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

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