Why does triacylglycerides with longer fatty acids have lower saponification number than those shorter fatty acids?

Answer 1

The saponification number is measure of the number of saponifiable sites (i.e. -esterified or unesterified- acyl groups, by reaction with OH-) per (AND HERE COMES THE CLUE)

1 gram of tested sample.

Just imagine you have 2 samples: L(ong) and S(hort) with the SAME amount of saponifiable sites per molecule (or mole), so they both react with the same amount of reactant (KOH or NaOH).

If it were 'per mole' then the saponification numbers would be equal, won't they?

BUT saponification number is per 1 gram, and as the Long chained triacylglycerides in sample L do have a higher molecular mass, there are less molecules (or moles, or sapon. sites) of them in 100 grams. Thus sample L has a lower saponification number than sample S.

In this way the saponification number is a measure of the average molacular mass (also of average chain length) of triacylglicerides (i.e. normal fats) though under certain restrictions.

Answer 2

Triacylglycerides with longer fatty acids have lower saponification numbers because longer fatty acids contain more carbon-carbon double bonds, which reduce the number of free fatty acid molecules available for saponification. The presence of double bonds makes the molecule less accessible to the base used in saponification reactions.