Download Alkanes Names Formula Mechanism Cheat Sheet and more Cheat Sheet Organic Chemistry in PDF only on Docsity!
ALKANE NAMES (Memorize) (Sections 3.2)
# C’s Name Formula Bp (ºC) Structure
1 Methane CH 4 - 162 H-(CH 2 )-H
2 Ethane C 2 H 6 - 89 H-(CH 2 ) 2 - H
3 Propane C 3 H 8 - 42 H-(CH 2 ) 3 - H
4 Butane C 4 H 10 0 H-(CH 2 ) 4 - H
5 Pentane C 5 H 12 36 H-(CH 2 ) 5 - H
6 Hexane C 6 H 14 69 H-(CH 2 ) 6 - H
7 Heptane C 7 H 16 98 H-(CH 2 ) 7 - H
8 Octane C 8 H 18 126 H-(CH 2 ) 8 - H
9 Nonane C 9 H 20 151 H-(CH 2 ) 9 - H
10 Octane C 10 H 22 174 H-(CH 2 ) 10 - H
Industrial Alkanes (Sections 3.5)
Name # C’s Boiling Range Use
Natural Gas C 1 - C 3
(70% methane)
Gas Fuel
“Petroleum Gas” C 2 - C 4 <30º Heating, Gas
Propane C 3 - 42º Propane tanks,
camping, etc.
Gasoline C 4 - C 9 30 - 180º Car fuel
Kerosene C 8 - C 16 160 - 230º Jet fuel
Diesel C 10 - C 18 200 - 320º Truck fuel
Heavy Oils C 16 - C 30 300 - 450º
Motor Oils High temp
Paraffin Vacuum
Asphalt Never Distills
Coke Never Distills
Nomenclature of Alkanes (Sections 3.3)
Systematic IUPAC Rules for Branched and Substituted Alkanes (Section 3.3B)
1. Longest continuous C-chain “core name”
2. Number core chain from an end nearest a substituent
3. Name substituents as “alkyl” groups:
4. Specify the location of substituents using numbers (hyphenate the #’s)
• If >2 substituents, list alphabetically
• Use di-, tri-, tetra- if the same substituent is repeated. (But ignore
these in alphabetizing).
Punctuation Notes:
• Hyphenate numbers
• Do not put a space between substituents and the core name
Special Names for Some 3 or 4-carbon Substituents
Memorize
H 3 C
CH
H 3 C
Isopropyl
C
CH 3
H 3 C
CH 3
t-butyl or tert-butyl
Others
H 3 C C
H 2
H 2
C
n-propyl (n for "normal")
H 3 C C
H 2
H 2
C C
H 2
n-butyl
H 3 C CH^ C
H 2
isobutyl
CH 3
H 3 C C
H 2
CH CH
3 s-butyl
Another Classification System
Primary (1º): with one attached carbon
Secondary (2º): with two attached carbons
Tertiary (3º): with three attached carbons
C
H
C
H 1 º
C
C
C
H 2 º
C
C
C
C 3 º
Very Complex Substituents (Not responsible)
Substituent: ( 1 - ethyl- 2 , 3 - dimethylpentyl) Overall: 9 - ( 1 - ethyl- 2 , 3 - dimethylpentyl)nonadecane
Structural Isomer Problems
- Check formula first. Is it an acyclic molecule, or not? (Cyclic alkane or an
alkene or something…)
- Be systematic. Try the longest possible chain (or largest ring size) first, then
systematically shorten it and find the branched isomers.
- Avoid duplicates!
- Beware of things that look different but are really the same thing.
1. Draw all structural isomers of C 7 H 16. (Be systematic; no duplicates!)
Formula proves acyclic alkane
7
6
5
4
2. Draw all structural isomers of C 7 H 14. (Be systematic; no duplicates!)
Formula proves either a cyclic alkane or an alkene. In addition to the 27 cycloalkanes
shown, there are at least another couple dozen alkenes. Notice that these are 27 cycloalkane
structure isomers; many of them could also have cis/trans issues, so where I drew just one,
you could perhaps actually draw both a cis version and a trans version.
Cyclic C 7 H 14
Structure, Conformations of Acyclic Alkanes (3.7)
A. “Conformations” = “Conformers” = “Rotamers” = different 3 - D arrangements
resulting from rotation around a single bond
H
H
H
H
H
H
H H
H H
H (^) H H H
H^ H^ H H
Normal zig-zag "sawhorse"
H
H (^) H H
H H
"Newman Projection"
B. “Newman Projections”: look straight down one C-C bond
If both bonded carbons are tetrahedral, there will be three bonds extending from
the front carbon, and three more bonds extending from the back carbon
Terms:
o Dihedral angle : angle between a bond on the front atom relative to a bond
on the back atom
o Eclipsed : when bonds are aligned. 0º, 120º, 240º, 360º dihedral angles
o Staggered : when bonds are as far apart as possible: 60º, 180º, 300º
o Skew : anything else in between the eclipsed and staggered extremes
H
H^ H HH
H* H
H H H
H H*
H
H^ H H*H
H H
H H H*
H H
H
H^ H HH
H H
H H H*
*H H
0 º 60 º 120 º 180 º 240 º 300 º eclipsed staggered eclipsed staggered eclipsed staggered
H
H^ H (^) HH
H*
360 º eclipsed
Energy: Staggered best, eclipsed worst
Why: Torsional strain. Repulsion between bonding electron pairs is reduced
in the staggered conformation, and is worst in the eclipsed conformation.
