Organic chemistry
Electrophilic and nucleophilic addition
Understand how electrophiles add to π-rich alkenes and how nucleophiles add to polarized π bonds such as carbonyls, including regiochemical and stereochemical outcomes.
IntuitionIntuition: add across a π bond
A π bond is more exposed than a σ bond and can be replaced by two new σ bonds. In an alkene, an electrophile often engages the π electrons first; in a carbonyl, the polarized carbon is electrophilic and accepts a nucleophile.
SchoolSchool level: recognize the reaction pattern
Definition: Addition reaction
Two atoms or groups become attached across a multiple bond, reducing its bond order without loss of atoms from the main reactants.
For an unsymmetrical alkene, addition may produce different constitutional isomers. Markovnikov’s rule is a useful trend for many ionic HX additions, not a universal law: radical conditions, rearrangements, and substrate effects can change the outcome.
UndergraduateUniversity: polarity directs electron flow
| Substrate π bond | Initially attacked by | Typical first event | — | — |
|---|---|---|---|---|
| C=C (alkene) | electrophile | π attack; carbocation or bridged intermediate may form | ||
| C=O (carbonyl) | nucleophile | attack at electrophilic carbon; tetrahedral intermediate |
Example
Predict the major constitutional product of propene plus HBr under ordinary ionic conditions, and state the key mechanistic reason.
Solution
2-Bromopropane is typically major: protonation in the orientation that gives the more stable secondary carbocation is favored, followed by bromide capture. This prediction assumes conditions without peroxide-initiated radical chemistry.
AdvancedAdvanced: intermediates and stereochemical control
Carbocation stability, neighboring-group participation, and ion-pair return can influence electrophilic additions; a bridged halonium ion often explains anti addition without a free carbocation. Carbonyl addition depends on nucleophile hardness, carbonyl substitution, coordination, and proton-transfer timing. A planar carbonyl attacked from either face can create a stereocenter, with facial selectivity set by the molecular environment.