# Butyl group
**Butyl group** is a four-carbon alkyl substituent with the general formula −C₄H₉, obtained by removing one hydrogen atom from either of the two isomers of butane, C₄H₁₀ (n-butane or 2-methylpropane, also called isobutane). The term covers four distinct structural forms, commonly called n-butyl, sec-butyl, isobutyl, and tert-butyl, and in its narrowest sense refers specifically to the straight-chain n-butyl group, −CH₂CH₂CH₂CH₃.[1](https://www.chem.ucla.edu/~harding/IGOC/B/butyl_group.html) • [2](https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/butyl-group)
| Key fact | Detail |
|---|---|
| Formula | −C₄H₉, a monovalent alkyl substituent derived from butane (C₄H₁₀) by loss of one hydrogen[2](https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/butyl-group) |
| Number of forms | Four isomeric butyl groups: n-butyl, sec-butyl, isobutyl, and tert-butyl[3](https://www.uky.edu/~rbgros1/CHE230/lecnotes/Klein_4_alkanes%2Bisomerism.pdf) |
| Abbreviations | n-Bu, s-Bu, i-Bu, and t-Bu; when just Bu is written, it refers to n-Bu[3](https://www.uky.edu/~rbgros1/CHE230/lecnotes/Klein_4_alkanes%2Bisomerism.pdf) |
| IUPAC preferred names | butan-1-yl (butyl), butan-2-yl (1-methylpropyl), 2-methylpropyl, and 2-methylpropan-2-yl (tert-butyl, unsubstituted)[4](https://iupac.qmul.ac.uk/BlueBook/Papp2.html) |
| Classification | n-Butyl and isobutyl attach through a primary carbon, sec-butyl through a secondary carbon, tert-butyl through a tertiary carbon[5](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Basic_Principles_of_Organic_Chemistry_%28Roberts_and_Caserio%29/03%3A_Organic_Nomenclature/3.02%3A_Alkanes) |
| Corresponding alcohols | Four butyl alcohols exist, one for each butyl group: 1-butanol, 2-butanol, 2-methyl-1-propanol, and 2-methyl-2-propanol[1](https://www.chem.ucla.edu/~harding/IGOC/B/butyl_group.html) |
## The four isomers
The two parent butanes give rise to the four groups. From n-butane, removing a hydrogen from a terminal carbon gives the n-butyl group, −CH₂CH₂CH₂CH₃, while removing a hydrogen from an internal carbon gives the sec-butyl group, −CH(CH₃)CH₂CH₃. From 2-methylpropane, removing a hydrogen can give two different butyl groups: loss of a hydrogen from a methyl carbon yields isobutyl, −CH₂CH(CH₃)₂, and loss of the single hydrogen from the central carbon yields tert-butyl, −C(CH₃)₃.[2](https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/butyl-group) • [3](https://www.uky.edu/~rbgros1/CHE230/lecnotes/Klein_4_alkanes%2Bisomerism.pdf)
Each group attaches to the rest of a molecule through a different kind of carbon, which is the basis of the primary, secondary, and tertiary labels. Alkyl groups are classified by how many other carbons attach to the linking carbon: one bond to a single carbon makes the group primary, bonds to two carbons make it secondary, and bonds to three make it tertiary.[5](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Basic_Principles_of_Organic_Chemistry_%28Roberts_and_Caserio%29/03%3A_Organic_Nomenclature/3.02%3A_Alkanes) The names sec-butyl and tert-butyl reflect this directly: they connect to the main chain from a secondary and a tertiary carbon, respectively.[6](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_I_%28Cortes%29/07%3A_Introduction_to_Organic_Chemistry/7.03%3A_Overview_of_Functional_Groups_Based_on_Atom_Hybridization)
The four groups also define the four butyl alcohols, each formed by adding a hydroxyl group to one of the four butyl structures: 1-butanol (n-butyl), 2-butanol (sec-butyl), 2-methyl-1-propanol (isobutyl), and 2-methyl-2-propanol (tert-butyl).[1](https://www.chem.ucla.edu/~harding/IGOC/B/butyl_group.html)
## Nomenclature status
The trivial names for the four butyl groups are very commonly used in practice, along with the abbreviations n-Bu, s-Bu, i-Bu, and t-Bu.[3](https://www.uky.edu/~rbgros1/CHE230/lecnotes/Klein_4_alkanes%2Bisomerism.pdf) Under current IUPAC rules, however, their standing differs from group to group.
