Locant

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Alpha and beta carbon
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In the nomenclature of organic chemistry, a locant is a term to indicate the position of a functional group or substituent within a molecule.[1]

Numeric locants

3-pentanone

The

parent hydrocarbon chain and assigning the carbon atoms based on their substituents in order of precedence. For example, there are at least two isomers of the linear form of pentanone, a ketone that contains a chain of exactly five carbon atoms. There is an oxygen atom bonded to one of the middle three carbons (if it were bonded to an end carbon, the molecule would be an aldehyde
, 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 an atom 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 either

pentan-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.

Greek letter locants

carbonyl
has two β-hydrogens and five α-hydrogens.
Skeletal formula of butyric acid with the alpha, beta, and gamma carbons marked
Skeletal formula of butyric acid with the α, β, and γ-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

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.[2]

The 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

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.[3]

  • Nitrostyrene
    Nitrostyrene
  • Phenethylamine
    Phenethylamine

Proteins and amino acids

In

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

picometers
) apart.

Enols and enolates

The α-carbon is important for

silyl chlorides, bromides, and iodides, where the oxygen acts as the nucleophile to produce silyl enol ether
.

See also

References

  1. ISBN 978-0-85404-182-4
    . Retrieved 14 December 2022.
  2. ^ Hackh's Chemical Dictionary. 1969. p. 95.
  3. ^ "Nomenclature". Ask Dr. Shulgin Online. Center for Cognitive Liberty & Ethics. Retrieved August 5, 2010.
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