Varrentrapp reaction
Encyclopedia
The Varrentrapp reaction is an organic reaction
involving the chemical decomposition
of an α,β-unsaturated
acid
into two other acid fragments by action of molten alkali
. This reaction pioneered by F. Varrentrapp in 1840 has been of some importance in structure elucidation of certain fatty acid
s but has no practical synthetic use.
The original 1840 Varrentrapp reaction concerned the conversion of oleic acid
to palmitic acid
, acetic acid
and hydrogen
gas:
the reaction conditions are harsh: medium molten potassium hydroxide
at temperatures in the range of 250 to 300 °C. Likewise cinnamic acid
is converted to benzoic acid
.
The reaction mechanism
for this reaction is outlined below. Although the alkene
group in the fatty acid can be located at more than one position in the chain, this does not affect the nature of the reaction products. This is because the double bond always migrates to the position next to the carboxylic acid group prior to cleavage. The next step is nucleophilic addition
of a hydroxyl
anion to the double bond of 1 followed by proton migration to intermediate 3. The next step is cleavage to enolate 4a and aldehyde
4b. The enolate on acidic workup will form acetic acid. The aldehyde continues to react with another equivalent of base forming carboxylate 7 together with hydrogen. The hydride transfer is akin the Cannizzaro reaction
.
Organic reaction
Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, photochemical reactions and redox reactions. In organic synthesis,...
involving the chemical decomposition
Chemical decomposition
Chemical decomposition, analysis or breakdown is the separation of a chemical compound into elements or simpler compounds. It is sometimes defined as the exact opposite of a chemical synthesis. Chemical decomposition is often an undesired chemical reaction...
of an α,β-unsaturated
Saturation (chemistry)
In chemistry, saturation has six different meanings, all based on reaching a maximum capacity...
acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
into two other acid fragments by action of molten alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
. This reaction pioneered by F. Varrentrapp in 1840 has been of some importance in structure elucidation of certain fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
s but has no practical synthetic use.
The original 1840 Varrentrapp reaction concerned the conversion of oleic acid
Oleic acid
Oleic acid is a monounsaturated omega-9 fatty acid found in various animal and vegetable fats. It has the formula CH37CH=CH7COOH. It is an odorless, colourless oil, although commercial samples may be yellowish. The trans isomer of oleic acid is called elaidic acid...
to palmitic acid
Palmitic acid
Palmitic acid, or hexadecanoic acid in IUPAC nomenclature, is one of the most common saturated fatty acids found in animals and plants. Its molecular formula is CH314CO2H. As its name indicates, it is a major component of the oil from palm trees . Palmitate is a term for the salts and esters of...
, acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
gas:
the reaction conditions are harsh: medium molten potassium hydroxide
Potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...
at temperatures in the range of 250 to 300 °C. Likewise cinnamic acid
Cinnamic acid
Cinnamic acid is a white crystalline organic acid, which is slightly soluble in water.It is obtained from oil of cinnamon, or from balsams such as storax. It is also found in shea butter and is the best indication of its environmental history and post-extraction conditions...
is converted to benzoic acid
Benzoic acid
Benzoic acid , C7H6O2 , is a colorless crystalline solid and the simplest aromatic carboxylic acid. The name derived from gum benzoin, which was for a long time the only source for benzoic acid. Its salts are used as a food preservative and benzoic acid is an important precursor for the synthesis...
.
The reaction mechanism
Reaction mechanism
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...
for this reaction is outlined below. Although the alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
group in the fatty acid can be located at more than one position in the chain, this does not affect the nature of the reaction products. This is because the double bond always migrates to the position next to the carboxylic acid group prior to cleavage. The next step is nucleophilic addition
Nucleophilic addition
In organic chemistry, a nucleophilic addition reaction is an addition reaction where in a chemical compound a π bond is removed by the creation of two new covalent bonds by the addition of a nucleophile....
of a hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
anion to the double bond of 1 followed by proton migration to intermediate 3. The next step is cleavage to enolate 4a and aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
4b. The enolate on acidic workup will form acetic acid. The aldehyde continues to react with another equivalent of base forming carboxylate 7 together with hydrogen. The hydride transfer is akin the Cannizzaro reaction
Cannizzaro reaction
The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction that involves the base-induced disproportionation of an aldehyde lacking a hydrogen atom in the alpha position...
.