Question

1. Explain the chemistry with reaction equations and structures for both the saponification and the sodium methoxide transesterification of oleic acid. Explain the difference between a saponification and transesterification reaction, what is the difference in type and amount of reagents that determines which reaction will occur?

2. Comment how many mole equivalents of NaOH and MeOH are reacted per mole of unused (virgin) vegetable oil for the biodiesel reaction and why NaOH is much less than one equivalent and why MeOH is much more than one equivalent.

Answer 1

1)

Saponification of stearic acid methyl ester

C17H35CO2CH3 + NaOH --> C17H35CO2Na + CH3OH

Here carboxylic acid ester is hydrlyzed with base NaOH to form a salt of sodium and alcohol.

Transesterification of steraic acid ethyl ester with sodium methoxide

C17H35CO2CH2CH3 + NaOCH3 --> C17H35CO2CH3 + CH3CH2ONa

Here the ethyl group of ester is replaced with the methyl group from sodium methoxide.

With exces base, saponification is the way to go for the reaction.

2)

three moles of base NaOH or KOH are needed for compete saponification of vegetable oil (biodiesel). Used vegetable oil, has higher degree of ester groups present as it is heated which makes more ester bonds. Thus it required higher value of base NaOH to completely saponify (hydrolyze) the ester linkages.

Titration helps us determine the amount of free acid groups present in oil.