Más contenido relacionado La actualidad más candente (20) Similar a fy name reactions sem ii poc i (20) 2. 06
React to convert an
aromatic aldehyde
and an anhydride to
an α,β- unsaturated
carboxylic acid
PERKIN
CONDENSATION
05BENZOIN
CONDENSATION
carbon-carbon bond is
formed when two
molecules of aldehydes
(particularly benzaldehy
-de) reacts with each
other to form a
condensed product
called Benzoin.
INTRODUCTIO
N
04
CROSSED
CANNIZZARO
REACTION
If two different aldehyde
having no –Hydrogen
reacts with each other, it
is called Crossed
Cannizzaro Reaction..
INTRODUCTIO
N
03CANNIZZARO
REACTION
The disproportionation
(self-redox) of aldehydes
lacking α-hydrogen atoms
in presence of strong base
to form salt of an acid and
a primary alcohol is known
as Cannizzaro reaction.
INTRODUCTIO
N
02CROSSED ALDOL
CONDENSATION
An aldol condensation
between two different
aldehydes produces a cross
‐ aldol condensation. If both
aldehydes possess α
hydrogens, a series of
products will form.
INTRODUCTIO
N
01ALDOL
CONDENSATION
Aldehydes that have α hydrogens react
with themselves when mixed with a
dilute aqueous acid or base. The
resulting compounds, β‐hydroxy
aldehydes, are referred to as aldol
compounds because they possess both
an aldehyde and alcohol functional
group.
INTRODUCTION
4. Mechanism
Reaction
© Dr. Atul R. Bendale
Aldol Condensation, in organic chemistry, is a condensation reaction of enolate and carbonyl compound. When enol ions react with
carbonyl compound it forms a β-hydroxyketone or β-hydroxyaldehyde. This process is followed by dehydration which results in conjugated
enone.
Step 1: Removal of α-hydrogen from
the base- enolate ion
Step 2: The first step results in the
formation of carbanion which undergoes
a nucleophilic addition reaction with the
carbonyl group present in the second
molecule of the aldehyde (in this case
ethanal). The second step results in the
formation of the condensation product
Step 3: Protonation of alkoxide ion will
occur due to reaction with water.
Animation
Step 4: Heating of the aldol compound in
the basic solution will help in dehydrating
the product to form α β‐unsaturated
aldehyde compound.
6. Reaction
© Dr. Atul R. Bendale
Cross aldol condensation refers to the aldol condensation reaction that occurs in between two different organic compounds, in this case,
two different aldehydes and ketones. Therefore, the reaction leads to the formation of a series of product. If the α-hydrogen atoms are
present in both aldehyde and ketone, the reaction will produce a mixture of four compounds.
The optimum result of cross aldol condensation reaction is possible if an aldehyde compound contains α-hydrogen and another aldehyde
compound does not contain α-hydrogen. Refer to the example below to see the reaction with the mixture of ethanal and propanal.
The success of these mixed aldol reactions is due to two factors. First, aldehydes are more reactive acceptor electrophiles than ketones, and formaldehyde is
more reactive than other aldehydes. Second, aldehydes lacking alpha-hydrogens can only function as acceptor reactants, and this reduces the number of
possible products by half. Mixed aldols in which both reactants can serve as donors and acceptors generally give complex mixtures of both dimeric (homo)
aldols and crossed aldols. Because of this most mixed aldol reactions are usually not performed unless one reactant has no alpha hydrogens.
Step 1: The hydroxide ion deprotonates
the enolizable aldehyde reversibly & it is
preferentially adds to the non-enolizable
aldehyde, which has the sterically less
hindered and, therefore, more
accessible carbonyl carbon.
Step 2: Alkoxide ion is protonated by
water, Aldol 3 is an enolizable aldehyde. A
small amount of it is converted to the
corresponding enolate ion by the
hydroxide ion.
Step 3: Enolate ion loses a
hydroxide ion.
Mechanism
8. Mechanism
Reaction
The disproportionation (self-redox) of aldehydes lacking αα-hydrogen atoms in presence of strong base to form salt of an acid and a
primary alcohol is known as Cannizzaro reaction.
- Thus, aromatic aldehydes, formaldehydes, trialkyl acetaldehyde, heterocyclic aldehydes undergo Cannizaro reaction.
Step 1 A nucleophile such as a hydroxide ion
is used to attack the carbonyl group of the
given aldehyde, causing a disproportionation
reaction and giving rise to an anion carrying
2 negative charges
Step 2: This resulting intermediate can
now function as a hydride reducing agent.
Due to its unstable nature, the
intermediate releases a hydride anion.
This hydride anion proceeds to attack
another aldehyde molecule. Now, the
doubly charged anion is converted into a
carboxylate anion and the aldehyde is
converted into an alkoxide anion.
Step 3: In this final step, water offers a
proton to the alkoxide anion which
alcohol product. The reaction can
proceed since the alkoxide is more basic
than water. Now, the carboxylate ion
gives carboxylic acid product when acid
workup is used (the acid workup is
required since carboxylate is less basic
than water and therefore cannot obtain a
proton from water).
