Alkyl Halides

Many alkyl halides consist of alkane molecules with halogen atoms attached. Examples from every day life include Teflon (non-stick coatings), DDT (insecticide), CFCs (coolant for air conditioners), PCB's (fluid for electrical transformers).

A quick glance will show you that many of these compounds can be extemely toxic as well as extremely useful.

Physical Properties:

Due to the presence of the large halide ions, alkyl halides tend to be more polar than their alkane counterparts.

Consistent with this increased polarity, are their increased boiling points and increased ability to disolve in polar solvents, relative to hydrocarbons of similar size.

Uses and Applications:

Alyl halides are very useful in the synthesis of organic compounds.

Halogens, F, Cl, Br, and I readily substitute with the hydrogen atoms attached to an alkane chain in, order to form an alkyl halides. The pair of halogen atoms in a molecule "attack" an attached hydrogen in such a way that one halogen atom takes the place of the attached hydrogen and the other pairs with the released hydrogen to form a halogen halide.

CH₃-CH₃ + Cl₂ --> CH₃-CH₂-Cl + HCl

This new alkyl halide has a similar ability to "attack" attached hydrogens, only this time the halogen pairs of with the hydrogen to form a hydrogen halide and the alkyl group attaches itself to the point where the hydrogen atom was released.

C₆H₆ + CH₃-CH₂-Cl ---> C₆H₅-CH₂-CH₃ + HCl

By careful selection of the location of the halide ions on the alkane chains, we can build up the organic compounds we need for almost any purpose.

Laboratory Preparation:

Alkyl halides can be synthesized with relative ease using addition reactions with selected alkenes.

Halogonation

In the following reaction the double bond in the ethene molecule is opened up to allow absorbtion of an bromine atom (Br₂), by placing one Bromine ion on each side of the previous double carbon bond.

CH₂=CH₂ + Br₂ ā†’ CH₂Br-CH₂Br

Hydrohalogenation

This process describes the absorption of a hydrogen halide molecule, such as HBr or HCl by an alkene, again by the opening up of the double carbon carbon bond to allow in the hydrogen and halide ions.

CH₃-CH=CH₂ + HBr ā†’ CH₃-CHBr-CH₃

The placement of the halide ion is governed by Markovnikov's Rule which states that the hydrogen ion will be placed on the carbon which had the most hydrogens prior to the reaction.

Aromatic Halogenation

When benzine is exposed to a halogen in the presence of heat or UV light a substitution can take place, where one halogen ion replaces one hydrogen ion on the benzene ring. The other halogen atom then combines with the newly released hydrogen ion to form a halogen halide.

C₆H₆ + Br₂ --> C₆H₅Br + HBr

Bibliography:

Article on the reactions of alkyl halides

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