**Retained preferred names.** The name butyl itself is retained as a preferred prefix and means butan-1-yl, the straight-chain group.[4](https://iupac.qmul.ac.uk/BlueBook/Papp2.html) The name tert-butyl is likewise retained as a preferred prefix, but only for the unsubstituted group −C(CH₃)₃, equivalently named 2-methylpropan-2-yl or 1,1-dimethylethyl.[4](https://iupac.qmul.ac.uk/BlueBook/Papp2.html)
**Names no longer recommended.** The retained names isobutyl and sec-butyl are no longer recommended; the preferred prefixes are 2-methylpropyl and butan-2-yl (also expressible as 1-methylpropyl), respectively.[7](https://iupac.qmul.ac.uk/BlueBook/P5.html) The Blue Book appendix marks butan-2-yl explicitly as "not sec-butyl; not but-2-yl".[4](https://iupac.qmul.ac.uk/BlueBook/Papp2.html)
The prefix "sec" is otherwise unused in systematic nomenclature; because a chemical name must specify only one compound, sec appears essentially only in sec-butyl.[8](https://student.uoh.edu.iq/admin/ebooks/98465-organic-chemistry-4th-ed---paula-bruice.pdf)
## Related names
The butyl family extends to oxygen- and carbonyl-containing substituents with parallel naming. Butoxy (not butyloxy) is the retained name for the n-butoxy group, −OCH₂CH₂CH₂CH₃; sec-butoxy is expressed as (butan-2-yl)oxy; and tert-butoxy (unsubstituted) equals (2-methylpropan-2-yl)oxy or 1,1-dimethylethoxy.[4](https://iupac.qmul.ac.uk/BlueBook/Papp2.html) For acyl groups, butyryl equals the preferred name butanoyl, −COCH₂CH₂CH₃.[4](https://iupac.qmul.ac.uk/BlueBook/Papp2.html)
## References
1. [Butyl group, Illustrated Glossary of Organic Chemistry, UCLA](https://www.chem.ucla.edu/~harding/IGOC/B/butyl_group.html)
2. [Butyl group, Encyclopedia.com](https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/butyl-group)
3. [Grossman, CHE 230 lecture notes, Alkanes and isomerism, University of Kentucky](https://www.uky.edu/~rbgros1/CHE230/lecnotes/Klein_4_alkanes%2Bisomerism.pdf)
4. [IUPAC Blue Book, Appendix 2: retained prefix names](https://iupac.qmul.ac.uk/BlueBook/Papp2.html)
5. [Alkanes, Roberts and Caserio, Basic Principles of Organic Chemistry, Chemistry LibreTexts](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Basic_Principles_of_Organic_Chemistry_%28Roberts_and_Caserio%29/03%3A_Organic_Nomenclature/3.02%3A_Alkanes)
6. [Overview of Functional Groups, Chemistry LibreTexts](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_I_%28Cortes%29/07%3A_Introduction_to_Organic_Chemistry/7.03%3A_Overview_of_Functional_Groups_Based_on_Atom_Hybridization)
7. [IUPAC Blue Book, chapter P-5, retained prefixes no longer recommended](https://iupac.qmul.ac.uk/BlueBook/P5.html)
8. [Bruice, Organic Chemistry, 4th ed., Section 2.1](https://student.uoh.edu.iq/admin/ebooks/98465-organic-chemistry-4th-ed---paula-bruice.pdf)
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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Hydrocarbons and aromatic systems › Alkanes*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
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