14.9: Halogen-Containing Compounds

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Learning Objectives

Identify and name a simple alkyl halide.

The presence of a halogen atom (F, Cl, Br, or I; also, X is used to represent any halogen atom) is one of the simplest functional groups. Organic compounds that contain a halogen atom are called alkyl halides. We have already seen some examples of alkyl halides when the addition of halogens across double and triple bonds was introduced in Section 16.3 - "Branched Hydrocarbons;" the products of these reactions were alkyl halides.

A simple alkyl halide can be named like an ionic salt, first by stating the name of the parent alkane as a substituent group (with the -yl suffix) and then the name of the halogen as if it were the anion. So CH₃Cl has the common name of methyl chloride, while CH₃CH₂Br is ethyl bromide and CH₃CH₂CH₂I is propyl iodide. However, this system is not ideal for more complicated alkyl halides.

The systematic way of naming alkyl halides is to name the halogen as a substituent, just like an alkyl group, and use numbers to indicate the position of the halogen atom on the main chain. The name of the halogen as a substituent comes from the stem of the element's name plus the ending -o, so the substituent names are fluoro-, chloro-, bromo- and iodo-. If there is more than one of a certain halogen, we use numerical prefixes to indicate the number of each kind, just as with alkyl groups. For example, this molecule

Structure of 2-bromobutane.

is 2-bromobutane, while this molecule

Structure of 2,3-dichloropentane.

is 2,3-dichloropentane. If alkyl groups are present, the substituents are listed alphabetically. Numerical prefixes are ignored when determining the alphabetical ordering of substituent groups.

Example \(\PageIndex{1}\)

Name this molecule.

A five carbon chain with a methyl group on the third carbon and two chlorines, one each on the second and third carbons.

Solution

The longest carbon chain has five C atoms, so the molecule is a pentane. There are two chlorine substituents located on the second and third C atoms, with a one-carbon methyl group on the third C atom as well. The correct name for this molecule is 2,3-dichloro-3-methylpentane.

Exercise \(\PageIndex{1}\)

Name this molecule.

A three carbon chain with two bromines on the first carbon and another bromine on the second carbon.

Answer: 1,1,2-tribromopropane

Most alkyl halides are insoluble in H₂O. Smaller alcohols, however, are very soluble in H₂O because these molecules can engage in hydrogen bonding with H₂O molecules. For larger molecules, however, the polar OH group is overwhelmed by the nonpolar alkyl part of the molecule. While methanol is soluble in H₂O in all proportions, only about 2.6 g of pentanol will dissolve in 100 g of H₂O. Larger alcohols have an even lower solubility in H₂O.

One reaction common to alcohols and alkyl halides is elimination, the removal of the functional group (either X or OH) and an H atom from an adjacent carbon. The general reaction can be written as follows:

where Z represents either the X or the OH group. The biggest difference between elimination in alkyl halides and elimination in alcohols is the identity of the catalyst: for alkyl halides, the catalyst is a strong base; for alcohols, the catalyst is a strong acid. For compounds in which there are H atoms on more than one adjacent carbon, a mixture of products results.

Example \(\PageIndex{3}\)

Predict the organic product(s) of this reaction.

2-propanol reacts with an acid.

Solution

Under these conditions, an HOH (otherwise known as H₂O) molecule will be eliminated, and an alkene will be formed. It does not matter which adjacent carbon loses the H atom; in either case the product will be

Structure of propene.

which is propene.

Exercise \(\PageIndex{3}\)

Predict the organic product(s) of this reaction.

2-chlorobutane reacts with a base.

Answer: 1-butene and 2-butene

Key Takeaways

Alkyl halides have a halogen atom as a functional group.
Alcohols have an OH group as a functional group.
Nomenclature rules allow us to name alkyl halides and alcohols.
In an elimination reaction, a double bond is formed as an HX or an HOH molecule is removed.