Haloalkanes

1. Introduction

Haloalkanes (alkyl halides) are compounds formed by replacing one or more hydrogen atoms of an alkane by halogen atoms ($\ce{-F, -Cl, -Br, -I}$). A monohaloalkane contains one halogen: general formula $\ce{C_nH_{2n+1}X}$, i.e. $\ce{R-X}$. Examples: chloromethane $\ce{CH3Cl}$, bromoethane $\ce{C2H5Br}$.

The $\ce{C-X}$ bond is polar ($\ce{C^{\delta+}-X^{\delta-}}$) because halogens are more electronegative than carbon. The electron-poor carbon is attacked by nucleophiles, so nucleophilic substitution is the characteristic reaction of haloalkanes.

2. Nomenclature, Isomerism and Classification

2.1 Classification

By the carbon bearing the halogen:

• Primary (1 degree): halogen carbon joined to one other carbon, e.g. $\ce{CH3CH2Cl}$.
• Secondary (2 degree): joined to two carbons, e.g. $\ce{(CH3)2CHCl}$.
• Tertiary (3 degree): joined to three carbons, e.g. $\ce{(CH3)3CCl}$.

2.2 Isomerism

• Chain isomerism (1-chlorobutane vs 1-chloro-2-methylpropane).
• Position isomerism (1-chloropropane vs 2-chloropropane).
• Optical isomerism when the halogen carbon is a chiral centre (e.g. 2-chlorobutane).

2.3 Nomenclature

IUPAC: name as haloalkane, halogen as a prefix (chloro-, bromo-) with the lowest locant.

3. Preparation of Monohaloalkanes

3.1 From alkanes (free-radical halogenation)

$\ce{CH4 + Cl2 ->[hv] CH3Cl + HCl}$

(gives a mixture; poor control).

3.2 From alkenes (addition of HX; Markovnikov)

$\ce{CH3-CH=CH2 + HBr -> CH3-CHBr-CH3}$

(hydrogen adds to the carbon with more hydrogens; the halogen goes to the more substituted carbon).