Alkyl group

In organic chemistry, an alkyl group is an alkane missing one hydrogen.^[1] The term alkyl is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of −C_nH_2n+1. A cycloalkyl group is derived from a cycloalkane by removal of a hydrogen atom from a ring and has the general formula −C_nH_2n−1.^[2] Typically an alkyl is a part of a larger molecule. In structural formulae, the symbol R is used to designate a generic (unspecified) alkyl group. The smallest alkyl group is methyl, with the formula −CH₃.^[3]

Isopropyl group

Methyl group

Related concepts

Alkylation is the addition of alkyl groups to molecules, often by alkylating agents such as alkyl halides.

Alkylating antineoplastic agents are a class of compounds that are used to treat cancer. In such case, the term alkyl is used loosely. For example, nitrogen mustards are well-known alkylating agents, but they are not simple hydrocarbons.

In chemistry, alkyl is a group, a substituent, that is attached to other molecular fragments. For example, alkyl lithium reagents have the empirical formula Li(alkyl), where alkyl = methyl, ethyl, etc. A dialkyl ether is an ether with two alkyl groups, e.g., diethyl ether O(CH₂CH₃)₂.

In medicinal chemistry

In medicinal chemistry, the incorporation of alkyl chains into some chemical compounds increases their lipophilicity. This strategy has been used to increase the antimicrobial activity of flavanones and chalcones.^[4]

Alkyl cations, anions, and radicals

Usually, alkyl groups are attached to other atoms or groups of atoms. Free alkyls occur as neutral radicals, as anions, or as cations. The cations are called carbocations. The anions are called carbanions. The neutral alkyl free radicals have no special name. Such species are usually encountered only as transient intermediates. However, persistent alkyl radicals with half-lives "from seconds to years" have been prepared.^[5] Typically alkyl cations are generated using superacids and alkyl anions are observed in the presence of strong bases. Alkyl radicals can be generated by a photochemical reaction or by homolytic cleavage.^[6] Alkyls are commonly observed in mass spectrometry of organic compounds. Simple alkyls (especially methyl) are observed in the interstellar space as well.

Nomenclature

Alkyl groups form homologous series. The simplest series have the general formula −C_nH_2n+1. Alkyls include methyl, (−CH₃), ethyl (−C₂H₅), propyl (−C₃H₇), butyl (−C₄H₉), pentyl (−C₅H₁₁), and so on. Alkyl groups that contain one ring have the formula −C_nH_2n−1, e.g. cyclopropyl and cyclohexyl. The formula of alkyl radicals are the same as alkyl groups, except the free valence "−" is replaced by the dot "•" and adding "radical" to the name of the alkyl group (e.g. methyl radical •CH₃).

The naming convention is taken from IUPAC nomenclature:

Number of carbon atoms	1	2	3	4	5	6	7	8	9	10	11	12	13	14
Prefix	meth-	eth-	prop-	but-	pent-	hex-	hept-	oct-	non-	dec-	undec-	dodec-	tridec-	tetradec-
Group name	Methyl	Ethyl	Propyl	Butyl	Pentyl	Hexyl	Heptyl	Octyl	Nonyl	Decyl	Undecyl	Dodecyl	Tridecyl	Tetradecyl

The prefixes taken from IUPAC nomenclature are used to name branched chained structures by their substituent groups, for example 3-methylpentane:

The structure of 3-methylpentane is viewed as consisting of two parts. First, five atoms comprise the longest straight chain of carbon centers. The parent five-carbon compound is named pentane (highlighted blue). The methyl "substituent" or "group" is highlighted red. According to the usual rules of nomenclature, alkyl groups are included in the name of the molecule before the root, as in methylpentane. This name is, however, ambiguous, as the methyl branch could be on various carbon atoms. Thus, the name is 3-methylpentane to avoid ambiguity: The 3- is because the methyl is attached to the third of the five carbon atoms.

If there is more than one of the same alkyl group attached to a chain, then the prefixes are used on the alkyl groups to indicate multiples (i.e., di, tri, tetra, etc.)

This compound is known as 2,3,3-trimethylpentane. Here three identical alkyl groups attached to carbon atoms 2, 3, and 3. The numbers are included in the name to avoid ambiguity about the position of the groups, and "tri" indicates that there are three identical methyl groups. If one of the methyl groups attached to the third carbon atom were instead an ethyl group, then the name would be 3-ethyl-2,3-dimethylpentane. When there are different alkyl groups, they are listed in alphabetical order.

In addition, each position on an alkyl chain can be described according to how many other carbon atoms are attached to it. The terms primary, secondary, tertiary, and quaternary refer to a carbon attached to one, two, three, or four other carbons respectively.

