Classification and naming of halogen compounds

Start with methane

Mix methane with chlorine in sunlight. A new gas appears, and hydrogen chloride comes with it. One hydrogen atom of the methane has been replaced by a chlorine atom. The new gas is chloromethane, $\text{CH}_{3}\text{Cl}$.

A compound in which one or more hydrogen atoms of a hydrocarbon are replaced by halogen atoms (fluorine, chlorine, bromine or iodine) is a halogen derivative. We split the family in two:

• A haloalkane has the halogen on a carbon of a chain. Another name is alkyl halide. A-Level books write halogenoalkane.
• A haloarene has the halogen on a carbon of a benzene ring. Another name is aryl halide.

We write haloalkanes as $\text{R}\text{-}\text{X}$, where R is an alkyl group and X is a halogen. This lesson works with haloalkanes first. The haloarenes come in the third course of this chapter.

Sorting by the number of halogen atoms

One halogen atom gives a mono compound, two give a di compound, three a tri compound, and more give a poly compound.

• Mono: $\text{CH}_{3}\text{CH}_{2}\text{Cl}$, chloroethane.
• Di: $\text{CH}_{2}\text{Cl}_{2}$ and $\text{ClCH}_{2}\text{CH}_{2}\text{Cl}$.
• Tri: $\text{CHCl}_{3}$, the compound called chloroform.

Two halogen atoms can sit in two typical ways. In a gem-dihalide (gem means twin) both are on the same carbon, as in $\text{CH}_{3}\text{CHCl}_{2}$. In a vicinal dihalide (vicinal means neighbouring) they are on adjacent carbons, as in $\text{ClCH}_{2}\text{CH}_{2}\text{Cl}$.

Sorting by the carbon that holds the halogen

This is the more useful way. Look at the carbon bonded to the halogen and ask two questions. How is it hybridised? How many other carbons does it touch?

The halogen is on an sp3 carbon.

• Primary (1°): the carbon holds one other carbon. $\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{Cl}$ is an example.
• Secondary (2°): the carbon holds two other carbons, as in $\text{CH}_{3}\text{CHClCH}_{2}\text{CH}_{3}$.
• Tertiary (3°): the carbon holds three other carbons, as in $(\text{CH}_{3})_{3}\text{CCl}$.
• Allylic: the sp3 carbon is next to a C=C double bond, as in $\text{CH}_{2}=\text{CHCH}_{2}\text{Cl}$.
• Benzylic: the sp3 carbon is next to a benzene ring, as in $\text{C}_{6}\text{H}_{5}\text{CH}_{2}\text{Cl}$.

The halogen is on an sp2 carbon.

• Vinylic: the carbon is one of the two carbons of a C=C double bond, as in $\text{CH}_{2}=\text{CHCl}$.
• Aryl: the carbon is part of a benzene ring, as in $\text{C}_{6}\text{H}_{5}\text{Cl}$.

This sorting predicts behaviour. Primary, secondary and tertiary halides differ in how quickly they react, and you will see why in the next course. Vinylic and aryl halides are far less reactive than the others, because the halogen sits on an sp2 carbon.

Move the slider from 2 to 6. Four carbons give four chloroalkanes: two primary, one secondary and one tertiary. Look at 5 carbons. There are 8 structural isomers, but 11 once mirror-image forms are counted, because three of the eight have a carbon with four different groups.

Practise with new numbers

Try it in Python

The program sorts five haloalkanes. It finds the carbon that holds the halogen, counts its carbon neighbours and names the class. It also counts the hydrogen atoms on the neighbouring carbons. We shall need that count for elimination later.

Common names and IUPAC names

Two systems are in use.

Common names name the alkyl group, then the halide: $\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{Cl}$ is n-propyl chloride, $(\text{CH}_{3})_{2}\text{CHCl}$ is isopropyl chloride and $(\text{CH}_{3})_{3}\text{CBr}$ is tert-butyl bromide. They are short, and they are still used for simple compounds.

IUPAC names treat the halogen as a prefix on the parent alkane. The steps are these.

