(iv) Urea and uric acid have derived their names
from urine in which both are present.
(v) The liquid obtained by the destructive
distillation of wood was named as wood spirit. Later
on, it was named methyl alcohol (Greek : methu =
spirit; hule = wood).
(vi) Names like glucose (sweet), pentane (five),
hexane (six), etc. were derived from Greek words
describing their properties or structures.
(vii) Methane was named as marsh gas because it
was produced in marshes. It was also named as fire
damp as it formed explosive mixture with air.
Table : 22.6 Common or trivial names of some organic compounds.
Compound Common name Compound Common name
CH 4 Methane CHCl 3 Chloroform
C 2 H 2 Acetylene CHI 3 Iodoform
H 3 CCH 2 CH 2 CH 3 n- Butane CH 3 CN Acetonitrile
( H 3 C ) 2 CHCH 3 Isobutane CH 3 COOH Acetic acid
( H 3 C ) 4 C Neopentane C 6 H 6 Benzene
HCHO Formaldehyde C 6 H 5 CH 3 Toluene
( H 3 C ) 2 CO Acetone C 6 H 5 NH 2 Aniline
CH 3 CH 2 OH Ethyl alcohol C 6 H 5 OH Phenol
CH 3 CONH 2 Acetamide C 6 H 5 OCH 3 Anisole
CH 3 OCH 3 Dimethyl ether C 6 H 5 COCH 3 Acetophenone
( CH 3 CH 2 ) 2 O Diethyl ether C 6 H 5 CONH 2 Benzamide
(2) IUPAC system : In order to rationalise the
system of naming, an International Congress of
Chemists was held in Geneva in 1892. They adopted
certain uniform rules for naming the compounds.
The system of nomenclature was named as Geneva
system. Since then the system of naming has been
improved from time to time by the International Union
of Pure and Applied Chemistry and the new system is
called IUPAC system of naming. This system of
nomenclature was first introduced in 1947 and was
modified from time to time. The most exhaustic rules
for nomenclature were first published in 1979 and later
revised and updated in 1993. The rules discussed in the
present chapter are based on guide books published by
IUPAC in 1979 ( Nomenclature of Organic Chemistry
by J. Rigandy and S.P. Klesney) and 1993 ( A Guide to
IUPAC Nomenclature for Organic Chemistry by R.
Panico, W.H. Powell and J.C. Richer). With the help of
this system, an organic compound having any number
of carbon atoms can be easily named.
IUPAC System of Naming Organic Compounds :
In the IUPAC system, the name of an organic compound
consist of three parts : (i) Word root (ii) Suffix (iii)
Prefix
(i) Word root : The word root denotes the number
of carbon atoms present in the chain.
- Cl Chloro
- Br Bromo
- I Iodo
- NO Nitroso
- N = N – Diazo
- OCH 3 Methoxy
- OC 2 H 5 Ethoxy
- NO 2 Nitro
- NH 2 Amino
- OH Hydroxo
Thus, a complete IUPAC name of an organic
compound may be represented as:
Prefix + word root + Primary suffix + Secondary
suffix
Word root : Pent (five C – C – C – C – C )
Primary suffix : ene (double bond at C – 2)
Secondary suffix : oic acid (– COOH group)
Prifix : Bromo (– Br group at C – 4)
IUPAC name : Bromo + pent + ene + oic acid or 4-
Bromopent - 2 - en- 1 - oic acid
Classification of carbon atoms in organic
compounds
The carbon atoms in an alkane molecule may be
classified into four types as primary (
o ), secondary
o ), tertiary (
o ) and quaternary (
o ). The carbon
atoms in an organic compound containing functional
group can be designated as , , , .
3
1
|
3
3 1
3
1
3
1
|
|
4 2
2 3
1
CH
CH CH
CH
CH
CH CH C
o
o o
o
o
o o o
CH CH CH CH OH
3 2 2 2
CH CH CHO
CH
CH CH
2 2
3
|
3
Alkyl groups
These are univalent groups or radicals obtained
by the removal of one hydrogen atom from a molecule
of a paraffin. The symbol ' R ' is often used to represent
an alkyl group.
( ) ( )
(Alkane) 2 2 2 1 (Alkylgroup)
R H R
C H CnHn
H n n
Alkyl groups are named by dropping-ane from the
name of corresponding paraffin and adding the ending–
yl.
