2-Methylbutanoic acid, also known as 2-methylbutyric acid is a branched-chain

alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl group is derived from a cy ...

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...

with the

chemical formula A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...

CH₃CH₂CH(CH₃)CO₂H, classified as a

short-chain fatty acid Short-chain fatty acids (SCFAs) are fatty acids of two to six carbon atoms. The SCFAs' lower limit is interpreted differently, either with one, two, three or four carbon atoms. Derived from intestine, intestinal microbe, microbial fermentation of ...

. It exists in two

enantiomer In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''), also known as an optical isomer, antipode, or optical antipode, is one of a pair of molecular entities whi ...

ic forms, (''R'')- and (''S'')-2-methylbutanoic acid. (''R'')-2-methylbutanoic acid occurs naturally in

cocoa bean The cocoa bean, also known as cocoa () or cacao (), is the dried and fully fermented seed of ''Theobroma cacao'', the cacao tree, from which cocoa solids (a mixture of nonfat substances) and cocoa butter (the fat) can be extracted. Cacao tree ...

s and (''S'')-2-methylbutanoic occurs in many fruits such as apples and apricots, as well as in the scent of the orchid '' Luisia curtisii''.

History

2-Methylbutanoic acid is a minor constituent of ''

Angelica archangelica ''Angelica archangelica'', commonly known as angelica, garden angelica, wild celery, and Norwegian angelica, is a biennial plant from the family Apiaceae, a subspecies of which is cultivated for its sweetly scented edible stems and roots. Like ...

'' and the perennial flowering plant valerian (''

Valeriana officinalis Valerian (''Valeriana officinalis'', Caprifoliaceae) is a perennial flowering plant native to Eurasia. It produces a catnip-like response in cats. Crude extracts of valerian root may have sedative and anxiolytic effects; however, the clinical ...

''), where it co-occurs with

valeric acid Valeric acid or pentanoic acid is a straight-chain alkyl carboxylic acid with the chemical formula . Like other low-molecular-weight carboxylic acids, it has an unpleasant odor. It is found in the perennial flowering plant '' Valeriana offici ...

and

isovaleric acid Isovaleric acid, also known as 3-methylbutanoic acid or β-methylbutyric acid, is a branched-chain alkyl carboxylic acid with the chemical formula (CH3)2CHCH2CO2H. It is classified as a short-chain fatty acid. Like other low-molecular-weight car ...

. The dried root of this plant has been used medicinally since antiquity. The chemical identity of all three compounds was first investigated in the 19th century by oxidation of the components of

fusel alcohol Fusel alcohols or fuselol, also sometimes called fusel oils in Europe, are mixtures of several higher alcohols (those with more than two carbons, chiefly amyl alcohol) produced as a by-product of alcoholic fermentation. The word ''Fusel'' is G ...

, which includes the five-carbon

amyl alcohol Amyl alcohols are alcohols with the formula C5H11OH. Eight are known. A mixture of amyl alcohols (also called amyl alcohol) can be obtained from fusel alcohol. Amyl alcohol is used as a solvent and in esterification, by which is produced amy ...

s. Among the products isolated was a compound which gave a (+)

rotation Rotation or rotational/rotary motion is the circular movement of an object around a central line, known as an ''axis of rotation''. A plane figure can rotate in either a clockwise or counterclockwise sense around a perpendicular axis intersect ...

polarised light , or , is a property of transverse waves which specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. One example of a polarize ...

, indicating it to be the (2''S'') isomer.

Preparation

Racemic In chemistry, a racemic mixture or racemate () is a mixture that has equal amounts (50:50) of left- and right-handed enantiomers of a chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds are produced industrially as r ...

2-methylbutanoic acid can readily be prepared by a

Grignard reaction The Grignard reaction () is an organometallic chemical reaction in which, according to the classical definition, carbon alkyl, allyl, vinyl, or aryl magnesium halides (Grignard reagent) are added to the carbonyl groups of either an aldehyde or ...

using 2-chlorobutane and

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

. It was the target of the first enantioselective synthesis in 1904 when the German chemist W. Marckwald heated ethyl(methyl)malonic acid with the chiral base

brucine Brucine is an alkaloid closely related to strychnine, most commonly found in the ''Strychnos nux-vomica'' tree. Brucine poisoning is rare, since it is usually ingested with strychnine, and strychnine is more toxic than brucine. In chemical synth ...

and obtained an optically active product mixture. Either enantiomer of 2-methylbutanoic acid can now be obtained by

asymmetric hydrogenation Asymmetric hydrogenation is a chemical reaction that adds two atoms of hydrogen to a target (substrate) molecule with three-dimensional spatial selectivity. Critically, this selectivity does not come from the target molecule itself, but from oth ...

tiglic acid Tiglic acid is a monocarboxylic unsaturated organic acid. It is found in croton oil and in several other natural products. It has also been also isolated from the defensive secretion of certain beetles. Properties and uses Tiglic acid has a doubl ...

using a

ruthenium Ruthenium is a chemical element; it has symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is unreactive to most chem ...

BINAP BINAP (2,2′-bis(diphenylphosphino)-1,1′-binaphthyl) is an organophosphorus compound. This Optical isomerism, chiral diphosphines, diphosphine ligand is widely used in chiral synthesis, asymmetric synthesis. It consists of a pair of 2-diphe ...

catalyst.

Reactions

The compound and its enantiomers react as typical carboxylic acids: they can form

amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula , where R, R', and R″ represent any group, typically organyl functional group, groups or hydrogen at ...

ester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...

anhydride An acid anhydride is a type of chemical compound derived by the removal of water molecules from an acid (chemistry), acid. In organic chemistry, organic acid anhydrides contain the functional group . Organic acid anhydrides often form when one ...

, and

chloride The term chloride refers to a compound or molecule that contains either a chlorine anion (), which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond (). The pr ...

derivatives. The acid chloride is commonly used as the intermediate to obtain the others.

Uses

Racemic 2-methylbutanoic acid is a slightly volatile, colorless liquid with a pungent cheesy

odor An odor (American English) or odour ( Commonwealth English; see spelling differences) is a smell or a scent caused by one or more volatilized chemical compounds generally found in low concentrations that humans and many animals can perceive ...

. The smell differs significantly between the two enantiomeric forms. (''S'')-2-Methylbutyric acid has a pleasantly sweet, fruity odor while (''R'')-2-methylbutanoic acid has a pervasive, cheesy, sweaty odor. The main use of the materials, and their esters, is therefore as flavours and food additives. The compounds' safety in this application was reviewed by an

FAO The Food and Agriculture Organization of the United Nations; . (FAO) is a List of specialized agencies of the United Nations, specialized agency of the United Nations that leads international efforts to defeat hunger and improve nutrition ...

and

WHO The World Health Organization (WHO) is a specialized agency of the United Nations which coordinates responses to international public health issues and emergencies. It is headquartered in Geneva, Switzerland, and has 6 regional offices and 15 ...

panel, who concluded that there were no concerns at the likely levels of intake.

Biology

Since 2-methylbutanoic acid and its esters are natural components of many foods, they are present in mammals including humans.

References

External links

{{DEFAULTSORT:Methylbutanoic acid, 2- Flavors Alkanoic acids