Properties of Alkanes

Long, unbranched alkanes tend to have higher melting points, boiling points, and enthalpies of vaporization than their branched isomers

Cycloalkanes: ring alkanes; made up of CH2 groups

General formula: CnH2n

cyclohexane

“boat” “chair”

Unsaturated Aliphatic Hydrocarbons

Alkenes: carbon-carbon double bond (sp2 hybridized)

Alkynes: carbon-carbon triple bond (sp hybridized)

CH2=CH2

(ethylene)

CHCH

(acetylene)

IUPAC Nomenclature for Aliphatic Hydrocarbons

Straight-chain alkanes - name ends in -ane

Branched alkane - side chain is a “substituent”

name the substituent formed by the removal of one H atom from an alkane by changing the ending from -ane to -yl

name of the alkane is derived from the longest continuous carbon chain

to indicate the position of the substituent, the C atoms in the longest chain are numbered, starting at the end that will give the lowest number for the position of the first attached group

use prefixes di-, tri-, tetra-, penta-, etc. to indicate the number

substituents are listed in alphabetical order (disregard the prefix)

H3C CH

CH3

CH2 C

CH3

CH3

CH3

123452,2,4-trimethylpentane

(sum of the numbers is lowest)

CH3H3C

CH2CH3 2-ethyl-1,1-dimethylcyclohexane

Alkenes and Alkynes

Double bonds - change the “ane” suffix to “ene”

Triple bonds - change the “ane” to “yne”

Position of the multiple bond is given by the number of the first C atom in the multiple bond

CH3-CH2-CH=CH-CH3 2-pentene

CH3-CH2-CH2-CCH3 1-pentyne

CH2=CH-CH=CH2 1,3-butadiene

Reactions of Alkanes

Alkanes are not very reactive

Strong C-C and C-H bonds

mean bond enthalpy (kJ/mol)

C-C 348

C-H 412

1) Oxidation Reactions

CH4(g) + 2 O2(g) CO2(g) + 2H2O(g) Ho = -890 kJ

Break the strong C-H bond, but replaced by two C=O bonds

(mean bond enthalpy of C=O is 743 kJ/mol). Also O-H bond is strong (463 kJ/mol)

2) Substitution Reactions

CH4(g) + Cl2(g) CH3Cl(g) + HCl(g)light or heat

Cl-Cl 2 Cl initiation steplight or heat

Cl + CH4 CH3 + HCl propagation steps

CH3 + Cl2 CH3Cl + Cl

Cl + Cl Cl2 termination steps

CH3 + CH3 CH3CH3

CH3 + Cl CH3Cl

Alkenes

Prepration - Elimination Reactions

1) From alkanes by dehyrogenation

CH3CH3(g) CH2=CH2(g) + H2(g)catalyst

2) From haloalkanes - dehydrohalogenation

CH3CH2Cl + KOH CH2=CH2 + KCl + H2O

3) Dehydration of alcohols

CH3CH2OH CH2=CH2(g) + H2OH2SO4

H2C

Cl

CH2

H

+ K OH H2C CH2 + KCl + H2O

http://www.whfreeman.com/chemicalprinciples/con_index.htm?18

Reactions

1) Addition reactions

Double bonds are more reactive than single bonds

C C + Br BrCCl4 C C

Br

Br

C C + H Hcatalyst

C C

H H

http://www.whfreeman.com/chemicalprinciples/con_index.htm?18

2-chloropropane is the product

The H atom always goes to the C atom of the double bond that already has the most H atoms - Markovnikov addition

C C

H3C

H H

H

+ HX

H3C CH CH3

Cl

H3C CH2 CH2Cl

1-chloropropane

2-chloropropane

Markovnikov’s rule holds - 2-propanol is favored

1-propanol

2-propanol

C C

H3C

H H

H

H3C CH CH3

OH

H3C CH2 CH2OH

+ HOH

Polymerization reactions

n CH2=CH2 -[CH2-CH2]-ncatalyst

H2C C

CH3

CH CH2

H2C C

CH3

CH CH2

n

C C

H3C

H2C H2C

H

CH2

C

H3C

C

H

H2C CH2

C

H3C

C

H

CH2

isoprene(2-methyl-1,3-butadiene)

rubber

cis geometry

C C

H3C

H2C H

H2C

H3C

C

CH2

C

H

H2C CH2

C

H3C

C

CH2

H

gutta-percha

trans-geometry

Aromatic Hydrocarbons

Parent compound of aromatic hydrocarbons - benzene (C6H6)

C is sp2 hybridized, ring is planar

As a substituent - phenyl (C6H5)

Phenol (C6H5OH)

OH

CH3

CH3

NO2

NO2

O2N

Toluene2,4,6-trinitrotoluene (TNT)

Resonance Stablization

-bonding electrons are delocalized over all C atoms

Resonance imparts stability to benzene with respect to hydrogenation and oxidation

Addition (Br2) none rapid none

cyclohexane cyclohexene benzene

Substitution Reactions - -bonds in the ring are left intact; substituent replaces an H atom

+ Br2FeBr3

Br

+ HBr

H2SO4

NO2

+ H2O+ HNO3

SO3

SO3H

+ H2O+ H2SO4

AlBr3

CH3

+ HBr+ CH3Br

Nitration

Sulfonation

Akylation