Leaving group stability when treated with grignard

Why does $\ce{-NH2}$ not leave in $\ce{RCONH2}$ while in $\ce{RCONR2}$ $\ce{NR2}$ leaves when treated with Grignard?
I thought $\ce{NH2}$ should leave as there negative charge is more stable on $\ce{N}$ compared to $\ce{NR2}$ where the alkyl groups have a +I effect already.




$\ce{RCONH\_2}$ is a protic acid, and even though it's weak the Grignard reagent is a powerful enough proton base to react. You get deprotonation of the amide instead of nucleophilic attack on the carbonyl group.


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