What is the structure of benzene? (C6H6)

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!

O

O

O

orHBr

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