The anion product can take a proton from the water or alcohol solvent.
A-level brings up two reducing reagents, lithium aluminium hydride and sodium borohydride. The mechanism above is how they work.
You should remember that Li[AlH4] is a stronger reducing agent. The use of aluminium and lithium both increase the hydride-donating ability of the ion. So sodium aluminium hydride or lithium borohydride would both be of intermediate strength.
A freely dissociated H- does not actually act as a nucleophile, since the 1s orbital isn't large enough to overlap with a pi* orbital by itself. The carbonyl hydride attack requires a weak sigma bond to overlap with the pi* - which is provided by the hydride donor.
Hence the ionic sodium hydride is only useful as a base, not as a nucleophile:
Orbital sizes also explain why sodium hydride is such a good base - the 1s orbital of the hydride overlaps well with H lobe of a typical H-X bond's sigma* orbital.
The oxyanion produced by the reaction of borohydride with a ketone can also be stabilized by boron's empty P orbital:
Which gives a new tetravalent boron anion. In theory this process can happen to all four hydrogens:
Hydrolysis of this molecule would give four alcohols and B(HO)4-, hence B(OH)3 and NaOH if we we pay attention to the counter ion.
In practice the process is rarely as efficient. So for a water solvent we would in total get alcohol + NaOH + B(OH)3
+ B(OH)2H + B(OH)H2 + BH3 + the starting ketone/aldehyde. An alcohol solvent would give sodium alkoxide in place of NaOH.
But the yield of the alcohol is pretty good even if things do not fully react, and it can be separated out of of the mixture.
Sodium Borohydride does not react with less reactive carbonyls such as esters and amides. Hence it is a good way of attacking a specific carbonyl:
It does not attack nitro groups or alkyl halides either:
Lithium aluminium hydride is powerful enough to reduce esters:
It can also attack nitro groups and alkyl halides. It is so powerful that it will even reduce water - explosively and violently - while giving off flammable hydrogen:
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