What is the structure of benzene? (C 6 H 6 ) first ideas:

or, it could be this...

actual structure (refer to models)

orbitals and electron densities:

8-membered version (cyclo-octa-tetra-ene): not in same plane – less delocalization, single/double bonds

Resonance contributors

four  electrons in three p orbitals

Rules for resonance: 1: Only electrons move – never atoms 2: Only  electrons and lone pair electrons can move – never  electrons 3: Number of electrons never changes. Net charge is conserved

carbocations two  electrons in three p orbitals

Relative stability (importance) of resonance contributors A is more stable – no sep of charge. A is closer to reality

equal stability

neg on carbon, pos on oxygen

rules for stability: contributor is less stable if it has: 1: atom(s) with incomplete octet 2: neg charge NOT on most electronegative atom 3: charge separation

which is more stable?

resonance delocalization gives stability! which is more stable carbocation?

conjugated dienes more stable:

heats of hydrogenation: Why? conjugated p electrons are delocalized (refer to model) also – sp2-sp2 overlap is closer than sp2-sp3 – meaning closer  overlap too

cumulated double bonds - allene

stability of allylic / benzylic carbocations

rate of reaction with HBr?

MO theory

1,3-butadiene LUMO HOMO

1,3,5-hexatriene HOMO-LUMO gap getting smaller!

Benzene don’t worry about pictures of higher energy MO’s (p. 311 fig)

Resonance delocalization and acid/base strength

what about ortho? meta?

explain regioselectivity:

1,2 vs 1,4 addition

Thermodynamic vs kinetic control

kinetic control: low temps

thermodynamic control: higher temps

more sub alkene – more stable

kinetic control: why is 1,2 product formed faster? consider the reaction:

proximity effect:

kinetic control, but no thermodynamic control:

Diels-Alder

increases reactivity of dienophile

MO picture:

prediciting regiochem:

prefered:

conformation of diene:

in general, endo favored

stereochem of Diels-ALder