Chemical properties of haloalkanes

Introduction

Haloalkanes, also known as alkyl halides, exhibit diverse chemical properties due to the presence of a polar carbon-halogen bond (C-X). This bond makes them reactive towards nucleophiles, bases, and other reagents, leading to various substitution, elimination, and other reactions. Below are the major chemical properties of haloalkanes, with examples and equations.

Chemical reactions

i. Nucleophilic substitution reactions ($SN_1$ and $SN_2$)

Haloalkanes undergo nucleophilic substitution reactions where the halogen atom is replaced by a nucleophile. The reaction mechanism can follow:

1. $SN_1$ Mechanism: A two-step process involving the formation of a carbocation intermediate. Favored by tertiary haloalkanes.
2. $SN_2$ Mechanism: A one-step bimolecular process where the nucleophile attacks as the leaving group departs. Favored by primary haloalkanes.

Example: Reaction with aqueous alkali to form alcohols:

R-X + OH⁻ → R-OH + X⁻

e.g.,

CH₃CH₂Cl + NaOH → CH₃CH₂OH + NaCl

ii. Elimination reactions

Haloalkanes can undergo elimination reactions in the presence of strong bases, resulting in the formation of alkenes. This reaction is also called β-elimination or dehydrohalogenation.

Example: Reaction with ethanolic potassium hydroxide (KOH):

CH₃CH₂Cl + alc. KOH → CH₂=CH₂ + KCl + H₂O

The product depends on the structure of the haloalkane and follows Saytzeff's rule, favoring the formation of the more substituted alkene.

iii. Reaction with ammonia

Haloalkanes react with excess ammonia to form primary amines. The reaction involves a nucleophilic substitution where the halogen is replaced by an amino group (-NH₂).

R-X + NH₃ → R-NH₂ + HX

e.g.,

CH₃Cl + NH₃ → CH₃NH₂ + HCl

iv. Reaction with silver cyanide (AgCN)

Haloalkanes react with silver cyanide to form alkyl isocyanides (R-NC). This reaction proceeds via a nucleophilic substitution mechanism.

R-X + AgCN → R-NC + AgX

e.g.,

CH₃Br + AgCN → CH₃NC + AgBr

v. Reaction with sodium cyanide (NaCN)

Haloalkanes react with sodium cyanide to form alkyl cyanides (nitriles). The product contains a C≡N group and is useful in organic synthesis.

R-X + NaCN → R-C≡N + NaX

e.g.,

CH₃CH₂Br + NaCN → CH₃CH₂CN + NaBr

vi. Reaction with Grignard reagents

Haloalkanes react with magnesium in the presence of dry ether to form Grignard reagents (R-MgX). These are important intermediates in organic synthesis.

R-X + Mg → R-MgX

e.g.,

CH₃Br + Mg → CH₃MgBr

vii. Wurtz reaction

In the Wurtz reaction, haloalkanes react with sodium metal in dry ether to form higher alkanes. This reaction is useful for synthesizing symmetrical alkanes.

2R-X + 2Na → R-R + 2NaX

e.g.,

2CH₃Cl + 2Na → CH₃-CH₃ + 2NaCl

viii. Reaction with aqueous silver nitrate

Haloalkanes react with aqueous silver nitrate to form silver halide (AgX) and an alcohol. This reaction tests the reactivity of different halogens (Cl, Br, I).

R-X + AgNO₃ → R-OH + AgX

e.g.,

CH₃CH₂Br + AgNO₃ → CH₃CH₂OH + AgBr

ix. Reaction with halogens (Finkelstein reaction)

Haloalkanes undergo halogen exchange when reacted with a different halide ion (e.g., NaI in acetone). This is known as the Finkelstein reaction.

R-Cl + NaI → R-I + NaCl

e.g.,

CH₃CH₂Cl + NaI → CH₃CH₂I + NaCl