aldehyde + benzylaldehyde reaction (with pic)

[TXpredent]

I ran into a weird dehydration reaction between an aldehyde and a benzaldehyde. So basically what's not shown in the picture is that an aldehyde and benzylaldehyde are added together in the presence of acid.

I understand the first step, the alpha carbon on the aldehyde attacks the electrophilic carbon of the benzylaldehyde and boom they get stuck together. The dehydration is what confuses me. I'm hoping the picture I post shows up. Can anyone explain what is going on? What does it mean "if it is near a benzene ring". I mean i understand that the group leaving is "near" one but why? and do all reactions that end up near a benzene ring dehydrate? also, it mentions that "with or without heat" it will do this, does that mean with heat other reactions will dehydrate too?

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[aznnytezx]

Dehydration reaction is another step to aldol reaction. Where you see that is the beta alcohol, and you just take it alpha hydrogens. It is a condensation where the water is leaving, and it will form a double bond where the alpha and beta carbons are at. Hope that helps!

 

[SoulPower]

I understand the first step, the alpha carbon on the aldehyde attacks the electrophilic carbon of the benzylaldehyde and boom they get stuck together. The dehydration is what confuses me. I'm hoping the picture I post shows up. Can anyone explain what is going on? What does it mean "if it is near a benzene ring". I mean i understand that the group leaving is "near" one but why? and do all reactions that end up near a benzene ring dehydrate? also, it mentions that "with or without heat" it will do this, does that mean with heat other reactions will dehydrate too?

I think near the benzene ring means that the dble bond occurs on the 2nd bond from the benzene. This allows resonance throughout the benzene ring up to the the c=o, which means this is very stable and likely to happen.

and do all reactions that end up near a benzene ring dehydrate?
I do not know if this is common it's been awhile since I took ochem, I think it'd be a rarity.

Most reactions are heated up to "dehydrate" something. So yes this reaction is unique in that sense that no heat is necessary, usually you do add heat.

 

[DAT12]

It's an aldol
If in acid automatically lose the OH and H to form double bond. HOWEVER if there is ever a ring if in acid or base, it automatically dehydrates.
Your welcome.

 

[biomolecmed]

When you are next to a benzene ring and dehydrate, you extend conjugation, and therefore make the molecule more stable.