PART 1 Carbocations Flashcards
Hammond Postulate
Stability: the more substituents on the carbocation, the more stable the carbocation, and the faster it can form
what can stabilise a carbocation?
i
o Inductive effects – 1o, 2o and 3o
o Conjugation – allylic and benzylic
o Heteroatoms – lone pair donation, oxonium/iminium ion
what is sigma congugation
sigma bond between c-h can donate e density to a parallel empty p orbital on the carbocation
non bonding allyic structure
p——p LUMO
bonding allyic structure
HOMO P-P-P
Pi systems
p empty orbital overlaps with pi system + e can be delocalised
resonance
allows + energy to be delocalise across the system, stabilising it
heteroatoms
group 7 or any group with a lone pair can form a pi bond which stabilises the carbocation.
inductive effect
, inductive effects occur when the electrons in covalent bonds are shifted towards an nearby atom with a higher electronegativity. In this case, the positively charged carbocation draws in electron density from the surrounding substituents thereby gaining stabilization by slightly reducing its positive charge
how do carbocation react
1,2 hydride/alkyl shift
nucleophilic attack
pi bonds
wagner meerwein
Relief of strain in the ring expansion from a four- to a five-membered ring makes the alkyl migration
favourable. Despite going from a tertiary carbocation to a secondary one.
what is an aromatic compound?
- Structure must be cyclic.
- Each atom in the ring must have an unhybridized p-orbital.
- The structure must be planar to allow for continuous overlap of parallel p-orbitals.
- Delocalization of the π-electrons over the ring must result in a lowering of the electronic energy – follows
Huckel’s rule.
4n + 2pi
aromatic compound
huckel rule
planar
unless with heteroatom, as long it is sp2
antiaromatic
4n not huckels law
