7 junio, 2024

Neopentil: structure, characteristics, nomenclature, formation

He neopentyl or neopentyl is an alkyl substituent, group or radical, whose formula is (CH3)3CCH2-, and is derived from neopentane, one of several structural isomers of alkanepentane. It is one of the pentyl groups that is most found in nature or in countless organic compounds.

The prefix new It comes from the word ‘new’, alluding perhaps to the time of the discovery of this type of structure among 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.

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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 were to lose 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».

Structure and characteristics

Neopentyl is a bulky substituent, even more so than tertbutyl; in fact, both share something in common, and that is that they have a 3rd carbon linked to three CH3. Ironically, neopentyl and tertbutyl are more similar than tertpentyl (1,1-dimethylpropyl) and tertbutyl.

The structure of tertbutyl is (CH3)3C-, while that of neopentyl is (CH3)3CCH2-; that is, they only differ because the latter has a methylene group, CH2, next to the 3rd carbon. Thus, neopentyl exhibits all of the bulky and structural character of tert-butyl, with a carbon chain longer by just one additional carbon atom.

Neopentyl is even bulkier and causes greater steric hindrance than tertbutyl. Its end resembles fan blades or a three-fingered leg, the CH and CC links of which vibrate and rotate. In addition to this, we must now add the proper rotations of CH2, which increases the molecular space that neopentyl monopolizes.

Apart from what has been explained, neopentyl shares the same characteristics as the other alkyl substituents: it is hydrophobic, nonpolar, and does not present unsaturations or aromatic systems. It is so voluminous that as a substituent it begins to wane; 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 (CH3)3CCH2R. Being such a large group, fewer examples are available where it is found as a substituent, or where it is considered a small fraction of a molecular structure.

halides

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

-FCH2C(CH3)3

-ClCH2C(CH3)3

-BrCH2C(CH3)3

-ICH2C(CH3)3

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

Neopentylamine

When R is replaced by OH, neopentyl alcohol is obtained; but if it is NH2 instead, then we will have neopentylamine (or 2,2-dimethylpropylamine), (CH3)3CCH2NH2. Once again, this compound is a liquid, and there is not much bibliographic information available about it that explains its properties.

neopentyl glycol

Neopentyl glycol, or 2,2-dimethylpropane-1,3-diol, is a unique case in which the pentyl group has two substitutions (top image). Note that the neopentyl is recognized in the center of the structure, in which another of the CH3 loses an H to link 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 more hydrogen bonds.

lithium neopentyl

Substituting R for a lithium atom we obtain an organometallic compound called neopentyl lithium, C5H11Li or (CH3)CCH2Li (upper image), in which a covalent C-Li bond stands out.

Neopentyl tosylate

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

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

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

Finally, we have another unusual example: neopentyl 2,3,4,5,6-pentafluorobenzoate (image above). In it, again, the neopentyl group is found as a substituent, with 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 “paw or tail” appearance.

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

References

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

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