Parent saturated
hydrocarbon
Name of
the alkyl
group
Structure
Methane Methyl CH 3 –
Ethane Ethyl CH 3 – CH 2 –
Propane n- Propyl CH 3 – CH 2 – CH 2 –
Butane n - Butyl CH 3 – CH 2 – CH 2 – CH 2
Alkyl groups derived from saturated hydrocarbons
having three or more carbon atoms exist in isomeric
forms.
Similarly, removal of different H atoms in
pentane gives the following radicals :
nPentyl
CH CHCHCH CH
3 2 2 2 2
Isopenty l
3
3 2 2 |
CH
CH CHCH CH ;
Neopenty l
3
3
3 2
|
CH
CH
CH CCH ;
secPenty l
3 2 2 3 |
CHCHCHCH CH ;
Penty l
3
3 2 3
CH
|
|
tert
CHCCH CH
Table : 22.11 Unsaturated groups or radicals
Group Common
name
IUPAC name
CH 2 CH Vinyl Ethenyl
2 1
CH 2 CH CH
Allyl 2 - Propenyl
CH CH CH
1
3
HC C Acetylide Ethynyl
1
2
2
HC C CH
Propargyl 2 - Propynyl
General rules for naming organic compounds
In the common system, all the isomeric alkanes
(having same molecular formula) have the same parent
name. The names of various isomers are distinguished
by prefixes. The prefix indicates the type of branching
in the molecule. For example,
Prefix (^) Pri.
suffix
OH
O
CH CH C
Br
CH CH
3 2 1 ||
|
5 4
3
Sec.
suffix
Functional
group
Butane
3 2 2 3
n
CHCHCHCH
CH 3 CH 2 CH 2 CH 2 n Butyl
CH s ec .Butyl 3
3 2
CH
CHCH
(1) Prefix n- ( normal ) is used for those alkanes in
which all the carbon atoms form a continuous chain
with no branching.
Butane
3 2 2 3 n
CHCHCH CH^ ;
Pentane
3 2 2 2 3 n
CHCHCHCH CH
(2) Prefix iso is used for those alkanes in which
one methyl group is attached to the next-to-end carbon
atom (second last) of the continuous chain.
Isobutane
3
|
3 3
CH
CH CHCH
Isopentane
3
|
3 2 3
CH
CH CH CHCH
Isohexane
3
|
3 2 2 3
CH
CH CHCHCHCH
(3) Prefix neo is used for those alkanes which
have two methyl groups attached to the second last
carbon atom of the continuous chain.
Neopentane
3
3 |
|
3 3
CH
CH
CH C CH
Neohexane
3
3 |
|
3 2 3
CH
CH
CH C CH CH
IUPAC system of nomenclature of complex
compounds
The naming of any organic compound depends on
the name of normal parent hydrocarbon from which it
has been derived. IUPAC system has framed a set of
rules for various types of organic compounds.
(1) Rules for Naming complex aliphatic
compounds when no functional group is present
(saturated hydrocarbon or paraffins or Alkanes)
(i) Longest chain rule : The first step in naming
an organic compound is to select the longest continuous
chain of carbon atoms which may or may not be
horizontal (straight). This continuous chain is called
parent chain or main chain and other carbon chains
attached to it are known as side chains (substituents).
Examples :
3
3 |
|
2 3
3
|
3 2 2
CH
CH
C CH CH
CH
CH CH CH CH
If two different chains of equal length are
possible, the chain with maximum number of side
chains or alkyl groups is selected.
(ii) Position of the substituent : Number of the
carbon atoms in the parent chain as 1, 2, 3,....... etc.
starting from the end which gives lower number to the
carbon atoms carrying the substituents. For examples,
A(Correct)
5 4 3 2 | 1
C C C C C
X
B(Wrong)
1 2 3 4 | 5
C C C C C
X
The number that indicates the position of the
substituent or side chain is called locant.
2 Methy lpentane
3
|
3
2 1
2
3
2
4
3
5
CH
CH CH CH CH CH
3 Ethy lhexane
2 2 3
4 | 5 6
3
3
2
2
3
1
CH CH CH
CH CH CH CH CH
(iii) Lowest set of locants : When two or more
substituents are present, then end of the parent chain
which gives the lowest set of the locants is preferred
for numbering.
This rule is called lowest set of locants. This
means that when two or more different sets of locants
are possible, that set of locants which when compared
term by term with other sets, each in order of
increasing magnitude, has the lowest term at the first
point of difference. This rule is used irrespective of the
nature of the substituent. For example,
Set oflocants:2,3, 5 (Correct)
3
|
3
2 1
3
|
3
2
4
3
|
6 5
3
CH
CH CH
CH
CH CH
CH
H C CH
Set oflocants:2,4, 5 (Wrong)
3
|
3
5 6
3
|
4
2
3
3
|
1 2
3
CH
CH CH
CH
CH CH
CH
H C CH
The correct set of locants is 2, 3, 5 and not 2, 4, 5.
