HALOGENOALKANES

What is a primary halogenoalkane?

has only one carbon attached to the carbon atom adjoining the halogen

What is a secondary halogenoalkane?

has 2 carbons attached to the carbon atom adjoining the halogen

What is a tertiary halogenoalkane?

has 3 carbons attached to the carbon atom adjoining the halogen

What reactions can halogenoalkanes undergo?

• nucleophilic substitution

• elimination

What happens in a substitution reaction?

swapping a halogen atom for another atom or group of atoms

What does :Nu represent?

any nucleophiles as they always have a lone pair and act as electron pair donators

What is the mechanism for a nucleophilic substitution reaction between X-ethane and :Nu?

What does the rate of substitution reactions depend upon?

• the strength of the C-X bond

• the weaker the bond, the easier it is to break and the faster the reaction

What halogenoalkane is the fastest to substitute?

iodoalkanes

What is hydrolysis?

the splitting of a molecule by a reaction with water

What happens when aqueous silver nitrate is added to a halogenoalkane?

• halide leaving group combines with a silver ion to form a silver halide precipitate

• rate of precipitate formation can be used to compare the reactivity of different halogenoalkanes

• quicker the precipitate is formed, the faster the substitution reaction and the more reactive the halogenoalkane

What colour precipitate does AgI, AgBr and AgCl form?

• AgI is a yellow precipitate

• AgBr is a cream precipitate

• AgCl is a white precipitate

What occurs in nucleophilic substitution with aqueous hydroxide ions?

• forms an alcohol

• reagent - potassium hydroxide

• conditions - in aqueous solution, warm

• mechanism - nucleophilic substitution

What is the mechanism for hydroxide ions undergoing nucleophilic substitution with bromo-methane?

How do tertiary halogenoalkanes undergo nucleophilic substitution?

• this is due to the tertiary carbocation being stabilised by the electron releasing methyl groups around it

• bulky methyl groups prevent the hydroxide ion from attacking the halogenoalkane

What occurs in nucleophilic substitution with cyanide ions?

• forms a nitrile

• reagent - KCN dissolved in ethanol/water mixture

• conditions - heating under reflux

• mechanism - nucleophilic substitution

What is the mechanism between bromo-ethane and :CN?

How do you name nitriles?

• nitrile groups are on carbon 1

• number the chain from carbon 1

• e.g. butanenitrile

What occurs in nucleophilic substitution with ammonia?

• forms an amine

• reagent - NH3 dissolved in ethanol

• conditions - heating under pressure in a sealed tube

• mechanism - nucleophilic substitution

What is the mechanism for nucleophilic substitution betwee 1-bromopropane and ammonia?

How do you prevent further substitution reactions between the halogenoalkane and the amines formed?

use excess ammonia

What occurs in an elimination reaction between alcoholic hydroxide ions?

• forms an alkene

• reagents - potassium hydroxide

• conditions - in ethanol, heat under reflux

• mechanism - elimination

What is the mechanism for the elimination reaction between 2-bromopropane and hydroxide ions?

What can happen with unsymmetrical secondary and tertiary halogenoalkanes?

two different structural isomers can be formed

What type of reaction often occurs in primary halogenoalkanes?

substitution

What type of reaction often occurs in tertiary halogenoalkanes?

elimination

Why has using halogenoalkanes for solvents stopped?

due to the toxicity and their detrimental effect on the atmosphere

What can halogenoalkanes be used as?

• refrigerants

• pesticides

• aerosol propellants

What is the benefit of naturally occuring ozone?

it filters out the suns harmful UV radiation

What is the issue with ozone in the lower atmosphere?

its a pollutant and contributes towards the formation of smog

How are chlorine radicals formed in the upper atmosphere?

energy from ultraivolet radiation causes Cl-Cl bonds in chlorofluorocarbons to break

What do chlorine free radicals cause?

• catalyse the decomposition of ozone as they’re regenerated

• contributes to the formation of a hole in the ozone layer

What are the 2 half equations for the break down of the ozone layer?

• Cl. + O3 → ClO. + O2

• ClO. + O3 → 2O2 + Cl.

What is the overall equation for the breakdown of the ozone layer?

2O3 → 3O2