NEOPENTIL: STRUCTURE, CHARACTERISTICS, NOMENCLATURE, FORMATION - CHEMISTRY - 2024

The neopentyl or neopentyl is a substituent group or alkyl moiety, whose formula is (CH ₃) ₃ CCH ₂ -, and neopentane drift, one of several structural isomers having alkane pentane. 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 referring to the time of the discovery of this type of structure among the isomers of pentane. Currently its use has been rejected by IUPAC recommendations; however, it is still used frequently, in addition to being applied for other similar alkyl substituents.

Structural formula of neopentyl. Source: Benjah-bmm 27.

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 with the general formula (CH ₃) ₃ CCH ₂ R, where R is an aliphatic side chain (although it can also be aromatic, Ar), a functional group, or a heteroatom.

If we change R for the hydroxyl group, we get neopentyl alcohol, (CH ₃) ₃ CCH ₂ OH. In the example of this alcohol, the neopentyl encompasses almost the entire structure of the molecule; when this is not the case, it is said to be a fraction or just a fragment of the molecule, so it behaves simply as one more substituent.

Nomenclature and training

Formation of neopentyl from neopentane. Source: Gabriel Bolívar via Mol View.

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

At first 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 CH _{3 were to} lose any of its hydrogens (in red circles), the neopentyl radical, (CH ₃) ₃ CCH ₂ · would immediately form.

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

Structure and characteristics

Neopentyl is a bulky substituent, even more so than tert-butyl; in fact, both share something in common, and that is that they have a 3rd carbon linked to three CH _3s. Ironically, neopentyl and tert-butyl are more similar than tertpentyl (1,1-dimethylpropyl) and tert-butyl.

The structure of tert-butyl is (CH ₃) ₃ C-, while that of neopentyl is (CH ₃) ₃ CCH ₂ -; that is, they only differ because the latter has a methylene group, CH ₂, next to the 3 ° carbon. Thus, neopentyl exhibits all the bulky and structural character of terbutyl, with a carbon chain longer by just one additional carbon atom.

Neopentyl is even more bulky and causes greater steric hindrance than tert-butyl. Its end resembles the blades of a fan or a three-toed leg, whose CH and CC links vibrate and rotate. In addition to this, we must now add the proper rotations of CH ₂, which increases the molecular space that the neopentyl occupies.

Apart from what has been explained, neopentyl shares the same characteristics as the other alkyl substituents: it is hydrophobic, apolar, and does not present unsaturations or aromatic systems. It is so voluminous that as a substituent it begins to diminish; that is to say, it is not as frequent to find it as it happens with terbutyl.

Examples

Compounds with neopentyl are obtained by varying the identity of R in the formula (CH ₃) ₃ CCH ₂ R. Being such a bulky group, there are fewer examples available in which it is found as a substituent, or where it is considered a fraction small of a molecular structure.

Halides

If we substitute a halogen atom for R, we will obtain a neopentyl (or 2,2-dimethylpropyl) halide. Thus, we have neopentyl fluoride, chloride, bromide and iodide:

-FCH ₂ C (CH ₃) ₃

-ClCH ₂ C (CH ₃) ₃

-BrCH ₂ C (CH ₃) ₃

-ICH ₂ C (CH ₃) ₃

All of these compounds are liquids, and are probably used as halogen donors for certain organic reactions, or to perform quantum chemistry calculations or molecular simulations.

Neopentylamine

When R is substituted for OH, the neopentyl alcohol is obtained; but if it is NH ₂ instead, then we will have neopentylamine (or 2,2-dimethylpropylamine), (CH ₃) ₃ CCH ₂ NH ₂. Again, this compound is a liquid, and there is not much bibliographic information available to explain its properties.

Neopentyl glycol

Structural formula of neopentyl glycol. Source: Emeldir via Wikipedia.

Neopentyl glycol, or 2,2-dimethylpropane-1,3-diol, is a unique case in which the pentyl group has two substitutions (upper image). Note that the neopentyl is recognized in the center of the structure, in which another of the CH ₃ loses an H to bind to a second OH group, thus differentiating itself from neopentyl alcohol.

As expected, this compound has stronger intermolecular interactions (a higher boiling point) because it can establish a greater number of hydrogen bonds.

Lithium neopentyl

Structural formula of lithium neopentyl. Source: Edgar 181 via Wikipedia.

Substituting R for a lithium atom we obtain an organometallic compound called lithium neopentyl, C ₅ H ₁₁ Li or (CH ₃) CCH ₂ Li (upper image), in which a C-Li covalent bond stands out.

Neopentyl Tosylate

Neopentyl tosylate molecule. Source: Gabriel Bolívar via Mol View.

In the upper image we have the neopentyl tosylate molecule, represented with a model of spheres and bars. This is an example where R is an aromatic segment, Ar, consisting of the tosylate group, which is toluene with a sulfate in the -para position. It is a rare compound in the literature.

In neopentyl tosylate the neopentyl does not take precedence, but is found as a substituent, and not the central part of the structure.

Neopentyl 2,3,4,5,6-pentafluorobenzoate

Neopentyl 2,3,4,5,6-pentafluorobenzoate. Source: Gabriel Bolívar via Mol View.

Finally, we have another unusual example: neopentyl 2,3,4,5,6-pentafluorobenzoate (upper image). In it, again, the neopentyl group is found as a substituent, the benzoate, together with the five fluorine atoms linked in the benzene ring, being the essential part of the molecule. Note how neopentyl is recognized by its "leg or cross" appearance.

Neopentyl compounds, being this very bulky, are less abundant compared to others with smaller alkyl substituents; such as methyl, cyclobutyl, isopropyl, tert-butyl, etc.

References

1. Morrison, RT and Boyd, R, N. (1987). Organic Chemistry. 5 ^ta Edition. Editorial Addison-Wesley Interamericana.
2. Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
3. Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. (10 ^th edition.). Wiley Plus.
4. Steven A. Hardinger. (2017). Illustrated Glossary of Organic Chemistry: Neopentyl group. Recovered from: chem.ucla.edu
5. Wikipedia. (2020). Pentyl Group. Recovered from: en.wikipedia.org