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CONTENTIntroduction to benzoin synthesis
Mechanism of synthesis
Results and precautions
Applications and modern synthetic procedure3
soluble in warm alcohol
insoluble in water
It is chiral and exists as pair of enantiomers:
It is an alpha hydroxy ketone attachedto two phenyl groups
The condensation of two moles of benzaldehyde to form a new carbon-carbon bond is known as
the Benzoin condensation.
Catalysts that promote this reaction are namely ;
It has pyrimidine portion
The active portion is thiazole portion
The thiazolium ion has the right balance of
Ability to stabilize the intermediate anion
Has leaving group properties
Thiamine catalyzes three important types of
Nonoxidative Decarboxylations of alpha keto
Oxidative Decarboxylations of alpha keto acids
Formation of acyloins (alpha hydroxy ketones)
Carbonyl functional group is electrophilic and therefore attacked by a nucleophile such as cyanide ion
The nucleophilic attack leads to the reversal of the normal polarisation of the Carbonyl.
This permits the subsequent condensation with another molecule of Benzaldehyde in benzoin synthesis and loss of the cyanide ion
Such a reaction is known as an impolungarrangement
Mechanism of synthesis
Cool in ice bath
5M NaOH solution
Cool for several minutes and add 96% ethanol followed by NaOHsolution to yield a yellow solution
Solid thiamine HCL + 4ml of water
Swirl continuously (solution becomes yellow and then, clear)
Add 1.3ml benzaldehyde to the yellow solution
Suction filter crude product and wash cold water and ethanol (2:1)
Stopper flask and place in cabinet for 24hrs
Allow to dry in oven
Recrystallize from hot ethanol
1.5g of NaCN + 15 ml of water
Add boiling chips
Cyanide solution + 30ml of ethanol +16g of Benzaldehyde
Remove flask from steam and cool in ice bath to form crystals
Reflux in boiling water for 30 minutes
Wash in 50% ethanol and more water
Dry in oven
Filter to obtain crystals
Recrystallize with 40ml of hot ethanol
RESULTS AND PRECAUTIONS
%YIELD ,RECOVERY AND ERROR
The %yield measures the reaction efficiency.
The theoretical yield is estimated based on the limiting reagent. This allows researchers to determine how much product can actually be formed based on the reagent present at the beginning of the reaction.
Actual yield can not be 100% due to limitations
Given that for benzoin,the weight after recrystallization was x
the weight before recrystallization was y
Percentage recovery= weight after recrystallization x 100%
weight before recrystallization
Percentage recovery = z%
The %recovery gives an indication of how much of the pure substance you were able to extract (recover) from the crude
i.e., it gives an indication of the efficiency of the recrystallization process. Thus a higher recovery may indicate a good recrystallization.
The converse is true for a lower recovery. However, due to high level of impurities, even a good recrystallization may still give a lower recovery.
Percentage yield obtained w%
The melting point of the pure crystals are determined.
The standard melting point for benzoin is (132-137)C.
Melting point is an indication of purity.
Impurities lower melting point and increase the range.
Other methods like absorbance, NMR ,and UV may also be used to assess the purity
Major impurities in benzoin may be unreacted benzaldehyde and thiamine
The benzaldehyde used should be free from benzoic acid
After the NaOH was prepared, it should be kept on ice to cool, this is because NaOH can react vigorously with water liberating heat which can decompose the benzoin compound formed.
Minimum amount of solvent should be used for the recrystallization so that a supersaturated solution would be formed for benzoin to precipitate out of the solution.
The reaction mixture should not be heated above 65C.This is because thiamine degrades at high temperature and so can be destroyed due to high temperature.
OTHER SYNTHETIC METHOD AND APPLICATIONS OF BENZOIN CONDENSATION
REDUCTION OF BENZIL
NaBH4 in ethanol
Furfural to furoin Anisaldehyde to Anisoin
KCN/EtOHKCN, EtOH, H2O
P.,D. L. Lampman, G. M.Chriz, Introduction to organic lab technique; CollegePublishing, New York,1982 experiment no 40
Bruice, T. C. and Benkovic, S. Bioorganic Mechanism. Vol. 2. New York: W. A. Benjamin, 1966.Chap. 8, Thiamine pyrophosphate and Pyridoxal-5-Phosphate.
Bernhard, S. The Structure and Function of Enzymes. New York: W. A. Benjamin, 1968. Chap. 7,Coenzymes and Cofactors
Ajibola et al, Essential inorganic and pharmaceutical chemistry, 2nd edition, Hope publication, Ibadan, Nigeria, page 274-276