Electrophiles / Nucleophiles. Nucleophile – A Lewis Base with a pair of unshared electrons that seeks a positive part of an atom. Electrophile - A Lewis Acid seeking an electron pair. Nucleophilic Substitution. Nucleophilic Substitution
Nucleophile – A Lewis Base with a pair of unshared electrons that seeks a positive part of an atom.
Electrophile - A Lewis Acid seeking an electron pair
The 2 electrons in filled sp3 orbitals of alkyl groups can overlap with empty p-orbitals of positively charged adjacent Carbon atoms that produces a stabilizing affect on the Carbocation (Hyperconjugation).
Chiral molecules possess handedness; thus, are capable of existing as enantiomers (stereoisomers of each other)
Achiral molecules lack handedness, thus, are incapable of existing as enatiomers
If Q is an Activator - an electron-releasing (donating) group - relative to hydrogen, the reaction occurs faster than substitution on an unsubstituted ring. Resonance forms favor o,p-directing
If Q is a Deactivator- electron-withdrawing (accepting) group – the reaction occurs slower than substitution on an unsubstituted aromatic ring. Resonance forms favor m-directing.
The Ortho/Para vs. Meta directing effect of the first substituted group on the ring is accounted for by the interplay of two factors working simultaneously, either of which can be dominate.
Induction Effect / Resonance Effect
Effects of Deactivating (Electron Withdrawing) Groups on the Orientation of Substituted Electrophiles
CF3 is strongly electron withdrawing (deactivating) leaving the ring electron deficient with a developing
positive charge at the end of the dipole, i.e., on the
ring carbon atom adjacent to the Deactivating Group.
This deactivates (destabilizes) the ring toElectrophilic substitution
As the positively charge Electrophile (E+) attacks
the Ortho/Para resonance structures it attempts
to add additional positive charge to the carbon adjacent to the positively charged withdrawing group, further destabilizing the ring; thus Ortho/ Para substitution are not favored.
In Meta attack the carbon atom bonded to thedeactivating CF3 group does not share as muchof the positive charge of the ring with the Deactivating Group, i.e., the positive charge is spread more evenly about the ring. Thus, the ring is less deactivated than the Ortho/Para attack and therefore is more susceptible to substitution at the Meta position.
Effects of Activating (Electron Donating) Groups on the Orientation of Substituted Electrophiles
The Amino group is a strong activating group.
Even though the amino group is moreElectronegative than the attached Carbon theelectron releasing resonance effect is much
more dominant that the Deactivatingelectron Withdrawing effect.
In O or P attack the nonbonding pair ofelectrons from the Nitrogen can form, through resonance, an extra bond to the Carbon atom completing the outer octet of Electrons. This stabilizes one of the Arenium ion (sigma complex) resonance structures.
In Meta attack of a ring with a substituted activating group, none of the resonance structures are able to take advantage of the resonance contribution from the non-bonding pair of electrons. Therefore, the structure remains less stable and less subject to substitution at the Meta Position.
Ortho & Para Directors
Activators (Donate, Release Electrons)
Methyl & Alkyl groups are activating because of the stabilizing effect of sp2 hybridization (hyperconjugation) of an unbonded electron in methyl radical.
Halogens have a net withdrawing effect through induction, but there is sufficient competition from resonance to make them o,p directors.