Preparation and physical properties of alkyl halides

PREPARATION AND PHYSICAL PROPERTIES OF ALKYL HALIDES

Key Notes

Preparation

Alkenes are converted to alkyl halides by reaction with hydrogen halides. Treatment with halogens results in dihaloalkanes. Tertiary alcohols can be converted to alkyl halides on treatment with hydrogen halides, whereas pri-mary and secondary alcohols are best converted by using thionyl chloride or phosphorus tribromide.

Structure

Alkyl halides consist of an alkyl group linked to a halogen. The carbon linked to the halogen is sp³ hybridized and tetrahedral. The carbon–halogen bond length increases and the bond strength decreases as the halogen increases in size.

Bonding

The C–halogen bond (C–X) is a polar σ bond where the halogen is slightly negative and the carbon is slightly positive. Intermolecular bonding is by weak van der Waals interactions.

Properties

Alkyl halides have a dipole moment. They are poorly soluble in water, but dissolve in organic solvents. They react as electrophiles at the carbon center.

Reactions

Alkyl halides undergo nucleophilic substitution reactions and elimination reactions.

Spectroscopic analysis

The presence of a halogen atom can be shown by IR spectroscopy (C–X stretching absorptions) as well as by mass spectroscopy. The latter shows a characteristic pattern of peaks for the molecular ion that matches the number and ratio of naturally occurring isotopes of the halogen. Elemental analysis also demonstrates the presence of halogens.

Preparation

Alkenes can be treated with hydrogen halides (HCl, HBr, and HI) or halogens (Cl₂ and  Br₂)  to  give  alkyl  halides  and  dihaloalkanes  respectively  . Anextremely useful method of preparing alkyl halides is to treat an alcohol with ahydrogen halide (HX = HCl, HBr, or HI). The reaction works best for tertiaryalcohols. Primary and secondary alcohols can be converted to alkylhalides more effectively by treating them with thionyl chloride (SOCl₂) or phosphorus tribromide (PBr₃). The conditions are less acidic and less likely to cause acid-catalyzed rearrangements.

Structure

Alkyl halides consist of an alkyl group linked to a halogen atom (F, Cl, Br, or I) by a single (σ) bond. The carbon atom linked to the halogen atom is sp³  hybridized and has a tetrahedral geometry with bond angles of approximately 109 . The carbon–halogen bond length increases with the size of the halogen atom and this is associated with a decrease in bond strength. For example, C–F bonds are shorter and stronger than C–Cl bonds.

Bonding

The carbon–halogen bond (referred to as C–X from here on) is a bond. The bond is polar since the halogen atom is more electronegative than carbon, resulting in The halogen being slightly negative and the carbon being slightly positive. Intermolecular hydrogen bonding or ionic bonding is not possible between alkyl halide molecules and the major intermolecular bonding force consists of weak van der Waals interactions.

Properties

The polar C–X bond means that alkyl halides have a substantial dipole moment. Alkyl halides are poorly soluble in water, but are soluble in organic solvents. They have boiling points which are similar to alkanes of comparable molecular weight. The polarity also means that the carbon is an electrophilic center and the halogen is a nucleophilic center. Halogens are extremely weak nucleophilic centers and therefore, alkyl halides are more likely to react as electrophiles at the carbon center.

Reactions

The major reactions undergone by alkyl halides are (a) nucleophilic substitution where an attacking nucleophile replaces the halogen (Fig. 1a), and (b) elimination where the alkyl halide loses HX and is converted to an alkene (Fig. 1b).

Spectroscopic analysis

The IR spectra of alkyl halides usually show strong C–X stretching absorptions. The position of these absorptions depends on the halogen involved i.e. the absorptions for C–F, C–Cl, C–Br and C–I occur in the regions 1400–1000, 800–600, 750–500 and 500 cm⁻¹ respectively.

The presence of a halogen can sometimes be implicated by the chemical shifts of neighboring groups in the nmr spectra. For example the chemical shifts in the ¹H nmr spectrum for CH₂I, CH₂Br and CH₂Cl are 3.2, 3.5 and 3.6 respectively.

Good evidence for the presence of a halogen comes from elemental analysis and mass spectroscopy. In the latter, there are characteristic peak patterns associated with particular halogens as a result of the natural abundance of various isotopes. For example, bromine has two naturally occurring isotopes of 79 and 81 that occur in a ratio of 1 : 1. This means that two peaks of equal intensity will be present for any organic compound containing bromine. For example, the mass spectrum for ethyl bromide has two peaks of equal intensity at m/e 108 and 110 for the molec-ular ions ¹²C₂¹H₅⁷⁹Br and ¹²C₂¹H₅⁸¹Br respectively.

In contrast, chlorine occurs naturally as two isotopes (³⁵Cl and ³⁷Cl ) in the ratio 3 : 1. This means that the mass spectrum of a compound containing a chlorine atom will have two peaks for the molecular ion. These peaks will be two mass units apart with an intensity ratio of 3 : 1.