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 is derived from a cycloalk ...

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...

with the

chemical formula In chemistry, 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, ...

CH₃CH₂CH(CH₃)CO₂H, classified as a

short-chain fatty acid Short-chain fatty acids (SCFAs) are fatty acids with fewer than six carbon atoms. Derived from intestinal microbial fermentation of indigestible foods, SCFAs are the main energy source of colonocytes, making them crucial to gastrointestinal health. ...

. It exists in two

enantiomer In chemistry, an enantiomer ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical ant ...

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

cocoa bean The cocoa bean (technically cocoa seed) or simply cocoa (), also called the cacao bean (technically cacao seed) or cacao (), is the dried and fully fermented seed of ''Theobroma cacao'', from which cocoa solids (a mixture of nonfat substances ...

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 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 several oth ...

'' and the perennial flowering plant valerian (''

Valeriana officinalis Valerian (''Valeriana officinalis'', Caprifoliaceae) is a perennial flowering plant native to Europe and Asia. In the summer when the mature plant may have a height of , it bears sweetly scented pink or white flowers that attract many fly specie ...

''), 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 officina ...

and isovaleric acid. 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 Ger ...

, which includes the five-carbon

amyl alcohol An amyl alcohol is any of eight alcohols with the formula C5H12O. A mixture of pentyl, 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 amyl a ...

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

rotation Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...

polarised light Polarization (also polarisation) is a property applying to transverse waves that 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 ...

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

Preparation

Racemic In chemistry, a racemic mixture, or racemate (), is one that has equal amounts 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 racemates. ...

2-methylbutanoic acid can readily be prepared by a

Grignard reaction The Grignard reaction () is an organometallic chemical reaction in which alkyl, allyl, vinyl, or aryl-magnesium halides ( Grignard reagent) is added to a carbonyl group in an aldehyde or ketone. This reaction is important for the formation of ...

using 2-chlorobutane and

carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

. 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 synthetic ch ...

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 othe ...

of tiglic acid using a

ruthenium Ruthenium is a chemical element with the Symbol (chemistry), 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 inert to ...

BINAP BINAP (2,2′-bis(diphenylphosphino)-1,1′-binaphthyl) is an organophosphorus compound. This chiral diphosphine ligand is widely used in asymmetric synthesis. It consists of a pair of 2-diphenylphosphinonaphthyl groups linked at the 1 and 1� ...

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 compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...

ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...

anhydride An organic acid anhydride is an acid anhydride that is an organic compound. An acid anhydride is a compound that has two acyl groups bonded to the same oxygen atom. A common type of organic acid anhydride is a carboxylic anhydride, where the pa ...

, and

chloride 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 ...

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 (English in the Commonwealth of Nations, Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is caused by one or more volatilized chemical compounds ...

. 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)french: link=no, Organisation des Nations unies pour l'alimentation et l'agriculture; it, Organizzazione delle Nazioni Unite per l'Alimentazione e l'Agricoltura is an intern ...

and

WHO Who or WHO may refer to: * Who (pronoun), an interrogative or relative pronoun * Who?, one of the Five Ws in journalism * World Health Organization Arts and entertainment Fictional characters * Who, a creature in the Dr. Seuss book '' Horton He ...

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