Ketene: Structure, Properties, Preparation & Applications

An organic compound with the formula R′R′′C=C=O and two arbitrary monovalent chemical groups R and R' is recognized as a ketene (or two different replacement sites in the same molecule). It's also possible that the name refers to the simplest ketene, ethenone H2C=C=O. Also, Ketenes are unsaturated ketenes, according to their name, but their chemistry is similar to carboxylic acid anhydrides. 

Ketene is a highly reactive compound that reacts with compounds that have an easily substituted hydrogen atom to produce acetic acid derivatives. The reaction of ketene with acetic acid to form acetic anhydride is the only significant industrial use of ketene.

[Image will be Uploaded Soon]

History of Ketene Chemistry

Acetone, acetic acid, or acetic anhydride are pyrolyzed, or acetyl chloride is treated with a nonprotic nucleophile to produce ketene. It can be used to acetylate nucleophiles to produce esters, amides, and other compounds that are difficult to create with other reagents. Ketenes were used in the early antibiotics penicillin and amoxicillin's synthesis.

Hermann Staudinger, a Nobel Prize-winning German chemist, discovered the ketene family of organic compounds in the early twentieth century. This ketene chemistry history.

Acetone, acetic acid, or acetic anhydride are pyrolyzed, or acetyl chloride is treated with a nonprotic nucleophile to produce ketene.

Preparation of Ketenes  

Ketenes are compounds with cumulated carbonyl and carbon-carbon double bonds that, as one would imagine, have interesting and unusual properties. Aldoketenes are made up of the ketene CH2=C=OCH2=C=O and its monosubstitution products RCH=C=ORCH=C=O (R=R= alkyl or aryl), while ketoketenes are made up of disubstituted ketenes R2C=C=OR2C=C=O.

Ketenes can be made in a variety of ways, but there are only a few popular ones. The most straightforward method is to treat a -bromoacyl bromide with zinc, but the yields are generally low.

The preparation of ketene itself can be done in a variety of ways. Passing 2-propanone vapour over a coil of resistance wire heated electrically to a dull red heat is the most convenient laboratory preparation; the air is omitted to prevent easy combustion:

C-CC-C bonds are the weakest, and fragmentation at 750o yields a methyl radical and an ethanoyl radical.

Methane and ketene are formed when a hydrogen atom is transferred (i.e., disproportionation). Ketene is better made industrially by dehydrating ethanoic acid.

Properties and Ketene Uses

Because of their sp character, ketenes are extremely electrophilic at the carbon bonded to the heteroatom. Ketene can be made with a variety of heteroatoms attached to the sp carbon, like O, S, or Se, and is known as ketene, thioketene, or selenoketene.

Each of the double bonds in ethenone, the simplest ketene, has a different experimental length: the C=O bond is 1,160 meters long, while the C=C bond is 1,314 meters long. The angle formed by the two H atoms is 121.5 degrees, which is close to the theoretically ideal angle formed by the sp² carbon atom and H substituents in alkenes.

Ketenes are inherently unstable and thus cannot be processed. Ethenone dimerizes to give -lactone, a cyclic ester, in the absence of nucleophiles with which to react. The dimerization product of a disubstituted ketene is a substituted cyclobutadione. Dimerization of monosubstituted ketenes can yield either the ester or the diketone product.

The ketene uses are to acetylate nucleophiles to produce esters, amides, and other compounds that are difficult to create with other reagents. Ketenes were used in the early antibiotics penicillin and amoxicillin's synthesis.

Synthesis of Ketene 

Ethenone can be produced by pyrolysis (thermal cracking) of acetone (dimethyl ketene):

CH3−CO−CH3 → CH2=C=O + CH4

This reaction is named the Schmidlin ketene synthesis.

Other ketenes can be made of acyl chlorides by an exclusion reaction in which HCl is lost:

[Image will be Uploaded Soon]

In this reaction, an acidic proton alpha is removed from the carbonyl group by a base, normally triethylamine, resulting in the formation of a carbon-carbon double bond and the loss of a chloride ion:

[Image will be Uploaded Soon]

Ketenes can also be made from α-diazoketones by Wolff rearrangement.

Flash vacuum thermolysis (FVT) with 2-pyridylamines is another way to make ketenes. In 1997, Plüg and Wentrup improved on FVT reactions to generate ketenes with a stable FVT that is moisture insensitive under mild conditions (480°C). N-pyridylamines are produced by combining R-malonates with N-amino (pyridene) and using DCC as a solvent.

Carbonylation of metal-carbenes and in situ reaction of the resulting highly reactive ketenes with suitable reagents such as imines, amines, or alcohols is a more robust approach for preparing ketenes. This method uses Co(II)–porphyrin metalloradicals to catalyze the carbonylation of diazocarbonyl compounds and a variety of Ntosylhydrazones, resulting in ketenes, which then react with a variety of nucleophiles and imines to form esters, amides, and lactams. This method can be used with a wide range of substrates, including carbene precursors, nucleophiles, and imines.