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Hydrocarbons question

2020 · 7 Jan · Shift 1 · Q9
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Hydrocarbons question

2020 · 7 Jan · Shift 1 · Q9

JEE MainChemistryHydrocarbonsMCQ+4 / −1
Consider the following reaction : (CH3)3CCH(OH)CH3→conc.   H2SO4(CH3)2CHCH(Br)CH3→alc.  KOH(CH3)2CHCH(Br)CH3→(CH3)3OΘK⊕(CH3)2C∣OH−CH2−CHO→Δ{\left( {C{H_3}} \right)_3}CCH\left( {OH} \right)C{H_3}\xrightarrow{conc.\,\,\,{H_2}S{O_4}} {\left( {C{H_3}} \right)_2}CHCH\left( {Br} \right)C{H_3}\xrightarrow{alc.\,\,KOH} {\left( {C{H_3}} \right)_2}{\rm{ }}CHCH\left( {Br} \right)C{H_3}\xrightarrow{{{(C{H_3})}_3}{O^\Theta }{K^ \oplus }} {\left( {C{H_3}} \right)_2}\mathop C\limits_{\mathop |\limits_{OH} } - C{H_2} - CHO\xrightarrow{\Delta}(CH3​)3​CCH(OH)CH3​conc.H2​SO4​​(CH3​)2​CHCH(Br)CH3​alc.KOH​(CH3​)2​CHCH(Br)CH3​(CH3​)3​OΘK⊕​(CH3​)2​OH∣​C​−CH2​−CHOΔ​ Which of these reaction(s) will not produce Saytzeff product ?
  1. A
    (d) only
  2. B
    (b) and (d)
  3. C
    (a), (c) and (d)
  4. D
    (c) only
View written solutionFree

Correct answer: D

  1. Interpret the sequence as four separate reactions

Let us label the steps as:

  • (a) Dehydration of alcohol with conc. H2SO4H_2SO_4H2​SO4​
  • (b) Elimination of alkyl bromide with alc. KOH
  • (c) Elimination with bulky base ttt-BuOK
  • (d) Heating of the alcohol obtained after hydroboration/oxidation-like addition

The question asks: which reaction(s) will not give the Saytzeff product?


  1. Step (a): Dehydration of
(CH3)3CCH(OH)CH3(CH_3)_3CCH(OH)CH_3(CH3​)3​CCH(OH)CH3​

This is a secondary alcohol adjacent to a quaternary carbon. Under conc. H2SO4H_2SO_4H2​SO4​, dehydration proceeds by E1 mechanism.

First, protonation of OHOHOH and loss of water gives:

(CH3)3C−C+H−CH3(CH_3)_3C-\overset{+}{C}H-CH_3(CH3​)3​C−C+H−CH3​

Now, this carbocation is adjacent to a quaternary carbon, so a 1,2-methyl shift occurs to form a more stable tertiary carbocation.

After rearrangement, elimination gives the more substituted alkene as major product.

So (a) gives Saytzeff product.


  1. Step (b): Reaction of alkyl bromide with alcoholic KOH

The bromide shown is:

(CH3)2CHCH(Br)CH3(CH_3)_2CHCH(Br)CH_3(CH3​)2​CHCH(Br)CH3​

This is 222-bromo-333-methylbutane.

Possible eliminations:

  • Remove β\betaβ-H from the right methyl carbon:

    (CH3)2CHCH=CH2(CH_3)_2CHCH=CH_2(CH3​)2​CHCH=CH2​

    less substituted alkene

  • Remove β\betaβ-H from the left CHCHCH carbon:

    (CH3)2C=CHCH3(CH_3)_2C=CHCH_3(CH3​)2​C=CHCH3​

    more substituted alkene

Alcoholic KOH is a small strong base, so in ordinary E2E2E2 elimination it favors the Saytzeff alkene.

Thus (b) gives Saytzeff product.


  1. Step (c): Reaction with ttt-BuOK

Now the same bromide is treated with bulky base:

(CH3)3O−K+(CH_3)_3O^-K^+(CH3​)3​O−K+

Bulky base abstracts the less hindered β\betaβ-hydrogen, so elimination favors the less substituted alkene (Hofmann product).

Thus from

(CH3)2CHCH(Br)CH3(CH_3)_2CHCH(Br)CH_3(CH3​)2​CHCH(Br)CH3​

major product is:

(CH3)2CHCH=CH2(CH_3)_2CHCH=CH_2(CH3​)2​CHCH=CH2​

not the Saytzeff alkene.

So (c) does not produce Saytzeff product.


  1. Step (d): The product shown after this step is
(CH3)2C(OH)−CH2−CHO(CH_3)_2C(OH)-CH_2-CHO(CH3​)2​C(OH)−CH2​−CHO

which on heating undergoes dehydration / rearrangement depending on the substrate pattern.

But the key point in the given options is to identify the step(s) that definitely do not give Saytzeff orientation in elimination.

Among the earlier elimination steps, only (c) is the standard case where bulky base forces Hofmann elimination, i.e. not Saytzeff.

Hence the correct choice is:

(c) only\boxed{\text{(c) only}}(c) only​
  1. Option matching
  • A: (d) only — incorrect
  • B: (b) and (d) — incorrect
  • C: (a), (c) and (d) — incorrect
  • D: (c) only — correct

  1. Comparison with stored answer

Stored correct answer: D

Our derived answer: D

So they agree.

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