[djvan]

Hi Everyone,

I'm moving through this material quickly, and I've made my way into acetal/ketal formation.  The conversion of hemiacetals to acetals is catalyzed by acid; the hydroxyl must be protonated to make a good leaving group.  However, they then show that the addition of acid to the formed acetal will cause it to revert back to aldehyde.

If this it true, what's preventing both reactions from occurring simultaneously in solution?   How do we get the reaction to stop at the protected, acetal stage?  Temperature, acid concentration, stoichiometric measures?

They mention that the acetals are unreactive towards bases; do bases typically undergo nucleophilic addition to the carbonyl carbon in aldehydes/ketones?  Can anyone give me an example of where acetal/ketal blocking is used?    Or, possibly a biochemical pathway?  If I can relate this back to some common household material, I might be able to remember this...heh.

[kriggy]

I was asked once the same question at exam. I didnt know the anwer but I was told that when you are making acetal, you are working in acidic non-aqueous condition. Then the deprotection is done by aqueous solution of acid, usualy solution of HCl.

It depends on which base we are talking about, but for example amines do add to carbonyl carbon. I dont think there is such biochemical pathway involvling protection of carbonyl group, just because those reactions are catalyzed by enzymes which are very selective. 
About example of this protection, look here:

[rwiew]

When you make an acetal, you simply have to be actively removing water - for example by distillation. It's an equilibrium, needs pushing to one side by either removing or adding water.

For biochemical pathways, not a protection, but have a look at the pyridoxal phosphate mediated pathways, the Schiff base formations in transaminations, decarboxylations and racemizations will be of interest.