Halogenoalkanes

AH ChemistryUnit 3(b)

Background• Also known as haloalkanes or alkyl

halides

• Rare in the natural world

• Widely used

• Synthesised in the lab

Nomenclature practise• 2,3-dichloropentane

• 3-bromobut-1-ene

• 2,3-dibromo-1-chloro-2-methylbutane

Primary, secondary, tertiary

• Draw a primary halogenoalkane and name it

• Draw a secondary halogenoalkane and name it

• Draw a tertiary halogenoalkane and name it

Reactions• Depends on two factors…

– Type of halogen atom– Position in molecule

• Two characteristic reactions:– Nucleophilic substitution– Elimination

Nucleophilic substitution

hydroxide ion with bromoethane

ethanolCH3CH2Br + OH- CH3CH2OH + Br-(aqueous)

Nucleophilic substitution

mechanism

hydroxide ion with 2-bromo,2-methylpropane

2-methylpropan-2-ol(CH3)3CBr + OH- (CH3)3COH + Br-(aqueous)

mechanism

+ -CH3

HBrC

H-OH

CH3

HHO C H

Br-

hydroxide ion with bromoethane (SN2)

Nucleophilic substitution mechanism

ethanol

reaction equation

2(species reacting in the slowest step)

SN2

S (substitution) N(nucleophilic)

CH3

HBrC

HHO

-

+ -CH3

CH3

BrCCH3

-OH

Br-OH- ion with 2-bromo,2-methylpropane (SN1)

Nucleophilic substitution mechanism

2-methylpropan-2-olreaction equation

1(species reactingin the slowest step)

SN1

S (substitution) N(nucleophilic)

Br-

CH3

CH3

CCH3

+ CH3

CH3

OHCCH3

General rule• Primary and secondary

halogenoalkanes tend to undergo SN2 reactions

• Tertiary halogenoalkanes tend to undergo SN1 reactions

• Can you suggest why?

Important substitution reactions

Water or aqueous alkali

Alcohols Can then be converted to aldehydes, ketones or carboxylic acids

Ammonia Amines

Alcoholic alkoxides Ethers A sodium alkoxide is produced in the reaction of sodium with a dry alcohol

Potassium cyanide (ethanolic solution)

Nitriles Increases length of carbon chain

Nucleophilic substitution

propanenitrileCH3CH2I (ethanol) + CN-(aq) CH3CH2CN + I-cyanide ion with iodoethane

cyanide ion with 2-bromo,2-methylpropane

2,2-dimethylpropanenitrile

(CH3)3CBr (ethanol) + CN-

(CH3)3CCN + Br-

(aqueous)mechanism

+ -CH3

CH3

BrCCH3

Br-CN- ion with 2-bromo,2-methylpropane (SN1)

Nucleophilic substitution mechanism

2,2-dimethylpropanenitrile

1(species reactingin the slowest step)

SN1

S (substitution) N(nucleophilic)

Br-

CH3

CH3

CCH3

+ CH3

CH3

CNCCH3

CN-

reaction equation