Reaction of synthetic importance

Importance
Hemang Bhatt
Student (B.Pharm) (2nd Year)
Smt.N.M.Padalia Pharmacy College , Ahemdabad

Here we are studying total 9 topic.
Which is also known as synthetic reactions and also
further using in various type of synthesis.
 Metal Hydride Reduction
 Clemmensen Reduction
 Birch Reduction
 Wolf Kishner Reduction
 Oppenauer Oxidation
 Schmidt Reduction
(To Be Continued..)

 Dakin Reaction
 Beckmanns Rearrangement
 Claisen – Schmidt Condensation.
This all reactions are very important in end
semester exam.
It should be asked as a long question and short
question which having 6 marks & 4 marks.
Also from this chapter comments have to be
asked in 6 marks question.
(To Be Continued..)

1) Metal Hydride Reduction
 This reaction use for doing reduction of carbonyl compounds (Like ketone &
aldehyde)
 Reduction of carbonyl compounds using reagent that transfer a hydride from Boron
or Aluminum.
 Most commonly used reagent:
1) LiAlH4 & ) NaBH4
 Properties of reagent :
A) Sodium Borohydride(NaBH4):-
- Mild reducing agent & rapidly reacts with aldehydes/ketones.
- It is stable in all polar solvents and can be used in water and alcohol.
B) Lithium aluminum hydride(LiAlH4):-
- Strong hydride donor and rapidly reacts with esters, amides, acids, nitriles, aldehydes and ketones.
- It is strong basic and reacts violently with water & alcohol to release hydrogen.
- So, LiAlH4 is generally used in anhydrous solvents, ethers or Tetrahydrofuran(THF)

Contd…
 Mechanism Of Reduction :-
- It involves activation of the carbonyl group by
coordination with a metal cation and nucleophilic
transfer of hydride to the carbonyl group.
- Lewis acid character of boron and aluminum
becomes an important factor.

Contd…
 Common Rection :-
a)Halogen/Sulphonate: this type leaving group are
undergo replacement by hydride.

Contd…
b) Epoxides ring opening :- epoxides gets reduces to
alcohols by LiAlH4.
c) Alkynes gets reduced to E-alkenes by LiAlH4.

2) Clemmensen Reduction
 This reaction named after Erik Christian Clemmensen.
 It is a chemical reaction of reduction aldehyde/ketone
into alkanes by using Zn(Hg) and H2SO4.
 This reaction generally used for the reduction of aryl-
alkyl ketones.
 Aliphatic and cyclic ketones are reduced by using Zinc
metal.
 Carboxylic group can’t be reduced by this method.

Mechanism:-
a) Carbanionic mechanism:
- In this the zinc directly attacks the protonated carbonyl
group.

Contd…
b) Carbenoid mechanism:
- It is a radical process and the reduction process
occurs on the surface of the zinc metal.
- Some examples are :-

3) Birch Reduction
 It is an organic reaction where aromatic rings
undergo 1,4 reduction to give unconjugated
cyclohexadienes .
 It is generally done by using sodium , lithium, liquid
ammonia & alcohol.
 Also known as Metal-Ammonia reduction.
 Primary alkyl amines and can also work at higher
temperatures.
 This method of reduction is a major contribution to
various organic reactions.

Mechanism
a)It is start with a single electron transfer from the
metal to the aromatic ring forming a radical anion.
b)The radical anion the picks up a proton from alcohol
which results in the formation of a neutral radical
intermediate.

c) There occurs again a single electron transfer and
picking up of a proton from the final product.

 Regioselectivity:-
1) In aromatic rings with electron donors like methoxyl
or alkyl group the product will have the residual
double bonds having maximum number of substituent.
2) Acetylenes readily gives olefinic compounds. The
dialkyl acetylenes gives trans olefines.
3) For electron withdrawing groups the double bonds of
the product have avoided the substituents.

