esterification

Question

我想問吓ester係點prepare 係咪有兩個方法?

1
將akanol+alkanoic acid HEATING UNDER REFLUEX
但係有d 先將akanol+alkanoic acid放進test tube + con. sul

2
然後將test tube 放在hot water
然後將mixture pour into cold water
ester will float on the water

究竟兩個有咩分別?
同埋點解HEATING UNDER REFLUEX 果個 closed

HEATING UNDER REFLUEX
係幾時先close?
其實有幾water condenser 都得
仲close 黎做咩?
定係冇用water condenser 就要close?
我看書果個好似用HEATING UNDER REFLUEX的方法 to obtain alkanoic acid from alkaol好似唔使close

Answer 1

First of all - a refulx system is usually CLOSED otherwise the solvent will evaporate over time! Reflux systems can be closed because there's a water condenser attached, which will condense the vapour of the solvent back into the reaction and hence, preventing the loss of products (if your product is volitale)
The reflux method will give you better yield of the final prodcut because of the above reason AND also, because of temp. of the reaction. To product ester from alcohol and carboxylic acid needs enough temp. to heat up otherwise the nucleophilic attack at the carboxyl carbon cannot be completed (not enough energy).
The test tube method is not hot enough and the alcohol starting material will be evapourated, meaning that the equilibrium cannot be pushed all the way to the product side. And not enough alcohol to react with the acid won't give you a good yield.
Hope that helps.

2006-10-31 04:43:14 補充：
volatile...

2006-11-01 00:55:31 補充：
reflux...spelling mistake againClosed systems are usually used in organic reactions, we close it up with a constant N2(g) flow to keep the water out of the reaction.

2006-11-01 00:55:53 補充：
Likewise, the reaction of forming ester would probably done in a closed system in the lab - the water condenser will cool the vapour down back into the reaction.

2006-11-01 00:56:53 補充：
You can't reflux something WITHOUT a condenser, then that's dangerous because under heat, the volitale solvent and products with heat up ripidly and blow up!

2006-11-01 00:58:37 補充：
The reaction CAN be done in an open-top reflux system BUT your reactants will be more likely to evapourate over time due to air...unless if you have the heat on low to do the reaction, which in that case the reaction will be slow and incomplete.

2006-11-01 00:59:33 補充：
When I say water, it's in fact the moisture from air Iøm talking about

Answer 2

HEATING UNDER REFLUX 可以防止化合物流失
而方法2就可能會有揮發