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Concept
Α-Halo ketone
Summary
In organic chemistry, an α-halo ketone is a functional group consisting of a ketone group or more generally a carbonyl group with an α-halogen substituent. α-Halo ketones are alkylating agents. Prominent α-halo ketones include phenacyl bromide and chloroacetone.
The general structure is RR′C(X)C(=O)R where R is an alkyl or aryl residue and X any one of the halogens.
The preferred conformation of a halo ketone is that of a cisoid with the halogen and carbonyl sharing the same plane as the steric hindrance with the carbonyl alkyl group is generally larger.
Halo ketones and halo carbonyl compounds in general are synthesized by reaction of carbonyl compounds with sources of X+ (X = halogen), which is provided using halogens:
RC(O)CH3 + X2 → RC(O)CH2X + HX
Specialized sources of electrophilic halogenating agents include N-Bromosuccinimide and 1,3-dibromo-5,5-dimethylhydantoin (DBDMH). In the Nierenstein reaction an acyl chloride reacts with diazomethane
Efforts are reported in asymmetric synthesis of halo carbonyls through organocatalysis. In one study an acid chloride is converted into an α-halo ester with a strong base (sodium hydride), a bromine donor and an organocatalyst based on proline and quinine:
In the proposed reaction mechanism the base first converts the acid chloride to the ketene, the organocatalyst then introduces chirality through its quinonoid tertiary amine, forming a ketene adduct.
Halo ketones take part in several reaction types, especially since they are bifunctional, with two electrophilic sites (α-carbon and carbonyl carbon). In one manifestation of this duality, they are precursors to heterocycles. Aminothiazoles are produced by reaction of chloroketones with thiourea and with thioamides. Pyrroles may be synthesized by reaction of halo ketones with dicarbonyls and ammonia in the Hantzsch pyrrole synthesis.
Illustrative of their alkylating activity are reactions with potassium iodide in acetone, chloroacetone reacts faster than 1-chloropropane by a factor of 36,000.
Official source
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