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Page 1
______________________________________________________________ ___
1. Learning Outcomes
After studying this module, you shall be able to:
? Know what are Benzoin condensation and Stobbe condensation reactions
? Learn mechanism of Benzoin and Stobbe condensation reactions
? Know about the role of CN
-
ion in Benzoin condensation
? Identify the products of reduction and oxidation of Benzoin condensation
? Understand the product formation in Benzoin and Stobbe condensation.
2. Introduction
A condensation reaction, also commonly referred to as dehydration synthesis, is a
chemical reaction in which two molecules or moieties (functional groups) combine to
form a larger molecule, together with the loss of a small molecule. Generally the small
molecules lost are water, hydrogen chloride, methanol, or acetic acid but most commonly
in a biological reaction it is water.
In organic synthesis, by far the most important activating groups are the carbonyl and
carboxylic ester groups. Removal of a proton from the a-carbon atom of a carbonyl
compound with base produces the corresponding a-carbanion, which are resonance
stabilized via enolate anion. These enolate ions are involved in base catalysed reactions
of carbonyl compounds.
Amongst these base catalysed reactions, the Stobbe reaction between dialkyl succinate
and aldehyde or a ketone is a good method for making carbon-carbon bonds.
Another important reaction for C-C bond formation is Benzoin condensation, in which
aromatic aldehydes (with no a- hydrogens) in the presence of CN
-
condensed to form
Page 2
______________________________________________________________ ___
1. Learning Outcomes
After studying this module, you shall be able to:
? Know what are Benzoin condensation and Stobbe condensation reactions
? Learn mechanism of Benzoin and Stobbe condensation reactions
? Know about the role of CN
-
ion in Benzoin condensation
? Identify the products of reduction and oxidation of Benzoin condensation
? Understand the product formation in Benzoin and Stobbe condensation.
2. Introduction
A condensation reaction, also commonly referred to as dehydration synthesis, is a
chemical reaction in which two molecules or moieties (functional groups) combine to
form a larger molecule, together with the loss of a small molecule. Generally the small
molecules lost are water, hydrogen chloride, methanol, or acetic acid but most commonly
in a biological reaction it is water.
In organic synthesis, by far the most important activating groups are the carbonyl and
carboxylic ester groups. Removal of a proton from the a-carbon atom of a carbonyl
compound with base produces the corresponding a-carbanion, which are resonance
stabilized via enolate anion. These enolate ions are involved in base catalysed reactions
of carbonyl compounds.
Amongst these base catalysed reactions, the Stobbe reaction between dialkyl succinate
and aldehyde or a ketone is a good method for making carbon-carbon bonds.
Another important reaction for C-C bond formation is Benzoin condensation, in which
aromatic aldehydes (with no a- hydrogens) in the presence of CN
-
condensed to form
_______________________________________________________________ ___
benzoin. In majority of simple cases, as a result of this
reaction, double the number of carbon atoms are present in the final product as compared
to the starting molecule.
3. Benzoin Condensation
The benzoin condensation is a reaction (often called a condensation reaction, for
historical reasons) between two aromatic aldehydes, particularly benzaldehyde. The
reaction is catalyzed by a nucleophile such as the cyanide anion or an N-heterocyclic
carbene. The reaction product is an aromatic acyloin with benzoin as the parent
compound.
An early version of the reaction was developed in 1832 by Justus von Liebig and
Friedrich Woehler during their research on bitter almond oil. The catalytic version of the
reaction was developed by Nikolay Zinin in the late 1830s, and the reaction mechanism
for this organic reaction was proposed in 1903 by A. J. Lapworth.
3.1 Mechanism of Benzoin Condensation
In the first step, the cyanide anion (as sodium cyanide) attacks the carbonyl carbon of the
aldehyde in a nucleophilic addition. Rearrangement of the intermediate results in polarity
reversal of the carbonyl group, which then adds to the carbonyl group of the other
aromatic aldehyde (instead of H
+
transfer as that of Cannizzaro reaction). Proton transfer
and elimination of the cyanide ion results in benzoin as the product.
Mechanism involved:
Page 3
______________________________________________________________ ___
1. Learning Outcomes
After studying this module, you shall be able to:
? Know what are Benzoin condensation and Stobbe condensation reactions
? Learn mechanism of Benzoin and Stobbe condensation reactions
? Know about the role of CN
-
ion in Benzoin condensation
? Identify the products of reduction and oxidation of Benzoin condensation
? Understand the product formation in Benzoin and Stobbe condensation.