The first set is lower than the second set because at the
first difference 3 is less than 4. (Note that first locant
is same in both sets 2; 2 and the first difference is with
the second locant 3, 4. We can compare term by term as
2 - 2, 3 - 4 (first difference), 5 - 5. Only first point of
difference is considered for preference. Similarly for
the compounds,
3
|
3
2 1
2
3
3
|
2
4
2
5
2
7 6
3
|
8
2
9
3
10
CH
CH CH CH
CH
CH CH CH CH
CH
C H CH CH
Set of locants : 2, 7, 8 (Correct)
3
|
3
9 10
2
8
3
|
2
7
2
6
2
4 5
3
|
3
2
2
3
1
CH
CH CH CH
CH
CH CH CH CH
CH
C H CH CH
Set of locants : 3, 4, 9 (Wrong)
First set of locants 2, 7, 8 is lower than second set
3, 4, 9 because at the first point of difference 2 is lower
than 3.
Lowest sum rule : It may be noted that earlier,
the numbering of the parent chain containing two or
more substituents was done in such a way that sum of
Substituen
ts
Substituen
ts
Parent
chain
3
3
3
2
| (^12)
3
3
|
3
2 3
2
| 1
3
1
2
2
2
3
2
| (^54)
2
6
2
7
2
8
3
9
CH
CH C CH CH
CH
CH CH CH
CH CH CH CH C CH CH CH CH
The substituent dimethyl is cited first because it
is alphabetized under d. Similarly,
3
3
10
2
9
2
| 7 8
2 5 3
2
6
2
3 | 4 | 5
2
2
3
1
CH
CH CH CH CH
CH CH
C H CH CH C CH CH
When the names of two or more complex
substituents are composed of identical words, priority
for citation is given to the substituent which has lowest
locant at the first cited point of difference within the
complex substituent. For example,
3
1
2
2
2
3
2
| (^54)
2
7 6
2
8
2
9
2
10
2
11
3
12
C H CH CH CH CH CH CH CH CH CH CH C H
- 5(1-methyl butyl)- 7 - (2-methyl butyl) dodecane
The substituent (1-methylbutyl) is written first
because it has lower locant than the substituent (2-
methylbutyl).
When the same complex substituent (substituted
in the same way) occurs more than once, it is indicated
by the multiplying prefix bis (for two), tris (for three),
tetra kis (for four) etc.
3
|
3
2 1
2
3
2
| 5 4
2
6
2
7
2
8
2
9
3
10
CH
C H CH CH CH CH C CH CH CH CH
(viii) Cyclic hydrocarbons : These compounds
contain carbon chain skeletons which are closed to
form rings. The saturated hydrocarbons with ring of
carbon atoms in the molecule are called cycloalkanes.
These have the general formula Cn H 2 n.
The cyclic compound is named by prefixing cyclo
to the name of the corresponding straight chain alkane.
For example,
If side chains are present, then the rules given in
the previous section are applied. For example,
When more than one side chains are present, the
numbering is done beginning with one side chain so
that the next side chain gets the lower possible
number. For example,
When a single ring system is attached to a single
chain with a greater number of carbon atoms or when
more than one ring system is attached to a single chain,
then it is named as cycloalkylalkanes. For example,
Complex
substituent
( 2-methylpropyl) 5 - (1, 1-Dimethylpropyl) – 5 - (2-methylpropyl)
nonane
Complex
substituent
( 1, 1-
dimethylpropyl)
2 - (^3) methylbutyl
|
3
4
2
2 3
2
| 1
CH
C H CH CH CH
3
4
2
3
2
2
3 | 1
CH CH C H
CH
C H
1 -
methylbutyl
Complex
substituent
( 1, 1-
4 - (1, 1-Dimethylpropyl) – 3 - ethyl dimethylpropyl)-4, 7-
dimethyldecane
3
3
| 2
| 1
3 CH
CH
H C C
1,
dimethylpropyl
5, 5-Bis (1, 1-dimethylpropyl)- 2 -
methyldecane
1,
dimethylpropyl
3
3
| 1
2
2
3
3
CH
C H CH C CH
3
|
3
1 |
2
2
3
3
CH
C H CH C CH
C 3 H 6 ,
Cyclopropane
C 4 H 8 , Cyclobutane
C 5 H 10 ,
Cyclopentane
C 6 H 12 ,
Cyclohexane
CH 3
Methylcyclohexa
ne
C 2 H 5
Ethylcyclopentan
e
CH 2 CH 2 CH 2
CH 3
1, 3 - Bis (2-methylcyclopropyl) CH 3
propane
Cyclohexyl
cyclohexane
CH 2 CH 3
2
3
CH 3 CH 3
1
3 - Ethyl-1, 1-dimethylcyclohexane
(Not 1 - Ethyl-3,3-
dimethylcyclohexane)
CH 3
CH 3
1.3-
Dimethylcyclobutane
1 2
(^43)
CH 2 CH 2 CH 3
6 2
3
4
5
CH 3
1 - Methyl- 3 - propylcyclohexane
(Not 5 - Methyl- 1 - propyl
cycloalkane)
1
3
4
2
3
2
2
2
1
C H CH CH C H
1 - Cyclopropyl butane
CH 2 CH 2 CH 2 CH 2 CH 3
1 -
Cyclobutylpentane
In case of substituted cycloalkenes, the double
bond is given the lowest possible number and
numbering is done in such a way that the substituents
get the lowest number.
