Question

Which of the following hydrocarbons reacts easily with MeMgBr to give methane?

• A.
• B.
• C.
• D.

Hint:

Using a simple frame or just bolding for the box Key Points: Grignard reagents (like MeMgBr) are strong bases. They react with acidic protons (H$^+$) to form alkanes (methane in this case). Acidity of C-H bonds is determined by the stability of the resulting carbanion (conjugate base). Aromatic stabilization significantly increases the acidity of precursor C-H bonds. Cyclopentadiene's CH$_2$ protons are acidic (pKa $\approx$ 16) because the cyclopentadienyl anion is aromatic.

Answer 1

The Correct Option is B

The Grignard reagent, methylmagnesium bromide (MeMgBr), acts as a very strong base (due to the carbanionic character of the methyl group, CH₃^-) in addition to being a nucleophile. It readily reacts with compounds containing acidic hydrogen atoms (protons, H⁺) to form methane (CH₄) via an acid-base reaction:
R-H (acidic) + CH₃MgBr → R^-MgBr⁺ + CH₄ We need to identify which of the given hydrocarbons possesses the most acidic C-H bond capable of reacting with the Grignard reagent. Acidity of C-H bonds is enhanced if the resulting carbanion (conjugate base) is stabilized. Let's evaluate the options:

• (A) Cyclopropene (C₃H₄): Contains vinylic hydrogens and sp³ hydrogens. While C-H bonds involving sp² carbons are slightly more acidic than those with sp³ carbons, they are generally not acidic enough to react readily with Grignard reagents unless significantly activated. Ring strain might influence acidity, but typically not enough.
• (B) Cyclopentadiene (C₅H₆): Contains vinylic hydrogens and hydrogens on a saturated CH₂ group. The CH₂ hydrogens are exceptionally acidic (pKa ≈ 16) compared to typical alkane/alkene C-H bonds. This is because removal of one of these protons generates the cyclopentadienyl anion (C₅H₅^-), which is aromatic (cyclic, planar, 6 π electrons) and highly stabilized by resonance. This compound readily reacts with strong bases like Grignard reagents.
• (C) Benzene (C₆H₆): The C-H bonds are attached to sp² carbons in an aromatic ring. These hydrogens are not significantly acidic (pKa ≈ 43) and do not react with Grignard reagents.
• (D) Cyclooctatetraene (C₈H₈): The C-H bonds are attached to sp² carbons in a non-aromatic, non-planar ring. These hydrogens are not significantly acidic.

Based on the principle of conjugate base stability, cyclopentadiene (B) possesses the most acidic hydrogens due to the aromaticity of its conjugate base. It should react readily with MeMgBr to produce methane.