What is the complete mechanism of reaction for sodium benzoate and ascorbic acid?
From Benzene production from decarboxylation of benzoic acid in the presence of ascorbic acid and a transition-metal catalyst:

$\ce {Cu^{2+} + H_2Asc \rightarrow Cu^+ + HAsc^.}$

$\ce {Cu^+ + O_2 \rightarrow Cu^{2+} + O_2^-}$

$\ce {2O_2^- + 2H^+ \rightarrow H_2O_2 + O_2}$

$\ce {Cu^+ + H_2O_2 \rightarrow Cu^{2+} + OH^- + OH^.}$

Then the hydroxyl radical reacts with benzonic acid as opposed to benzoate.