Cannizzaro Reaction Mechanism details the method to get one
molecule of alcohol and one molecule of carboxylic acid from two
molecules of a given aldehyde. The reaction is executed by a
nucleophilic acyl substitution on an aldehyde where the leaving group
attacks another aldehyde. A tetrahedral intermediate results from the
attack of hydroxide on a carbonyl. This tetrahedral intermediate
collapses, thereby reforming the carbonyl and transferring a hydride
which attacks another colony.
Now, a proton is exchanged by acid and alkoxide ions. When a base of
high concentration is introduced, the aldehyde forms an anion which
has a charge of 2. From this, a hydride ion is transferred to a second
molecule of the aldehyde, forming carboxylate and alkoxide ions. The
alkoxide ion also obtains a proton from the solvent for the reaction. © Dr. Atul R. Bendale
10. 0
Mechanism
Reaction
© Dr. Atul R. Bendale
Aldehydes with alpha hydrogen atoms undergo deprotonation due
to the strongly alkaline conditions of the reaction, leading to
enolates and/or aldol reactions of these enolates where beta
hydroxy aldehydes or ketones are obtained. Therefore, it is not
surprising that the reaction produces only 50% of the required
alcohol and carboxylic acid at ideal conditions. This is why the
crossed Cannizzaro reaction is more commonly used. A sacrificial
aldehyde is combined with a more valuable chemical and
formaldehyde is used as a reductant, oxidizing it to sodium
formate. The required alcohol is obtained from the reduction of
the other aldehyde chemical. Since 2 different aldehydes can be
completely converted into the required product, the yield of the
valuable chemical is increased.
Step 1 A nucleophile such as a hydroxide ion
is used to attack the carbonyl group of the
given aldehyde, causing a disproportionation
reaction and giving rise to an anion carrying
2 negative charges
Step 2: This resulting intermediate can
now function as a hydride reducing agent.
Due to its unstable nature, the
intermediate releases a hydride anion.
This hydride anion proceeds to attack
another aldehyde molecule. Now, the
doubly charged anion is converted into a
carboxylate anion and the aldehyde is
converted into an alkoxide anion.
Step 3: In this final step, water offers a
proton to the alkoxide anion which
alcohol product. The reaction can
proceed since the alkoxide is more basic
than water. Now, the carboxylate ion
gives carboxylic acid product when acid
workup is used (the acid workup is
required since carboxylate is less basic
than water and therefore cannot obtain a
proton from water).
In general, a mixture of aldehydes undergoes a Cannizzaro reaction to yield all possible products. If one of the aldehydes is formaldehyde,
the reaction yields almost exclusively salt of formic acid and the alcohol. corresponding to the other aldehyde. Such a reaction is called a
Crossed Cannizzaro Reaction. If two different aldehyde having no -Hydrogen, it is called Crossed Cannizzaro Reaction
12. 0
Reaction
© Dr. Atul R. Bendale
Benzoin Condensation is an important reaction in which carbon-carbon bond is formed when two molecules of aldehydes (particularly
benzaldehyde) reacts with each other to form a condensed product called Benzoin.
- This reaction is catalyzed by cyanide ion. Benzoin condensation reaction is nothing but the coupling reaction that occurs between
aldehydes for the formation of parent benzoin. In this case, the benzaldehyde is involved in this homocoupling process.
In the third phase, rearrangement occurs and also the removal of the cyanide
ions occurs resulting in the formation of benzoin. The Rearrangement process
results in the reversal of polarity of the carbonyl group, later adding to the
second group of carbonyl atom in the second nucleophilic addition
The first phase of the process includes the reaction of cyanide ions with the
benzaldehyde to form the product called cyanohydrin. In this step, the
cyanide ion or sodium cyanide takes part in the nucleophilic addition reaction,
and it is a reversible reaction. The cyanide ion helps the reaction to occur by
acting as a nucleophile and facilitating the abstraction of protons, thus
forming cyanohydrin. The cyanide ions serve as a catalyst in the reaction.
The second step is the condensation reaction that occurs between the
cyanohydrin and the benzaldehyde.
Mechanism
In the reaction, a strong base deprotonates the former carbonyl C-atom and the second kind of an equivalent aldehyde reacts with the carbanion, thus eliminating
the catalyst and regenerating the carbonyl compound at the end of the condensation reaction. In this Benzoin Condensation reaction, the two different aldehydes
serve two purposes. One aldehyde in the reaction donates the protons and whereas the other aldehyde accepts the protons.
14. Mechanism
Reaction
© Dr. Atul R. Bendale
The Perkin reaction is an organic reaction developed by English chemist William Henry Perkin that is used to make cinnamic acids. It gives
an α,β-unsaturated aromatic acid by the aldol condensation of an aromatic aldehyde and an acid anhydride, in the presence of an alkali
salt of the acid. The alkali salt acts as a base catalyst, and other bases can be used instead
This product is now hydrolyzed to
finally give alpha, beta-
unsaturated acid
Under the influence of the base, the
anhydride gives the carbanion. This
carbanion now attacks the carbonyl
carbon of the aldehyde.
This attack yields an intermediate.
The abstraction of a proton from
the active methyl group of the
intermediate by the given base and
the subsequent elimination of the
hydroxyl group gives unsaturated
anhydride.
