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Aldehydes Ketones and Carboxylic Acids question

2019 · 9 Apr · Shift 2 · Q6
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Aldehydes Ketones and Carboxylic Acids question

2019 · 9 Apr · Shift 2 · Q6

JEE MainChemistryAldehydes Ketones and Carboxylic AcidsMCQ+4 / −1
In the following reaction JEE Main 2019 (Online) 9th April Evening Slot Chemistry - Aldehydes, Ketones and Carboxylic Acids Question 191 English Rate of the reaction is the highest for :
  1. A
    Propanal as substrate and methanol in stoichiometric amount.
  2. B
    Acetone as substrate and methanol in stoichiometric amount
  3. C
    Propanal as substrate and methanol in excess
  4. D
    Acetone as substrate and methanol in excess
View written solutionFree

Correct answer: C

  1. Identify the reaction involved

The question compares the rate for reaction of a carbonyl compound with methanol. This is the acetal/hemiacetal formation reaction of aldehydes/ketones with alcohol.

In general:

  • Aldehydes react faster than ketones toward nucleophilic addition.
  • Increasing concentration of alcohol shifts the equilibrium and also increases the effective rate of forward addition.
  1. Compare aldehyde vs ketone

We compare:

  • Propanal: an aldehyde
  • Acetone: a ketone

Aldehydes are more reactive because:

  1. They have less steric hindrance around the carbonyl carbon.
  2. They have only one alkyl group, so the +I+I+I effect is less; hence the carbonyl carbon is more electrophilic.

Therefore,

Propanal reacts faster than acetone.\text{Propanal reacts faster than acetone.}Propanal reacts faster than acetone.

So among the options, those with propanal will be faster than those with acetone.

  1. Effect of methanol amount

The reaction involves attack of methanol on the carbonyl carbon. If methanol is present in excess, its concentration is higher, so the forward reaction occurs faster.

Also, acetal formation is an equilibrium process, and excess alcohol favors product formation.

Thus,

Rate is higher with methanol in excess than with stoichiometric methanol.\text{Rate is higher with methanol in excess than with stoichiometric methanol.}Rate is higher with methanol in excess than with stoichiometric methanol.
  1. Evaluate all options

Option A

Propanal + methanol in stoichiometric amount

  • Aldehyde: fast
  • But methanol not in excess

Option B

Acetone + methanol in stoichiometric amount

  • Ketone: slower
  • Methanol not in excess

Clearly slower than A.

Option C

Propanal + methanol in excess

  • Aldehyde: fast
  • Methanol in excess: further increases forward rate

This should be the fastest.

Option D

Acetone + methanol in excess

  • Methanol excess helps
  • But ketone is still less reactive than aldehyde

So D is slower than C.

  1. Final conclusion

The reaction rate will be highest for:

C: Propanal as substrate and methanol in excess\boxed{\text{C: Propanal as substrate and methanol in excess}}C: Propanal as substrate and methanol in excess​
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