The addition of a proton to the double bond of an alkene results in a product with the acidic proton bonded to the carbon atom that already holds the greater number of hydrogens.
extended: In an electrophilic addition to the alkene, the electrophile adds in such a way that it generates the most stable intermediate.
-- This rule applies to reactions between alkenes and other electrophiles as well
Markovnikov's rule:
Carbocations are
reactive intermediates
the mechanism of the reaction:
The complete description of a reaction pathway, including any reactive intermediates, that describes electron movement, the energy profile, kinetics, orbital descriptions, and stereochemistry
Two steps of a single halide addition reaction for alkene
1. The e- pair in alkene is donated to the proton in H-X. The resulting species is called a carbocation.
2. The halide ion (X-) is a nucleophile and donates a lone pair of electrons to the electron deficient carbocation (electrophile) forming a new bond.
(Markovinikov)
carbocation stability
primary < secondary < tertiary
Carbocations are (molecular geometry and hybridization)
trigonal planar → sp2-hybridized
The inductive electron-donating effects from alkyl groups are
additive
Hyperconjugation
The overlap of bonding electrons from the adjacent s-bond with the unoccupied p-orbital of the carbocation (NOT formation of a covalent bond)
is an additional factor of stabilization for a carbocation.
can be shown w/ resonance structures
minor (Cl on 2°). major (Cl on 3°)
Steps in the reaction mechanism
hydride shift
transition states:
- define
- location on energy diagram?
- are they observable?
- Forward and reverse reaction have
represents an energy barrier to the interconversion between reactants and products
transition states are maxima in energy diagrams
very short lifetimes; partially formed bonds – impossible to observe
Forward and reverse reaction have same transition state (‡).
Two factors govern the intrinsic reaction rate:
1. The energy barrier.
2. The Temperature (reactions are faster at higher temperatures ).
Rate-determining step (RDS):
- define
- draw energy diagram
The slowest step in a chemical reaction; controls the reaction rate.
* corresponds to the highest energy barrier
Hammond’s Postulate
says that a transition state more closely resembles whichever of the two states on either side of it is closest to the transition state in energy. If there’s an intermediate, this is usually closest in energy to the TS.