Halogenoalkanes: Nucleophilic Substitution - Mechanisms

Question 1

Do halogenoalkanes have polar bonds?

Answer 1

• Halogenoalkanes contain polar bonds
• All the halogens are more electronegative than carbon
• Carbon has δ+ charge
• Halogen has δ- charge

Question 2

What are nucleophiles?

Answer 2

• Nucleophiles are lone pair donors (to an electron deficient carbon)
• A negatively charged ion/δ− atom

Question 3

What common nucleophiles undergo reactions with halogenoalkanes?

Answer 3

• :OH– hydroxide ion
• :CN– cyanide
• :NH3 ammonia

Question 4

What type of reaction do nucleophiles undergo with halogenoalkanes?

Answer 4

• Nucleophilic substitution reactions
- A reaction where a nucleophile donates a lone pair of electrons to δ+ C atom, δ− atom leaves molecule (replaced by nucleophiles)

Question 5

What is the general formula for a nucleophilic substitution of halogenoalkanes?

Answer 5

• RX + Nu– → RNu + X–
• Nu is the nucleophile
• X is halogen

Question 6

Identify the nucleophile, reagents and conditions for hydroxide ion substitution

Answer 6

• Nucleophile
- Hydroxide ions (OH–)

• Reagents

• Potassium hydroxide (KOH)
• OH– don’t exist on their own

• Conditions
- Warm aqueous solvent

Question 7

What is the general formula for nucleophilic substitution reactions with hydroxide ions?

Answer 7

• R–X + –OH → ROH + X–

• R is alkyl group
• X is halogen

• Reaction between aqueous potassium hydroxide and 1-bromopropane
- CH3CH2CH2Br + KOH → CH3CH2CH2OH + KBr

Question 8

Identify the nucleophile, reagents and conditions for cyanide ion substitution

Answer 8

• Nucleophile
- Cyanide ions (CN–)

• Reagents
- Potassium cyanide (KCN)

• Conditions

• Ethanol solvent
• Reflux

Question 9

Example of formula for nucleophilic substitution reactions with cyanide ions

Answer 9

• Reactions between potassium cyanide and 1-bromopropane
- CH3CH2CH2Br + KCN → CH3CH2CH2CN + KBr

• Forms butanenitrile
• Nitriles made in general

Question 10

Identify the nucleophile, reagents and conditions for ammonia substitution

Answer 10

• Nucleophile
- Ammonia (NH3)

• Reagents
- Ammonia (NH3)

• Conditions
- Excess ammonia in a warm ethanol solvent in a sealed tube

Question 11

Example of formula for nucleophilic substitution reactions with ammonia

Answer 11

• Reaction between aqueous ammonia and 1-bromopropane
- CH3CH2CH2Br + 2NH3 → CH3CH2CH2NH2 + NH4Br

• Makes propan-1-amine and ammonium bromide
• Makes amines and ‘ammonium halogen’

Question 12

Describe the mechanism of nucleophilic substitution reactions

Answer 12

• Carbon-halogen bond is polar, so there is δ+ on carbon and δ− on halogen
• :Nu– is nucleophile
• Lone pair on Nu attacks δ+ - curly arrow from :Nu to carbon atom
• Lone pair of electrons create a new bond between Nu and carbon
• Carbon only has 4 bonds
• Addition of Nu breaks carbon-halogen bond - curly arrow from bond to halogen
• Pair of electrons from carbon-halogen bond are taken by halogen and become a lone pair
• Halogen has negative charge

Question 13

Describe the mechanism of nucleophilic substitution reactions with ammonia

Answer 13

• Ammonia substitutes halogen
• This gives nitrogen a positive charge and 4 bonds: 3 to hydrogen, 1 to carbon
• Also forms :X–
• Second ammonia molecule removes a hydrogen from NH3 group to form an ammonium ion (NH4+) and an amine
• Curly arrow goes from :NH3 to hydrogen
• Curly arrow goes from same hydrogen to N+
• Ammonium ion reacts with halogen ion to form an ‘ammonium halogen’

Question 14

Outline the mechanism of the nucleophilic substitution reaction between halogenoalkanes and ammonia

Answer 14

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Question 15

Outline the mechanism of the nucleophilic substitution reaction between halogenoalkanes and hydroxide ions

Answer 15

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Question 16

Outline the mechanism of the nucleophilic substitution reaction between halogenoalkanes and cyanide ions

Answer 16

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