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Acids and BasesAcids and Bases
Acidity Constant (K Acidity Constant (K aa ))
¾ Acids differ in their H+^ donating ability
¾ Measured based on their ability to
donate H +^ to water
H-A + H 2 O A-^ + H 3 O+
¾Position of the eq. relates to acid strength
- equilibrium favors the rhs ⇒ strong acid
- equilibrium favors the lhs ⇒ weak acid
Acids and Bases Acids and Bases
¾ quantified by measuring the eq. constant
¾ often expressed as pKa (= -log Ka )
H-A + H 2 O A -^ + H 3 O +
Keq =
[H 3 O
] [A
]
[HA] [H 2 O]
Ka =
[H 3 O
] [A
]
[HA]
3
Acids and BasesAcids and Bases
Relative Acid Strengths Relative Acid Strengths
CH 3 CH 2 OH
H 2 O
CH 3 COOH
HNO 3
HCl
Acid pKa
increasing acid strength
K a
K (^) a = [H^3 O
4
Acids and Bases Acids and Bases
The Lewis DefinitionThe Lewis Definition
¾ a Lewis acidLewis acid is an electron pair acceptoracceptor.
¾ a Lewis baseLewis base is an electron pair donordonor.
Example
H
+ NH 3 H NH 3
lewis acid lewis base
5
Acids and BasesAcids and Bases
¾¾^ Lewis acidsLewis acids require a unfilled low energy
orbital:
O
H
CH 3
N
H H
H
S
H
CH 3
¾ ¾Lewis basesLewis bases require a lone pair of
electrons
Only six electrons, B ∴ empty 2p orbital
F
F
H F
+ No electrons,
∴ empty s orbital
6
Reaction Types
What kind of Reactions can we
carry out!
Reaction Types
What kind of Reactions can we
carry out!
Reaction TypesReaction Types
1. Addition Reactions 1. Addition Reactions
¾ Two reactants add to form one product
A + B → C
Example
C C
H
H
H
H
+ HBr C C
Br
H
H
H
H
H
Starting materials (^) Product
Reaction TypesReaction Types
2. Elimination Reactions2. Elimination Reactions
¾ One reactant splits apart into two products
A → B + C
Example
C C
H
H
H
H
C C + HBr
Br
H H
H
H H
NaOH
9
Reaction TypesReaction Types
3. Substitution Reactions 3. Substitution Reactions
¾ Two reactants exchange parts
A-B + C-D → A-C + B-D
Example
H H C H
H Cl^ Cl
H H C H
Cl H Cl
hv
10
Reaction TypesReaction Types
4. Rearrangement Reactions4. Rearrangement Reactions
¾ One reactant ‘rearranges’ its bonds to
yield a different product
A → B
Example
C C
CH 3 CH 2
H
H
H
C C
CH 3
CH 3
H
H
acid
11
ReactionsReactions
ReactantsReactants
¾ Nucleophiles (nucleus loving)
- electron rich species (eg. – OH, :NH 3 )
C
H
H H I
δ−
δ+
¾ Electrophiles (electron loving)
- electron deficient species (eg. H+, )
¾ Radicals
- species with an unpaired electron (eg. Cl•)
12
Reactions Reactions
Reaction MechanismsReaction Mechanisms
¾ Step by step description of a reaction
¾ Shows which bonds are broken and
formed
¾ Shows the order in which bonds are
breaking and forming
¾ Accounts for all reactants and products
¾ ¾ Draw using curly arrows to showDraw using curly arrows to show
direction of electron flowdirection of electron flow
HydrocarbonsHydrocarbons
Introduction Introduction
¾ Alkanes
¾ Alkynes
¾ Alkenes
¾ Aromatic
Hydrocarbons
C C
H
H
H
H
HH
C C
H H
H H
H C C H
AlkanesAlkanes
General FormulaGeneral Formula
Industrial SourcesIndustrial Sources
Petroleum
Natural Gas C 1 -C 4
Gasoline C 5 -C 11
Kerosene C 11 -C (^14)
Gas Oil C 14 -C 25
Grease Wax
Asphalt
21
AlkanesAlkanes
Structure Structure
Geometry:
- bond angle = 109.5 o
- sp 3
- tetrahedral
eg. Methane
Look for the 3D structure!
22
Importance Importance
¾ ¾ Crude OilCrude Oil
- Petroleum, tar, paraffin etc.
