Haloalkanes

Definition of haloalkanes:

Haloalkanes are also known as halogenalkanes and (less commonly) as alkyl halides.

More about Haloalkanes

Alkanes are one of the simplest types of organic compounds and consist only of carbon and hydrogen atoms linked together exclusively by single bonds (i.e. alkanes are saturated hydrocarbons). Haloalkanes are similar to alkanes but one or more of the hydrogen atoms in the corresponding alkane is replaced by a halogen atom(s) in the haloalkane.

What are halogens ?

Halogens are the elements in Group 7 (Group VII) of the periodic table, specifically: Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I) and Astatine (At). The halogens most commonly found in, and of greatest importance in organic chemistry, are fluorine, chlorine, bromine and iodine.

Is there a general formula for haloalkanes ?

A simple general formula that describes many (but not all) of the haloalkanes usually included in basic chemistry courses is :

C_nH_2n+1X

where the letter n represents the number of carbon atoms in each molecule of the compound and the letter X represents a particular halogen atom. An example of a real chemical described by this formula is fluoromerhane (also known as methyl fluoride), whose molecules have just one carbon atom (so n=1) and includes the halogen fluorine (so X=F), hence it has the chemical formula CH₃F.

Examples of Haloalkanes

The simplest way to convey what haloalkanes are and how they differ from alkanes is using molecular diagrams of some simple haloalkanes together with the corresponding alkane, for comparison. (The names of the chemicals whose structures are shown below are explained further down this page.)

	Example of Haloalkane		Corresponding Alkane
1	Fluoromethane (also known as methylfluoride and as methyl fluoride)		Methane
2	Bromoethane (also known as ethyl bromide)		Ethane
3	Chloropropane (also known as 1-chloropropane)		Propane
4	Iodobutane (also known as 1-iodobutane and as n-butyl iodide)		Butane

The above molecular structures have been drawn was simply as possible.
In all cases it is easy to see that the hydrogen atom represented by the H on the extreme right-hand-side of the alkane on the right has been replaced by the halogen atom in each of the haloalkanes on the left. As indicated by these examples, simple haloalkanes are named according to the specific halogen element (F, Cl, Br or I) in the molecule and the number of carbon atoms (using the same system of naming as applies to alkanes). More information about structures, names and classification of haloalkanes follows below.

The simple molecular structures shown above all represent the halogen atoms as being located on the right-hand-side of the molecule. This is just to make clear the pattern and difference between these haloalkanes and their corresponding linear alkanes. In the case of halomethanes and haloethanes, all the possible positions at which the halogen atom may be attached to the rest of the molecule are equivalent, that is - there is only one structural isomer of halomethanes and haloethanes.
This shown in the diagram below of all the possible ways of drawing the four halomethane molecules CH₃F, CH₃Cl, CH₃Br and CH₃I.

Name of Halomethane	Four different ways to represent exactly the same molecular structure:
fluoromethane CH3F
chloromethane CH3Cl
bromomethane CH3Br
iodomethane CH3I
Note: The halogen atoms appear in green (above) for clarity.

However, in the cases of the larger haloalkanes, different chemicals result from arranging the atoms in different ways.
The following diagrams show, as examples, the two structural isomers of bromopropane:

6 different ways to draw 1-bromopropane (one of the structural isomers of bromopropane) Note that all of the representations on the right are equivalent: They are all ways of describing, via simple 2-dimensional simplifications, the same 3-dimensional molecule.
2 different ways to draw 2-bromopropane (the other structural isomer of bromopropane)
The 2 diagrams of 2-bromopropane (above) are equivalent to each other and different from the six diagrams of 1-bromopropane (above-top).

This simple example shows that it matters exactly how the atoms that form haloalkane molecules are arranged.

The simplest examples of structural isomerism of haloalkanes are the halopropanes, of which bromopropane (above) is an example. The larger the haloalkane molecule, the more different ways in which the atoms can be arranged to form molecules.

Classification of Haloalkanes

There are two main ways of classifying haloalkanes.
They are, according to:

• the type of halogen (e.g. fluorine, chlorine, bromine and iodine), or
• the structure of the molecules (the position of the csarbon atom(s) to which the halogen or halogens are attached).

