How to make the equilibrium favour the product you want The direct formation of esters
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص244-245
2025-05-15
473
The formation and hydrolysis of esters was discussed in Chapter 10 where we established that acid and ester are in equilibrium and that the equilibrium constant is about 1. Since the position of the equilibrium favours neither the starting materials nor the products, how can we manipulate the conditions of the reaction if we actually want to make 100% ester?
The important point is that, at any one particular temperature, the equilibrium constant is just that—constant. This gives us a means of forcing the equilibrium to favour the products (or reactants) since the ratio between them must remain constant. Imagine what happens if we add more methanol to the reaction above. [MeOH] increases, but the overall value of K has to stay the same. The only way this can happen is if more of the ester converts to the acid. Alternatively, imagine removing water from the equilibrium. [H2O] goes down, so to bring K back to the value of 1, the concentrations of acid and methanol are going to have to go down too, by converting themselves to ester and water.
This is exactly how the equilibrium is manipulated in practice. One way to make esters in the laboratory is to use a large excess of the alcohol and remove water continually from the system as it is formed, for example by distilling it out. This means that in the equilibrium mixture there is a tiny quantity of water, lots of the ester, lots of the alcohol, and very little of the carboxylic acid; in other words, we have converted the carboxylic acid into the ester. We must still use an acid catalyst, but the acid must be anhydrous since we do not want any water present—commonly used acids are toluenesulfonic acid (tosic acid, TsOH), concentrated sulfuric acid (H2SO4), or gaseous HCl. The acid catalyst does not alter the position of the equilibrium; it simply speeds up the rate of the reaction, allowing equilibrium to be reached more quickly. This is an important point that we will come back to shortly.
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