Halogenoalkanes

What is a nucleophile?

A chemical which always has a lone pair of electrons that it donates to an electron deficient atom

How do nucleophiles react with haloalkanes?

Nucleophile attacks the positive carbon atom. The carbon has a small positive charge because of the electronegativity difference between the carbon and the halogen

Conditions for nucleophilic substitution with hydroxide ions.(OH-)

• Reagent: KOH

• Conditions In aq solution,warm

Conditions for nucleophilic substitution with cyanide ions (CN-)

• Reagent : KCN

• Conditions aq and warm

• heat under reflux

Conditions for nucleophilic substitution with ammonia (NH3)

Reagent: NH3 disoolved in ethanol

Condition: Heating under pressure (in a sealed tube )

Excess NH3

Why should excess ammonia be used?

• Because in the first stage, the product amine still has a lone pair of electrons on the nitrogen atom.

• This can act as a nucleophile which will keep reacting with haloalkane.

• Excess ammonia makes it more likely that the haloalkane will react with ammonia rather than the product amine.

What does the halogenoalkane become in elimination

Alkene

What reagents are used in elimination?

Sodium hydroxide or potassium hydroxide

Conditions for elimination

Haloalkane is dissolved in ethanol (alcohol)only and is heated

What is happening in an elimination reaction?

A hydrogen atom and halogen atom is being removed.

Mechanism for elimination

what effects the alkene formed in elimination

elimination can happen starting from any hydrogen atom as long as it is not connected to the same carbon atom the halogen is attached to. So position of double bond can change

mechanism for substitution with ammonia for bromoethane

What reactions do halogenoalkanes undergo

• Nucleophilic substitution or elimination

What do the rate of substiuoon reactions depnd on

• strength of c-halogen bond

• Iodoalakens are the fastest to substitute and fluoroalkenes are the slowest

• The strength of the c-f bond is very high so its very unreactive