[RainbowMoo]

Why is epoxide ring opening under basic conditions carried out using reflux while reflux is not used under acidic conditions?

I think it has something to do with the solvent and acid interacting, but I don't know if that's correct and I don't know how the heat affects the reaction. When the acid protonates the epoxide...is this step affected and a different group other than an alcohol/the intended nucleophile add to the starting epoxide?

[ziqquratu]

Do you know the mechanisms of acid- and base-catalysed epoxide opening reactions? If you compare the two, can you imagine why one may need more energy than the other to take place? (Hint - consider the reactivity of the protonated epoxide species in the acid-catalysed reaction, versus the lack of such a species in the base-catalysed pathway).

[RainbowMoo]

Do you know the mechanisms of acid- and base-catalysed epoxide opening reactions? If you compare the two, can you imagine why one may need more energy than the other to take place? (Hint - consider the reactivity of the protonated epoxide species in the acid-catalysed reaction, versus the lack of such a species in the base-catalysed pathway).

When the epoxide is protonated under acidic conditions, the OH is a good leaving group. Under basic conditions, the O of the epoxide is not a good leaving group and so it needs more energy for the reaction to occur, hence the high heat. But I don't understand how high heat affects the acid catalyzed reaction. High heat puts more energy into the system and would make the epoxide more reactive, does this mean that an OH and the nucleophile will not form as substituents? Would the OH leave as water??

[ziqquratu]

I'm not sure I understand your second post. In the first, I thought you were asking why a base-promoted hydrolysis may require more heat than an acid catalysed one. For that, you're right - protonation of the epoxide activates the epoxide to attack by a nucleophile, with "R-OH" being the (very good) leaving group; for the base-promoted reaction, "R-O^-" is a poor leaving group, so more energy is required to drive it along.

I'm going to try to help with what I think is your question - please clarify if I've misinterpreted you and I can try again!

I think you may be asking "why not heat up the acid-promoted reaction too?". Certainly, increasing the temperature of a reaction should increase the rate. However, heating a reaction will increase the rate of ALL possible reactions - including undesirable side reactions! In the case of 1,2-diols, the pinacol rearrangement is conducted by heating in the presence of acid!

Hence, experimental conditions are often a balancing act between enough energy (heat) to enable a desired reaction to take place, but not so much that side reactions become problematic.

[RainbowMoo]

I'm not sure I understand your second post. In the first, I thought you were asking why a base-promoted hydrolysis may require more heat than an acid catalysed one. For that, you're right - protonation of the epoxide activates the epoxide to attack by a nucleophile, with "R-OH" being the (very good) leaving group; for the base-promoted reaction, "R-O^-" is a poor leaving group, so more energy is required to drive it along.

I'm going to try to help with what I think is your question - please clarify if I've misinterpreted you and I can try again!

I think you may be asking "why not heat up the acid-promoted reaction too?". Certainly, increasing the temperature of a reaction should increase the rate. However, heating a reaction will increase the rate of ALL possible reactions - including undesirable side reactions! In the case of 1,2-diols, the pinacol rearrangement is conducted by heating in the presence of acid!

Hence, experimental conditions are often a balancing act between enough energy (heat) to enable a desired reaction to take place, but not so much that side reactions become problematic.

That is the main part of my question - the side reactions. I am having trouble working out what side reaction would take place and what the product(s) would be.

[ziqquratu]

Ahh, very good. Well, as I said, the most common side reaction in the acid-catalysed opening of an epoxide would be the pinacol rearrangement. Have a look at that and see if you can work out the product you would obtain.

If you have trouble, post the structure of your epoxide and I can help you.