¾ ¾ Medicinal ChemistryMedicinal Chemistry
No VDW Interactions: moderate binding Many VDW Interactions: Strong Binding
N CO 2
O 2 C N N O CO^2
O 2 C N
Hydrophobic pocket
ACE (Angiotensin Converting Enzyme) inhibitors
- treat hypertension
- bind to enzyme active site captopril Enalapril
23
PreparationPreparation
Hydrogenation of Alkenes Hydrogenation of Alkenes
¾ Alkene is treated with hydrogen gas
and a metal catalyst
C C
H H
H H
H (^2)
Pd/C
C (^) nH (^) 2n C (^) nH (^) 2n+
C C
H
H
H H
H H
Catalyst Reagent
24
ReactionsReactions
1. Combustion
CH 4 + 2O 2 → CO 2 + 2H 2 O
2. Substitution
CH 4 + Cl 2 → CH 3 Cl + HCl
3. Pyrolysis
Cracking
ReactionsReactions
2. Substitution (Halogenation 2. Substitution (Halogenation))
¾ Free radical halogenation
Example
¾ Methane + chlorine
H C Cl (^2) or hν H—Cl
H
H
H
H C
Cl
H
H
Light
HalogenationHalogenation
Free Radicals Free Radicals
¾ Very reactive species with an odd (unpaired)
electron (a total of only 7)
¾ sp 2 hybridised
¾ Trigonal planar
H C
H
H
27
Halogenation Halogenation
Mechanism Mechanism
1. Initiation
¾ Production of free radicals
Cl Cl Cl Cl
hυ
28
HalogenationHalogenation
2. Propagation
¾ Major process
H 3 C H Cl H^3 C^ +^ H^ Cl
H 3 C Cl Cl H 3 C Cl + Cl
29
Halogenation Halogenation
3. Termination
¾ Destruction / removal of free radicals
Cl Cl Cl Cl
H 3 C CH 3 H 3 C CH 3
H 3 C Cl H 3 C Cl
30
HalogenationHalogenation
ProblemProblem
¾ Substitution by >1 halogen (difficult to
control!)
CH 4 → CH 3 X → CH 2 X 2 → CHX 3 → CX 4
Reason:
[CH 4 ]↓ and the [CH 3 X]↑
AlkanesAlkanes
Conformational Isomers Conformational Isomers
¾ Ethane (CH 3 -CH 3 )
observer
Rotations around single bonds
AlkanesAlkanes
Conformational IsomersConformational Isomers
¾ Ethane
Eclipsed (unfavoured)
Staggered (favoured)
H
H H
H
H H H
H
H H
H
H
NewmanNewman
ProjectionsProjections
39
AlkanesAlkanes
Conformational Isomers Conformational Isomers
¾ Butane (CH 3 CH 2 CH 2 CH 3 )
observer
40
AlkanesAlkanes
Butane
¾ Large groups near one another cause stericsteric
strainstrain
¾ Arises when such groups try to occupy the
same space
¾ Apparent in the gauchegauche and eclipsedeclipsed
conformations
41
AlkanesAlkanes
Conformational Isomers Conformational Isomers
anti (^) gauche (^) eclipsed
42
Conformational IsomersConformational Isomers
Cycloalkanes Cycloalkanes
¾ Most cyclic organic molecules have large
strain energies
¾ Cyclohexane is nearly strain free
¾ Three types of strain:
- angle strain
- torsional strain
Conformational Isomers Conformational Isomers
Angle strain
¾ Strain due to expansion or compression of
bond angles
Steric Strain
¾ Strain due to repulsive interactions when
atoms approach each other too closely
Torsional Strain
¾ Strain due to eclipsing of bonds.
- Bond = electron pair
- Electron pairs repel!
Conformational IsomersConformational Isomers
Cyclopropane
¾ C-C bond angle is only
60 o^ (normally 109.5o)
¾ Cyclopropane must be
planar
45
Conformational Isomers Conformational Isomers
Cyclohexane
¾ Exists mainly in the
‘chair’ conformation
¾ All angles are 109.5o
(no angle strain)
¾ Fully staggered
(no torsional strain)
H H
H
H
H H
H
H
46
Conformational IsomersConformational Isomers
Chair Cyclohexane
¾ Axial and equatorial substituents
H
H
H H
H H H
H
H H H
H
H
H
H H
H H H
H
H H H
H
Axial (^) Equatorial
47
Conformational Isomers Conformational Isomers
Chair Cyclohexane
¾ Undergoes ring flips
H
H
H H
H
H
H
H
H
H
H
H
48
Conformational IsomersConformational Isomers
Boat Cyclohexane
¾ All C-C angles 109.5o
¾ Fully eclipsed
H
H
H
H H H
H
H H
H
H H H
H
H H
H H H
H
H H H
H
H H
Limonene
oranges lemons
N NO O N
O O O
H H
O H N
OH O
Thalidomide
foetal abnormalities tranquiliser
Vs.
P. Guiry UCD
StereoisomersStereoisomers
Types of chiral moleculesTypes of chiral molecules
¾ Enantiomers
¾ Diastereomers
- configurational
- cis/trans or E/Z (alkene)
¾ Meso Compounds
57
3D Structure 3D Structure
Enantiomers Enantiomers
¾ Mirror images (just like our hands!):
Cl
C Br
F H
Cl
C Br
H F
¾ One enantiomer active, the other potentially harmful
58
59
3D Structure 3D Structure
Enantiomers Enantiomers
¾ Non-superimposable, mirror images
eg.
