Editing Locant (section)

==Greek letter locants==
[[Image:Alpha and beta positions of benzylacetone-structure.svg|thumb|α- and β-carbons in the [[skeletal formula]] of [[benzylacetone]]. The [[carbonyl]] has two β-hydrogens and five α-hydrogens.]]
[[File:Butyric acid carbons 2.svg|250px|thumb|[[Skeletal formula]] of [[butyric acid]] with the α, β, and γ-carbons marked|alt=Skeletal formula of butyric acid with the alpha, beta, and gamma carbons marked]]
Another common system uses [[Greek letter]] prefixes as locants, which is useful in identifying the relative location of carbon atoms as well as hydrogen atoms to other functional groups.

The '''α-carbon''' (''alpha''-carbon) refers to the first [[carbon]] atom that attaches to a [[functional group]], such as a [[carbonyl]]. The second carbon atom is called the '''β-carbon''' (''beta''-carbon), the third is the '''γ-carbon''' (''gamma''-carbon), and the naming system continues in alphabetical order.<ref>{{cite book |title=Hackh's Chemical Dictionary |date=1969 |page=95}}</ref>

The [[Chemical nomenclature|nomenclature]] can also be applied to the [[hydrogen]] atoms attached to the carbon atoms. A hydrogen atom attached to an α-carbon is called an '''α-hydrogen''', a hydrogen atom on the β-carbon is a '''β-hydrogen''', and so on.

Organic molecules with more than one functional group can be a source of confusion. Generally the functional group responsible for the name or type of the molecule is the 'reference' group for purposes of carbon-atom naming. For example, the molecules [[nitrostyrene]] and [[phenethylamine]] are quite similar; the former can even be [[redox|reduced]] into the latter. However, nitrostyrene's α-carbon atom is adjacent to the [[phenyl]] group; in phenethylamine this same carbon atom is the β-carbon atom, as phenethylamine (being an amine rather than a styrene) counts its atoms from the opposite "end" of the molecule.<ref>{{Cite web 
|url=http://www.cognitiveliberty.org/shulgin/adsarchive/nomenclature.htm
|publisher=Center for Cognitive Liberty & Ethics
|title=Nomenclature
|work=Ask Dr. Shulgin Online
|access-date=August 5, 2010}}</ref>

<gallery>
File:Beta-nitrostyrene.svg|Nitrostyrene
File:Fenyloetyloamina.svg|Phenethylamine
</gallery>

===Proteins and amino acids===
In [[protein]]s and [[amino acid]]s, the α-carbon is the backbone carbon before the carbonyl carbon atom in the molecule. Therefore, reading along the backbone of a typical protein would give a sequence of –[N—Cα—carbonyl C]<sub>n</sub>– etc. (when reading in the N to C direction). The α-carbon is where the different substituents attach to each different amino acid. That is, the groups hanging off the chain at the α-carbon are what give amino acids their diversity. These groups give the α-carbon its [[stereogenic]] properties for every amino acid except for [[glycine]]. Therefore, the α-carbon is a [[stereocenter]] for every amino acid except glycine. Glycine also does not have a β-carbon, while every other amino acid does.

The α-carbon of an amino acid is significant in [[protein folding]]. When describing a protein, which is a chain of amino acids, one often approximates the location of each amino acid as the location of its α-carbon. In general, α-carbons of adjacent amino acids in a protein are about 3.8 [[ångström]]s (380 [[picometer]]s) apart.

===Enols and enolates===
The α-carbon is important for [[enol]]- and [[enolate]]-based [[carbonyl]] chemistry as well. Chemical transformations affected by the conversion to either an enolate or an enol, in general, lead to the α-carbon acting as a [[nucleophile]], becoming, for example, [[alkylated]] in the presence of primary [[haloalkane]]. An exception is in reaction with [[silyl]] [[chlorosilane|chlorides]], [[bromide]]s, and [[iodide]]s, where the [[oxygen]] acts as the nucleophile to produce [[silyl enol ether]].