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
