15.1 halogenoalkanes

State the ways in which halogenoalkanes can be produced

• free-radical substitution of alkanes by Cl, or Br,

• electrophilic addition of an alkene with a halogen, or hydrogen halide

• substitution of an alcohol

Explain the free radical substitution of alkanes to form halogenoalkanes

Alkane + halogen in presence of UV light

Addition of HX

addition of hydrogen halides (HX) or halogens (X₂) at room temperature to alkenes

RTP

Substitution of alcohols

Alcohol group is replaced by a halogen to form a halogenoalkane

• HX (or KBr with H₂SO₄ or H₃PO₄ to make HX)

• PCl₃ and heat

• PCl₅ at room temperature

• SOCl₂

Why are halogenoalkanes more reactive than alkanes?

Due to the presence of electronegative halogens

halogen-carbon bond is polar causing the carbon to carry a partial positive and the halogen a partial negative charge

State the substitution reactions that occur with halogenoalkanes

1. Reaction with NaOH

2. Reaction with KCN

3. Reaction with NH3

4. Reaction with aq silver nitrate

Reaction with NaOH

CH₃CH₂Br + :OH^– → CH₃CH₂OH + :Br^–

• halogenoalkane with aqueous alkali results in the formation of an alcohol

• The halogen is replaced by the OH^-

• The aqueous hydroxide (OH^- ion) behaves as a nucleophile by donating a pair of electrons to the carbon atom bonded to the halogen

• Nucleophilic substitution

Reaction with KCN

• Ethanolic solution of is KCN heated under reflux with the halogenoalkane

• Produces nitrile

• Nucleophilic substitution

• Nucleophile = CN-

Reaction with ammonia

• nucleophilic substitution

• Nucleophile is: ammonia

• ethanolic solution of excess ammonia is heated under pressure with halogenoalkane

• Forms primary amine

Why must the ammonia be in excess when reacting it with halogenoalkane

As primary amine can react with halogenoalkane to form secondary amine

Reaction of halogenoalkane with aqueous silver nitrate

water in aqueous silver nitrate will hydrolyse the halogenoalkane to form alcohols

With which halogenoalkane does the fastest and slowest nucleophilic substitution occur

• The fastest nucleophilic substitution reactions take place with the iodoalkanes as the C-I bond is the weakest (longest)

• The slowest nucleophilic substitution reactions take place with the fluoroalkanes as the bond is the strongest (shortest)

Why is pure water not used for the hydrolysis of halogenoalkanes?

hydrolysis with water is much slower than with the OH^- ion in alkalis

• The hydroxide ion is a better nucleophile than water as it carries a full negative charge

State the elimination reaction that occurs with halogenoalkanes

• Halogenoalkanes are heated with ethanolic sodium hydroxide

• Under these anhydrous conditions, elimination occurs:

  • The C–X bond (where X = halogen) breaks heterolytically

  • A halide ion (X⁻) is released

  • A double bond forms, producing an alkene

Write an equation for the elimination of bromoethane:

C₂H₅Br + NaOH (ethanol + heat) → C₂H₄ + NaBr + H₂O

Draw the mechanism for SN1

Draw the mechanism for SN2

Describe the mechanism for SN1

• In the first step, the C-X bond breaks heterolytically and the halogen leaves the halogenoalkane as an X^- ion (this is the slow and rate-determining step)

• This forms a tertiary carbocation

• In the second step, the tertiary carbocation is attacked by the nucleophile

Describe the mechanism for SN2

one-step reaction

• The nucleophile donates a pair of electrons to the delta positive carbon atom to form a new bond

• At the same time, the C-X bond is breaking and the halogen (X) takes both electrons in the bond (heterolytic fission)

• The halogen leaves the halogenoalkane as an X^- ion

What type of nucleophilic substitution do primary halogenoalkanes undergo?

SN2

What type of nucleophilic substitution do tertiary halogenoalkanes undergo?

SN1

What type of nucleophilic substitution do secondary halogenoalkanes undergo?

SN1 ands SN2