4.4 Bonding in Alcohols and Alkyl Halides

1
4.4Bonding in Alcoholsand Alkyl Halides
2
Dipole Moments

• alcohols and alkyl halides are polar

d

H
d
d
d
d
H
H
m 1.9 D
m 1.7 D
3
Dipole Moments

• alcohols and alkyl halides are polar

m 1.9 D
m 1.7 D
4
Dipole-Dipole Attractive Forces
d d
d d
d d
d d
d d
5
Dipole-Dipole Attractive Forces
d d
d d
d d
d d
d d
6
4.5Physical Properties of Alcohols and Alkyl
HalidesIntermolecular Forces

• Boiling point
• Solubility in water
• Density

7
Effect of Structure on Boiling Point
CH3CH2CH3
CH3CH2OH
CH3CH2F
Molecularweight Boilingpoint,
C Dipolemoment, D

• 44 48 46
• -42 -32 78
• 0 1.9 1.7

8
Effect of Structure on Boiling Point
CH3CH2CH3
Molecularweight Boilingpoint,
C Dipolemoment, D
Intermolecular forcesare weak. Only
intermolecular forces are induced dipole-induced
dipole attractions.

• 44
• -42
• 0

9
Effect of Structure on Boiling Point
CH3CH2F
A polar moleculetherefore dipole-dipoleand
dipole-induceddipole forces contributeto
intermolecular attractions.
Molecularweight Boilingpoint,
C Dipolemoment, D

• 48
• -32
• 1.9

10
Effect of Structure on Boiling Point
CH3CH2OH
Highest boiling pointstrongest
intermolecularattractive forces. Hydrogen
bonding isstronger than other dipole-dipole
attractions.
Molecularweight Boilingpoint,
C Dipolemoment, D

• 46
• 78
• 1.7

11
Figure 4.4 Hydrogen bonding in ethanol
d
d
d
d
12
Figure 4.4 Hydrogen bonding in ethanol
13
Boiling point increases with increasingnumber of
halogens
Compound Boiling Point

• CH3Cl -24C
• CH2Cl2 40C
• CHCl3 61C
• CCl4 77C

Even though CCl4 is the only compound in this
list without a dipole moment, it has the highest
boiling point. Induced dipole-induced dipole
forces are greatest in CCl4 because it has the
greatest number of Cl atoms. Cl is more
polarizable than H.
14
But trend is not followed when halogenis fluorine
Compound Boiling Point

• CH3CH2F -32C
• CH3CHF2 -25C
• CH3CF3 -47C
• CF3CF3 -78C

15
But trend is not followed when halogenis fluorine
Compound Boiling Point

• CH3CH2F -32C
• CH3CHF2 -25C
• CH3CF3 -47C
• CF3CF3 -78C

Fluorine is not very polarizable and induced
dipole-induced dipole forces decrease with
increasing fluorine substitution.
16
Solubility in water

• Alkyl halides are insoluble in water.
• Methanol, ethanol, isopropyl alcohol
  arecompletely miscible with water.
• The solubility of an alcohol in waterdecreases
  with increasing number of carbons (compound
  becomesmore hydrocarbon-like).

17
Figure 4.5 Hydrogen Bonding Between Ethanol and
Water
18
Density

• Alkyl fluorides and alkyl chlorides areless
  dense than water.
• Alkyl bromides and alkyl iodides are more dense
  than water.
• All liquid alcohols have densities of about 0.8
  g/mL.