The Blanc reaction , also known as Blanc chloromethylation , is a name reaction in organic chemistry and named after the French chemist Gustave Louis Blanc (1872–1927). They can introduce a chloromethyl group (-CH 2 Cl) into aromatics .
Grassi-Cristaldi and Maselli were pioneers of this reaction around 1898, but Gustave Louis Blanc developed this reaction further around 1923 and it has been known as the Blanc reaction ever since. Temperatures of 105–127 ° C are required for the reaction and it can also be carried out with various Lewis acids (AlCl 3 , NiCl 2 etc.). In addition to hydrochloric acid , phosphoric acid can also be used. Instead of formaldehyde, its trimerized form, i.e. trioxane , can also be used. However, a subsequent reduction should be carried out in order to obtain good yields (between 69 and 92%). This reaction is particularly important for the production of resins , as it represents an alternative to conventional polymerizations.
Proposed reaction mechanism
The reaction proceeds in the sense of an S E Ar reaction ( electrophilic aromatic substitution ). In the presence of acidic catalysts , formaldehyde can also be converted with less reactive aromatics. First, an electrophilic carbocation is formed from the formaldehyde with the help of zinc chloride . This then enters into a bond with an aromatic in 3 after the S E Ar reaction already mentioned . By rearomatization 4 and splitting off the zinc chloride 5 we obtain a benzyl alcohol 6 . The hydroxy group in 6 is protonated to form the oxonium ion 7 . By splitting off water and attacking the chloride ion, benzyl chloride 8 is formed as the end product in a substitution reaction :
Although Blanc's chloromethylation is a very efficient method, it has a serious drawback: the reaction of formaldehyde with hydrogen chloride produces a small amount of highly carcinogenic bis (chloromethyl) ether . Therefore, their necessity should be critically reconsidered when planning the synthesis .
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- Zerong Wang: Comprehensic Organic Name Reactions and Reagents , Volume 1, Wiley, 2009, p. 429, ISBN 978-0-471-70450-8 .
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- External identifiers of or database links to bis (chloromethyl) ether : CAS number: 542-88-1, EC number: 208-832-8, ECHA InfoCard: 100.008.030 , GESTIS substance database : 37690 , PubChem : 10967 , ChemSpider : 21106500 , Wikidata : Q2283186 .
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