1. Find the longest carbon chain that holds the halogen atom. That is the parent alkane.
2. Number the chain from the end that gives the lowest locant to the first substituent met, whether it is a halogen or an alkyl group.
3. Write the substituents as prefixes in alphabetical order: bromo, chloro, fluoro, iodo, methyl and so on. Prefixes such as di and tri do not count when you sort.
4. If two numberings give the same set of locants, the substituent that comes first in the alphabet gets the lower number.

Four examples:

• $(\text{CH}_{3})_{2}\text{CHCH}_{2}\text{Cl}$ is 1-chloro-2-methylpropane. Its common name is isobutyl chloride.
• $\text{CH}_{3}\text{CH}_{2}\text{CHBrCH}_{3}$ is 2-bromobutane. Its common name is sec-butyl bromide.
• $(\text{CH}_{3})_{3}\text{CI}$ is 2-iodo-2-methylpropane, the tert-butyl iodide.
• $\text{CH}_{3}\text{CHClCH}_{2}\text{CHBrCH}_{3}$ is 2-bromo-4-chloropentane. Both ends give the locants 2 and 4, so the alphabet decides: bromo comes first and gets 2.

An unsaturated chain is numbered from the end that gives the double bond the lowest number. $\text{CH}_{2}=\text{CHCH}_{2}\text{Cl}$ is 3-chloroprop-1-ene, called allyl chloride in common use.

For haloarenes the halogen is a prefix on benzene. $\text{C}_{6}\text{H}_{5}\text{Cl}$ is chlorobenzene. With two substituents we use numbers or the old words ortho, meta and para. $\text{C}_{6}\text{H}_{4}\text{Cl}_{2}$ with the chlorine atoms next to each other is 1,2-dichlorobenzene, also called o-dichlorobenzene.

Compounds with special names

Some compounds are better known by a common name. You must know both.

• $\text{CH}_{2}\text{Cl}_{2}$ is dichloromethane, called methylene chloride.
• $\text{CHCl}_{3}$ is trichloromethane, called chloroform.
• $\text{CHI}_{3}$ is triiodomethane, called iodoform.
• $\text{CCl}_{4}$ is tetrachloromethane, called carbon tetrachloride.
• $\text{CH}_{2}=\text{CHCl}$ is chloroethene, called vinyl chloride.
• $\text{C}_{6}\text{H}_{5}\text{CH}_{2}\text{Cl}$ is (chloromethyl)benzene, called benzyl chloride.

Common mistakes

• Counting the halogen as a carbon. The class depends on the carbons that touch the carbon holding the halogen. The halogen does not count.
• Choosing the longest chain without the halogen. The chain must include the carbon that holds the halogen, if a choice exists.
• Numbering from the wrong end. The lowest locant goes to whichever substituent comes first from either end, not always to the halogen.
• Mixing up allylic and vinylic. In an allylic halide the halogen is one carbon away from the double bond. In a vinylic halide it is on the double bond.

Remember these

• A haloalkane has X on an sp3 carbon. A haloarene has X on a ring carbon.
• Primary, secondary and tertiary mean one, two or three carbon neighbours of the carbon that holds X.
• Allylic and benzylic halides have an sp3 carbon next to C=C or a ring. Vinylic and aryl halides have X on an sp2 carbon.
• A gem-dihalide has both halogens on one carbon. A vicinal dihalide has them on neighbouring carbons.
• In IUPAC names the halogen is a prefix, and prefixes go in alphabetical order.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Classify $(\text{CH}_{3})_{3}\text{CCH}_{2}\text{Br}$ as primary, secondary or tertiary.

2. Name $(\text{CH}_{3})_{2}\text{CHCH}_{2}\text{CH}_{2}\text{Br}$.

3. Which is allylic and which is vinylic: $\text{CH}_{3}\text{CH}=\text{CHCl}$ and $\text{CH}_{2}=\text{CHCH}_{2}\text{Br}$?

4. Why do 1,1-dichloroethane and 1,2-dichloroethane belong to different classes of dihalide?

References

• NCERT, Chemistry Part II for Class XII, the chapter Haloalkanes and Haloarenes, on classification and nomenclature.
• IUPAC naming of substituted alkanes: OpenStax, Organic Chemistry, Chapter 10 (Organohalides).