Halogenoalkanes AH Chemistry Unit 3(b). Background Also known as haloalkanes or alkyl halides Rare...
-
Upload
gillian-greer -
Category
Documents
-
view
220 -
download
0
description
Transcript of Halogenoalkanes AH Chemistry Unit 3(b). Background Also known as haloalkanes or alkyl halides Rare...
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