Secondary (chemistry)

This article is about general bonding patterns. For local three-dimensional motifs of proteins and nucleic acids, see secondary structure.

Secondary is a term used in organic chemistry to classify various types of compounds (e. g. alcohols, https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Alcohols/Nomenclature_of_Alcohols alkyl halides, amines) or reactive intermediates (e. g. alkyl radicals, carbocations). An atom is considered secondary if it has two 'R' Groups attached to it. An 'R' group is a carbon containing group such as a methyl (CH3). A secondary compound is most often classified on an alpha carbon (middle carbon) or a nitrogen. The word secondary comes from the root word 'second' which means two.

Red highlighted central atoms in various groups of chemical compounds.

Secondary central atoms compared with primary, tertiary and quaternary central atoms.

primary secondary tertiary quaternary
Carbon atom in an alkane


This nomenclature can be used in many cases and further used to explain relative reactivity. The reactivity of molecules varies with respect to the attached atoms. Thus, a primary, secondary, tertiary and quaternary molecule of the same function group will have different reactivities.

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