I join to collegues opinion about possibility of acetal and hemiacetal formation, but...

But please note that in presence of water in huge excess, simple hydrate of aldehyde is expected. It is significant for benzaldehyde; electron-withdrawing substituents in Ph rise this effect. So, transformation Ar-CHO -> Ar-CH(OH)2 will occure, maybe with some Ar-CH(OH)OMe.

In contrary to acetal formation which need in acid catalysis, hydrate formation could be possible by both acidic and basic catalysis - so you can suppress acetal formation in basic condition.

It may be the dimethyl acetal, 2,4-dichloro-1-(dimethoxymethyl)benzene, though one would not expect the compound to be stable in aqueous media. Still, it may be present in equilibrium with its hemiacetal and the parent aldehyde.

Dear Mr Vojtisek, maybe you have obtained hemiacetals and acetals between aldehyde group in 2,4 dichlorobenzaldehyde and methanol. This reaction is catalyzed by acid. See the link: http://www.masterorganicchemistry.com/2010/05/28/on-acetals-and-hemiacetals/. This link explained these substances. I hope to have helped.

I do not think that any reaction has occurred by just placing 2,4-dichlorobenzaldehyde in the homogeneous mixture of water-methanol without adding anything else & without changing conditions for the 3 materials.

Look at it from a different angle: The 2 Cl atoms withdraw electrons from the benzene ring, so the ring will withdraw electrons from the carbon of the aldehyde (which is already subjected to electron withdrawing from the oxygen). So this will have an effect on the C-H bond in the aldehyde group rendering the H slightly to be susceptible for donation, i.e., there will be weak acidity just like the acidity of the terminal alkynes.

Tell us what you observed that led you to conclude a reaction occurred. And what tests have you made on the reaction product or mixture?

I join to collegues opinion about possibility of acetal and hemiacetal formation, but...

But please note that in presence of water in huge excess, simple hydrate of aldehyde is expected. It is significant for benzaldehyde; electron-withdrawing substituents in Ph rise this effect. So, transformation Ar-CHO -> Ar-CH(OH)2 will occure, maybe with some Ar-CH(OH)OMe.

In contrary to acetal formation which need in acid catalysis, hydrate formation could be possible by both acidic and basic catalysis - so you can suppress acetal formation in basic condition.