Halogenoalkanes

Based on original alkane, with a prefix indicating halogen atom: Fluoro for F; Chloro for Cl; Bromo for Br; Iodo for I. Substituents are listed alphabetically

Halogenoalkanes can be classified as primary, secondary or tertiary depending on the number of carbon atoms attached to the C-X functional group.

One carbon attached to the carbon atom adjoining the halogen

Two carbons attached to the carbon atom adjoining the halogen

Three carbons attached to the carbon atom adjoining the halogen

Halogenoalkanes undergo either substitution or elimination reactions

Substitution: swapping a halogen atom for another atom or groups of atoms Nucleophile: electron pair donator e.g. :OH-, :NH3, CN- :Nu represents any nucleophile – they always have a lone pair and act as electron pair donators

The nucleophiles attack the positive carbon atom carbon has a small positive charge because of the electronegativity difference between the carbon and the halogen curly arrow will always start from a lone pair of electrons or the centre of a bond

The weaker the bond, the easier it is to break and the faster the reaction. The iodoalkanes are the fastest to substitute and the fluoroalkanes are the slowest. The strength of the C-F bond is such that fluoroalkanes are very unreactive

Hydrolysis is defined as the splitting of a molecule ( in this case a halogenoalkane) by a reaction with water Water is a poor nucleophile but it can react slowly with halogenoalkanes in a substitution reactio

Aqueous silver nitrate is added to a halogenoalkane. The halide leaving group combines with a silver ion to form a silver halide precipitate. The precipitate only forms when the halide ion has left the halogenoalkane and so the rate of formation of the precipitate can be used to compare the reactivity of the different halogenoalkanes. CH3CH2X + H2O  CH3CH2OH + X- + H+

CH3CH2I + H2O —> CH3CH2OH + I- + H+ Ag+ (aq) + I- (aq) —> AgI (s) - yellow precipitate The iodoalkane forms a precipitate with the silver nitrate first as the C-I bond is weakest and so it hydrolyses the quickest

The quicker the precipitate is formed, the faster the substitution reaction and the more reactive the halogenoalkane. The rate of these substitution reactions depends on the strength of the C-X bond. The weaker the bond, the easier it is to breal and the faster the reaction.

Change in functional group: halogenoalkane —> alcohol Reagent: potassium (or sodium) hydroxide Conditions: In aqueous solution; warm Mechanism: Nucleophilic substitution Type of reagent: Nucleophile, OH- The aqueous conditions needed is an important point. If the solvent is changed to ethanol an elimination reaction occurs.

Tertiary halogenoalkanes undergo this mechanism as the tertiary carbocation is stabilised by the electron releasing methyl groups around it. Also the bulky methyl groups prevent the hydroxide ion from attacking the halogenoalkane in the same way as the mechanism above. The Br first breaks away from the halogenoalkane to form a carbocation intermediate. The hydroxide nucleophile then attacks the positive carbon

Change in functional group: halogenoalkane —> nitrile Reagent: KCN dissolved in ethanol/water mixture Conditions: Heating under reflux Mechanism: Nucleophilic substitution Type of reagent: Nucleophile, :CN- This reaction increases the length of the carbon chain (which is reflected in the name) In the above example butanenitrile includes the C in the nitrile group

Nitrile groups have to be at the end of a chain. Start numbering the chain from the C in the CN.

Change in functional group: halogenoalkane —> amine Reagent: NH3 dissolved in ethanol Conditions: Heating under pressure (in a sealed tube) Mechanism: Nucleophilic substitution Type of reagent: Nucleophile, :NH3

In the above example propylamine, the propyl shows the 3 C’s of the carbon chain. Sometimes it is easier to use the IUPAC naming for amines e.g. Propan-1-amine

Further substitution reactions can occur between the halogenoalkane and the amines formed leading to a lower yield of the amine. Using excess ammonia helps minimise this.

Elimination: removal of small molecule (often water) from the organic molecule Change in functional group: halogenoalkane —> alkene Reagents: Potassium (or sodium) hydroxide Conditions: In ethanol ; heat Mechanism: Elimination Type of reagent: Base, OH-

Aqueous - substitution Alcoholic - elimination The structure of the halogenoalkane also has an effect on the degree to which substitution or elimination occurs in this reaction. Primary tends towards substitution Tertiary tends towards elimination

Chloroalkanes and chlorofluoroalkanes can be used as solvents. CH3CCl3 was used as the solvent in dry cleaning. Halogenoalkanes have also been used as refrigerants, pesticides and aerosol propellants Many of these uses have now been stopped due to the toxicity of halogenoalkanes and also their detrimental effect on the atmosphere.

The naturally occurring ozone (O3) layer in the upper atmosphere is beneficial as it filters out much of the sun’s harmful UV radiation. Ozone in the lower atmosphere is a pollutant and contributes towards the formation of smog.