19 julio, 2024

Secbutil: structure, characteristics, nomenclature, formation

He secbutyl or secbutyl is a group, radical or alkyl substituent, which derives from no-butane, the straight-chain structural isomer of butane. It is one of the butyl groups, along with isobutyl, tertbutyl, and no-butyl, being of all of them the one that most often goes unnoticed by those who study organic chemistry for the first time.

This is because secbutyl causes confusion when trying to memorize it, or identify it, by looking at any structural formula. Therefore, more importance is given to tertbutyl or isobutyl groups, which are easier to understand. However, knowing that it derives from no-butane, and with a little practice, you learn to take it into account.

In the upper image you can see the secbutyl linked to an R side chain. Note that it is like having a group no-butyl, but instead of joining R by the carbon atoms at its ends, it is linked to the penultimate carbons, which are 2nd. This is, then, where the prefix comes from sec-, of secondary either secondaryalluding to this 2nd carbon.

A secbutyl compound can be represented by the general formula CH3CH(R)CH2CH3. For example, if R is an OH functional group, then we would have 2-butanol or secbutyl alcohol, CH3CH(OH)CH2CH3.

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Nomenclature and formation

secbutyl is known to be derived from no-butane, the linear isomer of butane, C4H10 (left of top image). Because R bonds to either of its two penultimate carbons, only carbons 2 and 3 are considered. These carbons have to lose one of their two hydrogens (circled in red), breaking the CH bond and forming the secbutyl radical.

Once this radical is incorporated or linked to another molecule, it will become the secbutyl group or substituent (right of the image).

Note that R could be attached to either carbon 2 or carbon 3, both positions being equivalent; that is, there would be no structural difference in the result, apart from the fact that the two carbons are secondary or 2nd. Thus, placing R on carbon 2 or 3, the compound of the first image will be generated.

On the other hand, it is worth mentioning that the name ‘secbutyl’ is the one by which this group is best known; however, according to systematic nomenclature, its correct name is 1-methylpropi. or 1-methylpropyl. That’s why the secbutyl carbons in the image above (on the right) are numbered up to 3, with carbon 1 being the one that bonds to R and to CH3.

Structure and characteristics

Structurally the no-butyl and secbutyl are the same group, the only difference being that they are attached to R with different carbons. He no-butyl is bonded using primary or 1st carbons, while secbutyl is bonded to a secondary or 2nd carbon. Outside of this, both are linear and similar.

secbutil is confusing the first few times because its 2D representation is not easy to memorize. Instead of trying the method of having it in mind as a striking figure (a cross, a Y, or a leg or fan), it is enough to simply know that it is exactly the same as the no-butyl, but attached with a 2nd carbon.

Thus, when you see any structural formula, and see a linear butyl chain, you will know at once whether it is the no-butyl or secbutyl.

secbutyl is slightly less bulky than no-butyl, since its bond on the 2nd carbon prevents it from covering more molecular space. One consequence of this is that its interactions with another molecule are less efficient; therefore, a compound with the secbutyl group is expected to have a lower boiling point than one with the group no-butyl.

Otherwise, secbutyl is hydrophobic, nonpolar, and a group with no unsaturations.

examples

Changing the identity of R, in the compound CH3CH(R)CH2CH3, by any functional group, heteroatom, or a given molecular structure, different examples of compounds containing secbutyl are obtained.

halides

When we replace R with a halogen atom X, we have a secbutyl halide, CH3CH(X)CH2CH3. In the upper image we see, for example, secbutyl chloride or 2-chlorobutane. Note that below the chlorine atom is the linear chain of butane, but attached to the chlorine with a secondary carbon. The same observation applies to secbutyl fluoride, bromide, and iodide.

secbutyl alcohol

Now substituting OH for R we get secbutyl alcohol or 2-butanol (above). Again, we have the same observation as for secbutyl chloride. This representation is equivalent to that of the first image, differing only in the orientation of the structures.

The boiling point of secbutyl alcohol is 100ºC, while that of butyl alcohol (or 1-butanol) is almost 118ºC. This partly demonstrates what was commented in the structures section: that intermolecular interactions are weaker when secbutyl is present, compared to those found with no-butyl.

secbutylamine

Similar to secbutyl alcohol, substituting NH2 for R we have secbutylamine or 2-butanamine.

secbutyl acetate

Substituting R for the acetate group, CH3CO2, we have secbutyl acetate (top image). Note that secbutyl is represented not linear but with folds; however, it is still linked to the acetate with a 2nd carbon. The secbutyl begins to lose chemical importance because it is linked to an oxygen group.

So far the examples have consisted of liquid substances. The following will deal with solids where secbutyl is just a fraction of the molecular structure.

Bromacil

In the image above we have bromacil, a herbicide used to combat weeds, whose molecular structure contains secbutyl (on the right). Notice that the secbutyl now looks small next to the uracil ring.

Pravastatin

Finally, in the image above we have the molecular structure of pravastatin, a drug used to reduce cholesterol levels in the blood. Can the secbutyl group be located? First find out if there is a four-carbon chain, which is located to the left of the structure. Second, note that it bonds to a C=O using a 2nd carbon.

Again, in pravastatin the secbutyl looks small compared to the rest of the structure, so it is said to be a simple substituent.

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: Sec-butyl. Retrieved from: chem.ucla.edu
James Ashenhurst. (2020). Don’t Be Futyl, Learn The Butyls. Recovered from: masterorganicchemistry.com
Wikipedia. (2020). Category:Sec-butyl compounds. Retrieved from: commons.wikimedia.org

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