Resonance carbocation reactivity, electron-withdrawing

Through resonance, halogen tends to stabilise the carbocation and the effect is more pronounced at ortho- and para- positions. The inductive effect is stronger than resonance and causes net electron withdrawal and thus causes net deactivation. The resonance effect tends to oppose the Inductive effect for the attack at ortho- and para-positions and hence makes the deactivation less for ortho- and para-attack. Reactivity Is thus controlled by the stronger Inductive effect and orientation Is controlled by resonance effect. 

Alkenes that are directly attached to lone-pair-bearing heteroatoms, especially N and O, are particularly reactive toward electrophiles. A resonance structure that shows a formal negative charge on the /3 C can be drawn, and the intermediate carbocation is especially well stabilized. Conversely, alkenes substituted with electron-withdrawing groups are less reactive toward electrophiles. 

The monosubstituted alkenes 2, 3 and 5 are aU less reactive than 1 because their corresponding mercurinium ions involve resonance structures with a secondary carbocation, thus resulting in higher energy than the mercurinium ion obtained from compoimd 1. Alkene 3 reacts slower than 2 due to electron-withdrawal from the -OMe group, which would destabilize the mercurinium ion by further reducing the electron density of the resulting secondary carbocation resonance contributor. A similar inductive effect would also destabilize the mercurinium ion from alkene 5, but an... 

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