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While reading organic texts, I have come across authors referring to "Activation of alkene" what does that mean !? Does it mean to include the alkene in resonance or what else exactly ?

I saw this notation, while reading about Heck reaction:

The palladium-catalyzed C-C coupling between aryl halides or vinyl halides and activated alkenes in the presence of a base is referred as the "Heck Reaction"

I also observed this while reading about nucleophilic conjugate addition:

Ordinary nucleophilic additions or 1,2-nucleophilic additions deal mostly with additions to carbonyl compounds. Simple alkene compounds do not show 1,2- reactivity due to lack of polarity, unless the alkene is activated with special substituents.

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Activation of an alkene just means that the double bond has a higher electron density than that of a normal isolated double bond. That is, the electron density in the double bond is greater than the one observed in ethene $\ce{CH2=CH2}$.

Activation, in organic chemistry, generally means the compound displays a greater nucleophilic nature than it normally should due to increased electron density. For example, $\ce{-OCH3}$ group activates benzene when it forms toluidine. This makes the benzene ring more electron rich and so make it easier to react in nucleophilic reactions.

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  • $\begingroup$ "Activation of an alkene just means that the double bond has a higher electron density" Does it really? One C atom could have partial negative and the other positive - still more reactive; or two positive - enables it to act as electrophile - why exactly nucleophilic character would be only important? $\endgroup$ – Mithoron Apr 16 at 23:15
  • $\begingroup$ The term was used to refer to an $\alpha, \beta$ unsaturated carbonyl in my notes for the Michael reaction. Doesn't that mean it's an electron deficient bond? $\endgroup$ – Harry Holmes Apr 30 at 9:00
  • $\begingroup$ Also, the wiki on the Michael reaction has the phrase 'The R" substituent on the activated alkene'. Seems to imply it's electron-deficiency that's meant. $\endgroup$ – Harry Holmes Apr 30 at 9:05
  • $\begingroup$ @SafdarFaisal: please respond if you're available. $\endgroup$ – Harry Holmes May 2 at 14:38

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