[palmtreez]

I am reacting concentrated sulfuric acid with a benzyl alcohol and a carboxylic acid to make an ester (i.e. esterification). Now when i begin to add sulfuric acid it instantly reacts with benzyl alcohol why? what can i do so that i can do esterification with sulfuric acid?  

[Arkcon]

You're not performing the procedure properly for one thing.  You need to mix the carboxylic acid with an excess of alcohol, then add sulfuric acid slowly, perhaps drop-wise, with stirring.

http://en.wikipedia.org/wiki/Ester#Preparation

You can use other drying agents as well.

[palmtreez]

Oh i c, so i have to add alot more benzyl alcohol but what should i do to get rid of the water that is made, use 4A molecular seives? So say I get rid of the water, how can i purify the ester ( it looks like some type of polymer is formed)? (also i was using toluene as a solvent )

[Arkcon]

It really depends on the physical properties of the ester you're making.  I suspect vacuum distillation would be the typical way.  Wikipedia says molecular sieves should work, as long as you're sure reactants and products won't enter the molecular sieves.

[palmtreez]

How can i use vacuum distillation if i have to add acid drop wise to the rxn? Also benzyl alcohol forms an azeotrope with water the boils at 100C.  After heating the reaction it will make all of the benzyl alcohol boil over...

[Doc Oc]

I would strongly encourage you not to use excess benzyl alcohol.  It has a very high boiling temp (~230), is highly soluble in organic solvents, and doesn't azeotrope very well.

Rather than doing the acid catalyzed esterification, you may find it beneficial to use peptide coupling reagents instead.

[Honclbrif]

Would it be possible to use the carboxylate to attack benzyl bromide, maybe with a pinch of KI added to help things along?

[sjb]

Would it be possible to use the carboxylate to attack benzyl bromide, maybe with a pinch of KI added to help things along?

I've seen silver salts used with iodides to form esters, at least on paper, so this may be a goer. No references though

[Honclbrif]

Probably no need to get Ag involved. A quick structure search shows plenty of benzyl esters being formed from acids and BnBr. Most with good yields under boring conditions.