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The alpha carbon in organic chemistry
Organic chemistry

Organic chemistry is a discipline within chemistry which involves the science study of the structure, properties, composition, chemical reaction, and preparation of chemical compounds that contain carbon....
 refers to the first carbon that attaches to a functional group
Functional group

In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules....
 (the carbon is attached at the first, or alpha, position). By extension, the second carbon is the beta carbon, and so on. This nomenclature can also be applied to the hydrogen atoms attached to the carbons. A hydrogen attached to an alpha carbon is called an "alpha-hydrogen" (a-hydrogen), a hydrogen on the beta-carbon is a beta-hydrogen, and so on.

This naming standard is sometimes considered to be not in compliance with IUPAC nomenclature
IUPAC nomenclature

IUPAC nomenclature is a system of naming chemical compounds and of describing the science of chemistry in general. It is developed and kept up to date under the auspices of the International Union of Pure and Applied Chemistry ....
 (which encourages that carbons be identified by number, not by Greek letter); but it nonetheless remains very popular, particularly because it is useful in identifying the relative location of carbons to other functional groups (often a carbonyl
Carbonyl

In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double bond to an oxygen atom : C=O.The term carbonyl can also refer to carbon monoxide as a ligand in an inorganic or organometallic complex ; in this situation, carbon is triple-bonded to oxygen : C=O....
).

In the diagram above, the alpha and beta carbons to the left of the carbonyl group are labeled.

Proteins and amino acids
a-carbon is also a term that applies to protein
Protein

Proteins are organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid Residue ....
s and amino acid
Amino acid

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent....
s.






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The alpha carbon in organic chemistry
Organic chemistry

Organic chemistry is a discipline within chemistry which involves the science study of the structure, properties, composition, chemical reaction, and preparation of chemical compounds that contain carbon....
 refers to the first carbon that attaches to a functional group
Functional group

In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules....
 (the carbon is attached at the first, or alpha, position). By extension, the second carbon is the beta carbon, and so on. This nomenclature can also be applied to the hydrogen atoms attached to the carbons. A hydrogen attached to an alpha carbon is called an "alpha-hydrogen" (a-hydrogen), a hydrogen on the beta-carbon is a beta-hydrogen, and so on.

This naming standard is sometimes considered to be not in compliance with IUPAC nomenclature
IUPAC nomenclature

IUPAC nomenclature is a system of naming chemical compounds and of describing the science of chemistry in general. It is developed and kept up to date under the auspices of the International Union of Pure and Applied Chemistry ....
 (which encourages that carbons be identified by number, not by Greek letter); but it nonetheless remains very popular, particularly because it is useful in identifying the relative location of carbons to other functional groups (often a carbonyl
Carbonyl

In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double bond to an oxygen atom : C=O.The term carbonyl can also refer to carbon monoxide as a ligand in an inorganic or organometallic complex ; in this situation, carbon is triple-bonded to oxygen : C=O....
).

Alpahbeta
In the diagram above, the alpha and beta carbons to the left of the carbonyl group are labeled.

Examples


Proteins and amino acids


a-carbon is also a term that applies to protein
Protein

Proteins are organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid Residue ....
s and amino acid
Amino acid

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent....
s. It is the backbone carbon next to the carbonyl carbon. Therefore, reading along the backbone of a typical protein would give a sequence of carbonyl C, a-C, N, carbonyl C, a-C, N, and so on (when reading in the C to N direction). The a-carbon is where the different substituents attach to each different amino acid. That is, the groups hanging off the chain at the a-carbon are what give amino acids their diversity. These groups give the a-carbon its stereogenic properties for every amino acid except for glycine
Glycine

Glycine is the organic compound with the chemical formula NH2CH2COOH. It is the smallest of the 20 amino acids commonly found in proteins, coded by codons GGU, GGC, GGA and GGG....
. Therefore, the a-carbon is a stereocenter
Stereocenter

A stereocenter, or stereogenic center, is any point, though not necessarily an atom, in a molecule bearing groups such that an interchanging of any two groups leads to a stereoisomer ....
 for every amino acid except glycine.

The a-carbon of an amino acid is significant in protein folding
Protein folding

Protein folding is the physical process by which a polypeptide folds into its characteristic and functional protein structure.Each protein begins as a polypeptide, translated from a sequence of mRNA as a linear chain of amino acids....
. 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 a-carbon. In general, a-carbons of adjacent amino acids in a protein are about 3.8 ångströms
Ångström

An ?ngstr?m or angstrom is an internationally recognized non-SI unit of length equal to 0.1 nanometre or 1 metres. It is sometimes used in expressing the sizes of atoms, lengths of chemical bonds and optical spectrum, and dimensions of parts of integrated circuits....
 (380 picometers) apart.

Enols and enolates


The a-carbon is important for enol and enolate based carbonyl
Carbonyl

In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double bond to an oxygen atom : C=O.The term carbonyl can also refer to carbon monoxide as a ligand in an inorganic or organometallic complex ; in this situation, carbon is triple-bonded to oxygen : C=O....
 chemistry as well. Chemical transformations effected by the conversion to either an enolate or enol generally lead to the a-carbon acting as a nucleophile becoming, for example, alkyated in the presence of primary haloalkane
Haloalkane

The haloalkanes are a group of chemical compounds, consisting of alkanes, such as methane or ethane, with one or more halogens linked, such as chlorine or fluorine, making them a type of organic halide....
. An exception is in reaction with silyl- chlorides, -bromides, and -iodides, where the oxygen acts as the nucleophile to produce silyl enol ether
Silyl enol ether

Silyl enol ethers in organic chemistry are a class of organic compounds sharing a common functional group composed of an enolate bonded through its oxygen terminus to an organosilicon group....
.

In ketone
Ketone

In organic chemistry, a ketone is a type of organic compound which contains a carbonyl group bonded to two other carbon atoms in the form:Neither of the substituents R1 and R2 may be equal to hydrogen ....
s (a type of carbonyl) with acidic alpha hydrogen atoms on either side of the carbonyl carbon, selectivity of deprotonation
Deprotonation

Deprotonation is a chemistry term that refers to the removal of a proton from a molecule, forming the conjugate base. The relative ability for a molecule to give up a proton is measured by a pKa value....
 may be achieved under select conditions. At low temperatures (-78°C, i.e. dry ice bath), in aprotic solvents, and with bulky non-equilibrating bases (e.g. LDA
Lithium diisopropylamide

Lithium diisopropylamide is the chemical compound with the formula [2CH]2NLi. Generally abbreviated LDA, it is a strong base used in organic chemistry for the deprotonation of weakly acidic compounds....
) the "kinetic" proton may be removed. The "kinetic" proton is the one which is sterically most accessible. Under thermodynamic conditions
Thermodynamic reaction control

Thermodynamic reaction control or kinetic reaction control in a chemical reaction can decide the composition in a reaction product mixture when competing pathways lead to different products and the reaction conditions influence the selectivity....
 (warmer temperatures, weak base, and protic solvent) equilibrium is established between the ketone and the two possible enolates, the enolate favoured is termed the "thermodynamic" enolate and is favoured because of its lower energy level than the other possible enolate. Thus, by choosing the "correct" conditions to generate an enolate one can increase the yield of the desired product and minimize the formation of the undesired product.