C4H9

C4H9 is a chemical formula that denotes a four-carbon alkyl group, commonly called a butyl group. It is not a single compound but rather a family of substituents derived from butane by removing one hydrogen. The parent hydrocarbon with four carbons is butane, C4H10.

There are four constitutional isomers of the C4H9 group, corresponding to the four ways to arrange four carbon atoms in a chain or branched skeleton. These are n-butyl (butan-1-yl), sec-butyl (butan-2-yl), isobutyl (2-methylpropyl), and tert-butyl (2-methylprop-2-yl). Each isomer attaches to other molecules at different positions, affecting the properties and reactivity of the resulting compound.

The C4H9 moiety has an approximate molecular weight of 57.1 g/mol. As substituents, butyl groups are generally nonpolar and hydrophobic, and their physical properties depend on the specific isomer and the rest of the molecule they are part of. In chemical reactions, these groups can influence reaction pathways, site selectivity, and the stability of intermediates.

Applications and relevance: Butyl groups are common in organic synthesis, medicinal chemistry, and polymer science. They are used to modify solubility, boiling points, and steric hindrance. In protecting-group strategies, tert-butyl (t-butyl) is especially important in Boc protection of amines. Isobutyl and n-butyl groups appear in esters, halides, alcohol derivatives, and various polymers, contributing to material properties and reactivity.

Safety and handling: As with many aliphatic hydrocarbons, compounds bearing C4H9 groups are typically flammable and should be handled with standard laboratory precautions.