a series, a progression sequenced by the number of carbon atoms involved. In that progression, Butyl (containing 4 carbon atoms) is the fourth, and the last with preferred IUPAC name derived from its history. The word "butyl" is derived from butyric acid, a four-carbon carboxylic acid found in rancid butter. The name "butyric acid" comes from Latin butyrum, butter. Subsequent preferred IUPAC names for alkyl radicals in the series are simply named from the Greek number that indicates the number of carbon atoms in the group: pentyl, hexyl, heptyl, etc. tert-Butyl effect The tert-butyl substituent is very bulky and is used in chemistry for kinetic stabilization, as are other bulky groups such as the related trimethylsilyl group. The effect of the tert-butyl group on the progress of a chemical reaction is called the Thorpe–Ingold effect illustrated in the Diels-Alder reaction below. Compared to a hydrogen substituent, the tert-butyl substituent accelerates the reaction rate by a factor of 240. The tert-butyl effect is an example of steric hindrance. Tert-butyl as Protecting group Tert-butyl (tBu) ether is an acid-labile protecting group for alcohols. Protection & formation A traditional way to introduce tBu group to hydroxyl group is by treating the compound with isobutylene in the presence of Bronsted acid or Lewis acid. Deprotection & Cleavage References Alkyl groups