[jkulier]

Hi there,

in order to determine the type of reaction that occurs, I have to figure out whether or not mixing H2O and CH3CN would produce a good or poor nucleophile.

I have considered H2O giving up a proton to CH3CN, but that would not make much sense because there is no place for the proton to attach to. The other option is for CH3CN to lose a proton to water, but the same problem arises.

Any guidance would be great, because I am very lost.
Thanks in advance 

[UG]

I think that the nitrile carbon will undergo attack by water to give an sp²-hybridised intermediate imine anion. Someone please correct me if I'm wrong  .

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By the way, isn't the product ethanoic acid?

[Heory]

H₂O doesn't react with MeCN at all, so neither a good nor a poor new nucleophile would be obtained by mixing the two materials.
the O of H₂O is not nucleophilic enough to attack the C of MeCN unless the MeCN's N atom is protonated by which the electrophilicity of the MeCN is enhanced.

[orgopete]

I don't think the answer is found in the interaction of the two solvents. I think you should determine what effect pure acetonitrile and water has on the reaction. Acetonitrile is a classic dipolar aprotic solvent and improves nucleophilicity in some reactions. Water also effects a reaction. I think the answer is to compare whether a nucleophile is enhanced in acetonitrile or water and what effect water will on the overall reaction.

By the wording of the question ("type of reaction"), it would appear that a broader answer than nucleophilicity might be being sought.