Cn	Trivial name	Symbol	Condensed formula	IUPAC status	Preferred IUPAC name	Skeletal formula
С1	methyl	Ме	−CH3		methyl
С2	ethyl	Et	−CH2−CH3		ethyl
С3	propyl, n-propyl	Pr, nPr, n-Pr	−CH2−CH2−CH3		propyl
	isopropyl	iPr, i-Pr,iPr	−CH(−CH3)2		2-propyl
С4	n-butyl	Bu, n-Bu, nBu	−CH2−CH2−CH2−CH3		butyl
	isobutyl	iBu, i-Bu, iBu	−CH2−CH(−CH3)2		2-methylpropyl
	sec-butyl	sBu, s-Bu, sBu	−CH(−CH3)−CH2−CH3		2-butyl
	tert-butyl	tBu, t-Bu, tBu	−C(−CH3)3		tert-butyl
С5	n-pentyl, amyl	Pe, Am, nPe, n-Pe, nPe, nAm	−CH2−CH2−CH2−CH2−CH3		pentyl
	tert-pentyl	tPe, t-Pe, tPe	−C(−CH3)2−CH2−CH3	No longer recommended	2-methylbutan-2-yl (aka 1,1-dimethylpropyl)
	neopentyl		−CH2−C(−CH3)3	No longer recommended	2,2-dimethylpropyl
	isopentyl, isoamyl		−CH2−CH2−CH(−CH3)2	No longer recommended	3-methylbutyl
	sec-pentyl	sPe, s-Pe, sPe	−CH(−CH3)−CH2−CH2−CH3		pentan-2-yl(or (1-Methylbutyl))
	3-pentyl		−CH(−CH2−CH3)2		pentan-3-yl (also known as (1-Ethylpropyl))
	sec-isopentyl, sec-isoamyl, siamyl	Sia	−CH(−CH3)−CH(−CH3)2		3-methylbutan-2-yl (or (1,2-Dimethylpropyl))
	active pentyl		−CH2−CH(−CH3)−CH2−CH3		2-methylbutyl

Etymology

The first named alkyl radical was ethyl, named so by Liebig in 1833 from the German word "Äther" (which in turn had been derived from the Greek word "aither" meaning "air", for the substance now known as diethyl ether) and the Greek word ύλη (hyle), meaning "matter".^[7] This was followed by methyl (Dumas and Peligot in 1834, meaning "spirit of wood"^[8]) and amyl (Auguste Cahours in 1840^[9]). The word alkyl was introduced by Johannes Wislicenus in or before 1882, based on the German word "Alkoholradikale" and then-common suffix -yl.^[10][11]

See also

• Alkane
• Alkene
• Alkyne
• Aryl
• Hydrocarbon
• IUPAC nomenclature of organic chemistry

References

1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "alkyl groups". doi:10.1351/goldbook.A00228
2. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "cycloalkyl groups". doi:10.1351/goldbook.C01498
3. Virtual Textbook of Organic Chemistry Naming Organic Compounds Archived 2016-05-21 at the Portuguese Web Archive
4. Mallavadhani UV, Sahoo L, Kumar KP, Murty US (2013). "Synthesis and antimicrobial screening of some novel chalcones and flavanones substituted with higher alkyl chains". Medicinal Chemistry Research. 23 (6): 2900–2908. doi:10.1007/s00044-013-0876-x. S2CID 5159000.
5. Griller, David; Ingold, Keith U. (1976-01-01). "Persistent carbon-centered radicals". Accounts of Chemical Research. 9 (1): 13–19. doi:10.1021/ar50097a003. ISSN 0001-4842.
6. Crespi, Stefano; Fagnoni, Maurizio (2020-09-09). "Generation of Alkyl Radicals: From the Tyranny of Tin to the Photon Democracy". Chemical Reviews. 120 (17): 9790–9833. doi:10.1021/acs.chemrev.0c00278. ISSN 0009-2665. PMC 8009483. PMID 32786419.
7. Rocke, Alan (2012). From the Molecular World: A Nineteenth-Century Science Fantasy. Springer. p. 62. ISBN 978-3-642-27415-2. Ethyl radicals (named by Liebig in 1833 from the Greek and German word "aether" plus Greek "hyle"
8. Rocke, Alan (2012). From the Molecular World: A Nineteenth-Century Science Fantasy. Springer. p. 62. ISBN 978-3-642-27415-2. The methyl radical ... named from Greek roots, by Dumas and Peligot in 1834: methyl = methy + hyle ("spirit" + "wood")
9. Rocke, Alan (2012). From the Molecular World: A Nineteenth-Century Science Fantasy. Springer. p. 62. ISBN 978-3-642-27415-2. Amyl radicals ("amilène" was coined by Auguste Cahours in 1840, to designate a substance from potato starch after fermentation and distillation
10. Rocke, Alan (2012). From the Molecular World: A Nineteenth-Century Science Fantasy. Springer. p. 62. ISBN 978-3-642-27415-2. "Alkyl" was coined without fanfare by Johannes Wislicenus, professor at Würzburg; an early use (perhaps not the first) is in his 1882 article [22, 244]. The word was derived from the first three letters of "Alkoholradicale" combined with the suffix -yl; it was (and is) a generic term for any of those radicals who bear the "first names" methyl, ethyl, propyl, butyl, amyl, etc.
11. Wisclicenus, Johannes (1882). "Ueber die Schätzung von Haftenenergien der Halogene und des Natriums an organischen Resten". Ann. Chem. 212: 239–250. doi:10.1002/jlac.18822120107.