Alkyl group

Inorganic chemistry,analkyl groupis analkanemissing onehydrogen.^[1] The termalkylis intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of−C_nH_2n+1.Acycloalkyl groupis derived from acycloalkaneby removal of a hydrogen atom from aringand has the general formula−C_nH_2n−1.^[2] Typically an alkyl is a part of a larger molecule. Instructural formulae,the symbol R is used to designate a generic (unspecified) alkyl group. The smallest alkyl group ismethyl,with the formula−CH₃.^[3]

Related concepts

Alkylationis the addition of alkyl groups to molecules, often by alkylating agents such asalkyl halides.

Alkylating antineoplastic agentsare a class of compounds that are used to treat cancer. In such case, the term alkyl is used loosely. For example,nitrogen mustardsare 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 reagentshave the empirical formula Li(alkyl), where alkyl = methyl, ethyl, etc. Adialkylether is anetherwith two alkyl groups, e.g., diethyl etherO(CH₂CH₃)₂.

In medicinal chemistry

Inmedicinal chemistry,the incorporation of alkyl chains into somechemical compoundsincreases theirlipophilicity.This strategy has been used to increase the antimicrobial activity offlavanonesandchalcones.^[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 calledcarbocations.The anions are calledcarbanions.The neutral alkylfree radicalshave no special name. Such species are usually encountered only as transient intermediates. However, persistent alkyl radicals withhalf-lives"from seconds to years" have been prepared.^[5]Typically alkyl cations are generated usingsuperacidsand alkyl anions are observed in the presence of strong bases. Alkyl radicals can be generated by aphotochemical reactionor byhomolytic cleavage.^[6]Alkyls are commonly observed inmass spectrometryoforganic compounds.Simple alkyls (especiallymethyl) are observed in theinterstellar spaceas well.

Nomenclature

Alkyl groups formhomologous series.The simplest series have the general formula−C_nH_2n+1.Alkyls includemethyl,(−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 fromIUPAC 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 theirsubstituentgroups, for example3-methylpentane:

The structure of3-methylpentaneis viewed as consisting of two parts. First, five atoms comprise the longest straight chain ofcarboncenters. The parent five-carbon compound is namedpentane(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 inmethylpentane.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 as2,3,3-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 termsprimary,secondary,tertiary,andquaternaryrefer to a carbon attached to one, two, three, or four other carbons respectively.

C_n	Trivial name	Symbol	Condensed formula	IUPAC status	Preferred IUPAC name
С₁	methyl	Ме	−CH₃		methyl
С₂	ethyl	Et	−CH₂−CH₃		ethyl
С₃	propyl,n-propyl	Pr,ⁿPr,n-Pr	−CH₂−CH₂−CH₃		propyl
С₃	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 byLiebigin 1833 from the German word "Äther" (which in turn had been derived from the Greek word "aither"meaning" air ", for the substance now known asdiethyl ether) and the Greek word ύλη (hyle), meaning "matter".^[7]This was followed by methyl (DumasandPeligotin 1834, meaning "spirit of wood"^[8]) and amyl (Auguste Cahoursin 1840^[9]). The word alkyl was introduced byJohannes Wislicenusin or before 1882, based on the German word "Alkoholradikale" and then-common suffix -yl.^[10]^[11]

References

^IUPAC,Compendium of Chemical Terminology,2nd ed. (the "Gold Book" ) (1997). Online corrected version: (2006–) "alkyl groups".doi:10.1351/goldbook.A00228
^IUPAC,Compendium of Chemical Terminology,2nd ed. (the "Gold Book" ) (1997). Online corrected version: (2006–) "cycloalkyl groups".doi:10.1351/goldbook.C01498
^Virtual Textbook of Organic ChemistryNaming Organic Compounds Archived2016-05-21 at the Portuguese Web Archive
^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.
^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.
^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.PMC8009483.PMID 32786419.
^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"
^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" )
^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
^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.
^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.

[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 ChemistryNaming Organic Compounds Archived2016-05-21 at the Portuguese Web Archive

[medchem1-4] 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.PMC8009483.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.

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