[chmstud101]

Say you were given a problem in which the reactants and products of a reaction are given and you are asked to find the mechanism for that reaction.  Would it be ok to find a mechanism that converts the products to reactants and then just reverse this mechanism so it goes from reactants to products?  Basically what I'm asking is if the mechanism for the forward reaction is the same as the reverse reaction except in the opposite order.

[tmartin]

I would look up the principle of microscopic reversibility for a good explanation to answer this question.

[orgopete]

What you describe can work with some reactions. Reactions which are controlled by equilibration, Fischer esterification and acid catalyzed ester hydrolysis, would work as described. However, that approach to many reactions will leave you trying to suggest energetically improbable reactions, for example the formation of a carboxylic acid from an acid chloride and sulfur dioxide.

For those that like to generalize, the mechanisms of chemical reactions can be described with a relatively small number of types, for example, radical (single electron transfer) or ionic. With ionic reactions (the dominant type in ordinary chemistry), there are relatively few types of reactions that actually take place. Once you begin to see the pattern of one reaction, you can begin to see the mechanistic similarity of a Fischer esterification and an acid catalyzed ketal formation.

A problem in which the reactants and products a given may be considered as hints to how a reaction takes place. When I asked questions of this type, it was intended as a hint to students that reaction mechanisms were an important aspect of learning organic chemistry and that students should make the effort to learn them. Et tu…?