Alkyl Group | Definition, Characteristics & Nomenclature – Video & Lesson Transcript | Study.com

Video Transcript

What Is an Alkyl Group?

What do the words alkylation, alkylate, and alkyl halide all have in common? Each contain the root word, ‘alkyl.’ From refining petroleum using the alkylation mechanism to binding with a halide group, alkyl molecules are widely found throughout organic chemistry.

The alkyl group is a type of functional group that has a carbon and hydrogen atom present in its structure. The general formula for an alkyl group is CnH2n+1, where n represents a number or integer. Wait a minute; you may be wondering what a functional group is? That’s a great question! A functional group is a group of atoms that are easily identified in a given compound. Let’s say you’re looking at the compound p-tert-butyl-cyclohexanol, shown here.

Example of the Compound P-Tert_Butyl-Cyclohexanol

The group of carbon and hydrogen atoms, termed butyl, represent the alkyl functional group for this compound. It’s worth noting that the alkyl group has a relative called alkane. Alkane is a functional group that has the general formula of CnH2n+2. As a cousin to the alkyl group, alkanes are different in that they’re missing one hydrogen atom from their chain. Here, you’ll see the difference between both functional groups.

Example of an Alkane and Alkyl Molecule

Although you should be careful not to confuse these relatives, it is helpful to know that alkanes can be used to not only create the name of alkyl group molecules but also determine their structure.

Naming an Alkyl Group

In organic chemistry, there’s a distinct set of rules you must follow when naming compounds. The nomenclature, or rulebook, used for naming molecules is called IUPAC, or the International Union of Pure and Applied Chemistry. If we start with alkane nomenclature you will see how easy it is to name alkyl molecules. Just remember that alkane and alkyl are two different functional groups based on the removal of one hydrogen atom.

With all functional groups, the first process in naming involves determination of the prefix. This is based on how many carbon atoms are present in the longest chain of a compound. Also, prefix is based on carbon count. Once you determine your prefix name, next up is the suffix ending. In the case of alkane functional groups, the suffix ending would be, ‘-ane.’ However, we are interested in the alkyl group. If an alkyl group is present, you simply change the alkane ending of ‘-ane’ to ‘-yl.’ Let’s look at this hexane molecule as an example.

Example of the Alkane Molecule Hexane

Can you identify what the prefix is? It’s, ‘hex-‘ because there are six carbon molecules present in the longest chain of this compound. What would the alkyl IUPAC name be? It would be hexyl. You can take the ending ‘-ane’ and drop it, attaching the ending, ‘-yl’ to form the word hexyl.

What Does an Alkyl Group Look Like?

Spotting an alkyl group is quite easy to do. If you can identify a chain of carbon and hydrogen atoms attached together in a portion of a molecule or compound, it’s most likely an alkyl group. Keep in mind that alkyl groups do not like to hang alone. Rather, they love to remain attached or branched to a larger group of atoms called the parent chain. This leads us to a great method for identifying the alkyl group.

First, locate the parent chain, which is the longest chain in a compound’s structure. Next, look for carbon and hydrogen atoms branching off of this parent chain. Those branched molecules that only contain carbon atoms that are fully saturated with hydrogen atoms are your alkyl groups. Saturation simply means you have a molecule that contains as many hydrogen atoms as possible. Look at these images of compounds containing alkyl groups.

Example One of an Alkyl Group in Acetonitrile

Example of an Alkyl Group in Propanoyl Chloride

Make sure you can easily identify the parent chain. If you can spot this chain, locating the alkyl group will be a breeze!

Lesson Summary

Alkyl groups are a type of functional group defined by the general formula of CnHn+1, where n equals a number or integer. Only carbon and hydrogen atoms are present in the structure of an alkyl group. When naming an alkyl group, the suffix to use is ‘-yl.’ Alkyl groups do not appear alone but rather are branched or attached to an existing parent chain. Identification of the alkyl group is achieved by locating the saturated branched molecule. That is, a molecule that has a carbon atom with as many hydrogen atoms attached as possible.