Blanc reaction

An electron-withdrawing substituent leads to a product where it is bound to a saturated carbon center. Benzoic acid 9 is reduced to the cyclohexa-2,5-diene carboxylic acid 10  

The introduction of a chloromethyl group on aromatic compounds (e.g. benzene 1) by reaction with formaldehyde 2 and gaseous hydrogen chloride in the presence of a catalyst is called the Blanc reaction  

In a first reaction step the formaldehyde 2 is protonated, which increases its reactivity for the subsequent electrophilic aromatic substitution at the benzene ring. The cationic species 4 thus formed loses a proton to give the aromatic hydroxymethyl derivative 5, which further reacts with hydrogen chloride to yield the chloromethylated product 3  

The rate-determining step is the electrophilic aromatic substitution as in the closely related Friedel-Crafts reaction. Both reactions have in common that a Lewis acid catalyst is used. For the Blanc reaction zinc chloride is generally employed, and the formation of the electrophilic species can be formulated as follows  

Electron-rich aromatic substrates can react without a catalyst present. Modern variants of the Blanc reaction use chloromethyl ether (e.g. (C1CH2)20, ClCH20Me) or methoxyacetyl chloride, since those reagents are more reactive and give higher yields. 

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An important side reaction is the formation of diaryl methane derivatives ArCHaAr. Moreover poly substituted products may be obtained as minor products. Aromatic compounds have been treated with formaldehyde and hydrogen bromide or hydrogen iodide instead of hydrogen chloride. The formaldehyde may be replaced by another aldehyde the term Blanc reaction however stands for the chloromethylation only. 

A functional group is introduced to the polystyrene 10 by chloromethylation Blanc reaction) in order to allow for reaction with the substrate 11. The polymer-bound diester is then treated with base to initiate the Dieckmann condensation. 

As an illustrating example for the application of the Friedel-Crafts acylation in the synthesis of complex molecules, its use in the synthesis of [2.2.2]cyclophane 13 by Cram and Truesdale shall be outlined. The reaction of [2.2]paracyclo-phane 10 with acetyl chloride gives the acetyl-[2.2]paracyclophane 11, which is converted into the pseudo-geminal disubstituted phane 12 by a Blanc reaction, and further to the triple bridge hydrocarbon 13 ... 

Methods to transform ketones into alcohols are based on the nse of alkali metals as electron donors. One of these methods is the Bouveanlt-Blanc reaction—a ketone is dissolved in alcohol and boiled with an excess of sodinm. Sometimes, a ketone-alcohol mixture is rapidly added to molten sodinm. 

The reaction of esters with sodium in ethanol is referred to as the Bouveault-Blanc reaction. Prior to the discovery of complex metal hydrides, this reaction was the only method for the reduction of esters to alcohols. The diesters shown in Figure 17.59 produce a diol in this way. 

Although lactones may be reduced electrochemically or via Bouveault-Blanc reactions to produce diols, such reactions are more frequently used to prepare lactols. Both cathodic (Hg or Pb) and Na/Hg reduction are useful in the preparation of alditols from aldonic acid y-lactones. The reductions may be easily stopped at the intermediate aldose stage. ... 

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