1How will Cl show -M effect ?
Would you please explain how can we say that 3p-2p delocalisation is more stable?
How can we account for the below Kavalues
a) Benzoic acid 6.3 x 10-5
b) p-nitro benzoic acid 30.6 x 10-5
c) p-chlorobenzoic acid 6.4 x 10-5
d) p-methoxy benzoic acid 3.3 x 10-5
e) p-methyl benzoic acid 4.2 x 10-5
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3 Answers
23-M of NO2 will decrease the e- density on COO- in the anion making it less basic and hence making the conjugate acid highly acidic
similarly -M of Cl shows same effect but due to 3p-2p delocalisation the resonance wont be highly effective hence the conjugate acid will be just slightly acidic than benzoic acid
methoxy grp will show +M effect wich will actually increase e- density on the COO- making it more basic and hence the conjugate acid lesser basic
methyl grp shos similar effect but with the help of +I and HC hence less effective than methoxy grp
hence the order
1
49in general halogens show +R effect weaker in comparison to their -I effect..
plus there can be dπ-pπ bonding..(though to small extent due to presence of 3 lone pairs already causing intense repulsion)
thus, dπ-pπ bonding + -I effect just wins over the +R effect slightly..:)