1Since its related to hydrogen, then hydride ion should be used in an SN1 Rkn...den it should be a strong Nucleophile.......
HYDRIDE ION IS A STRONG BASE OR A STRONG NUCLEOPHILE??
IF BOTH ..WHICH EFFECT IS STRONGER????
GIVEN A REACTION..WILL IT ACT AS A PROTON ACCEPTOR OR A NUCLEOPHILE??
PLZZ ANSWER SOMEONE..
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4 Answers
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39I don't remember...but this is what I think.
Depends on the conditions(solvent, temperature, etc). It is both a strong nucleophile as well as a Lewis base.
For example, if the solvent is polar, the hydride ions will be blocked off from nucleophilic action by solvent molecules. If the temperature is high, thermodynamically elimination will be favoured and if low substitution will be.
49Lets try to logic...
Hydrogen is a chunnu atom and in hydride ion it is burdened wit a negative charge...
As a result, there is hi-fi charge density on the bechara hydrogen...
So now, if we release this burdened baccha hydrogen atom in air (not solution) it would want to get rid of its burden and so it would behave as a "SUPER" base or nucleophile (i.e. both).
Also it needs mention that this hifi reactivity makes hydride ion impatient and so it will not be patient enough to wait for the formation of carbocation...and thus will undergo substitution or elimination by the bi-molecular pathway(i.e. SN2 or E2).
Thus, this will follow the general rule...while reacting with a super 1° or 1° substrate, generally the H- will act as a nucleophile while with a 3° it will act predominately as a base..!!
Now lets come to solutions...
In Polar protic solvents, hydrogen is attached to highly electronegative atom and thus is capable of charge stabilisation by hydrogen bonding (large ΔHsolvation)...
When H- is furnished in protic solvents, it causes formation of a BIG solvation sphere all round it...thus the charge diffuses out...(Imagine yourself stepping into a shopping mall putting on a LOTS of AXE...imagine the AXE EFFECT!!! LOL!!!)
Nucleophilic character decreases considerably...
Hydride acts as base but it does not react with the hydrocarbon substrate but reacts with the highly available and strongly acidic protic polar solvent!!
In polar aprotic solvent, the hydride ion is NOT solvated and may be treated SIMILAR to the first case...!!
Also points pointed out by pritish are true!!
The reader must believe in the explanation above at his own risk...there are high chances of it being wrong!! After all when do we innovate?? Its when we do not know exactly!! :P :P
1both kinetic and thermodynamic effects must be seen then only we can go for the action of it in the reaction.......