- A(d) only
- B(b) and (d)
- C(a), (c) and (d)
- D(c) only
View written solutionFree
Correct answer: D
- Interpret the sequence as four separate reactions
Let us label the steps as:
- (a) Dehydration of alcohol with conc.
- (b) Elimination of alkyl bromide with alc. KOH
- (c) Elimination with bulky base -BuOK
- (d) Heating of the alcohol obtained after hydroboration/oxidation-like addition
The question asks: which reaction(s) will not give the Saytzeff product?
- Step (a): Dehydration of
This is a secondary alcohol adjacent to a quaternary carbon. Under conc. , dehydration proceeds by E1 mechanism.
First, protonation of and loss of water gives:
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.
- Step (b): Reaction of alkyl bromide with alcoholic KOH
The bromide shown is:
This is -bromo--methylbutane.
Possible eliminations:
-
Remove -H from the right methyl carbon:
less substituted alkene
-
Remove -H from the left carbon:
more substituted alkene
Alcoholic KOH is a small strong base, so in ordinary elimination it favors the Saytzeff alkene.
Thus (b) gives Saytzeff product.
- Step (c): Reaction with -BuOK
Now the same bromide is treated with bulky base:
Bulky base abstracts the less hindered -hydrogen, so elimination favors the less substituted alkene (Hofmann product).
Thus from
major product is:
not the Saytzeff alkene.
So (c) does not produce Saytzeff product.
- Step (d): The product shown after this step is
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:
- Option matching
- A: (d) only — incorrect
- B: (b) and (d) — incorrect
- C: (a), (c) and (d) — incorrect
- D: (c) only — correct
- Comparison with stored answer
Stored correct answer: D
Our derived answer: D
So they agree.
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