Name: Jessica B.
Why is saponification a slow reaction?
"Fast" and "slow" are relative terms, depending upon the context. The saponification reaction is the
reaction of an ester: R'--CO2--R with water to produce an alcohol, ROH and a carboxylic acid R'--CO2--H.
If R is a so-called "good" leaving group, the reaction can be quite smooth requiring only minutes to an
hour or so. For "poor" leaving groups, for example, 2-methyl hexyl alcohol, the reaction can be quite
sluggish, on the minute/hour time scale. There is also a lot of slack in those times because the presence
catalyst for example, (H+) or (OH-), among many others cans reduce the reaction time. The temperature at
which the reaction is run is also important. In the case of saponification reactions higher temperatures
speed up the rate of reaction. The solvent also plays a contributing role. For example if the ester is
water soluble and water is the reaction solvent, the rate of the saponification reaction is greatly
increased. On the other hand, if the leaving alcohol group is R--OH, and the alcohol R--OH is used
as the solvent the rate of saponification will be substantially suppressed.
Saponification is the reaction between a strong base (such as sodium or potassium hydroxide) and a
natural fat. The products are glycerol and the alkali metal salts (soaps) of all the fatty acids that
comprised the fat. The reaction need not be "slow" if the concentration of the base is relatively high.
Hydrolysis of fats (a reaction between the fat and hot water) to produce glycerol and free fatty acids
released from the fat can be a relatively slow reaction because the fat and water are not mutually
soluble one in the other. Typically, hydrolysis of fats is accomplished by use of high pressure steam
rather than plain hot water.
Click here to return to the Chemistry Archives
Update: June 2012