2. Introduction
A condensation reaction, also commonly referred to as dehydration synthesis, is a
chemical reaction in which two molecules or moieties (functional groups) combine to
form a larger molecule, together with the loss of a small molecule. Generally the small
molecules lost are water, hydrogen chloride, methanol, or acetic acid but most commonly
in a biological reaction it is water.
In organic synthesis, by far the most important activating groups are the carbonyl and
carboxylic ester groups. Removal of a proton from the a-carbon atom of a carbonyl
compound with base produces the corresponding a-carbanion, which are resonance
stabilized via enolate anion. These enolate ions are involved in base catalysed reactions
of carbonyl compounds.
Amongst these base catalysed reactions, the Stobbe reaction between dialkyl succinate
and aldehyde or a ketone is a good method for making carbon-carbon bonds.
Another important reaction for C-C bond formation is Benzoin condensation, in which
aromatic aldehydes (with no a- hydrogens) in the presence of CN
-
condensed to form
_______________________________________________________________ ___
benzoin. In majority of simple cases, as a result of this
reaction, double the number of carbon atoms are present in the final product as compared
to the starting molecule.
3. Benzoin Condensation
The benzoin condensation is a reaction (often called a condensation reaction, for
historical reasons) between two aromatic aldehydes, particularly benzaldehyde. The
reaction is catalyzed by a nucleophile such as the cyanide anion or an N-heterocyclic
carbene. The reaction product is an aromatic acyloin with benzoin as the parent
compound.
An early version of the reaction was developed in 1832 by Justus von Liebig and
Friedrich Woehler during their research on bitter almond oil. The catalytic version of the
reaction was developed by Nikolay Zinin in the late 1830s, and the reaction mechanism
for this organic reaction was proposed in 1903 by A. J. Lapworth.
3.1 Mechanism of Benzoin Condensation
In the first step, the cyanide anion (as sodium cyanide) attacks the carbonyl carbon of the
aldehyde in a nucleophilic addition. Rearrangement of the intermediate results in polarity
reversal of the carbonyl group, which then adds to the carbonyl group of the other
aromatic aldehyde (instead of H
+
transfer as that of Cannizzaro reaction). Proton transfer
and elimination of the cyanide ion results in benzoin as the product.
Mechanism involved:
____________________________________________________________________________________________________
3.2 Characteristics of Benzoin Condensation
? This reaction is completely reversible, the reversibility is indicated by the fact that
benzoin is heated with another aromatic aldehyde mixed products are obtained.
? Cyanide ion catalyzes the reaction because:
(i) It is good nucleophile
(ii) It is good leaving group
(iii) It increases the acidity of the C-H bond and stabilizes the carbanion that
results from the loss of proton from C.
Page 4
______________________________________________________________ ___
1. Learning Outcomes
After studying this module, you shall be able to:
? Know what are Benzoin condensation and Stobbe condensation reactions
? Learn mechanism of Benzoin and Stobbe condensation reactions
? Know about the role of CN
-
ion in Benzoin condensation
? Identify the products of reduction and oxidation of Benzoin condensation
? Understand the product formation in Benzoin and Stobbe condensation.
2. Introduction
A condensation reaction, also commonly referred to as dehydration synthesis, is a
chemical reaction in which two molecules or moieties (functional groups) combine to
form a larger molecule, together with the loss of a small molecule. Generally the small
molecules lost are water, hydrogen chloride, methanol, or acetic acid but most commonly
in a biological reaction it is water.
In organic synthesis, by far the most important activating groups are the carbonyl and
carboxylic ester groups. Removal of a proton from the a-carbon atom of a carbonyl
compound with base produces the corresponding a-carbanion, which are resonance
stabilized via enolate anion. These enolate ions are involved in base catalysed reactions
of carbonyl compounds.
Amongst these base catalysed reactions, the Stobbe reaction between dialkyl succinate
and aldehyde or a ketone is a good method for making carbon-carbon bonds.
Another important reaction for C-C bond formation is Benzoin condensation, in which
aromatic aldehydes (with no a- hydrogens) in the presence of CN
-
condensed to form
_______________________________________________________________ ___
benzoin. In majority of simple cases, as a result of this
reaction, double the number of carbon atoms are present in the final product as compared
to the starting molecule.
3. Benzoin Condensation
The benzoin condensation is a reaction (often called a condensation reaction, for
historical reasons) between two aromatic aldehydes, particularly benzaldehyde. The
reaction is catalyzed by a nucleophile such as the cyanide anion or an N-heterocyclic
carbene. The reaction product is an aromatic acyloin with benzoin as the parent
compound.
