Heterolytic decarbonylation

Heterolytic decarbonylation is undergone by various carboxylic acids and carboxylic acid derivatives such as acid chlorides and esters provided that they are convertible by, e.g., very strong acids into acyl cations the cations then eliminate carbon monoxide provided that the group R is such that it can be stabilized by loss of a proton or that the electron deficiency resulting from elimination of carbon monoxide can be made good in some way  

This decarbonylation mechanism is involved in the following reactions  

Oxalic acid decomposes to carbon monoxide, carbon dioxide, and water when heated alone above 100° or when treated with concentrated sulfuric acid. 

Oxalyl chloride decomposes to carbon monoxide and phosgene when gently warmed with aluminum chloride 50 

Intermediates analogous to the carboxonium salt formulated in the preceding example are also formed during Friedel-Crafts acylations and these can undergo decarbonylation, so that reaction of an aromatic compound with an acid chloride may result in alkylation instead of acylation for example, 

---

The following observations should also be included among the heterolytic decarbonylations ... 

Photolysis of oc-oxo carboxylic esters takes an unusual course 73 in contrast to the result of heterolytic decarbonylation, this type of compound yields, alongside carbon monoxide, a ketone and an aldehyde, and not the carboxylic ester containing one fewer carbon atoms the reaction can be generalized as follows, where R = alkyl or aryl and R and R" = alkyl and/or H ... 

---