Alkanes
Physical and Chemical Properties
Attractive Forces Acting Between Ions & Molecules
“Secondary Forces”
Pure Electrostatic Attractions - chiefly ionic compounds
Dipole-dipole attractions - chiefly between molecules
δ+
δ−
δ+
δ+
δ−
δ−
δ+
δ+
δ−
δ−
δ+
δ+
δ−
δ−
δ+
δ−
Attractive Forces Acting Between Ions & Molecules
Dispersion Forces - Very weak, due to a temporary shift in
electron distribution dependent on the size of the molecule
Physical Properties of Alkanes
Alkanes are Nonpolar
Only dispersion forces act between molecules.
Most alkanes are insoluble in water.
Boiling points, melting points, and densities
generally increase with the size of the alkane:
Decane > Heptane > Butane
Physical Properties of Alkanes
Compound
B.P. ºC
CH3CH3
-89
CH3(CH2)3CH3
36
CH3(CH2)7CH3
151
CH3(CH2)11CH3
235
CH3(CH2)16CH3
316
Physical Properties of Alkanes
Boiling points generally decrease with increased branching
as a result of decreased surface area. Consider C9H20
CH3CH 2CH2CH2CH 2CH2CH2CH 2CH3
H3C
CHCH2CH2CH 2CH
CH3
CH3
H3C
CH3
CH3
C CH2 C
CH3
CH3
CH3
CH3
Physical Properties of Alkanes
3-D Structures of Isomeric Nonanes
n-nonane, bp 151ºC
2,6-dimethylheptane, bp 135ºC
2,2,4,4-tetramethylpentane, bp 122ºC
Physical Properties of Alkanes
The Basis for Fractional Distillation
Hot crude oil is fed in at
the
foot
of
the
fractionating column and
mixed with steam. Various
constituents, or fractions,
of the crude oil separate at
various points of the
column.
Physical Properties of Alkanes
The Basis for Fractional Distillation
# C’s
1-4
7
13
16
19
22
25+
Physical Properties of Alkanes
Comparison of Boiling Points for Alkanes and Alcohols
Compound
B.P. ºC
Compound
B.P. ºC
CH3CH3
-89
CH3-OH
65
CH3(CH2)3CH3
36
CH3(CH2)3-OH
118
CH3(CH2)7CH3
151
CH3(CH2)7-OH
194
CH3(CH2)11CH3
235
CH3(CH2)11-OH
255
CH3(CH2)16CH3
316
CH3(CH2)16-OH
308
Note: All organic molecules may have “Alkane-like” portions of their structures
which confer “hydrocarbon-like” properties on the molecules
Physical Properties of Alkanes
Comparison of the Boiling of Ethane (a hydrocarbon) and Methanol (an alcohol)
Boiling involves
breakdown of
dispersion forces
ethane, bp -89ºC
Boiling involves
breakdown of
dispersion forces +
polar forces.
methanol, bp +65ºC
Physical Properties of Alkanes
Comparison of the Boiling of a Hydrocarbon and an Alcohol
Liquid
Gas
Liquid
Gas
Boiling involves
breakdown of
dispersion forces
Boiling involves
breakdown of
dispersion forces +
polar forces.
However, the boiling
points are very
similar....Why ??
Physical Properties of Alkanes
Comparison of Boiling Points for Alkanes and Alcohols
Compound
B.P. ºC
Compound
B.P. ºC
CH3CH3
-89
CH3-OH
65
CH3(CH2)3CH3
36
CH3(CH2)3-OH
118
CH3(CH2)7CH3
151
CH3(CH2)7-OH
194
CH3(CH2)11CH3
235
CH3(CH2)11-OH
255
CH3(CH2)16CH3
316
CH3(CH2)16-OH
308
Note: All organic molecules may have “Alkane-like” portions of their structures
which confer “hydrocarbon-like” properties on the molecules
Chemical
Reactions of
Alkanes
Chemical Reactions of Alkanes
Combustion - Burning in Oxygen
“Propane burns in oxygen to yield carbon dioxide and water.”
C3H8 +
O2 --------------->
CO2 +
H2O
C3H8 +
? O2 --------------->
3 CO2 + 4 H2O
C3H8 +
5 O2 --------------->
3 CO2 + 4 H2O
Chemical Reactions of Alkanes
Combustion - Burning in Oxygen
“Butane burns in oxygen to yield carbon dioxide and water.”
C4H10 +
C4H10 +
2 C4H10
2 C4H10 +
+
O2 --------------->
CO2 +
H2O
? O2 --------------->
4 CO2 + 5 H2O
? O2 --------------->
8 CO2 + 10 H2O
13 O2 --------------->
8 CO2 + 10 H2O
Chemical Reactions of Alkanes
Halogenation - Reaction with Halogen + UV light
“Methane reacts with chlorine gas in the presence of ultraviolet
light to yield chloromethane and hydrogen chloride gas.”
H
H
C
H
uv light
+
H
Cl
H
Cl
C
Cl
+
H
+
HCl
H
H
uv light
CH4
+
Cl2
CH4
CH3Cl
Cl2
uv light
CH3Cl
Cl
Chemical Reactions of Alkanes
Halogenation - Reaction with Halogen + UV light
Cl2
uv light
CH4
H
H
H
C
H
CH3Cl
H
Cl2
uv light
H
C
H
A SUBSTITUTION REACTION
Cl
Chemical Reactions of Alkanes
Halogenation
“Ethane reacts with chlorine gas in the presence of ultraviolet
light to yield chloroethane and hydrogen chloride gas.”
H
H
H
C
C
H
H
C2H6
H
Cl2
uv light
Cl2
uv light
H
H
H
C
C
H
H
Cl
C2H5Cl
A SUBSTITUTION REACTION
Does it make any difference which hydrogen is exchanged in
the substutution reaction ?
Chemical Reactions of Alkanes
Note: It makes no difference which of the hydrogens in methane or
ethane is replaced because all of the hydrogens within each compound
are identical.
Methane
Ethane
Chemical Reactions of Alkanes
Structures of Halogenated Alkanes
chloromethane
1-chloropropane
chloroethane
2-chloropropane
Chemical Reactions of Alkanes
Halogenation
“Propane reacts with chlorine gas in the presence of ultraviolet
light to yield two different compounds with the formula C3H7Cl.”
H
H
H
H
C
C
C
H
H
H
H
Cl2
uv light
H
or
H
H
H
C
C
C
H
H
H 45%
Cl
bp 46ºC
1-chloropropane
H
H
H
H
C
C
C
H
Cl H
H
55%
2-chloropropane
bp 36ºC
Chemical Reactions of Alkanes
Structures of Halogenated Alkanes
chloromethane
1-chloropropane
chloroethane
2-chloropropane
Chemical Reactions of Alkanes
Halogenation - Chlorination
Chemical Reactions of Alkanes
Halogenation - Chlorination vs Bromination
H
H
H
H
C
C
C
H
H
H
H
Cl2
uv light
H
or
H
H
H
H
H
C
C
C
H
H
H
H
Br2
uv light
Bromine is more selective !!
H
or
H
H
H
H
C
C
C
H
H
H
H
H
H
C
C
C
H
Cl H
H
H
H
C
C
C
H
H
H
H
H
H
C
C
C
H
Br H
Cl
H
1-chloropropane
2-chloropropane
45%
55%
Br
1-bromopropane
3%
H
2-bromopropane
97%
Chemical Reactions of Alkanes
Halogenation - Bromination
Substitution takes place at the carbon
which has the fewest hydrogens in the
starting material !!