Halogenoalkane Reactions and Mechanisms

Halogenoalkanes

What type of reactions do halogenoalkanes commonly undergo?

Substitution reactions, where the halogen atom is replaced by another atom or group.

What is nucleophilic substitution?

A reaction in which an attacking nucleophile replaces an existing atom or group in a molecule.

What four substitution reactions of halogenoalkanes should you know?

Reaction with water, aqueous potassium hydroxide, potassium cyanide in ethanol, and ammonia.

What happens when a halogenoalkane reacts with water?

The halogen is replaced by an –OH group to form an alcohol.

What are the conditions for the reaction of a halogenoalkane with water?

Warm with water.

What is the general reaction of a halogenoalkane with water?

RX + H₂O → ROH + H⁺ + X⁻.

What happens when a halogenoalkane reacts with aqueous potassium hydroxide?

The halogen is replaced by an –OH group to form an alcohol.

What are the conditions for reacting a halogenoalkane with aqueous potassium hydroxide?

Heat under reflux with aqueous KOH.

What is the nucleophile in the reaction between a halogenoalkane and aqueous KOH?

OH⁻.

What is the ionic equation for the reaction of a halogenoalkane with OH⁻?

RX + OH⁻ → ROH + X⁻.

What type of reaction occurs between a halogenoalkane and aqueous OH⁻?

Nucleophilic substitution.

What does reflux allow you to do?

Heat a reaction mixture without losing volatile substances because vapours condense and return to the flask.

What happens when a halogenoalkane reacts with potassium cyanide?

The halogen is replaced by –CN to form a nitrile.

What are the conditions for reacting a halogenoalkane with potassium cyanide?

Heat under reflux with KCN dissolved in ethanol.

What is the nucleophile in the reaction with potassium cyanide?

CN⁻.

What is the general ionic equation for reaction with cyanide ions?

RX + CN⁻ → RCN + X⁻.

What is a nitrile?

An organic compound containing the C–CN group.

How does reaction with CN⁻ affect the carbon chain length?

It increases the carbon chain by one carbon atom.

Why is reaction with KCN useful in organic synthesis?

It provides a way of increasing the carbon-chain length when synthesising more complex compounds.

What happens when a halogenoalkane reacts with ammonia?

The halogen is replaced and a primary amine is formed.

What conditions are used for reacting a halogenoalkane with ammonia?

Heat with ammonia in a sealed tube.

Why is a sealed tube used when reacting a halogenoalkane with ammonia?

Ammonia is a gas and would otherwise escape before it could react.

What is the nucleophile in the reaction between a halogenoalkane and ammonia?

NH₃.

What does the nitrogen atom in NH₃ donate during nucleophilic substitution?

A lone pair of electrons.

Why is excess ammonia used when making a primary amine?

To increase the yield of the primary amine and reduce further substitution.

What are the final products when a halogenoalkane reacts with excess ammonia?

A primary amine and an ammonium halide.

What is the overall general equation for a halogenoalkane reacting with ammonia?

RX + 2NH₃ → RNH₂ + NH₄X.

What is a primary amine?

A compound containing the –NH₂ group attached to an alkyl group.

What happens in the first step of the reaction between a halogenoalkane and ammonia?

NH₃ donates its lone pair to the δ+ carbon while the C–X bond breaks, forming an alkylammonium ion and X⁻.

What happens in the second step of the reaction with ammonia?

Another NH₃ molecule acts as a base and removes H⁺ from the alkylammonium ion, forming the primary amine.

How does nucleophilic substitution with OH⁻ begin?

The lone pair on OH⁻ attacks the δ+ carbon bonded to the halogen.

What happens to the C–X bond during nucleophilic substitution?

The bonding pair of electrons moves to the halogen, causing the C–X bond to break and X⁻ to leave.

What type of bond breaking occurs in nucleophilic substitution of halogenoalkanes?

Heterolytic fission.

What is heterolytic fission?

Bond breaking in which both electrons from the covalent bond go to one of the bonded atoms.

What curly arrow should be drawn from OH⁻ in a nucleophilic substitution mechanism?

From the lone pair on the oxygen atom to the δ+ carbon atom.

What curly arrow should be drawn from the C–X bond in a nucleophilic substitution mechanism?

From the C–X bond to the halogen atom.

Why does a nucleophile attack the carbon atom in a halogenoalkane?

The C–X bond is polar, making the carbon δ+ and electron deficient.

What happens when a halogenoalkane is heated with ethanolic potassium hydroxide?

An elimination reaction occurs and an alkene is formed.

What are the conditions for elimination of a halogenoalkane?

Heat with ethanolic potassium hydroxide.

How does the role of OH⁻ change when ethanolic KOH is used instead of aqueous KOH?

OH⁻ acts as a base rather than as a nucleophile.

What does OH⁻ remove during an elimination reaction?

A hydrogen ion from a carbon adjacent to the carbon bonded to the halogen.

What happens during an elimination reaction of a halogenoalkane?

H and the halogen are removed from adjacent carbon atoms and a C=C double bond forms.

What is an elimination reaction?

A reaction in which a molecule loses atoms or groups attached to adjacent carbon atoms, forming a C=C double bond.

What products are formed when 2-bromopropane reacts with ethanolic KOH?

Propene, water and potassium bromide.

What is the equation for the elimination of 2-bromopropane with KOH?

CH₃CHBrCH₃ + KOH → CH₂=CHCH₃ + H₂O + KBr.

Why can some halogenoalkanes produce more than one alkene during elimination?

Hydrogen can be removed from different adjacent carbon atoms, producing different positions for the C=C double bond.

What determines whether OH⁻ causes substitution or elimination?

The solvent and reaction conditions.

What is favoured by aqueous KOH?

Nucleophilic substitution, producing an alcohol.

What is favoured by ethanolic KOH?

Elimination, producing an alkene.

Why is it important to distinguish aqueous KOH from ethanolic KOH?

The same reagent can produce different organic products depending on the solvent.

Can OH⁻ act as both a nucleophile and a base?

Yes. It acts as a nucleophile in aqueous substitution reactions and as a base in ethanolic elimination reactions.

Can ammonia act as both a nucleophile and a base?

Yes. It acts as a nucleophile in the first step and a base in the second step of amine formation.