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Name: Jessica B.
Status: student
Age: 15
Location: N/A
Country: N/A
Date: 6/9/2003


Question:
Why is saponification a slow reaction?


Replies:
"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 of a 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.

Vince Calder


Jessica,

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.

Regards,
ProfHoff 682



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