Halogenoalkanes

Alkane + Halogen = ?

Initiation Step

  • The initiation step involves the formation of chlorine radicals.

  • It occurs when chlorine molecules (Cl2) are exposed to heat or ultraviolet light, causing them to dissociate into two chlorine radicals (Cl·)

Propagation Step

  • In the propagation step, the chlorine radicals react with alkanes to form alkyl radicals and more chlorine radicals, allowing the chain reaction to continue.

  • The general reaction can be summarized in two main steps:

    1. A chlorine radical reacts with an alkane (R-H) to form an alkyl radical (R·) and hydrochloric acid (HCl)

      1. CH4 + Cl· → ·CH3 + HCl

    2. The newly formed alkyl radical then reacts with another chlorine molecule, producing a chlorinated alkane and regenerating a chlorine radical

      1. ·CH3 + Cl2 → CH3Cl + Cl·

Termination Step

  • The termination step occurs when two radicals combine to form a stable product, effectively stopping the chain reaction.

  • This can happen in several ways:

    1. Two chlorine radicals can combine to form chlorine molecules

      1. Cl· + Cl· → Cl2

    2. An alkyl radical can react with a chlorine radical to form a chlorinated alkane, or two alkyl radicals can combine to form a dimer

      1. ·CH3 + ·CH3 → CH3CH3

      2. ·CH3 + Cl· → CH3Cl

CFCs and Chloroalkanes

Uses

Polar molecules, so they can be used as solvents and tend to be unreactive and good ad dissolving other polar organic compounds.

Ozone Depletion

Initiation

CF2Cl2 → CF2Cl + Cl-

Propagation

O3+ Cl· → O2 + ClO·

O3 + ClO· → 2O2 + Cl·

Overall

2O3 → 3O2

Bonding in Halogenoalkanes

Halogen atoms are much more electronegative than C (except iodine) and therefore the pair of electrons between the C-X bond is displaced towards the halogen. Therefor ethe bond is polarised and has ionic character.

As the halogen gets larger, bond strength decreases as there is a greater distance and shielding between nucleus and shared pair of electrons, therefore there is a weaker attraction.

Reactivity depends on bond strength. The weaker the C-X bond, the more reactive the halogenoalkane.

Nucleophilic Substitution Reactions of Halogenoalkanes

Warm, Aqueous NaOH

Hydrolysis is breaking a bond using water

Bromoethane + sodium hydroxide → ethanol + sodium bromide

The above reaction is only able to proceed as the C-Br bond is polar due to the electronegativity difference between C and Br

Cyanide Ions

Conditions: Reflux in ethanolic solution

The :CN acts as a nucleophile and the mechanism shows that a C-CN bond is formed and simultaneously C-Br bond breaks.

Aqueous Ammonia

The halogenoalkane is heated with a concentrated solution of ammonia in ethanol. The reaction is carried out in a sealed tube. You couldn't heat this mixture under reflux, because the ammonia would simply escape up the condenser as a gas.

Halogenoalkanes - Reactivity of Halogenoalkanes (A-Level Chemistry ...

Reaction with ammonia is a poor method to make amines industrially. Further substitution occurs leading to a mixture of products forming which have to be separated

Excess ammonia minimises this chance. More efficient to produce amines in a 2-step process with a intermediate.

  1. Nucleophilic substitution: CH3Br + KCN → CH3CN+ KBr

  2. Reduction/Hydrogenation: CH3CN +2H2 → CH3CH2NH2

Uses of Propylamines

Pharmaceuticals, pesticides, and rubber chemicals.

Elimination Reactions of Halogenoalkanes

With HOT NaOH in ethanol; a different reaction takes place with OH ions acting as a proton acceptor (as a base).

For a given halogenoalkane, to favour elimination rather than substitution, use:

  • heat

  • a concentrated solution of sodium or potassium hydroxide

  • pure ethanol as the solvent

Production of elimination is ALWAYS an alkene

Lone pair on oxygen forms a coordinate bond with H+

The shared pair of electrons on C-H bond to form C=C

This repels electrons from C-Br bond onto Br and C-Br bond breaks.

Eliminate or Substitute?

In summary

For a given halogenoalkane, to favour elimination rather than substitution, use:

  • higher temperatures

  • a concentrated solution of sodium or potassium hydroxide

  • pure ethanol as the solvent

To favour substitution rather than elimination, use:

  • lower temperatures

  • more dilute solutions of sodium or potassium hydroxide

  • more water in the solvent mixture