An early version of the reaction was developed in 1832 by Justus von Liebig and
Friedrich Woehler during their research on bitter almond oil. The catalytic version of the
reaction was developed by Nikolay Zinin in the late 1830s, and the reaction mechanism
for this organic reaction was proposed in 1903 by A. J. Lapworth.
3.1 Mechanism of Benzoin Condensation
In the first step, the cyanide anion (as sodium cyanide) attacks the carbonyl carbon of the
aldehyde in a nucleophilic addition. Rearrangement of the intermediate results in polarity
reversal of the carbonyl group, which then adds to the carbonyl group of the other
aromatic aldehyde (instead of H
+
transfer as that of Cannizzaro reaction). Proton transfer
and elimination of the cyanide ion results in benzoin as the product.
Mechanism involved:
____________________________________________________________________________________________________
3.2 Characteristics of Benzoin Condensation
? This reaction is completely reversible, the reversibility is indicated by the fact that
benzoin is heated with another aromatic aldehyde mixed products are obtained.
? Cyanide ion catalyzes the reaction because:
(i) It is good nucleophile
(ii) It is good leaving group
(iii) It increases the acidity of the C-H bond and stabilizes the carbanion that
results from the loss of proton from C.
____________________________________________________________________________________________________
? The benzoin condensation is in effect a
dimerization and not a condensation because a small molecule like water is not
released in this reaction. For this reason the reaction is also called a benzoin
addition.
? In this reaction, the two aldehydes serve different purposes; one aldehyde donates
a proton and one aldehyde accepts a proton. In this way it is possible to synthesize
mixed benzoins, i.e. products with different groups on each half of the product.
? Rate law of Benzoin condensation, r = [ ArCHO]
2
[CN
-
]
? Benzoin is colorless solid (M.P. 157 C) which assumed to tautomerise to ene diol.
3.3 Reactions of Benzoin
3.3.1. Reduction of Benzoin:
Benzoin is sensitive to reduction and can be reduced to different products using
different reaction conditions and catalysts.
(i) Reduction with Na/ EtOH gives hydrobenzoin.
(ii) Reduction with Sn/HCl results in the formation of Deoxybenzoin.
(iii) Clemmensen reduction results in stilbene.
(iv) Complete reduction product dibenzyl is obtained on reducing in the presence of
Ni catalyst.
Page 5
______________________________________________________________ ___
1. Learning Outcomes
After studying this module, you shall be able to:
? Know what are Benzoin condensation and Stobbe condensation reactions
? Learn mechanism of Benzoin and Stobbe condensation reactions
? Know about the role of CN
-
ion in Benzoin condensation
? Identify the products of reduction and oxidation of Benzoin condensation
? Understand the product formation in Benzoin and Stobbe condensation.
2. Introduction
A condensation reaction, also commonly referred to as dehydration synthesis, is a
chemical reaction in which two molecules or moieties (functional groups) combine to
form a larger molecule, together with the loss of a small molecule. Generally the small
molecules lost are water, hydrogen chloride, methanol, or acetic acid but most commonly
in a biological reaction it is water.
In organic synthesis, by far the most important activating groups are the carbonyl and
carboxylic ester groups. Removal of a proton from the a-carbon atom of a carbonyl
compound with base produces the corresponding a-carbanion, which are resonance
stabilized via enolate anion. These enolate ions are involved in base catalysed reactions
of carbonyl compounds.
Amongst these base catalysed reactions, the Stobbe reaction between dialkyl succinate
and aldehyde or a ketone is a good method for making carbon-carbon bonds.
Another important reaction for C-C bond formation is Benzoin condensation, in which
aromatic aldehydes (with no a- hydrogens) in the presence of CN
-
condensed to form
_______________________________________________________________ ___
benzoin. In majority of simple cases, as a result of this
reaction, double the number of carbon atoms are present in the final product as compared
to the starting molecule.
3. Benzoin Condensation
The benzoin condensation is a reaction (often called a condensation reaction, for
historical reasons) between two aromatic aldehydes, particularly benzaldehyde. The
reaction is catalyzed by a nucleophile such as the cyanide anion or an N-heterocyclic
carbene. The reaction product is an aromatic acyloin with benzoin as the parent
compound.
An early version of the reaction was developed in 1832 by Justus von Liebig and
Friedrich Woehler during their research on bitter almond oil. The catalytic version of the
reaction was developed by Nikolay Zinin in the late 1830s, and the reaction mechanism
for this organic reaction was proposed in 1903 by A. J. Lapworth.
