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Malonic Ester, an Important Precursor of Carboxylic Acid Synthesis

By Edited Nov 10, 2015 1 0

Malonic Acid as a Carbanion

One of the most valuable method to prepare carboxylic acid is the use of diethyl malonate, CH2(COOC2H)2, and is called the malonic-ester synthesis. However this depends upon (a) the high acidity of the alpha hydrogen of the diethyl malonate, and (b) the extreme ease with which malonic acid and the substituted malonic acids undergo decarboxylation. Like acetoacetatic ester, malonic acid contains alpha hydrogens that are particularly acidic: they are alpha to two keto groups. When treated with sodium ethoxide in absolute alcohol, diethyl malonate is converted largely into its salt called sodium diethyl malonate.

H-CH(COOC2H5)2 (stronger acid) + Na+ -OC2H— CH(COOC2H5)2-Na+(sodium malonic ester) + HOC2H5(weaker acid)

Reaction of this salt with an alkyl halide yields a substituted malonic ester, an ethyl alkyl malonate, called as an alkyl malonic ester:

CH(COOC2H5)2-Na+ + RX —RCH(COOC2H5)2 (ethyl alkyl malonate)+ Na+X-

This reaction involves nucleophilic attack on the alkyl halide by the carbanion, CH(COOC2H5)2- and as we might expect gives highest yield with primary alkyl halides, lower yields with secondary alkyl halides and is worthless for tertiary alkyl halides and for aryl halides.

The alkylmalonic ester still contains one ionizable hydrogen and on treatment with sodium ethoxide it, too can be converted into its salt; this salt can react with any alkyl halide which may be the same as or different from, the first alkyl halide to yield a dialkyl malonate.

R-CH(COOC2H5)2 + Na+ -OC2H5 — R-C(COOC2H5)2Na+ + C2H5OH

R-C(COOC2H5)Na+ + R’X —RR’C(COOC2H5)2 (dialkylmalonic ester) + Na+X-

Conversion of malonic ester into substituted acetic acids

Conversion of malonic ester into substituted acetic acids

A malonic ester synthesis yields an acetic acid in which one or two hydrogens have been replaced by alkyl groups.

Acidic Character of α- Hydrogen Atoms of Malonic Ester

The acidity of diethyl malonate thus permits the preparation of substituted products containing one or two alkyl groups. Now how can these substituted malonic esters be used to make carboxylic acids? When heated above its melting point, malonic acid readily loses carbon dioxide to form substituted acetic acids. The monoalkylmalonic and dialkylmalonic esters we have prepared are readily converted into monocarboxylic acids by hydrolysisacidification, and heat:

RCH(COOC2H)2 H2O/OH heat RCH(COO)

A malonic ester synthesis yields an acetic acid in which one or two hydrogens have been replaced by alkyl groups.

Synthesis of Isocaproic Acid From the Malonic Ester

Synthesis of Isocaproic Acid From the Malonic Ester

use of isobutyl bromide as a alkylating agent to synthesize the isocaproic acid, which is also called as 4 methyl pentanoic acid

Planning for Malonic Ester Synthesis

In planning a malonic-ester synthesis, our problem is to select the proper alkyl halide or halides; to do this we have only to look at the structure of the acid we want. Isocaproic acid for example (CH3)3CHCH2CH2COOH can be considered as acetic acid in which one hydrogen has been replaced by an isobutyl group. To prepare this acid by the malonic ester synthesis, we would have to use isobutyl bromide as the alkylating agent:

An isomer of isocaproic acid alpha methylvaleric acid, CH3CH2CH2CH(CH3)COOH, can be considered as acetic acid in which one hydrogen has been replaced by a n propyl group and a second hydrogen has been replaced by a methyl group;we must therefore use two alkyl halides, n peopyl bromide, and methyl bromide.

Synthesis of α- Methylvaleric Acid

Synthesis of α- Methylvaleric Acid

To synthesize α methylvaleric acid which is also called as 2 methylpentanoic acid, two alkylating agents are used. Firts n propyl bromide is used while in succesive step methyl bromide is used

Modification of Malonic Ester Synthesis for Desired Products

The basic malonic-ester synthesis outlined above can be modified so as to obtain desired products. For example, potassium tert-butoxide; alkyl sulfonate instead of halides; polar aprotic solvents like DMSO or DMF.

In place of simple alkyl halides, certain other halogen-containing compounds may be used, in particular the readily available α- bromo ester, which yield substituted succinic acids by the malonic ester synthesis as shown in the figure (iv) below.

Malonic ester synthesis modification

Malonic ester synthesis modification

Synthesis of α methyl succinic acid by some modification of malonic ester synthesis in which ethyl α bromopropionate as alkylating agent

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