Chapter 4 Alkanes

• 1. For alkanes beyond butane, add -ane to the Greek root for the number of carbons.

• C-C-C-C-C-C = hexane• 2. Alkyl substituents: drop the -ane and

add -yl.

• -C2H5 is ethyl

Rules for Naming Alkanes

● identify the longest chain of carbon atoms. In the exampl e shown (Fig. 2a) , the longest chain consists of five carbon atoms and a pentane chain;● number the longest chain of carbons, starting from the end nearest the branch point (Fig. 2b);● identify the carbon with the branching group (number 2 in Fig. 2b);● identify and name the branching group. CH3 is called methyl and not methane.● name the structure by first identifying the substituent and its position in the chain, then naming the longest chain. 2-methylpentane

√x

5+3+3 = 113+5+5 = 13

Pentanes

CH

H

C

H

H

C

H

H

C

H

C

HH

H

H

H

n-pentane, C5H12

CHH

C CHH

C H

H

H

HH

CH

HH

isopentane, C5H12

=>

C

CH3

CH3H3C

CH3

neopentane, C5H12

构造异构体

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.

Longest Chain• The number of carbons in the longest chain

determines the base name: ethane, hexane. 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

=>

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

=>

Name Alkyl Groups

• CH3-, methyl

• CH3CH2-, ethyl

• CH3CH2CH2-, n-propyl

• CH3CH2CH2CH2-, n-butyl

CH3 CH CH2 CH3

sec-butyl

CH3 CH

CH3

CH2

isobutyl

CH3 CH CH3

isopropyl

CH3C

CH3

CH3

tert-butyl

=>

Propyl Groups

C

H

H

H

C

H

H

C

H

H

H

n-propyl

C

H

H

H

C

H

C

H

H

H

isopropyl

H

A primary carbon A secondary carbon

=>

Butyl Groups

C

H

H

H

C

H

C

H

H

C

H

H

H

C

H

H

H

C

H

C

H

HH

C

H

H

n-butyl sec-butyl

H

H

A primary carbon A secondary carbon

=>

Isobutyl Groups

CH

H

H

C

CH H

C

HH

H H

CH

H

H

C

CH H

C H

HH

H

H

H

A primary carbon A tertiary carbon

=>

isobutyl tert-butyl

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 =>

Higher Alkanes

• Anti conformation is lowest in energy.• “Straight chain” actually is zigzag.

CH3CH2CH2CH2CH3

CH C

CC

CH H H H

H H

H H

HH H =>

Constitutional isomers （构造异构体） are compounds which have the same molecular formula but have the atoms joined together in a different way. Constitutional isomers have different physical and chemical properties.

Physical Properties

• Solubility: hydrophobic• Density: less than 1 g/mL• Boiling points increase with increasing

carbons (little less for branched chains).

Melting points increase with increasing carbons (less for odd- number of carbons).

•

Hydrocarbons

mp -135oCbp-5oC

mp -145oCbp-10oC

mp -130oCbp+30oC

mp -160oCbp+28oC

mp -20oCbp+9oC

丁烷异构体

戊烷异构体

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

Major Uses of Alkanes

• C1-C2: gases (natural gas)

• C3-C4: liquified petroleum (LPG)

• C5-C8: gasoline

• C9-C16: diesel, kerosene, jet fuel

• C17-up: lubricating oils, heating oil

• Origin: petroleum refining =>

Conformational isomers

Conformational Analysis• Torsional strain: resistance to rotation.• For ethane, only 3.0 kcal/mol

=>

Cyclic Alkanes

•Carbon atoms can form rings containing only carbon-carbon single bonds.

•C3H6, C4H8, C6H12

Cyclopropane

• Large ring strain due to angle compression• Very reactive, weak bonds

=>

22_499

C C

C

109.5

60

No "head-on"overlap of atomic orbitals

(b)(a)

Chair Conformer

=>

Axial and Equatorial Positions

=>

Reactions of Alkanes

• Combustion

CH3CH2CH2CH3 + O2 CO2 + H2Oheat

8 10132

long-chain alkanes catalyst

shorter-chain alkanes

CH4 + Cl2 CH3Cl + CH2Cl2 CHCl3 CCl4+ +heat or light

=>

• Cracking and hydrocracking (industrial)

• Halogenation

Reactions of Alkanes

Radicals

Petroleum is a complex mixture of alkanes and cycloalkanes that can be separated by distillation

柴油

油脂沥青

40

C

H

H

H

H Cl+ C

H

H

H

+ H Cl

C

H

H

H

+ Cl Cl C

H

H

H

Cl + Cl

C

H

H

H

H + Cl Cl C

H

H

H

Cl + H Cl

Cl Cl + photon (h) Cl + Cl

自由基反应机理

Initiation链引发

Propagation（链增长）

Termination （链终止）CH3. + CH3

. CH3CH3

Reaction coordinate