# Reactivity comparison between benzaldehyde and acetaldehyde [duplicate]

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.

• 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. – andselisk Jul 20 '20 at 6:38
• $\ce{-CH3}$ 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 – Aniruddha Deb Jul 20 '20 at 7:23
• Be sure to check for resonances before inductives. Also, I'm not able to get the sense of "partial carbocation"? – Rahul Verma Jul 20 '20 at 10:12
• I'm confused in your initial lines where you write, "substitution nucleophillic addition elimination". I can't see which one are you asking? – Rahul Verma Jul 20 '20 at 10:17
• @andselisk Do you mean first-year college students or first grade elementary students (6 year olds)? ;-) – Karsten Theis Jul 28 '20 at 19:58