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The
alpha carbon in organic chemistry refers to the first carbon after the carbon that attaches to the functional group (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" (α-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 (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).
α-carbon is also a term that applies to proteins and
amino acids. 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, α-C, N, carbonyl C, α-C, N, and so on (when reading in the C to N 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.
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 angstroms (380 picometers) apart.
The α-carbon is important for enol and
enolate based carbonyl chemistry as well. Chemical transformations effected by the conversion to either an enolate or enol generally lead to the α-carbon acting as a nucleophile becoming, for example, alkyated in the presence of primary haloalkane. An exception is in reaction with silyl- chlorides, -bromides, and -iodides, where the oxygen acts as the nucleophile to produce silyl enol ether.
In
ketones (a type of carbonyl) with acidic
alpha hydrogen atoms on either side of the carbonyl carbon, selectivity of
deprotonation 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.
Lithium_diisopropylamide) the "kinetic" proton may be removed. The "kinetic" proton is the one which is sterically most accessible. Under
thermodynamic reaction control (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.
References
"Hackh's Chemical Dictionary", 1969, page 30.
"Hackh's Chemical Dictionary", 1969, page 95.
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The
alpha carbon in organic chemistry refers to the first carbon after the carbon that attaches to the functional group (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" (α-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 (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).
α-carbon is also a term that applies to proteins and
amino acids. 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, α-C, N, carbonyl C, α-C, N, and so on (when reading in the C to N 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.
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 angstroms (380
picometers) apart.
The α-carbon is important for enol and enolate based
carbonyl chemistry as well. Chemical transformations effected by the conversion to either an enolate or enol generally lead to the α-carbon acting as a nucleophile becoming, for example, alkyated in the presence of primary haloalkane. An exception is in reaction with silyl- chlorides, -bromides, and -iodides, where the oxygen acts as the nucleophile to produce silyl enol ether.
In
ketones (a type of carbonyl) with acidic
alpha hydrogen atoms on either side of the carbonyl carbon, selectivity of deprotonation 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. Lithium_diisopropylamide) the "kinetic" proton may be removed. The "kinetic" proton is the one which is sterically most accessible. Under thermodynamic reaction control (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.
References
"Hackh's Chemical Dictionary", 1969, page 30.
"Hackh's Chemical Dictionary", 1969, page 95.
ar:كربون ألفاur:الفا کاربن
Alpha carbon - Wikipedia, the free encyclopedia
The alpha carbon in organic chemistry refers to the first carbon that attaches to a functional group (the carbon is attached at the first, or alpha, position). [1] By extension ...
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Each amino acid consists of an alpha carbon atom to which is attached a hydrogen atom an amino group (hence "amino" acid) a carboxyl group (-COOH).
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Definition: alpha-oxidation from Online Medical Dictionary
A form of oxidation of fatty acids in which carbons are removed one at a time in the form of CO2; the alpha-carbon is first hydroxylated and then converted into a carbonyl; a deficiency ...
alpha carbon atom - definition of alpha carbon atom in the Medical ...
alpha carbon atom. carbon-2 of a molecule or the carbon atom next to the function group of a molecule, the carbon(s) of which are not included in the lettering.
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Protein: Bifunctional carbon monoxide dehydrogenase/acetyl-CoA synthase (CODH/ACS) alpha (ACS) subunit from Moorella thermoacetica [TaxId: 1525]
Amino acids general structure
... and general formula shows a distinctive side group (R) together with two functional groups (amino and carboxyl), and a H atom all bound to a carbon called the alpha-carbon from ...
