SBU

sBu, also written as sec‑butyl, denotes the sec‑butyl substituent in organic chemistry. The group has the formula –CH(CH₃)CH₂CH₃ and contains four carbon atoms arranged in a branched chain with the point of attachment at the secondary carbon. It is one of several butyl isomers, the others being n‑butyl (straight‑chain), isobutyl (2‑methylpropyl) and tert‑butyl (tert‑butyl). The “sec” prefix indicates that the substituent is attached to a secondary carbon atom, distinguishing it from the primary n‑butyl and the tertiary tert‑butyl groups.

The sec‑butyl group appears in a variety of chemical contexts. In alkylation reactions, sBu can be transferred from reagents such as sec‑butyl lithium (sBuLi) or sec‑butyl magnesium bromide (sBuMgBr), which serve as strong bases and nucleophiles. These organometallic reagents are employed in the formation of carbon–carbon bonds, metalation of acidic protons, and deprotonation of heteroatoms. The steric bulk of the sec‑butyl moiety influences reaction selectivity, often providing a balance between reactivity and hindrance that is useful in synthetic design.

Physical properties of sBu‑containing compounds reflect the branching of the group; they typically have lower boiling points than their n‑butyl analogues but higher than those of isobutyl derivatives. The sec‑butyl group also imparts moderate lipophilicity, which can affect the biological activity and membrane permeability of pharmaceutical molecules.

In nomenclature, the sec‑butyl substituent is indicated by the prefix “s‑Bu” or “sec‑Bu” preceding the parent name, for example sec‑butyl acetate or 2‑sec‑butylphenol. The group is widely recognized in the IUPAC system and in commercial laboratory literature.