what is the order of -I effect groups?give the complete and reliable answer.especially which has more -I effect NO₂  OR NH₃⁺   ?

NH3+ > NO2 > CN > (SO)₃H > CHO > COR> COOH >COCl > COOR > CONH₂ > F > Cl > Br > I > OH > OR > C₆H₅ > H

 

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NO2>F>COOH>Cl>Br>I>OH>OR>C6H5>H

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The extent of -I effect depends upon the electronegativity of the hetero atom or the substituent, greater the electronegativity stronger is the  -I effect. The decreasing order of some common substituents relative to hydrogen is as follows :

-NR3+ > -SR2+ > -NH3+ > -NO2 > -SO2R > -CN > -SO2Ar > -COOH >- F>-Cl>-Br >- I> -OC6H5 >-COOR >-OR>-COR>-SH> -SR> -OH >-Ctriple bond CR > -C6H5 >-CH=CR2 >-D >-H.

Relative inductive effects may also be measured by NMR spectroscopy. However, the results must be accepted with reverse, since the applied field itself may affect the electron distribution in the molecule.

This can also be done by comparing the acid strengths with these substituents.

With respect to NH3+ & NO2, in the first one the nitrogen carries a distinct positive charge which being more electronegative than carbon will be pulling electrons towards itself; in the second the bond between nirogen and one of the oxygen is dative ( formed by donation of electron pair) hence is not distinctly positive ( though sometimes shown as -O-N+=O), its -I effect is due to more electronegative oxygen attached to nitrogen by double bond, here oxygen pulls the pi cloud towards itself and thus nitrogen becomes deficient which inturn pulls the electrons towards it.