9/7/2010
Straight‐chain Alkanes: Melting/boiling points vs Molecular Mass
250
200
150
Temperature
100
50
0
-50
-100
-150
-200
-250
0
20
40
60
80
100
120
140
160
180
200
Molecular Mass
mp (ºC)
bp (ºC)
van der Waals attractions
1
9/7/2010
Boiling point and chain branching
150
Boiling Point (Celsius)
100
50
0
-50
-100
-150
-200
0
20
40
60
80
100
120
Molecular Mass
unbranched
one branch
two branches
Branched chains
 Chain branching decreases the surface area for a given formula
 Examples: C5H12 isomers
 Pentane
 Isopentane
 Neopentane
2
9/7/2010
Alkyl halides
R X
Boiling point vs. MW for alkanes and alkyl halides
300
Boiling Point (Celsius)
250
200
150
100
50
CH3I
0
CH3Cl
CH3Br
-50
CH3CH3
-100
-150
CH3F
CH4
-200
0
50
100
150
200
250
Molecular Mass
alkanes
fluorides
chlorides
bromides
iodides
3
9/7/2010
R
Amines
Alcohols and ethers
R
R
N
O
R
R
4
9/7/2010
MW 44
MW 46
MW 46
MW 45
MW 58
MW 60
MW 60
MW 59
Alcohols, ethers and amines
 Hydrogen bonds explain the trend in boiling points
 Alcohols can form strong dipole‐dipole attractions to each other via their polar O‐H bonds
 Primary and secondary amines can form (weaker) dipole‐dipole attractions to each other via their polar N‐H bonds
 Ethers and tertiary amines do not have X‐H bonds and cannot form hydrogen bonds
5
9/7/2010
van der Waals attractions
6
9/7/2010
Van der Waals vs. Dipole‐Dipole: O‐H, N‐H bonds attract strongly
200
150
Boiling Point, C
100
50
alkanes
alcohols
0
ethers
primary amines
‐50
secondary amines
‐100
tertiary amines
‐150
‐200
0
20
40
60
80
100
120
Molecular Mass
Van der Waals vs. Dipole‐Dipole: N‐H bonds are strong dipoles
140
120
Boiling Point, C
100
80
60
primary amines
secondary amines
40
tertiary amines
20
0
‐20
0
20
40
60
80
100
120
Molecular Mass
7
9/7/2010
Van der Waals vs. Van der Waals: ethers and 3° amines same as alkanes 150
100
Boiling Point, C
50
0
alkanes
‐50
ethers
tertiary amines
‐100
‐150
‐200
0
20
40
60
80
100
120
Molecular Mass
Dipole‐Dipole vs. Dipole‐Dipole: amines weaker than alcohols
180
160
Boiling Point, C
140
120
100
80
alcohols
60
primary amines
40
20
0
‐20
0
20
40
60
80
100
120
Molecular Mass
8