Why NO2 shows -M effect.
Why NO2 shows -M effect.
NO2, the nitro group is an EWG (electron-withdrawing group) and hence meta-directing. When you draw the resonance structures, you can see that the ortho and para positions are those that can hold the electron density or positive charge that a substituent group provides. Since NO2 is an EWG, it will pull the electron density away from the o- and p-positions, hence electrophilic sustituents tend to attack the relatively electron rich meta-position.
Analogously, for an ERG (electron-releasing group) like -CH3 or -NH2 or -OCH3, they push their own electron density towards the ortho and para positions, which hence become more rich in electron density and are readily attacked by electrophiles.
Halogens are exceptions, which are electron-withdrawing by nature, but owing to their lone pairs of electrons stabilise the o- and p-positions with their electron density. Hence though electron-withdrawing, they are ortho-para directing.
Hope it helps!
NO2, the nitro group is an EWG (electron-withdrawing group) and hence meta-directing. When you draw the resonance structures, you can see that the ortho and para positions are those that can hold the electron density or positive charge that a substituent group provides. Since NO2 is an EWG, it will pull the electron density away from the o- and p-positions, hence electrophilic sustituents tend to attack the relatively electron rich meta-position.
Analogously, for an ERG (electron-releasing group) like -CH3 or -NH2 or -OCH3, they push their own electron density towards the ortho and para positions, which hence become more rich in electron density and are readily attacked by electrophiles.
Halogens are exceptions, which are electron-withdrawing by nature, but owing to their lone pairs of electrons stabilise the o- and p-positions with their electron density. Hence though electron-withdrawing, they are ortho-para directing.
Hope it helps!
