Depto. Química Orgánica

ELECTROPHILICITY

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15/07/2017
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A chemical species or a given region of a molecule is ELECTROPHILIC when it has a defect of electronic density and thus tends to attack chemical species or regions of a molecule with an excess of electronic density.

One may understand the reactions between molecules as a function of their electron distribution. The electron density is to the molecules as the atmospheric pressure is to the earth. The latter can be understood by low (storm) and high (anticyclone) pressure zones. Electronegative atoms concentrate electron density and leave their surroundings with a lack of it.
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A ELECTROPHILIC species or region of a molecule will attack species or molecular regions with an excess of electron density.

ELECTROPHILE literally means 'friend of electrons', i.e. of negative charges.
Does this reaction make sense to you?Image
Image Electron density distribution in ethene (red = high; blue = low)
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The relatively high electron density at the double bond is electrophilically attacked by the proton of HCl.
A carbocation is thus produced that electrophilically attacks the chloride anion.

The same can be said in a complementary manner:

The relatively high electron density at the double bond nucleophilically attacks the proton of HCl. A carbocation is thus produced that is nucleophilically attacked by the chloride anion.

The two ways of discribing the reaction are correct.
Can you realize the subtlety of the language using the electrophile-nucleophile pair of terms?