In the
nomenclature
Nomenclature (, ) is a system of names or terms, or the rules for forming these terms in a particular field of arts or sciences. The principles of naming vary from the relatively informal naming conventions, conventions of everyday speech to the i ...
of
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J.; ...
, a locant is a term to indicate the position of a
functional group
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest ...
or
substituent
A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as ''side ...
within a
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
.
Numeric locants
The
International Union of Pure and Applied Chemistry
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
(IUPAC) recommends the use of numeric prefixes to indicate the position of substituents, generally by identifying the
parent hydrocarbon chain
In chemistry, a parent hydride in IUPAC nomenclature refers to a main group compound with the formula , where A is a main group element. The names of parent hydrides end with ''-ane'', analogous with the Alkane#Nomenclature, nomenclature for alkan ...
and assigning the carbon atoms based on their substituents in
order of precedence
An order of precedence is a sequential hierarchy of nominal importance and can be applied to individuals, groups, or organizations. Most often it is used in the context of people by many organizations and governments, for very formal and state o ...
. For example, there are at least two
isomer
In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers.
Iso ...
s of the linear form of
pentanone Pentanone may refer to the following ketones containing five carbon atoms:
* 2-Pentanone (Methyl propyl ketone, MPK)
** 3-Methyl-2-butanone (Methyl isopropyl ketone, MIPK)
* 3-Pentanone (Diethyl ketone, DEK)
See also
* Cyclopentanone
Cyclope ...
, a
ketone
In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bo ...
that contains a chain of exactly five
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
atoms. There is an
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
atom bonded to one of the middle three carbons (if it were bonded to an end carbon, the molecule would be an
aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
, 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
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas, and ...
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-2-one or
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
![Butyric acid carbons 2](https://upload.wikimedia.org/wikipedia/commons/8/8f/Butyric_acid_carbons_2.svg)
Another common system uses
Greek letter
The Greek alphabet has been used to write the Greek language since the late 9th or early 8th century BCE. It is derived from the earlier Phoenician alphabet, and was the earliest known alphabetic script to have distinct letters for vowels as w ...
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
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
atom that attaches to a
functional group
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest ...
, such as a
carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a ...
. 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.
The
nomenclature
Nomenclature (, ) is a system of names or terms, or the rules for forming these terms in a particular field of arts or sciences. The principles of naming vary from the relatively informal naming conventions, conventions of everyday speech to the i ...
can also be applied to the
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
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
nitrostyrene
β-Nitrostyrene is an aromatic compound and a nitroalkene used in the synthesis of indigo dye and the slimicide bromo-nitrostyrene.
Applications
β-Nitrostyrene is a chemical precursor for slimicides and dyes. Specifically bromo-nitrostyrene is ...
and
phenethylamine
Phenethylamine (PEA) is an organic compound, natural monoamine alkaloid, and trace amine, which acts as a central nervous system stimulant in humans. In the brain, phenethylamine regulates monoamine neurotransmission by binding to trace amin ...
are quite similar; the former can even be
reduced into the latter. However, nitrostyrene's α-carbon atom is adjacent to the
phenyl
In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C6 H5, and is often represented by the symbol Ph. Phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydrogen ...
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.
File:Beta-nitrostyrene.svg, Nitrostyrene
File:Fenyloetyloamina.svg, Phenethylamine
Proteins and amino acids
In
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s and
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s, 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 –
—Cα—carbonyl Csub>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 its
stereogenic
In stereochemistry, a stereocenter of a molecule is an atom (center), axis or plane that is the focus of stereoisomerism; that is, when having at least three different groups bound to the stereocenter, interchanging any two different groups cr ...
properties for every amino acid except for
glycine
Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid (carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinogeni ...
. Therefore, the α-carbon is a
stereocenter
In stereochemistry, a stereocenter of a molecule is an atom (center), axis or plane that is the focus of stereoisomerism; that is, when having at least three different groups bound to the stereocenter, interchanging any two different groups cr ...
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
protein folding
Protein folding is the physical process by which a protein chain is translated to its native three-dimensional structure, typically a "folded" conformation by which the protein becomes biologically functional. Via an expeditious and reproduci ...
. 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
ångström
The angstromEntry "angstrom" in the Oxford online dictionary. Retrieved on 2019-03-02 from https://en.oxforddictionaries.com/definition/angstrom.Entry "angstrom" in the Merriam-Webster online dictionary. Retrieved on 2019-03-02 from https://www.m ...
s (380
picometer
The picometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: pm) or picometer (American spelling) is a unit of length in the International System of Units (SI), equal to , or one trillionth of ...
s) apart.
Enols and enolates
The α-carbon is important for
enol
In organic chemistry, alkenols (shortened to enols) are a type of reactive structure or intermediate in organic chemistry that is represented as an alkene ( olefin) with a hydroxyl group attached to one end of the alkene double bond (). The te ...
- and
enolate
In organic chemistry, enolates are organic anions derived from the deprotonation of carbonyl () compounds. Rarely isolated, they are widely used as reagents in the synthesis of organic compounds.
Bonding and structure
Enolate anions are electr ...
-based
carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a ...
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 a
nucleophile
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
, becoming, for example,
alkylated
Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecting al ...
in the presence of primary
haloalkane
The haloalkanes (also known as halogenoalkanes or alkyl halides) are alkanes containing one or more halogen substituents. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely us ...
. An exception is in reaction with
silyl
Silylation is the introduction of one or more (usually) substituted silyl groups (R3Si) to a molecule. The process is the basis of organosilicon chemistry.
Of organic compounds
Alcohols, carboxylic acids, amines, thiols, and phosphates can be sily ...
chlorides
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts ...
,
bromide
A bromide ion is the negatively charged form (Br−) of the element bromine, a member of the halogens group on the periodic table. Most bromides are colorless. Bromides have many practical roles, being found in anticonvulsants, flame-retardant ...
s, and
iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. In everyday life, iodide is most commonly encountered as a component of iodized salt, which many governments mandate. Worldwide, iodine defic ...
s, where the
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
acts as the nucleophile to produce
silyl enol ether
Silyl enol ethers in organic chemistry are a class of organic compounds that share a common functional group composed of an enolate bonded through its oxygen end to an organosilicon group. They are important intermediates in organic synthesis.
Syn ...
.
See also
*
IUPAC nomenclature
A chemical nomenclature is a set of rules to generate systematic names for chemical compounds. The nomenclature used most frequently worldwide is the one created and developed by the International Union of Pure and Applied Chemistry (IUPAC).
The ...
References
{{Reflist
Chemistry prefixes
Organic chemistry