CHE-240Unit 1

Structure and Stereochemistry of Alkanes

CHAPTER THREE

Terrence P. Sherlock

Burlington County College

2004

Chapter 3 2

IUPAC Names• Find the longest continuous carbon chain.• Number the carbons, starting closest to

the first branch.• Name the groups attached to the chain,

using the carbon number as the locator.• Alphabetize substituents.• Use di-, tri-, etc., for multiples of same

substituent.

Chapter 3 3

Longest Chain• The number of carbons in the longest

chain determines the base name: ethane, hexane. (Listed in Table 3.2, page 81.)

• If there are two possible chains with the same number of carbons, use the chain with the most substituents.

C

CH3

CH2

CH3

CH CH2 CH2 CH3

CH CH2 CH3H3C

H3C

=>

Chapter 3 4

Number the Carbons

• Start at the end closest to the first attached group.

• If two substituents are equidistant, look for the next closest group.

1

2

3 4 5

6 7CHH3C

CH3

CH

CH2CH3

CH2 CH2 CH

CH3

CH3

=>

Chapter 3 5

Name Alkyl Groups

• CH3-, methyl

• CH3CH2-, ethyl

• CH3CH2CH2-, n-propyl

• CH3CH2CH2CH2-, n-butylCH3 CH CH2 CH3

sec-butyl

CH3 CH

CH3

CH2

isobutyl

CH3 CH CH3

isopropyl

CH3C

CH3

CH3

tert-butyl

=>

Chapter 3 6

Alphabetize

• Alphabetize substituents by name.

• Ignore di-, tri-, etc. for alphabetizing.

CHH3C

CH3

CH

CH2CH3

CH2 CH2 CH

CH3

CH3

3-ethyl-2,6-dimethylheptane =>

Chapter 3 7

Complex Substituents• If the branch has a branch, number the

carbons from the point of attachment.

• Name the branch off the branch using a locator number.

• Parentheses are used around the complex branch name.

12

31-methyl-3-(1,2-dimethylpropyl)cyclohexane =>

Chapter 3 8

Boiling Points of AlkanesBranched alkanes have less surface area contact,so weaker intermolecular forces.

=>

Chapter 3 9

Branched Alkanes

• Lower b.p. with increased branching

• Higher m.p. with increased branching

• Examples:H

CH3CH

CH3

CH2 CH2 CH3

bp 60°Cmp -154°C

CH3CH

CH3

CHCH3

CH3 bp 58°Cmp -135°C

=>

bp 50°Cmp -98°C

CH3 C

C 3

CH3

CH2 CH3

Chapter 3 10

Ethane Conformers

• Staggered conformer has lowest energy.

• Dihedral angle = 60 degrees

H

H

HH

H H

Newmanprojection

sawhorse

=>

model

Chapter 3 11

Ethane Conformers (2)• Eclipsed conformer has highest energy

• Dihedral angle = 0 degrees

=>

Chapter 3 12

Conformational Analysis

=>

Chapter 3 13

Cis-Trans Isomerism

• Cis: like groups on same side of ring

• Trans: like groups on opposite sides of ring =>

Chapter 3 14

Cycloalkane Stability

• 5- and 6-membered rings most stable

• Bond angle closest to 109.5• Angle (Baeyer) strain

• Measured by heats of combustion per -CH2 - =>

Chapter 3 15

Heats of Combustion Alkane + O2 CO2 + H2O

Long-chain

157.4 157.4

166.6 164.0158.7 158.6

=>

158.3

Chapter 3 16

Cyclopropane• Large ring strain due to angle compression

• Very reactive, weak bonds

=>

Chapter 3 17

Cyclopropane (2)

Torsional strain because of eclipsed hydrogens

=>

Chapter 3 18

Cyclobutane• Angle strain due to compression

• Torsional strain partially relieved by ring-puckering

=>

Chapter 3 19

Cyclopentane• If planar, angles would be 108, but all

hydrogens would be eclipsed.

• Puckered conformer reduces torsional strain.

=>

Chapter 3 20

Cyclohexane

• Combustion data shows it’s unstrained.• Angles would be 120, if planar.• The chair conformer has 109.5 bond

angles and all hydrogens are staggered.• No angle strain and no torsional strain.

=>

Chapter 3 21

Chair Conformer

=>

Chapter 3 22

Boat Conformer

=>

Chapter 3 23

Conformational Energy

=>

Chapter 3 24

Axial and Equatorial Positions

=>

Chapter 3 25

Monosubstituted Cyclohexanes

=>

Chapter 3 26

1,3-Diaxial Interactions

=>

Chapter 3 27

Disubstituted Cyclohexanes

=>

Chapter 3 28

Cis-Trans Isomers

Bonds that are cis, alternate axial-equatorial around the ring.

CH3

CH3

=>

Chapter 3 29

Bulky Groups• Groups like t-butyl cause a large energy

difference between the axial and equatorial conformer.

• Most stable conformer puts t-butyl equatorial regardless of other substituents.

=>

Chapter 3 30

POWER POINT IMAGES FROM “ORGANIC CHEMISTRY, 5TH EDITION”

L.G. WADEALL MATERIALS USED WITH PERMISSION OF AUTHOR

PRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGEORGANIC CHEMISTRY COURSE

BY:ANNALICIA POEHLER STEFANIE LAYMAN CALY MARTIN