In partial reduction alkynes are reduced to E alkenes by using Na metal with ammonia,My question is why then after donating of electrons by Na after formation of alkene it does not proceed ? why alkene is also not reduced by taking electrons from sodium metal again?

'x' gm of a compound A3B2C5 contains 'y' gm of A atoms

 

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how to represent the possible fischer projections for 2-bromohexane