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Basics of Organic Chemistry question

2022 · 25 Jul · Shift 2 · Q22
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Basics of Organic Chemistry question

2022 · 25 Jul · Shift 2 · Q22

JEE MainChemistryBasics of Organic ChemistryNumerical+4 / −1
The total number of monobromo derivatives formed by the alkanes with molecular formula C5H12C_5H_{12}C5​H12​ is (excluding stereo isomers) ‾\underline{\hspace{2cm}}​.
Numerical answer
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Correct answer: 8

  1. The molecular formula C5H12C_5H_{12}C5​H12​ has three alkane isomers:

    • nnn-pentane: CH3−CH2−CH2−CH2−CH3CH_3-CH_2-CH_2-CH_2-CH_3CH3​−CH2​−CH2​−CH2​−CH3​
    • isopentane (2-methylbutane): (CH3)2CH−CH2−CH3(CH_3)_2CH-CH_2-CH_3(CH3​)2​CH−CH2​−CH3​
    • neopentane (2,2-dimethylpropane): C(CH3)4C(CH_3)_4C(CH3​)4​

  2. We count distinct monobromo derivatives obtained by replacing one H by Br at non-equivalent positions. Stereo isomers are excluded.

(i) nnn-Pentane

Structure: CH3−CH2−CH2−CH2−CH3CH_3-CH_2-CH_2-CH_2-CH_3CH3​−CH2​−CH2​−CH2​−CH3​ By symmetry:

  • C1 and C5 are equivalent
  • C2 and C4 are equivalent
  • C3 is unique

So monobromination gives 3 distinct products:

  • 1-bromopentane
  • 2-bromopentane
  • 3-bromopentane

Hence, from nnn-pentane: 3 derivatives.

(ii) Isopentane (2-methylbutane)

Structure: CH3−CH(CH3)−CH2−CH3CH_3-CH(CH_3)-CH_2-CH_3CH3​−CH(CH3​)−CH2​−CH3​ Distinct hydrogen positions:

  • The two CH3CH_3CH3​ groups attached to C2 are equivalent → one type
  • The terminal CH3CH_3CH3​ at the other end is different → second type
  • The CH2CH_2CH2​ carbon is another type
  • The central CHCHCH carbon is another type

Thus total distinct substitution positions = 4.

So monobromination gives 4 distinct products.

(iii) Neopentane (2,2-dimethylpropane)

Structure: C(CH3)4C(CH_3)_4C(CH3​)4​ All four methyl groups are equivalent by symmetry. So replacing one H anywhere gives only 1 monobromo derivative.

Hence, from neopentane: 1 derivative.

  1. Total number of monobromo derivatives: 3+4+1=83+4+1=83+4+1=8

Therefore, the required number is: 8\boxed{8}8​

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