3.1 Mechanism of Benzoin Condensation
In the first step, the cyanide anion (as sodium cyanide) attacks the carbonyl carbon of the
aldehyde in a nucleophilic addition. Rearrangement of the intermediate results in polarity
reversal of the carbonyl group, which then adds to the carbonyl group of the other
aromatic aldehyde (instead of H
+
transfer as that of Cannizzaro reaction). Proton transfer
and elimination of the cyanide ion results in benzoin as the product.
Mechanism involved:
____________________________________________________________________________________________________
3.2 Characteristics of Benzoin Condensation
? This reaction is completely reversible, the reversibility is indicated by the fact that
benzoin is heated with another aromatic aldehyde mixed products are obtained.
? Cyanide ion catalyzes the reaction because:
(i) It is good nucleophile
(ii) It is good leaving group
(iii) It increases the acidity of the C-H bond and stabilizes the carbanion that
results from the loss of proton from C.
____________________________________________________________________________________________________
? The benzoin condensation is in effect a
dimerization and not a condensation because a small molecule like water is not
released in this reaction. For this reason the reaction is also called a benzoin
addition.
? In this reaction, the two aldehydes serve different purposes; one aldehyde donates
a proton and one aldehyde accepts a proton. In this way it is possible to synthesize
mixed benzoins, i.e. products with different groups on each half of the product.
? Rate law of Benzoin condensation, r = [ ArCHO]
2
[CN
-
]
? Benzoin is colorless solid (M.P. 157 C) which assumed to tautomerise to ene diol.
3.3 Reactions of Benzoin
3.3.1. Reduction of Benzoin:
Benzoin is sensitive to reduction and can be reduced to different products using
different reaction conditions and catalysts.
(i) Reduction with Na/ EtOH gives hydrobenzoin.
(ii) Reduction with Sn/HCl results in the formation of Deoxybenzoin.
(iii) Clemmensen reduction results in stilbene.
(iv) Complete reduction product dibenzyl is obtained on reducing in the presence of
Ni catalyst.
____________________________________________________________________________________________________
3.3.2. Oxidation of Benzoin:
Benzoin can be easily oxidised using various oxidising agents. One of the most important
product of benzoin oxidation is benzil.
(i) Oxidation with CrO3 results in the formation of an aromatic aldehyde and one
benzoin acid derivative.
(ii) Oxidation of benzoin with nitric acid gives benzil.
(iii) And benzoin oxidation
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FAQs on Benzoin Condensation and Stobbe Condensation - Chemistry Optional Notes for UPSC
| 1. What is the Benzoin Condensation? |  |
Ans. Benzoin condensation is a chemical reaction in which two molecules of benzaldehyde react with a catalytic amount of a base to form a benzoin compound. This reaction is a type of nucleophilic addition reaction and is commonly used in organic synthesis.
| 2. How does the Benzoin Condensation reaction occur? | |
Ans. The Benzoin Condensation reaction occurs when benzaldehyde molecules are treated with a base, such as sodium hydroxide, in the presence of a catalytic amount of thiamine hydrochloride. The base deprotonates one of the benzaldehyde molecules, creating a nucleophile that attacks the carbonyl carbon of another benzaldehyde molecule. This leads to the formation of an intermediate called a benzoin, which undergoes further reaction to form the final benzoin product.
| 3. What are the applications of Benzoin Condensation? | |
Ans. Benzoin condensation has several applications in organic synthesis. It is commonly used to synthesize benzoin derivatives, which can be further transformed into various compounds with pharmaceutical and biological activities. Benzoin condensation is also used in the synthesis of chalcones, which are important intermediates in the production of flavonoids.
| 4. What is Stobbe Condensation? | |
Ans. Stobbe condensation is a type of condensation reaction in organic chemistry. It involves the reaction of an ester with an α,β-unsaturated ketone or aldehyde in the presence of a base to form a β-keto ester. This reaction is named after the German chemist Hermann Stobbe, who first reported it in 1901.
| 5. How does the Stobbe Condensation reaction occur? | |
Ans. The Stobbe Condensation reaction occurs when an ester is treated with an α,β-unsaturated ketone or aldehyde in the presence of a base, such as sodium ethoxide. The base deprotonates the α-carbon of the α,β-unsaturated ketone or aldehyde, creating a nucleophile that attacks the carbonyl carbon of the ester. This leads to the formation of an intermediate, which undergoes further reaction to form the final β-keto ester product.
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