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Alcohols Phenols and Ethers question

2023 · 6 Apr · Shift 2 · Q22
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Alcohols Phenols and Ethers question

2023 · 6 Apr · Shift 2 · Q22

JEE MainChemistryAlcohols Phenols and EthersNumerical+4 / −1
Among the following, the number of compounds which will give positive iodoform reaction is ‾\underline{\hspace{2cm}}​ (a) 1-Phenylbutan-2-one (b) 2-Methylbutan-2-ol (c) 3-Methylbutan-2-ol (d) 1-Phenylethanol (e) 3,3-dimethylbutan-2-one (f) 1-Phenylpropan-2-ol
Numerical answer
View written solutionFree

Correct answer: 4

  1. Condition for positive iodoform test

A compound gives positive iodoform reaction if it contains either:

  • a methyl ketone group: R−CO−CH3\mathrm{R-CO-CH_3}R−CO−CH3​, or
  • a secondary alcohol of type: R−CH(OH)−CH3\mathrm{R-CH(OH)-CH_3}R−CH(OH)−CH3​

because such alcohols are oxidized to methyl ketones under the test conditions.

We now check each compound.

  1. Examination of each compound

(a) 1-Phenylbutan-2-one

Structure: Ph−CH2−CO−CH2−CH3\mathrm{Ph-CH_2-CO-CH_2-CH_3}Ph−CH2​−CO−CH2​−CH3​ The carbonyl carbon is attached to CH2Ph\mathrm{CH_2Ph}CH2​Ph on one side and CH2CH3\mathrm{CH_2CH_3}CH2​CH3​ on the other. It is not of the form R−CO−CH3\mathrm{R-CO-CH_3}R−CO−CH3​.

So, no iodoform reaction.

(b) 2-Methylbutan-2-ol

Structure: CH3−C(OH)(CH3)−CH2−CH3\mathrm{CH_3-C(OH)(CH_3)-CH_2-CH_3}CH3​−C(OH)(CH3​)−CH2​−CH3​ This is a tertiary alcohol. Tertiary alcohols are not oxidized to ketones under iodoform test conditions.

So, no iodoform reaction.

(c) 3-Methylbutan-2-ol

Structure: CH3−CH(OH)−CH(CH3)−CH3\mathrm{CH_3-CH(OH)-CH(CH_3)-CH_3}CH3​−CH(OH)−CH(CH3​)−CH3​ This is a secondary alcohol of type: R−CH(OH)−CH3\mathrm{R-CH(OH)-CH_3}R−CH(OH)−CH3​ with R=CH(CH3)−CH3\mathrm{R = CH(CH_3)-CH_3}R=CH(CH3​)−CH3​. On oxidation, it gives a methyl ketone.

So, gives positive iodoform test.

(d) 1-Phenylethanol

Structure: Ph−CH(OH)−CH3\mathrm{Ph-CH(OH)-CH_3}Ph−CH(OH)−CH3​ This is also a secondary alcohol of type: R−CH(OH)−CH3\mathrm{R-CH(OH)-CH_3}R−CH(OH)−CH3​ with R=Ph\mathrm{R = Ph}R=Ph. On oxidation, it forms acetophenone (Ph−CO−CH3)\mathrm{(Ph-CO-CH_3)}(Ph−CO−CH3​), which is a methyl ketone.

So, gives positive iodoform test.

(e) 3,3-dimethylbutan-2-one

Structure: CH3−CO−C(CH3)3\mathrm{CH_3-CO-C(CH_3)_3}CH3​−CO−C(CH3​)3​ This clearly contains the methyl ketone group: R−CO−CH3\mathrm{R-CO-CH_3}R−CO−CH3​ with R=C(CH3)3\mathrm{R = C(CH_3)_3}R=C(CH3​)3​.

So, gives positive iodoform test.

(f) 1-Phenylpropan-2-ol

Structure: Ph−CH2−CH(OH)−CH3\mathrm{Ph-CH_2-CH(OH)-CH_3}Ph−CH2​−CH(OH)−CH3​ This is a secondary alcohol of type: R−CH(OH)−CH3\mathrm{R-CH(OH)-CH_3}R−CH(OH)−CH3​ with R=CH2Ph\mathrm{R = CH_2Ph}R=CH2​Ph. On oxidation, it gives: Ph−CH2−CO−CH3\mathrm{Ph-CH_2-CO-CH_3}Ph−CH2​−CO−CH3​ which is a methyl ketone.

So, gives positive iodoform test.

  1. Counting positive cases

Positive iodoform reaction is shown by:

  • (c)
  • (d)
  • (e)
  • (f)

Total number of compounds = 444

  1. Comparison with stored answer

Derived answer = 444

Stored correct answer = 444

Hence, the derived answer agrees with the stored answer.

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