Halogenoalkanes Lecture Review

Vocabulary flashcards covering halogenoalkane definitions, reaction mechanisms, trends in bond strength and hydrolysis, and industrial uses.

Halogenoalkanes

Organic compounds where the hydrogen atom on an alkane has been replaced by a halogen atom.

Hydrolysis of Bromoethane

A fairly slow reaction given by $CH_3CH_2Br + H_2O \rightarrow CH_3CH_2OH + HBr$, where the resulting solution can be tested with $AgNO_3$ to determine the halogen present.

Nitrile Formation from Bromoethane

A nucleophilic substitution reaction where bromoethane is heated under reflux with $KCN$ in ethanol solvent: $CH_3CH_2Br + KCN \rightarrow CH_3CH_2CN + KBr$.

Primary Amine Formation from Bromoethane

A reaction sequence where bromoethane reacts with ammonia to form an alkyl ammonium salt ($CH_3CH_2Br + NH_3 \rightarrow CH_3CH_2NH_3Br$), which then reacts with excess ammonia to form a primary amine ($CH_3CH_2NH_3Br + NH_3 \rightarrow CH_3CH_2NH_2 + NH_4Br$).

Elimination Reaction of 2-Bromopropane

An elimination reaction where 2-bromopropane is refluxed with $KOH$ in ethanol (or conc. $NaOH$ solution) to form an alkene: $CH_3CHBrCH_3 + KOH \rightarrow CH_2=CHCH_3 + KBr + H_2O$.

$S_N1$ Nucleophilic Substitution Mechanism

A mechanism (e.g. $(CH_3)_3CCl + OH^- \rightarrow (CH_3)_3COH + Cl^-$) where in the first stage the halogenoalkane ionises to form the carbocation intermediate $(CH_3)_3C^+$ and $Cl^-$; the intermediate then immediately reacts with the hydroxide ion to form the product.

$S_N2$ Nucleophilic Substitution Mechanism

A one-step mechanism (e.g. $CH_3CH_2Br + OH^- \rightarrow CH_3CH_2OH + Br^-$) where the nucleophile attacks the carbon atom from the back side at the same time as the leaving group leaves, causing an inversion of the groups in the product.

Positive Inductive Effect

The effect created by electron-donating alkyl groups that push electrons towards the carbon atom they are bonded to, giving the alkyl group a slight positive charge and the carbon atom a slight negative charge.

Halogenoalkane Reaction Mechanisms by Class

Primary halogenoalkanes tend to react via the $S_N2$ mechanism, tertiary halogenoalkanes tend to react via the $S_N1$ mechanism, and secondary halogenoalkanes react via a mixture of the two depending on structure.

Relative Strengths of Carbon-Halogen Bonds

The carbon-halogen bond strength decreases going down Group 7 in the order $C-F > C-Cl > C-Br > C-I$.

Trend in Rate of Hydrolysis of Carbon-Halogen Bonds

The rate of hydrolysis of carbon-halogen bonds increases going down Group 7 because bond strength weakens down the group, requiring less energy to break the bond.

Uses of Fluoroalkanes and Fluorohalogenoalkanes

Chemically inert compounds that will not react with anything, used as refrigerants, in aerosols, and in making foamed plastics.