SECBUTIL: STRUCTURE, CHARACTERISTICS, NOMENCLATURE, FORMATION - CHEMISTRY - 2025

The secbutyl or sec. is a radical or substituent alkyl group derived from the n -butane, the structural isomer straight chain butane. It is one of the butyl groups, along with isobutyl, tert-butyl and n-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 secbutil causes confusion when trying to memorize it, or identify it, by observing any structural formula. Therefore, more importance is given to the terbutyl or isobutyl groups, which are easier to understand. However, knowing that it is derived from n-butane, and with a little practice, you learn to take it into consideration.

Structural formula of any compound with the sec-butyl substituent. Source: Pngbot via Wikipedia.

In the upper image you can see the sec-butyl linked to a side chain R. Note that it is like having an n-butyl group, but instead of joining R through the carbon atoms at its ends, it is linked to the penultimate carbons, which are 2nd. Hence, this is where the prefix sec -, from secondary or secondary, comes from, alluding to this 2nd carbon.

A sec-butyl compound can be represented by the general formula CH ₃ CH (R) CH ₂ CH ₃. For example, if R is an OH functional group, then we will have 2-butanol or sec-butyl alcohol, CH ₃ CH (OH) CH ₂ CH ₃.

Nomenclature and training

Formation of sec-butyl from n-butane. Source: Gabriel Bolívar via Mol View.

Secbutyl is known to be derived from n -butane, the linear isomer of butane, C ₄ H ₁₀ (left of upper image). Because R binds to either of its two penultimate carbons, only carbons 2 and 3 are considered. These carbons have to lose one of their two hydrogens (in red circles), 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 linked to either carbon 2 or carbon 3, both positions being equivalent; that is, there would be no structural difference in the result, in addition to the fact that the two carbons are secondary or 2nd. Thus, by placing R on carbon 2 or 3, the compound in the first image will be created.

On the other hand, it is worth mentioning that the name 'secbutyl' is by which this group is best known; however, according to systematic nomenclature, its correct name is 1-methylpropyl. or 1-methylpropyl. This is why the secbutyl carbons in the image above (right) are numbered through 3, with carbon 1 being the one that binds to R and a CH ₃.

Structure and characteristics

Structurally, n -butyl and secbutyl are the same group, with the only difference that they are linked to R with different carbons. The n -butyl bonds using primary or 1st carbons, while secbutyl bonds to a secondary or 2nd carbon. Outside of this, they are both linear and similar.

The secbutil is a cause for confusion the first few times because its 2D representation is not easy to memorize. Instead of trying the method of having it in mind as an eye-catching figure (a cross, a Y, or a leg or fan), just know that it is exactly the same as n-butyl, but bonded with a 2nd carbon.

Thus, when any structural formula is seen, and a linear butyl chain is appreciated, it will be possible to differentiate at once whether it is n-butyl or sec-butyl.

The sec-butyl is a little less bulky than the n-butyl, since its bond at 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 sec-butyl group is expected to have a lower boiling point than one with the n-butyl group.

Otherwise secbutyl is hydrophobic, apolar, and a non-unsaturated group.

Examples

By changing the identity of R, in the compound CH ₃ CH (R) CH ₂ CH ₃, by any functional group, heteroatom, or a given molecular structure, different examples of compounds containing secbutyl are obtained.

Halides

Structural formula of sec-butyl chloride. Source: Dschanz / Public domain

When we substitute a halogen atom X for R, we have a secbutyl halide, CH ₃ CH (X) CH ₂ CH ₃. In the image above we see, for example, sec-butyl chloride or 2-chlorobutane. Note that below the chlorine atom is the straight chain of butane, but attached to the chlorine with a secondary carbon. The same observation applies to sec-butyl fluoride, bromide, and iodide.

Sec-butyl alcohol

Structural formula of sec-butyl alcohol or 2-butanol. Source: Kado6450 / Public domain

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

The boiling point of sec-butyl alcohol is 100ºC, while that of butyl alcohol (or 1-butanol) is almost 118ºC. This partially demonstrates what was discussed in the section on structures: that intermolecular interactions are weaker when sec-butyl is present, compared to those found with n-butyl.

Secbutylamine

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

Secbutyl acetate

Structural formula of sec-butyl acetate. Source: Edgar181 / Public domain

Substituting R for the acetate group, CH ₃ CO ₂, we have secbutyl acetate (upper image). Note that secbutyl is represented not linear but with folds; however, it remains attached to the acetate with a 2nd carbon. The secbutyl begins to lose chemical importance because it is linked to an oxygenated group.

So far the examples have consisted of liquid substances. The following will be about solids where sec-butyl is only a fraction of the molecular structure.

Joker

Structural formula of bromacil. Source: Fvasconcellos via Wikipedia.

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

Pravastatin

Structural formula of pravastatin. Source: Edgar181 / Public domain

Finally, in the upper image we have the molecular structure of pravastatin, a drug used to reduce cholesterol levels in the blood. Can the secbutyl group be located? First look for a four-carbon chain, which is located to the left of the structure. Second, note that it binds 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

1. Morrison, RT and Boyd, R, N. (1987). Organic Chemistry. 5th 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. (10th edition.). Wiley Plus.
4. Steven A. Hardinger. (2017). Illustrated Glossary of Organic Chemistry: Sec-butyl. Recovered from: chem.ucla.edu
5. James Ashenhurst. (2020). Don't Be Futyl, Learn The Butyls. Recovered from: masterorganicchemistry.com
6. Wikipedia. (2020). Category: Sec-butyl compounds. Recovered from: commons.wikimedia.org