[zimrock]

Can -N=O group have +R effect or is it restricted to delocalisation on oxygen with the help of pi bond.
Hence, would this group be ortho para directing or meta directing on the benzene ring?

Also, which group would exert greater -I inductive effect, -OH or -OR?

among -OH and -OR , the -OH bond is more polar and hence, the electron density on oxygen is more and hence wouldnt -OR be a better -I group?

In -N=O, due to the presence of pi bond, wont it be only a -I group? how can it donate its lone pair to stabilise the sigma complex when the pi electrons are delocalised on oxygen?

[zimrock]

another doubt.
is phenyl a bulky group?
because its planar shouldnt it offer almost no steric hinderance?

[macman104]

For the N=O group.  You can draw resonance structures where the group is donating electron density and where it is removing electron density from the ring.  However, you are correct, if you look, the resonance structures where the negative charge is on the oxygen will be the major contributors, so it will be meta directing.

For the OH vs. OR, the alkyl group is going to donate electron density to the oxygen and make it a strong electron pusher into the ring.

When you say "offer almost no steric hindrance", in what sense?  In what regard are you worried about the phenyl groups size?

[zimrock]

by bulky i mean,

while comparing the relative rates of SN2 reaction of various compounds, do we take into effect bulkiness of phenyl group or only -I effect?
I was thinking, phenyl is planar and hence not bulky and hence the -I effect shuold increase rate of SN2 reaction
correct?

[macman104]

Are you talking about the bulkiness as an SN2 substrate or an SN2 attacking agent?  I don't believe you can perform SN2 *on* a phenyl ring.  However you can use it as an attacking group in something like a grignard.

[zimrock]

lets say we treat Ph-CH2-O-CH3 with anydrous HI. what will be the product? since the pathway is SN2, the answer depends on whether the phenyl group offers steric hinderance or not

[macman104]

Ah, ok.  For the benzylic group this cleavage should actually occur through an SN1 mechanism.  You protonate the ether then cleave leaving you with a benzylic carbocation which is then attacked by the I^-.  If you have a benzylic, allylic, or tertiary position, then the reaction usually proceeds through a SN1/E1 mechanism, secondary and primary usually proceed through a SN2 (which then if it's secondary or primary, USUALLY, the steric issue is not a problem).

[zimrock]

btw, deciding whether the group is o-p directing or m directing is decided by the stability of sigma complex and not where it increses electron density.

so im not convinced with your answer

[macman104]

And....what determines whether the sigma complex is stabilized?