EXEC statement inside case statement

I have a table TAB1 with the below structure
Index Function (This column contains the full function stored as string)
AAA Null
BBB fn_evaluate1()
CCC fn_evaluate2()
DDD Null

My requirement :-
1. When Function = null – Fetch rate for this Index from another table TAB2
2. If not null execute the function stored in the Function column of TAB1 and get the calculated value as rate.
I tried this –
SELECT A.Index,
CASE WHEN (A.Functions IS NULL)
THEN B.Rate
ELSE
Exec A.Function-- Need to execute the Function stored as string in the function column of TAB1
END Rate
FROM TAB1 A LEFT JOIN TAB2 B
on A.Index=B.Index

I could not execute the statement since it is giving a syntax error near exec.