I am confused about the stability of carbanion III and IV in the image, I think III is more unstable because of electron donating effect (+R) of -OCH3 group but my book says III is more stable than IV, please help. In compound III -OCH3 would donate its lone pair and would make the carbanion unstable but no such effect happens in compound IV. -OCH3 in compound III would also have -I effect to stabilise the carbanion but I have read that for -OR group +R effect is more than its -I effect i.e., its electron donating nature is more than its electron withdrawing nature through inductive effect, +R means electron donating by resonance(In -OR, R is any alkyl group)
I think in both (III) and (IV), Cross Conjugation is present which generally prevents resonance from taking place.
In (III), Both CH3O and CH2minus can give electrons to the C in middle.
In (IV), After resonance, CHminus(on left) and CH2minus(on right), can give electrons to the C in middle.
Supposedly, in (III), O will not prefer to give it's LP electrons because of high Electronegativity. In (IV), CHminus(on left) can comparatively give electrons more easily than O. Also +M effect of CH2minus is greater than O(with LP).
Therefore, CROSS CONJUGATION EXTENT in (III) is less than in (IV).
Hence, (III) is more stable than (IV).