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Q8.17(x):

Complete each synthesis by giving missing starting material, reagent or products.

Solution :

Reaction Analysis: Synthesis of Benzyl Alcohol from Benzaldehyde

The chemical transformation represented is the reduction of a carbonyl group ($C=O$) in an aromatic aldehyde to a primary alcohol ($-CH_2OH$).

Step 1: Identifying the Functional Transformation

The reactant is Benzaldehyde ($C_6H_5CHO$) and the product is Benzyl alcohol ($C_6H_5CH_2OH$). This is a nucleophilic addition followed by a proton transfer, categorized as a reduction reaction [Per the theory of Carbonyl Reductions].

Step 2: Selecting the Appropriate Reagent

To reduce an aldehyde to a primary alcohol without affecting the aromatic ring, we utilize metal hydride reducing agents. The most common and effective laboratory reagents are:

  • Sodium Borohydride ($NaBH_4$) in methanol or ethanol.
  • Lithium Aluminium Hydride ($LiAlH_4$) followed by acidic workup ($H_3O^+$).
  • Catalytic Hydrogenation ($H_2 / Pd$ or $Ni$).

Given the standard laboratory practice for this specific transformation, $NaBH_4$ is the preferred selective reagent [Refer to Grignard/Hydride addition mechanisms].

Step 3: Reaction Mechanism (Summary)

The hydride ion ($H^-$) acts as a nucleophile, attacking the electrophilic carbonyl carbon. The resulting alkoxide intermediate is then protonated by the solvent to yield the alcohol.

C6H5CHO NaBH4 C6H5CH2OH [Reduction]

Step 4: Chemical Equation

The balanced chemical equation for the synthesis is:

$C_6H_5CHO + 2[H] \xrightarrow{NaBH_4 / CH_3OH} C_6H_5CH_2OH$

Step 5: Tabular Summary of Synthesis

Component Chemical Identity
Starting Material Benzaldehyde ($C_6H_5CHO$)
Reagent Required $NaBH_4$ (Sodium Borohydride)
Product Benzyl Alcohol ($C_6H_5CH_2OH$)

Final Solution: The missing reagent for the conversion of Benzaldehyde to Benzyl alcohol is Sodium Borohydride ($NaBH_4$) in the presence of an alcoholic solvent like methanol ($CH_3OH$).


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