Alpha carbon


Alpha carbon

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).ref|Hackha By extension, the second carbon is the beta carbon,ref|Hackhb 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).

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

Examples

Proteins and amino acids

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 ångströms (380 picometers) apart.

Enols and enolates

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. LDA) the "kinetic" proton may be removed. The "kinetic" proton is the one which is sterically most accessible. Under thermodynamic conditions (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.


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Alpha — may refer to:The Greek letter alphaAlpha (letter), a letter in the Greek alphabet. α may be used as the symbol for: *Alpha channel, describing transparency and opacity in computer graphics *Angle of attack in aerodynamics *Common base current… …   Wikipedia

  • Alpha (letter) — Alpha (uppercase Α, lowercase α; el. Αλφα) is the first letter of the Greek alphabet. In the system of Greek numerals it has a value of 1. It was derived from the Phoenician letter Aleph .In both Classical Greek and Modern Greek, alpha represents …   Wikipedia

  • alpha oxidation — oxidation of a fatty acid at the alpha carbon, that adjacent to the carboxyl group, as occurs in the metabolism of phytanic acid. Written also α oxidation. Oxidation of a saturated fatty acid having (n + 4) carbon atoms; α oxidation… …   Medical dictionary

  • alpha-Methyltryptamine — α Methyltryptamine Systematic (IUPAC) name 2 (1H indol 3 …   Wikipedia

  • Alpha cleavage — Alpha cleavage, (α cleavage) in organic chemistry, refers to the act of breaking the carbon carbon bond,cite book |author=Hathaway, Bruce A. |authorlink= |editor= |others= |title=Organic chemistry the easy way |edition= |language=… …   Wikipedia

  • Alpha-olefin — Alpha olefins (or α olefins) are a family of organic compounds which are olefins or alkenes with a chemical formula CxH2x, distinguished by having a double bond at the primary or alpha (α) position. See the illustration below. This location of a… …   Wikipedia

  • alpha — [al′fə] n. [Gr < NW Sem: cf. Heb alef, ALEPH] 1. the first letter of the Greek alphabet (Α, α) 2. the beginning of anything 3. [A ] Astron. the name assigned to the brightest star in each constellation: followed by the constellation s name in… …   English World dictionary

  • Carbon star — A carbon star is a late type star similar to a red giant (or occasionally to a red dwarf) whose atmosphere contains more carbon than oxygen; the two elements combine in the upper layers of the star, forming carbon monoxide, which consumes all the …   Wikipedia

  • alpha-Linolenic acid — Not to be confused with linoleic acid. This article is about alpha linolenic acid. For other uses, see Linolenic acid. α Linolenic acid …   Wikipedia

  • Carbon monoxide poisoning — This article is about carbon monoxide poisoning. For general information on carbon monoxide, see carbon monoxide. Carbon monoxide poisoning Classification and external resources Spacefilling model of a carbon monoxide molecule …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.