Halogenoalkanes

Introduction

Halogenoalkanes are a class of organic compounds containing a carbon-halogen bond. They are important intermediates in organic synthesis due to the reactivity of the carbon-halogen bond. The reactivity of halogenoalkanes depends on the halogen atom and the alkyl group attached to the carbon bearing the halogen.

Naming

Halogenoalkanes are named using the IUPAC nomenclature. The prefix "halo-" indicates the presence of a halogen. The halogen is named as a prefix (e.g., fluoro-, chloro-, iodo-, bromo-). The alkane part of the name indicates the alkyl group attached to the carbon bearing the halogen.

Example: 1-chloropropane

Properties

Physical Properties

Halogenoalkanes are typically liquids at room temperature. Their boiling points are higher than those of corresponding alkanes due to dipole-dipole interactions. The presence of a halogen also increases the intermolecular forces.

Chemical Properties

The reactivity of halogenoalkanes is determined by the carbon-halogen bond. The electronegativity of the halogen atom makes the carbon atom partially positive, making it susceptible to nucleophilic attack.

Table of Physical Properties

Compound	Molecular Weight (g/mol)	Physical State	Boiling Point (°C)
CH3F (Fluoroethane)	69.08	Gas	-31
CH3Cl (Chloroethane)	67.42	Liquid	-47
CH3Br (Bromoethane)	108.91	Liquid	-61
CH3I (Iodoethane)	146.90	Liquid	-7.2

Reactions

Nucleophilic Substitution (SN1 and SN2)

Halogenoalkanes undergo nucleophilic substitution reactions. These reactions can proceed via two main mechanisms: SN1 and SN2.

SN1 Mechanism

SN1 reactions occur with tertiary and secondary halogenoalkanes. The reaction proceeds in two steps:

1. The carbon-halogen bond breaks, forming a carbocation intermediate.
2. The nucleophile attacks the carbocation, forming the substitution product.

The rate of SN1 reactions depends only on the concentration of the substrate. The order of reactivity for alkyl halides in SN1 reactions is: Tertiary > Secondary > Primary.

SN2 Mechanism

SN2 reactions occur with primary and some secondary halogenoalkanes. The reaction occurs in one step:

1. The nucleophile attacks the carbon atom bearing the halogen from the backside.
2. The carbon-halogen bond breaks, and the halogen leaves as a halide ion.

The rate of SN2 reactions depends on the concentration of both the substrate and the nucleophile. The order of reactivity for alkyl halides in SN2 reactions is: Primary > Secondary > Tertiary.

Elimination Reactions (E1 and E2)

Halogenoalkanes can also undergo elimination reactions, forming alkenes. These reactions can proceed via two main mechanisms: E1 and E2.

E1 Mechanism

E1 reactions occur with tertiary and secondary halogenoalkanes under acidic conditions. The reaction proceeds in two steps:

1. The carbon-halogen bond breaks, forming a carbocation intermediate.
2. A proton is removed from a carbon atom adjacent to the carbocation, forming an alkene.

The rate of E1 reactions depends only on the concentration of the substrate. The order of reactivity for alkyl halides in E1 reactions is: Tertiary > Secondary > Primary.

E2 Mechanism

E2 reactions occur with primary and secondary halogenoalkanes in the presence of a strong base. The reaction occurs in one step:

1. The base removes a proton from a carbon atom adjacent to the carbon bearing the halogen.
2. The carbon-halogen bond breaks, and the halogen leaves as a halide ion, forming an alkene.

The rate of E2 reactions depends on the concentration of both the substrate and the base. The order of reactivity for alkyl halides in E2 reactions is: Tertiary > Secondary > Primary.

Hydrolysis

Tertiary halogenoalkanes react with water under acidic conditions to form alcohols and alkyl halides. This is an example of hydrolysis.

Factors Affecting Reactivity

The reactivity of halogenoalkanes depends on several factors:

• Halogen Atom: The reactivity of the halogen atom increases down the group (F < Cl < Br < I). Iodine is the least reactive, while fluorine is the most reactive.
• Substituent Effects: Electron-donating groups increase the reactivity of the halogenoalkane towards SN2 reactions, while electron-withdrawing groups decrease the reactivity.
• Reaction Mechanism: The reaction mechanism (SN1, SN2, E1, E2) determines the rate of the reaction.

Reaction Mechanisms

Applications

Halogenoalkanes are used as solvents, refrigerants, and intermediates in the synthesis of other organic compounds.