Step 1: Nucleophilic attack of alcoxide ion RO- to carbon in the carbonyl group (-CO-) of the ester R*COO-R** forming tetrahedral ion as an intermediate.
Step 2: Proton moves from ROH to the oxygen atom of -OR** radical.
Step 3: Balance electron in the anion of ~O- moves toward carbon atom forming double bond of carbolyl group and removing R**OH.
Thus -OR** in R*-CO-OR**(Ester "A") is replaced with -OR to form R*-CO-OR (Ester "B") in all of the three steps.
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