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Basics of Organic Chemistry question

2010 · Shift 0 · Q2
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Basics of Organic Chemistry question

2010 · Shift 0 · Q2

JEE MainChemistryBasics of Organic ChemistryMCQ+4 / −1
The correct order of increasing basicity of the given conjugate bases (R = CH3CH_3CH3​) is
  1. A
    RCOO‾RCO\overline ORCOO< HC =C‾\overline CC<R‾\overline RR<N‾H2\overline NH_2NH2​
  2. B
    R‾\overline RR< HC =C‾\overline CC<RCOO‾RCO\overline ORCOO<N‾H2\overline NH_2NH2​
  3. C
    RCOO‾RCO\overline ORCOO<N‾H2\overline NH_2NH2​< HC =C‾\overline CC<R‾\overline RR
  4. D
    RCOO‾RCO\overline ORCOO< HC =C‾\overline CC<N‾H2\overline NH_2NH2​<R‾\overline RR
View written solutionFree

Correct answer: D

  1. Principle used

    The basicity of a conjugate base is inversely related to the acidity of its conjugate acid.

    If conjugate acid is very strong, its conjugate base is very weak.

    So we compare the conjugate acids of the given bases:

    • RCOO−RCOO^-RCOO− comes from RCOOHRCOOHRCOOH
    • HC≡C−HC \equiv C^-HC≡C− comes from HC≡CHHC \equiv CHHC≡CH
    • R−R^-R− comes from RHRHRH
    • NH2−NH_2^-NH2−​ comes from NH3NH_3NH3​

  2. Write approximate pKapK_apKa​ values of conjugate acids

    For R=CH3R = CH_3R=CH3​:

    • CH3COOHCH_3COOHCH3​COOH : pKa≈4.8pK_a \approx 4.8pKa​≈4.8
    • HC≡CHHC \equiv CHHC≡CH : pKa≈25pK_a \approx 25pKa​≈25
    • NH3NH_3NH3​ : pKa≈38pK_a \approx 38pKa​≈38
    • CH4CH_4CH4​ : pKa≈50pK_a \approx 50pKa​≈50

  3. Relate pKapK_apKa​ to basicity

    Higher pKapK_apKa​ of the conjugate acid ⇒\Rightarrow⇒ weaker acid ⇒\Rightarrow⇒ stronger conjugate base.

    Therefore increasing basicity follows increasing pKapK_apKa​ of the conjugate acids:

    RCOO−<HC≡C−<NH2−<R−RCOO^- < HC \equiv C^- < NH_2^- < R^-RCOO−<HC≡C−<NH2−​<R−

  4. Check with resonance/hybridization ideas

    • RCOO−RCOO^-RCOO− is highly stabilized by resonance, so it is the weakest base.
    • HC≡C−HC \equiv C^-HC≡C− has charge on an spspsp carbon, which is relatively stabilized, so it is less basic than the next two.
    • NH2−NH_2^-NH2−​ is very strong base.
    • R−R^-R− (alkyl carbanion) has charge on sp3sp^3sp3 carbon, least electronegative among these centers and least stabilized, so it is the strongest base.

  5. Correct order

    RCOO−<HC≡C−<NH2−<R−RCOO^- < HC \equiv C^- < NH_2^- < R^-RCOO−<HC≡C−<NH2−​<R−

    This matches Option D.

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