4.4 bonding in alcohols and alkyl halides

4.44.4Bonding in AlcoholsBonding in Alcohols

and Alkyl Halidesand Alkyl Halides

HH

HH

HH

alcohols and alkyl halides are polaralcohols and alkyl halides are polar

= 1.7 D= 1.7 D = 1.9 D= 1.9 D

HH

HH

CC OO

HH

HH

CC ClCl++

–– ––++

++

Dipole MomentsDipole Moments

alcohols and alkyl halides are polaralcohols and alkyl halides are polar

= 1.7 D= 1.7 D = 1.9 D= 1.9 D

Dipole MomentsDipole Moments

+ + – –

– – + +

+ + – – + + – – + + – –

Dipole-Dipole Attractive ForcesDipole-Dipole Attractive Forces

+ + – –

– – + +

+ + – – + + – – + + – –

Dipole-Dipole Attractive ForcesDipole-Dipole Attractive Forces

Boiling pointBoiling pointSolubility in waterSolubility in water

DensityDensity

4.54.5Physical Properties of Physical Properties of

Alcohols and Alkyl Halides:Alcohols and Alkyl Halides:Intermolecular ForcesIntermolecular Forces

4444 4848 4646

-42-42 -32-32 +78+78

00 1.91.9 1.71.7

CHCH33CHCH22CHCH33 CHCH33CHCH22FF CHCH33CHCH22OHOH

MolecularMolecularweightweight

BoilingBoilingpoint, °Cpoint, °C

DipoleDipolemoment, Dmoment, D

Effect of Structure on Boiling PointEffect of Structure on Boiling Point

4444

--4242

00

CHCH33CHCH22CHCH33

MolecularMolecularweightweight

BoilingBoilingpoint, °Cpoint, °C

DipoleDipolemoment, Dmoment, D

Intermolecular forcesIntermolecular forcesare weak.are weak.

Only intermolecularOnly intermolecularforces are inducedforces are induceddipole-induced dipoledipole-induced dipoleattractions.attractions.

Effect of Structure on Boiling PointEffect of Structure on Boiling Point

4848

-32-32

1.91.9

CHCH33CHCH22FF

MolecularMolecularweightweight

BoilingBoilingpoint, °Cpoint, °C

DipoleDipolemoment, Dmoment, D

A polar molecule;A polar molecule;therefore dipole-dipoletherefore dipole-dipoleand dipole-inducedand dipole-induceddipole forces contributedipole forces contributeto intermolecular to intermolecular attractions.attractions.

Effect of Structure on Boiling PointEffect of Structure on Boiling Point

4646

+78+78

1.71.7

CHCH33CHCH22OHOH

MolecularMolecularweightweight

BoilingBoilingpoint, °Cpoint, °C

DipoleDipolemoment, Dmoment, D

Highest boiling point;Highest boiling point;strongest intermolecularstrongest intermolecularattractive forces.attractive forces.

Hydrogen bonding isHydrogen bonding isstronger than otherstronger than otherdipole-dipole attractions.dipole-dipole attractions.

Effect of Structure on Boiling PointEffect of Structure on Boiling Point

++––

––++

Figure 4.4 Hydrogen bonding in ethanolFigure 4.4 Hydrogen bonding in ethanol

Figure 4.4 Hydrogen bonding in ethanolFigure 4.4 Hydrogen bonding in ethanol

Boiling point increases with increasingBoiling point increases with increasingnumber of halogensnumber of halogens

CHCH33Cl Cl -24°C-24°C

CHCH22ClCl22 40°C40°C

CHClCHCl33 61°C61°C

CClCCl44 77°C77°C

CompoundCompound Boiling PointBoiling Point

Even though CClEven though CCl4 4 is the only compound in this list without is the only compound in this list without a dipole moment, it has the highest boiling point.a dipole moment, it has the highest boiling point.

Induced dipole-induced dipole forces are greatest in CClInduced dipole-induced dipole forces are greatest in CCl44 because it has the greatest number of Cl atoms. Cl is more because it has the greatest number of Cl atoms. Cl is more polarizable than H. polarizable than H.

But trend is not followed when halogenBut trend is not followed when halogenis fluorineis fluorine

CHCH33CHCH22FF -32°C-32°C

CHCH33CHCHFF22 -25°C-25°C

CHCH33CCFF33 -47°C-47°C

CCFF33CCFF33 -78°C-78°C

CompoundCompound Boiling PointBoiling Point

But trend is not followed when halogenBut trend is not followed when halogenis fluorineis fluorine

CHCH33CHCH22FF -32°C-32°C

CHCH33CHCHFF22 -25°C-25°C

CHCH33CCFF33 -47°C-47°C

CCFF33CCFF33 -78°C-78°C

CompoundCompound Boiling PointBoiling Point

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

Solubility in waterSolubility in water

Alkyl halides are insoluble in water.Alkyl halides are insoluble in water.

Methanol, ethanol, isopropyl alcohol areMethanol, ethanol, isopropyl alcohol arecompletely miscible with water.completely miscible with water.

The solubility of an alcohol in waterThe solubility of an alcohol in waterdecreases with increasing number of decreases with increasing number of carbons (compound becomescarbons (compound becomesmore hydrocarbon-like).more hydrocarbon-like).

++––

––

++

––++

Figure 4.5 Hydrogen Bonding Figure 4.5 Hydrogen Bonding Between Ethanol and WaterBetween Ethanol and Water

DensityDensity

Alkyl fluorides and alkyl chlorides Alkyl fluorides and alkyl chlorides areareless dense than water.less dense than water.

Alkyl bromides and alkyl iodides are Alkyl bromides and alkyl iodides are more dense than water.more dense than water.

All liquid alcohols have densities of All liquid alcohols have densities of about 0.8 g/mL.about 0.8 g/mL.