4) Wolf Kishner Reduction
 It is a most common reaction used to convert
carbonyl groups into methyl groups.
 This reduction is generally used for removing a
carbonyl group after it has served its synthetic
purpose.
 It involves reduction of the hydrazones of the
aldehydes & Ketones in alkaline medium at high
temperature to give hydrocarbons.

a) First step involves the formation of hydrazone anion by
deprotonation of the terminal nitrogen by a base.
b) Hydrazone anion rearrangement and the carbon atom
gets protonated.
c) Again deprotonation of nitrogen occurs and carbon gets
prtonated.
Mechanism:-

5) Oppenauer Oxidation
 It is a useful method for oxidising secondary
alcohols into ketones. This reaction is opposite to
that Meerwein-Ponndrof-Verly reduction.
 Alcohol gets oxidised by using aluminum
isopropoxide in presence of excess acetone.
 Primary alcohols do not undergo oxidation as they
show competition with aldol condensation of
aldehyde products.

Mechanism:-
 Step 1: Alcohol forms co-ordination complex aluminum.
 Step 2: Complex gets deprotonated by an alkoxide ion to
produce an intermediate.
 Step 3: Both the acetones & alcohol gets bound to the
aluminum.
 Step 4: Aluminum catalyses hydride shift from α-carbon of
alcohol to the carbonyl carbon of acetone. There occurs a
formation of six-membered transition state.
 Step 5: Finally, ketone is formed after hydride transfer.

6) Schmidt Reaction
 This reaction involves addition of hydrazoic acid to
carboxylic acids, aldehydes and ketones, alcohols and
olefins. Most common is the reaction with carboxylic acids.
 Various catalysts like sulphuric acid and Lewis acid are
used. When R is aliphatic then good yields are obtained but
when R is aryl group the stearic factors will decide the
product.
 If reaction occurs between a ketone and a hydrazoic acid,
then it helps in the introduction of –NH between the
carbonyl group and ‘R’ group. Ketone gets converted into
an amide.

 R may be alkyl or aryl. Generally, dialkylketones and cyclic
ketones reacts more rapidly than alkyl aryl ketones. Cyclic
ketones gives lactmas.
 Mechanism :-
1) With carboxylic acids :

7) Dakin Reaction
 Phenols can be prepared from aryl aldehydes or
arylketones by oxidation with hydrogen peroxide in
the presence of base.
 Basic requirement:-
i) –OH or –NH2 group must be present at
ortho or Para position.
 Aryl formate or alkanote are formed as a
intermediate which gets saponified to give
substituted phenol product.
 Overall, the carbonyl group is oxidised and the
hydrogen peroxide is reduced.

 Mechanism:-
o Reaction starts with nucleophilic addition of
hydroperoxide anion to the carbonyl carbon & forms a
tetrahedral intermediate.
o Intermediate then breaks down to cause 1,2 aryl
migration, hydroxide elimination & formation of
phenyl ester which is then hydrolysed to form the
product.

8) Beckmanns Rearrangement
 Oximes on reaction with PCl5 or any other such reagents
gets rearranged to form substituted amides.(Known as
Beckmanns Rearrangement)
 Ex:- [other reagents]
-H2SO4 , -formic acid, -liquid SO2, -polyphosphoric acid,
thionyl chloride etc.
 Stereochemistry:
- Group which is anti to the hydroxyl group migrates.
- Oximes undergoes isomerisation before migration take
place.
- R and R’ may be alkyl or hydrogen.

- The oximes of cyclic ketones gives ring enlargement.
- For ex:-
 Mechanism :-
o Step 1 :
o Step 2 :

9) Claisen – Schmidt
Condensation
 It is also known as cross aldol condensation.
 It involves reaction b/w an aldehydes or ketone and an
aromatic carbonyl compound lacking an alpha
hydrogen.
 For Ex:- (Synthesis of benzylidenacetone)
- It is generally carried out in the presence of solvent using
sodium hydroxide as the base and benzaldehyde with
acetone.

 Mechanism :-
 Step 1 :- Base abstracts a proton from ketone to form a
carbanion.
 Step 2 :- Carbanion attack at carbonyl of an aldehydes to form
an addition product.
 Step 3 :- The addition product then looses a water molecule
to give the final product.

Reaction of synthetic importance

Reaction of synthetic importance

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Reaction of synthetic importance