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

Methylmagnesium bromide (MeMgBr), a Grignard reagent, acts as a strong base due to the carbanionic nature of its methyl group (CH₃^-). It reacts with compounds having acidic hydrogen atoms (protons, H⁺) to form methane (CH₄):
R-H (acidic) + CH₃MgBr → R^-MgBr⁺ + CH₄
The task is to identify the hydrocarbon with the most acidic C-H bond, which will readily react with the Grignard reagent. C-H bond acidity is enhanced when the resulting carbanion (conjugate base) is stabilized. Let's examine the options:

• (A) Cyclopropene (C₃H₄): Contains vinylic and sp³ hydrogens. C-H bonds with sp² carbons are slightly more acidic than those with sp³ carbons, but generally not acidic enough to react with Grignard reagents, even with ring strain.
• (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 because removing a proton generates the cyclopentadienyl anion (C₅H₅^-), which is aromatic (cyclic, planar, 6 \(\pi\) electrons) and highly resonance-stabilized. 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 conjugate base stability, cyclopentadiene (B) has the most acidic hydrogens due to its conjugate base's aromaticity. It should react readily with MeMgBr to produce methane.