What is a neopentyl group?

Neopentyl or neopentyl is a substituent, group or alkyl radical, whose formula is (CH3)3CCH2-, and derives from neopentane, one of several structural isomers that alkanepentane possesses. It is one of the pentyl groups that is most found in nature or in countless organic compounds.
The prefix neo comes from the word ‘new’, perhaps alluding to the time of the discovery of this type of structure between the pentane isomers. Currently its use has been rejected by the IUPAC recommendations. however, it is still frequently used, as well as being applied for other similar alkyl substituents.

In the upper image we have the structural formula of neopentyl. Any compound that contains it, as a main part of its structure, can be represented by the general formula (CH3)3CCH2R, where R is an aliphatic side chain (although it can also be aromatic, Ar), a functional group, or a heteroatom.
If we change R to the hydroxyl group, we get neopentyl alcohol, (CH3)3CCH2OH. In the example of this alcohol, neopentyl covers almost the entire structure of the molecule; when it is not like this, it is said that it is a fraction or just a fragment of the molecule, for which reason it behaves simply as one more substituent.

Nomenclature and formation

Neopentyl is the common name by which this pentyl substituent is known. But according to the nomenclature rules governed by the IUPAC, its name comes to be 2,2-dimethylpropyl (right of the image). It can be seen that there are indeed two methyl groups attached to carbon 2, which together with the other two make up the carbon skeleton of propyl.

With that said, the name of neopentyl alcohol becomes 2,2-dimethylpropan-1-ol. For this reason, the prefix neo- continues in force, since it is much easier to name as long as it is clear what it is referring to specifically.

At the beginning it was also said that neopentyl derives from neopentane, or more correctly: from 2,2-dimethylpropane (left of the image), which looks like a cross. If one of its four CH3’s lost any of its hydrogens (circled in red), the neopentyl radical, (CH3)3CCH2·, would immediately form.

Once this radical is attached to a molecule, it will become the neopentyl group or substituent. In this sense, the formation of neopentyl is “easy”.