[mrsme]

i consider the following reaction: beznyl chloride + sodium acetate -> beznyl acetate + sodium chloride. I think it is SN2 reaction, but the question is: why benzyl chloride in this reaction doesn't transform into carbocation? What is the simplest way to know if there will be no carbocation in reaction?

[ramboacid]

The formula of benzyl chloride is C₆H₅CH₂Cl, making it a primary halide. It would be very unfavorable to form a primary carbocation, as the charge wouldn't be delocalized very well.

In an S_N2 reaction, you wouldn't form a carbocation intermediate. In an S_N1 reaction, you do form a carbocation intermediate.

[Arctic-Nation]

That isn't entirely correct, ramboacid. Benzylic cations are much more stable than primary alkylic cations as they can delocalize the charge over the aromatic ring, and they can react through SN1 under the right conditions.

mrsme, in some cases you cannot a priori know whether a certain substitution reaction will be SN1 or SN2, and if the reaction rates are similar, both mechanisms might operate simultaneously. With benzyl halides you can assume that they will react through SN2 if the following conditions are met: a strong nucleophile, an electron-poor or electron-neutral electrophile, and a solvent that doesn't stabilize cations effectively. If the reaction conditions are the opposite, you'll get an SN1 reaction.

If the reaction conditions are mixed or inconclusive, use two out of three and your best judgment. But whatever you do, make sure that if you absolutely must create a carbocation, it is at least a little stabilized (para-nitrobenzyl chloride will never, ever, react through SN1).

In your case, acetate is a not a very strong nucleophile, but it certainly isn't weak, and benzyl chloride (as I understand it) has a weak preference for SN2 over SN1. I'd make the reaction an SN2.

[Schrödinger]

The benzylic position does stabilize a carbocation, but a transition state in an SN2 type reaction is also pretty much stable, thanks to the p-orbitals of the aromatic ring. Benzylic halides, for that matter undergo faster SN2 reactions than many other primary halides, despite the steric effect.

As  Arctic-Nation has already pointed out, it is the reaction conditions that you should pay careful attention to.