Bonding in Alkyl Halides Bonding in Alkyl Halides

HH

alkyl halides are polaralkyl halides are polar

= 1.9 D= 1.9 D

HH

HH

CC ClCl––++

Dipole MomentsDipole Moments

alkyl halides are polaralkyl halides are polar

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

Physical Properties of Physical Properties of Alkyl Halides:Alkyl Halides:

Intermolecular ForcesIntermolecular Forces

4444 4848

-42-42 -32-32

00 1.91.9

CHCH33CHCH22CHCH33 CHCH33CHCH22FF

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

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.

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.