This reaction was part of an organic scheme I was doing:


I thought that $\ce{POCl3}$ just converts the OH group into Cl, but looking at the chemical formula for A, I noticed that A has an extra carbon compared to the starting material.

It turns out that A is


I'm very confused as to how that aldehyde group was added to the benzene ring. Where is the additional C coming from?


It is a Vilsmeier–Haack reaction.

You are actually neglecting the effect of DMF (Dimethylformamide).

The Mechanism:

1. Formation of Iminium Cation:

The reaction of the DMC i.e. Dimethyl formamide (which is basically an amide) with $\ce{POCl3}$ produces an iminium cation.

2. Aromatic Electrophilic Substitution Reaction:

The formed iminium cation now act as an electrophile and does electrophilic aromatic substitution as a result of which we get an aryl ketone or an aryl aldehyde. So that extra aldehyde group which you are speaking about came from the electrophilic substitution reaction of your initial structure with iminium ion.

Here I have drawn the whole mechanism for you.

Mechanism of the aboce reaction

  • $\begingroup$ Thanks for the diagram :) I was just wondering, is there any way to predict the regiochemistry of the rxn since all three available C's are ortho/para to the two OMe groups? (In the scheme, I was given the final product after a series of rxns, so I can use that to guess the regiochemistry if there's no good way to predict it.) $\endgroup$ – carbenoid Mar 15 '17 at 23:21
  • $\begingroup$ I too used the given final product to decide the positon of the electrophilic attack. If I name the carbon where I attached the aldehyde group as 1, then moving clockwise, position 3 carbon is equivalent to position 1. So I could have choosed any one of those two carbons. Thinking about the 5th position, thats an unique position. But in forming -CHO inbetween the two methoxy groups can cause ineffective resonance i.e. SIR effect (Steric Inhebition to resonance). Inbetween 1 methoxy and 1 OH group, SIR effect is reduced and hence the resonance would be effective. $\endgroup$ – Yb609 Mar 16 '17 at 9:42

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