Aldehydes Clemmensen reduction

Clemmensen reduction Aldehydes and ketones may generally be reduced to the corresponding hydrocarbons by healing with amalgamated zinc and hydrochloric acid. 

Clemmensen reduction of aldehydes and ketones. Upon reducing aldehydes or ketones with amalgamated zinc and concentrated hydrochloric acid, the main products are the hydrocarbons (>C=0 —> >CHj), but variable quantities of the secondary alcohols (in the case of ketones) and unsaturated substances are also formed. Examples are ... 

Clemmensen reduction (Section 12 8) Method for reducing the carbonyl group of aldehydes and ketones to a methylene... 

Common catalyst compositions contain oxides or ionic forms of platinum, nickel, copper, cobalt, or palladium which are often present as mixtures of more than one metal. Metal hydrides, such as lithium aluminum hydride [16853-85-3] or sodium borohydride [16940-66-2] can also be used to reduce aldehydes. Depending on additional functionahties that may be present in the aldehyde molecule, specialized reducing reagents such as trimethoxyalurninum hydride or alkylboranes (less reactive and more selective) may be used. Other less industrially significant reduction procedures such as the Clemmensen reduction or the modified Wolff-Kishner reduction exist as well. 

By application of the Clemmensen reduction,aldehydes and ketones 1 can be converted into the corresponding hydrocarbons 2. As the reducing agent zinc amalgam, together with concentrated hydrochloric acid or gaseous hydrogen chloride, is used. 

Another important synthetic method for the reduction of ketones and aldehydes to the corresponding methylene compounds is the Woljf-Kishner reduction. This reaction is carried out under basic conditions, and therefore can be applied for the reduction of acid-sensitive substrates it can thus be regarded as a complementary method. The experimental procedure for the Clemmensen reduction is simpler however for starting materials of high molecular weight the Wolff-Kishner reduction is more successful. 

Reduction of saturated aliphatic aldehydes to alkanes was carried out by refluxing with amalgamated zinc and hydrochloric acid (Clemmensen reduction) [760, 758] (p. 28) or by heating with hydrazine and potassium hydroxide (Wolff-Kizhner reduction) [280, 759] (p. 34). Heptaldehyde gave heptane in 72% yield by the first and in 54% yield by the second method. 

Like any aldehydes aromatic aldehydes undergo Clemmensen reduction [758, 778] and Wolff-Kizhner reduction [759, 774] and give the corresponding methyl compounds, generally in good yields. The same effect is accomplished by conversion of the aldehydes to p-toluenesulfonyl hydrazones followed by reduction with lithium aluminum hydride (p. 106). 

Reduction of unsaturated aromatic aldehydes to unsaturated hydrocarbons poses a serious problem, especially if the double bond is conjugated with the benzene ring or the carbonyl or both. In Clemmensen reduction the a,)8-unsaturated double bond is usually reduced [160], and in Wolff-Kizhner reduction a cyclopropane derivative may be formed as a result of decomposition of pyrazolines formed by intramolecular addition of the intermediate hydrazones across the double bonds [280]. The only way of converting unsaturated aromatic aldehydes to unsaturated hydrocarbons is the reaction of... 

Conversion of unsaturated aromatic aldehydes to saturated hydrocarbons can be realized by Clemmensen reduction [160]. 

Selective reduction of aldehydes or ketones, either by Clemmensen reduction (see Section 5.7.17) or Woljf-Kishner reduction (see Section 5.7.18) yields alkanes. 

Aryl alkyl ketones are readily prepared by the Friedel-Crafts acylation process (see Section 6.11.1, p. 1006) and their Clemmensen reduction constitutes a more efficient procedure for the preparation of monoalkylbenzenes than the alternative direct Friedel-Crafts alkylation reaction (see below). Alternatively aldehydes and ketones may be reduced to the corresponding hydrocarbon by the Wolff-Kishner method which involves heating the corresponding hydrazone or semicarbazone with potassium hydroxide or with sodium ethoxide solution. 

The Clemmensen Reduction allows the deoxygenation of aldehydes or ketones, to produce the corresponding hydrocarbon. 

Several procedures can be used to convert the carbonyl group of an aldehyde or ketone to a methylene group. One reaction, known as the Clemmensen reduction, employs amalgamated zinc (zinc plus mercury) and hydrochloric acid as the reducing agent. An example is provided by the following equation ... 

Another reaction that can be used to accomplish the same transformation is the Wolff-Kishner reduction. In this procedure the aldehyde or ketone is heated with hydrazine and potassium hydroxide in a high boiling solvent. An example is provided in the following equation. (The mechanism for the Wolff-Kishner reduction is presented in Section 18.8.) The Clemmensen reduction and the Wolff-Kishner reduction are... 

In actual use, H2 is not a good reagent for deoxygenation of ketones and aldehydes. Deoxygenation can be accomplished by either the Clemmensen reduction (under acidic conditions) or the Wolff-Kishner reduction (under basic conditions). 

Clemmensen Reduction (Review) The Clemmensen reduction commonly converts acylbenzenes (from Friedel-Crafts acylation, Section 17-1 IB) to alkylbenzenes, but it also works with other ketones and aldehydes that are not sensitive to acid. The carbonyl compound is heated with an excess of amalgamated zinc (zinc treated with mercury) and hydrochloric acid. The actual reduction occurs by a complex mechanism on the surface of the zinc. 

A four-electron reduction that replaces the carbonyl oxygen atom of a ketone or aldehyde with two hydrogen atoms. The Clemmensen reduction and the Wolff-Kishner reduction are the two most common methods of deoxygenation, (p. 863)... 

Reduction of carbonyl group to methylene via thioacetals In contrast to the Clemmensen reduction (section 6.4.3) and Wolff-Kishner reduction (section 6.4.4), this method does avoid treatment with strong acid or base but requires two separate steps. The first step is to convert the aldehyde or ketone into a thioacetal. The second step involves refluxing an acetone solution of the thioacetal over a Raney nickel. This reduction method is known as Mozingo reduction. Hydrazine can also be used in the second step. 

Clemmensen reduction Aldehydes and ketones are reduced by heating with a solution of zinc metal in mercury (zinc amalgam) and hydrochloric acid. 

Aldehydes and ketones may be reduced to the corresponding primary and secondary alcohols by reagents such as lithium aluminium hydride, sodium borohydride, sodium and ethanol or hydrogen over a platinum catalyst. A ketone is reduced to a methylene group under more vigorous conditions with zinc amalgam and concentrated hydrochloric acid (the Clemmensen reduction) or treatment of the hydrazone with alkali (the Wolff-Kishner reduction)) (Scheme 3.39). 

Because of carbonium ion generation, aromatic aldehydes and ketones can usually be reduced more easily than the corresponding aliphatic compounds. However, a modified Clemmensen reduction is an effective method to reduce isolated aliphatic carbonyl groups directly to methylene groups, and typical examples are shown in equations (4)-(6)." ... 

In the case of aryl aldehydes and ketones, benzaldehyde afforded benzyl alcohol as the major product, but acetophenone and its para-substituted derivatives carrying such groups as OMe, Cl or OH provided ethylbenzene derivatives in good yields. As with Clemmensen reduction, the alcohol produced in this reduction cannot be further reduced, and the alcohol is not therefore an intermediate. Still uncertain in the reaction mechanism of electrolytic reduction, however, is the role of adsorbed hydrogen. ... 

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