1. Haloalkanes by Type of Halogen

Haloalkanes can be grouped, named, and described according to the particular halogen that has replaced a hydrogen atom in the corresponding alkane:

Halogen	Generic name of organic compounds that include this halogen (as a "functional group")	Examples of Haloalkanes: (simple examples, based on descriptions above)
Fluorine (F)	organofluorine compounds	fluoromethane fluoroethane 1-fluoropropane 2-fluoropropane
Chlorine (Cl)	organochlorine compounds	chloromethane chloroethane 1-chloropropane 2-chloropropane
Bromine (Br)	organobromine compounds	bromomethane bromoethane 1-bromopropane 2-bromopropane
Iodine (I)	organoiodine compounds	iodomethane iodoethane 1-iodopropane 2-iodopropane
More than one type of halogen: various combinations are possible. A well-known example is:
Fluorine (F) and Chlorine (Cl)	chlorofluorocarbon (CFC) compounds also known, simply as: chlorofluorocarbons (CFCs)	trichlorofluoromethane dichlorodifluoromethane chlorotrifluoromethane chlorodifluoromethane dichlorofluoromethane chlorofluoromethane

2. Haloalkanes by Structure (which carbon atom is the halogen attached to ?)

Haloalkanes are also grouped, named, and described according to the position in the molecule of the carbon atom(s) to which the halogen atom(s) is/are attached. Obviously there are a huge number of possibilities, especially in the cases of larger molecules. Only the simplest rules are mentioned here.

Primary, Secondary and Tertiary Haloalkanes
Structure	Description	Example of a molecular structure	Examples of other simple haloalkanes
Primary (1°) haloalkanes	The carbon atom to which the halogen atom is attached is only attached to one other carbon atom (or "alkyl group" = CH3).		1-fluoroethane 1-bromobutane 1-iodohexane 1-fluoropentane 1-chloro-methylpropane 1-iodo-methylbutane
Secondary (2°) haloalkanes	The carbon atom to which the halogen atom is attached is attached to two other carbon atoms, so forms two C-C (single) bonds.		2-chlorobutane 3-fluoropentane 2-iodoheptane 4-chloro-octane 2-fluoro-3-methylpentane 3-bromo-2-methylhexane
Tertiary (3°) haloalkanes	The carbon atom to which the halogen atom is attached, is attached to three other carbon atoms, so forms three C-C (single) bonds.		2-chloro-2-methylbutane 3-fluoro-3-methylpentane 3-iodo-3-methylheptane 2-fluoro-2-ethylpropane 2-fluoro-2-ethylhexane 4-iodo-4-methylheptane
Diagrams of Molecular Structures: The above examples of molecular structures are drawn out in detail for clarity. When describing increasing large and/or complicated structures it is common to use more compact representations, so each alkyl- group of a carbon atom attached to three hydrogen atoms can by written as simply CH3. Using this simpler representation the 3 examples above can be re-written as:
1-chloroethane 2-fluoropropane 2-bromo-2-methylpropane
Notes: This introduction to primary, secondary and tertiary haloalkanes has been kept simple by using only examples of molecules that contain ONE halogen atom. An obvious next question is, therefore: How are the haloalkanes grouped, named, and described when more than one halogen atom, possibly of more than one type of halogen e.g. chlorine and bromine form part of each molecule ? Organic Chemistry can become complicated rather quickly - check your course syllabus before getting too concerned about complexities.

Chemistry and Reactions of Haloalkanes (more advanced information)

The chemical reactions of haloalkanes generally involve the halogen part of the molecule (which distinguishes it from the corresponding alkane).

Haloalkanes are polar molecules, meaning that the carbon to which the halogen is attached is slightly electropositive and the halogen is slightly electronegative. This results in an electron deficient (electrophilic) carbon atom which, inevitably, attracts nucleophiles (electron-rich chemical reactants that are attracted by electron deficient compounds). Therefore haloalkanes tend to be reactive with nucleophiles, with which they undergo nucleophilic substitution reactions.

Common reactions of haloalkanes include:

• haloalkane with potassium hydroxide
• haloalkane with potassium cyanide
• haloalkane with ammonia
• haloalkane with a sodium salt of a carboxylic acid
• haloalkane with a alkoxide
• formation of Grignard reagents from haloalkanes

Relative Rates of Reactions:
Generally, iodoalkanes react more rapidly that bromoalkanes, which react more rapidly that chloroalkanes.

Note: This is one of many pages about types of organic compounds in our Organic Chemistry Section. Similar and related pages describe hydrocarbons, alkanes, haloalkanes, alkenes, alkynes, cycloalkanes, aromatic hydrocarbons (arenes), alcohols, esters, ethers, aldehydes, ketones, carboxylic acids, acid chlorides, acid anhydrides, azo dyes, amines, amides, nitriles, amino acids (chemistry), peptides, proteins (chemistry), polypeptides and others. If you need further information ask your chemistry tutor.