Unbranched alkanes are compounds containing only carbon and hydrogen atoms in continuous chains. Branched-chain alkanes, on the other hand, contain short carbon chains (known as alkyl groups) attached to longer carbon skeletons. The convention is to name the alkyl group using the Greek word for the number of carbon atoms present. Thus, the alkyl group with five carbons is referred to as a pentyl group, which is the name obtained by dropping the -ane suffix from the five-carbon unbranched chain (pentane, having the structural formula CH₃CH₂CH₂CH₂CH₃), and substituting a -yl ending.

The pentyl group (also known as an amyl group), therefore, corresponds to the five-carbon aliphatic group C₅H₁₁. This group has eight possible constitutional as isomeric arrangements (excluding optical isomers); the structure of the simplest of these, i.e., the n-pentyl group, is CH₃CH₂CH₂CH₂CH₂-(where the n- prefix is an abbreviation for "normal." Compounds containing pentyl groups usually occur as as mixtures of several isomers. Because the as boiling points of these compounds are close and their other properties are similar, the expense and difficulty of separating them is seldom warranted. Amyl compounds, i.e., those containing pentyl groups, exist in fusel oils; they can also be formed from petroleum pentanes.

The amyl alcohols (C₅H₁₁OH), which are among the simplest of compounds containing pentyl groups, have the following eight isomeric structures (excluding optical isomers): n-amyl alcohol, primary or CH₃(CH₂)₄OH; 2-meth yl-1-butanol or CH₃CH₂CH(CH₃ )CH₂OH; isoamyl alcohol, primary or (CH₃)₂CHCH₂C H₂OH; 2-pentanol or CH₃CH₂CH₂CH₂OCH₃, 3-pentanol or CH₃CH₂CHOHCH ₂CH₃; tert-amyl alcohol or (CH₃)₂C(OH)CH₂CH₃; sec-isoamyl alcohol or CH₃CHOHCH(CH₃)₂; and 2,2-dimethyl-1-propanol or (CH₃)₃CCH₂OH. The isomers n-amyl alcohol, primary; 2-pentanol; and 3-pentanol exist as linear, unbranched chains of carbon and hydrogen atoms, whereas the isomers 2 methyl-1-butanol;isoamyl alcohol, primary; tert-amyl alcohol; sec-isoamyl alcohol;and 2,2 dimethyl-1 propanol are branched compounds.The isomers 2-methyl-1-butanol; 2-pentanol; and sec-isoamyl alcohol are chiral and exhibit optical activity.