benzoin synthesis

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  • Benzoin synthesis

    Group 3H3

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  • CONTENTIntroduction to benzoin synthesis

    Mechanism of synthesis

    Synthetic procedure

    Results and precautions

    Applications and modern synthetic procedure3

  • INTRODUCTION

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  • Chemical name:

    2-hydroxy-1,2-diphenylethanone

    soluble in warm alcohol

    insoluble in water

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  • It is chiral and exists as pair of enantiomers:

    (R)-benzoin

    (S)- benzoin

    It is an alpha hydroxy ketone attachedto two phenyl groups

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  • 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 ;

    Cyanide ion

    Thiamine hydrochloride

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    It has pyrimidine portion

    Thiazole portion

    The active portion is thiazole portion

    The thiazolium ion has the right balance of

    nucleophilicity

    Ability to stabilize the intermediate anion

    Has leaving group properties

  • Thiamine catalyzes three important types of

    enzymatic reactions:

    Nonoxidative Decarboxylations of alpha keto

    Acids

    Oxidative Decarboxylations of alpha keto acids

    Formation of acyloins (alpha hydroxy ketones)

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  • UMPOLUNGNucleophilic addition

    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

  • POLARISED NATURE

  • Mechanism of synthesis

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    NaCN/EtOH

    Thiamine HCl

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  • Synthetic procedure

    Thiamine catalysed

    Cyanide catalysed

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  • 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

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  • Cyanide Solution

    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

    Cyanide

    catalysed

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  • RESULTS AND PRECAUTIONS

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  • %YIELD ,RECOVERY AND ERROR

    The %yield measures the reaction efficiency.

    Percentage yield=

    x 100%

    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

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  • 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

    x 100%

    Percentage recovery = z%

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  • 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.

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  • 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

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  • 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.

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  • OTHER SYNTHETIC METHOD AND APPLICATIONS OF BENZOIN CONDENSATION

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  • REDUCTION OF BENZIL

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    NaBH4 in ethanol

  • Furfural to furoin Anisaldehyde to Anisoin

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    KCN/EtOHKCN, EtOH, H2O

  • REFERENCES

    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

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  • Benzoin synthesis

    Group 3H3

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