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Alcohols Phenols and Ethers question

2021 · 20 Jul · Shift 1 · Q7
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  5. /2021 · 20 Jul · Shift 1 · Q7

Alcohols Phenols and Ethers question

2021 · 20 Jul · Shift 1 · Q7

JEE MainChemistryAlcohols Phenols and EthersMCQ+4 / −1
In the given reaction 3-Bromo-2, 2-dimethyl butane →C2H5OH′A′(Major Pr⁡oduct)\xrightarrow{{C_2}{H_5}OH} \mathop {'A'}\limits_{(Major\,\Pr oduct)}C2​H5​OH​(MajorProduct)′A′​ Product A is :
  1. A
    2-Ethoxy-3, 3-dimethyl butane
  2. B
    1-Ethoxy-3, 3-dimethyl butane
  3. C
    2-Ethoxy-2, 3-dimethyl butane
  4. D
    2-Hydroxy-3, 3-dimethyl butane
View written solutionFree

Correct answer: C

  1. Write the structure of the substrate

    The compound is 3-bromo-2,2-dimethylbutane.

    Start from butane: CH3−CH2−CH2−CH3CH_3-CH_2-CH_2-CH_3CH3​−CH2​−CH2​−CH3​

    Put two methyl groups at C-2 and bromine at C-3:

    CH3−C(CH3)2−CH(Br)−CH3CH_3-C(CH_3)_2-CH(Br)-CH_3CH3​−C(CH3​)2​−CH(Br)−CH3​

  2. Nature of reagent/solvent

    The reaction is carried out in ethanol (C2H5OH)\left(C_2H_5OH\right)(C2​H5​OH).

    In alcoholic medium, alkyl halides can undergo solvolysis/substitution. Here, since the carbon bearing Br is adjacent to a quaternary-like crowded center, ionization can be followed by rearrangement to form a more stable carbocation.

  3. Formation of carbocation

    Loss of Br−Br^-Br− from

    CH3−C(CH3)2−CH(Br)−CH3CH_3-C(CH_3)_2-CH(Br)-CH_3CH3​−C(CH3​)2​−CH(Br)−CH3​

    gives a secondary carbocation at C-3:

    CH3−C(CH3)2−CH+−CH3CH_3-C(CH_3)_2-CH^+-CH_3CH3​−C(CH3​)2​−CH+−CH3​

  4. Rearrangement of carbocation

    The adjacent carbon (C-2) is highly substituted. A 1,2-methyl shift from C-2 to the carbocation center (C-3) occurs, generating a more stable tertiary carbocation.

    After methyl shift, the positive charge moves to C-2:

    CH3−C+(CH3)−CH(CH3)−CH3CH_3-C^+(CH_3)-CH(CH_3)-CH_3CH3​−C+(CH3​)−CH(CH3​)−CH3​

    This is a tertiary carbocation.

  5. Attack by ethanol

    Ethanol attacks the tertiary carbocation at C-2, followed by deprotonation, giving the ether:

    CH3−C(OC2H5)(CH3)−CH(CH3)−CH3CH_3-C(OC_2H_5)(CH_3)-CH(CH_3)-CH_3CH3​−C(OC2​H5​)(CH3​)−CH(CH3​)−CH3​

  6. IUPAC name of the product

    Longest chain = butane.

    Substituents:

    • at C-2: ethoxy and methyl
    • at C-3: methyl

    Hence the product is:

    2-Ethoxy-2,3-dimethylbutane\boxed{\text{2-Ethoxy-2,3-dimethylbutane}}2-Ethoxy-2,3-dimethylbutane​

  7. Check options

    • A: 2-Ethoxy-3,3-dimethyl butane →\rightarrow→ incorrect structure
    • B: 1-Ethoxy-3,3-dimethyl butane →\rightarrow→ incorrect
    • C: 2-Ethoxy-2,3-dimethyl butane →\rightarrow→ correct
    • D: 2-Hydroxy-3,3-dimethyl butane →\rightarrow→ alcohol not expected with ethanol as nucleophile

Therefore, the major product is Option C.

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