Cl
C
Br
F H
Cl
C
H
F Br
60
3D Structure3D Structure
Enantiomers:Enantiomers:
¾ only differ in:
- the way they react or interact with other
chiral molecules
- the shape of the crystals they form
(sometimes)
- the way they interact with plane polarised
light (optical acivity)
¾ All other physical properties are
identical
StereoisomersStereoisomers
Optical ActivityOptical Activity
¾ Chiral compounds rotate plane polarised light
¾ Clockwise rotation ⇒ dextrorotatory ( D / +)
¾ Anticlockwise rotation ⇒ levorotatory ( L / -)
¾ Called optical activity
StereoisomersStereoisomers
The PolarimeterThe Polarimeter
¾ schematic
α
light source
polariser sample tube analyser observer containing organic molecules
63
Stereoisomers Stereoisomers
The Polarimeter The Polarimeter
Observed Rotation α
= the angle through which the plane of
light is rotated
¾ Depends on the nature of the sample
- the enantiomeric purity of the material
- concentration
- size of the polarimeter cell
- wavelength of light
64
StereoisomersStereoisomers
Specific Rotation [α]D
¾ Physical constant
¾ Use standard path length and conc.
[α]D = α / l x C
α = observed rotation (in degrees) l = path length (in decimeters) C = concentration (in g/mL)
65
enantiomersenantiomers
Enantiomeric excess Enantiomericexcess
- More of one enantiomer than the other
- Quoted as %ee
% ee =
moles of one − moles of the other total moles of both
x 100%
observed specific rotation known specific rotation of pure enantiomer
% ee = x 100%
- equal amounts of each enantiomer = 0%ee = racemic mixture
66
StereoisomersStereoisomers
To specify ConfigurationTo specify Configuration
Cahn-Ingold-Prelog Rules
¾ Used to describe the arrangement of
groups around a chiral centre
¾ Chiral centres are described as R or S
- Independent of D(+) and L(–)
StereoisomersStereoisomers
>More then one chiral centre >More then one chiral centre
¾ Consider the following molecules with twotwo
chiral centres:
- Structure with n chiral centres has total of 2 n^ stereoisomers:
H 2 N H
COOH
HO H CH 3
H NH 2
COOH
H OH CH 3
H NH 2
COOH
HO H CH 3
H 2 N H
COOH
H OH CH 3
EnantiomersEnantiomers
StereoisomersStereoisomers
>1 chiral centre>1 chiral centre
¾ Consider the following molecules:
H 2 N H
COOH
HO H
CH 3
H NH 2
COOH
H OH
CH 3
H NH 2
COOH
HO H
CH 3
H 2 N H
COOH
H OH
CH 3
EnantiomersEnantiomers
75
StereoisomersStereoisomers
>1 chiral centre >1 chiral centre
¾ Consider the following molecules
H 2 N H
COOH
HO H
CH 3
H NH 2
COOH
H OH
CH 3
H NH 2
COOH
HO H
CH 3
H 2 N H
COOH
H OH
CH 3
Diastereomers Diastereomers
76
StereoisomersStereoisomers
>1 chiral centre>1 chiral centre
¾ consider the following molecules
H 2 N H
COOH
HO H CH (^3)
H NH 2
COOH
H OH CH (^3)
H NH 2
COOH
HO H CH (^3)
H 2 N H
COOH
H OH CH (^3)
Diastereomers Diastereomers
2 pairs of enantiomers 2 pairs of diastereomers
Summary:
77
StereoisomersStereoisomers
Diastereomers Diastereomers
¾ Stereoisomers that are notnot mirror images
( i.e. not enantiomers)
¾¾ SomeSome (one or more) of the chiral centres
have opposite configuration
¾ Diastereomers have differentdifferent physical
properties.
¾ Can be separated by crystallisation,
distillation, etc.
78
StereoisomersStereoisomers
MesoMeso CompoundsCompounds
H OH
COOH
H OH COOH
HO H
COOH
HO H COOH
Plane of symmetry
- Not optically active
- Different groups attached but the
compound has got a plane of symmetry
(∴not chiral)
StereoisomersStereoisomers
Cis/Trans Cis/Trans DiastereomersDiastereomers
¾ Differ in their stereochemistry about a
double bond or ring
¾ Also called geometric isomers
H 3 C
H
H
CH 3
H 3 C
H
H
CH 3
Assign priority to each group attached to double bond or ring Highest priority groups on same side = cis ( Z for zusammen) opposite side = trans ( E for entgegen)
TransTrans EE
TransTrans EE
H 3 C
H
H
CH 3
H 3 C
H
H
CH 3
StereoisomersStereoisomers
Fischer ProjectionsFischer Projections
¾ Allow representation of 3D structures
on a 2D page
A B
C D
A B
D
C
Up/down groups into page Left/right groups out from page
81
3D Structure 3D Structure
Isomerism (Summary) Isomerism (Summary)
isomers
constitutional stereoisomers
enantiomers diastereomers cis/trans
conformational
82
PrioritiesPriorities
Straight from IUPACStraight from IUPAC
H C CH 2
H C CH 2 (C) (C)
(C) = atom with nothing else attached
C^ C^ C
H
Br
H
H 2 C
CH
C C
C
Br
H 2 C
CH
(C)
(C)
(C)
(C)
(C)(C)
H
H
1
4
3 2
*C *C
