1.

The Pk_(a) value of the C-H acids HC -= CH (I), MeCOCH_(2)COOEt (II), MeCOOEt (III) and PhMe (IV) decreases as

Answer»

`I gt III gt IV gt II`
`II gt IV gt III gt I`
`IV gt III gt I gt II`
`I gt IV gt III gt II`

Solution :The carbanion left after removal of a proton from MeCOOEt is more STABLE than that obtained from PhMe since in the latter case, the aromatic character of the benzene ring is DESTROYED. THUS, MeCOOEt is a stronger C-H acid than PhMe.
`CH_(3)-overset(O)overset(||)C_OEt overset("base")to overset(-)CH_(2)-overset(O)overset(||)C-OEt harr CH_(2)=overset(O^(-))overset(|)C-OEt harr "etc."`

Among `HC -= CH` and `MeCOCH_(2)COOEt` is a stronger acid than `HC -= CH` since the carbanion from `MeCOCH_(2)COOEt` is resonance stabilized by two carbonyl groups.
`CH_(3)-overset(O)overset(||)C-CH_(2)-overset(O)overset(||)C-OEt overset("base")to CH_(3)-overset(O)overset(||)C-CH-overset(O)overset(||)C-OEt harr CH_(3)-overset(O^(-))overset(|)C=CH-overset(O)overset(||)C_OEt harr CH_(3)-overset(O)overset(||)C_CH=overset(O)overset(|)C-OEt`
while the carbanion, i.e., `CH -= C^(-)` from `CH -= CH` is stabilized only by the greater electronegativity of the sp-carbon. Thus the overall acidity decreases in the order, `MeCOCH_(2)COOEt gt HC -= CH gt MeCOOEt gt PhMe`. Since a stronger acid has a lower `pK_(a)` value, `pK_(a)` VALUES decrease in the opposite order, i.e., `PhMe gt MeCOOEt gt HC -= CH gt MeCOCH_(2) COOEt`. In other words, `pK_(a)` values decrease in the order :
`IV gt III gt I gt II`


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