Rotation Barrier: energy gap between the best and worst conformation when you go
through a full 360º rotation (as would take place in a full bond rotation)
Draw in Entergy diagram:
Relative Energy (kcal/mol)
0
1
2
3
0 º 60 º 120 º 180 º 240 º 300 º 360 º Dihedral angle
Rotation Barrier
Summary
1. Anti < gauche < eclipsed < totally eclipsed
2. Steric and torsional reasons
3. The bulkier a substituent, the greater the steric strain in eclipsed and totally eclipsed
conformations
Skills. Be Able to:
1. predict relative rotation barriers
2. write a conformational analysis (rotation/energy diagram)
3. draw Newman pictures for any bond in any structure
4. identify anti/gauche/eclipsed/totally eclipsed conformations
Steps to Drawing Newman Structure:
1. Draw a circle (back carbon) with a dot in the middle
2. Add three sticks extending from the periphery of the circle, with one of them straight up
3. Add three sticks extending from the center dot (front carbon) to illustrate the bonds
radiating from the front carbon
CH 3
H H
CH 3
H H
CH 3
H H
Problems
1. Rank the rotation barriers for the following, relative to the indicated bonds
CH 3 - CH 3
They increase from left to right. The rotation barrier is basically dominated by how
bad things are in the worst possible, totally eclipsed conformation relative to a bond,
and the steric interaction involved. From left to right, the worst eclipsing is between:
H and H; CH 3 and H; CH 3 and CH 3 ; CH 3 and isopropyl; CH 3 and t-butyl
2. Draw Newman projections for the best and worst conformations of the structure shown,
relative to the indicated bond. Use the 3rd^ carbon in the back.
CH 3
H^ H HCH 3
CH 3
worst totally eclipsed
CH 3
H (^) CH CH 3 3
H H
best staggered
Ring Stability and Ring Strain (Section 3.12)
Ring Size
Total Ring
Strain
(kcal/mol)
Strain
Per
CH 2
Main
Source
Of Strain
3 28 9 Angle Strain
4 26 7 Angle Strain
5 7 1 Torsional Strain (eclipsing)
6 0 0 -- STRAIN FREE
7 6 1 Torsional Strain (eclipsing)
8 10 1 Torsional Strain (eclipsing)
Cyclohexane Chair Conformations (Section 3-13,14)
Cyclohexane has no angle strain or torsional strain
Cyclohexane has perfect 109º angles with staggered, non-eclipsed C-C bonds
Obviously it is not flat (natural angle for a flat cyclohexane would be 120º)
Chair Conformations:
A B C^ D
best easier^ to^ see^ "chair"
E
boat intermediate
o Chairs A and B are constantly interconverting via “boat” E
o A and B are best to draw and work with.
o But C / D make it easier to visualize why it’s called a “chair”: 4 carbons make the
seat of the chair, one makes backrest, one a footrest.
Process for Drawing Both Chairs:
"Right-handed chair"
"Left-handed chair"
1. Draw a 4-carbon zig-zag. It helps if your left-most carbon is a little lower than your
3 rd^ carbon
2. Add a 5th^ carbon and 6th^ carbon, but don’t have them exactly underneath the 2nd^ and
3 rd^ carbons.
3. Connect the 6th^ carbon to the orginal 1st^ carbon
For a “left-handed chair”, start up and zig-zag down.
Cis and Trans Disubstituted Cyclohexanes
Questions:
1. Draw both chair forms for cis- 2 - methyl- 1 - isopropylcyclohexane.
2. Which is the best chair for cis- 2 - methyl- 1 - isopropylcyclohexane?
3. Draw both chair forms and identify the best chair for trans- 2 - methyl- 1 -
isopropylcyclohexane.
4. Which is more stable, cis- or trans- 2 - methyl- 1 - isopropylcyclohexane?
5. Then answer the same questions for the 1,3- and 1,4- isomers.
DiSubbed
H
CH 3
H
cis- 1 H
H
CH 3
trans- 1
H
CH 3
H
H
H 3 C
H
A B H
H
H 3 C
CH 3
H
H
C D
Best cis is A > B
Best trans is C > D
Best of all is C (eq-eq), so trans-1 is
better than cis- 1
A > B and C > D because large
isopropyl group wants to be
equatorial
DiSubbed
H
cis- 2
H
CH (^3) trans- 2 H CH 3
H
H
A B
H H
CH 3 CH^3
H
H
C D
H CH 3
H H
H 3 C
Best cis is A (eq eq) > B (ax ax)
Best trans is C (eq ax) > D (ax eq)
Best of all is A (eq-eq), so cis-2 is
better than trans- 2
DiSubbed
H
cis- 3
CH 3
H
trans- 3 H
H
CH 3
H
A B
H
CH 3
H
H
CH 3
C D
H
H
H
CH 3
CH 3
H
Best cis is A (eq ax) > B (ax eq)
Best trans is C (eq eq) > D (ax ax)
Best of all is C (eq-eq), so trans-3 is
better than cis- 3
