In thenomenclature oforganic chemistry, alocant is a term to indicate the position of afunctional group orsubstituent within amolecule.[1]
TheInternational Union of Pure and Applied Chemistry (IUPAC) recommends the use of numeric prefixes to indicate the position of substituents, generally by identifying theparent hydrocarbon chain and assigning the carbon atoms based on their substituents inorder of precedence. For example, there are at least twoisomers of the linear form ofpentanone, aketone that contains a chain of exactly fivecarbon atoms. There is anoxygen atom bonded to one of the middle three carbons (if it were bonded to an end carbon, the molecule would be analdehyde, not a ketone), but it is not clear where it is located.
In this example, the carbon atoms are numbered from one to five, which starts at one end and proceeds sequentially along the chain. Now the position of the oxygen atom can be defined as on carbon atom number two, three or four. However, atoms two and four are exactly equivalent - which can be shown by turning the molecule around by 180 degrees.
The locant is the number of the carbon atom to which the oxygen atom is bonded. If the oxygen is bonded to the middle carbon, the locant is 3. If the oxygen is bonded to anatom on either side (adjacent to an end carbon), the locant is 2 or 4; given the choice here, where the carbons are exactly equivalent, the lower number is always chosen. So the locant is either 2 or 3 in this molecule.
The locant is incorporated into the name of the molecule to remove ambiguity. Thus the molecule is named eitherpentan-2-one orpentan-3-one, depending on the position of the oxygen atom.
Any side chains can be present in the place of oxygen and it can be defined as simply the number on the carbon to which any thing other than a hydrogen is attached.
Another common system usesGreek letter prefixes as locants, which is useful in identifying the relative location of carbon atoms as well as hydrogen atoms to other functional groups.
Theα-carbon (alpha-carbon) refers to the firstcarbon atom that attaches to afunctional group, such as acarbonyl. The second carbon atom is called theβ-carbon (beta-carbon), the third is theγ-carbon (gamma-carbon), and the naming system continues in alphabetical order.[2]
Thenomenclature can also be applied to thehydrogen atoms attached to the carbon atoms. A hydrogen atom attached to an α-carbon is called anα-hydrogen, a hydrogen atom on the β-carbon is aβ-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 moleculesnitrostyrene andphenethylamine are quite similar; the former can even bereduced into the latter. However, nitrostyrene's α-carbon atom is adjacent to thephenyl 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.[3]
Inproteins andamino acids, 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]n– 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 itsstereogenic properties for every amino acid except forglycine. Therefore, the α-carbon is astereocenter 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 inprotein 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 3.8ångströms (380picometers) apart.
The α-carbon is important forenol- andenolate-basedcarbonyl 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 anucleophile, becoming, for example,alkylated in the presence of primaryhaloalkane. An exception is in reaction withsilylchlorides,bromides, andiodides, where theoxygen acts as the nucleophile to producesilyl enol ether.