One of my teacher says that $\ce{Ph-CHO}$ is more reactive than $\ce{CH3-CHO}$ in performing nucleophillic addition reaction because $\ce{—Ph}$ is planar and thus offers less steric hindrance as compared to $\ce{—CH3}$ which is tetrahedral.

My other teacher gives the reason that the partial positive charge developed on the carbonyl carbon is resonance stabilised in $\ce{Ph-CHO}$ which makes the carbonyl carbon more prone to nucleophillic attacks.

What I think is that $\ce{—Ph}$ has its −I effect which deprives the carbonyl carbon of some amount of its electron density and thus makes it more prone to nucleophillic attacks.

I want to know which reason is more correct in explaining the reactivity of benzaldehyde over acetaldehyde or if there is some other reason please specify. The main thing I want to know by this is that whether planar substitutents offer less steric resistance than tetrahedral or not, and whether a partial charge on carbon can be stabilised by resonance or not.

  • 2
    $\begingroup$ The first reasoning reminded me of the prank often performed on first-graders. Shall one get a stuck glass joint during a lab practicum, the person is often told to soak the glassware in benzene, and here comes the sanity check. A first-grader ask why, and receives an explanation that planar benzene rings can easier penetrate the space between glass surfaces and wedge a joint at once. Student accepting such explanation usually don't do well during the first exam session. $\endgroup$
    – andselisk
    Jul 20 '20 at 6:38
  • 2
    $\begingroup$ $\ce{-CH3}$ has an A-value of 1.7 whereas $\ce{-Ph}$ has an A-value of 3. I'm also inclined to mark this as a duplicate of This answer $\endgroup$ Jul 20 '20 at 7:23
  • 2
    $\begingroup$ Be sure to check for resonances before inductives. Also, I'm not able to get the sense of "partial carbocation"? $\endgroup$ Jul 20 '20 at 10:12
  • 1
    $\begingroup$ I'm confused in your initial lines where you write, "substitution nucleophillic addition elimination". I can't see which one are you asking? $\endgroup$ Jul 20 '20 at 10:17
  • 1
    $\begingroup$ @andselisk Do you mean first-year college students or first grade elementary students (6 year olds)? ;-) $\endgroup$ Jul 28 '20 at 19:58