According to the IUPAC system of Nomenclature,
certain trivial or semi- systematic names may be used
for unsubstituted radicals. For example, the following
names may be used,
( CH (^) 3 ) 2 CH ^ Isopropyl
3
|
3 2
CH
CH CH CH Sec - Butyl
( CH 3 ) 2 CH CH 2 CH 2 Isopentyl
3
3 |
|
3 2
CH
CH
CH CH C
tert - Pentyl
( CH 3 ) 2 CH CH 2 Isobutyl
( CH (^) 3 ) 3 C tert-Butyl
( CH 3 ) 3 C CH 2 Neopentyl
( CH (^) 3 ) 2 CH CH 2 CH 2 CH Isohexyl
However, when these are substituted, these
names cannot be used as such. For example,
3 |
2
3 , 3 Diethy l 5 isopropy l 4 methy loctane
2 3
|
3
1
2
| 3 2
3
|
4
3
|
3
|
5
2
6
2
7
3
8
CH
CH
CH CH
C CH CH
CH
CH
CH
CH CH
CH CH CH CH
It may be noted that while writing the
substituent's name in alphabetical order, the prefixes
iso - and neo- are considered to be part of the
fundamental name. However, the prefixes sec - and tert-
are not considered to be the part of the fundamental
name.
(2) Rules for IUPAC names of polyfunctional
organic compounds
Organic compounds which contain two or more
functional groups are called polyfunctional compounds.
Their IUPAC names are obtained as follows,
(i) Principal functional group : If the organic
compound contains two or more functional groups, one
of the functional groups is selected as the principal
functional group while all the remaining functional
groups (also called the secondary functional groups)
are treated as substituents. The following order of
preference is used while selecting the principal
functional group.
Sulphonic acids > carboxylic acids > anhydrides >
esters > acid chlorides > acid amides > nitriles >
aldehydes > ketones > thiols > alcohols >alkenes >
alkynes.
All the remaining functional groups such as halo
(fluoro, chloro, bromo, iodo), nitroso (– NO ), – nitro (–
NO 2 ), amino (– NH 2 ) and alkoxy (– OR ) are treated as
substituents.
Table : 22.
Order of
preference
Preflx Suffix (ending)
- SO 3 H Sulpho Sulphonic acid
- COOH Carboxy – oic acid
- COOR Alkoxy carbonyl Alkyl alkanoate
- COX Haloformyl Oyl halide
- CONH 2 Carbamoyl – amide
- C N Cyano – nitrile
- CHO Formyl – al
C = O Keto – one
- OH Hydroxy – ol
- NH 2 Amine – amine
C = C – – ene
- C C – – – yne
- O – Epoxy –
- X Halo –
- NO 2 Nitro –
(ii) Selecting the principal chain : Select the
longest continuous chain of carbon atoms containing
the principal functional group and maximum number of
secondary functional groups and multiple bonds, if any.
(iii) Numbering the principal chain : Number the
principal chain in such a way that the principal
functional group gets the lowest possible number
followed by double bond and triple bond and the
substituents, i.e.
Principal functional group > double bond > triple
bond > substituents
(iv) Alphabetical order : Identify the prefixes and
the positional numbers (also called locants) for the
secondary functional groups and other substituents and
place them in alphabetical order before the word root.
CH 3
CH 3
3
2
1
2, 3 - Dimethylcyclopent-
1ene
CH 3
2
1
3 - Methylcyclohex- 1 - ene
