Chapter 17

Spring 2009

Benzene• Six-membered ring• Is planar• All C-C bond lengths are equal

Kekulé structure

Resonance description of benzeneImplication of

the geometry of the structure above

Shorter isolated double bonds

Longer single bonds Common representation of

benzene

BenzeneBenzene is the parent aromatic molecule.Benzene has the following characteristics that are common among all aromatic compounds•Benzene is cyclic and conjugated•Benzene is unusually stable in comparison to a molecule with three isolated pi bonds•Benzene is planar and has the shape of a regular hexagon. All bond angles are 120º, all carbon atoms are sp2 hybridized, and all carbon-carbon bond lengths are intermediate in length between a single and double bond. •Benzene undergoes substitution reactions rather than electrophilic addition reactions that would destroy its conjugation.

Larger aromatic hydrocarbons

http://www.cem.msu.edu/~reusch/VirtualText/react3.htm

Pictures of Benzene

Electrostatic potential surface

Benzene molecular orbitalIn top picture notice a p

orbital on each C atom.Bottom picture show the overlap of the p orbitals.

Orbital picture of benzene

each carbon atom is sp2 hybridized

The resonance picture indicates why each C-C bond is the same length.

Electrostatic potential surface

Electron-rich portion represents π electron cloud

For a compound to be classified as aromatic, it must fulfill both of the of the following criteria:

• It must have an uninterrupted cyclic cloud of π electrons above and below the plane of the molecule (this is often called the π cloud. Also, the π cloud must be cyclic, uninterrupted, and planar.

• The p cloud must contain an odd number of pairs of π electrons. (4n+2 rule)

Common names for aromatic compounds

IUPAC names

• To name benzene ring with one substituent, name the substituent and add the word benzene.

ethyl benzene

?

If the alkane chain is longer than six carbon atoms name the benzene ring as a phenyl substituent.

Benzyl

Benzyl substituent

Disubstituted benzenes

1,2-dimethylbenzene 1,3-dimethylbenzene 1,4-dimethylbenzene

IUPAC names

Same substituent

meta-dichlorobenzeneor, 1,3-dichlorobenzene

Different substituents

These two have common root names

Different substituents

These two have common root names

toluene

analine

Different substituents

These two have common root names

meta-chlorotoluene

ortho-chloroanaline

Different substituents

These don’t have a common root name so alphabetize the name of the substituents.

Different substituents

These don’t have a common root name so alphabetize the name of the substituents.

ortho-bromochlorobenzene

para-bromonitrobenzene

Heat of hydrogenation dataThe low heat of hydrogenation means that benzene is especially stable – this extra stability is characteristic of all aromatic compounds.

Reaction of aromatic compounds

• Benzene does not undergo addition reactions typical of other unsaturated hydrocarbons.

What makes a compound aromatic?

• A molecule must be cyclic

• Planar

• Completely conjugated (alternating single-double bonds)

• Have a particular number of π electrons

This molecule is non-aromatic

It is cyclic, completely conjugated, non-planar, and contains 8 π electrons

This molecule is anti-aromatic

Cyclic, planar, completely conjugated, and contain four π electrons.

Aromatic, anti-aromatic, or non-aromatic

• Aromatic: A cyclic planar, completely conjugated compound with 4n+2 π electrons

• Anti-aromatic: A cyclic, planar, completely conjugated compound with 4n π electrons

• Non-aromatic: A compound that lack one (or more) of the following requirements for aromaticity.

Aromatic compounds can have more than one ring.

Aromatic rings can contain a single ring with lots of alternating single-double bonds

atomic orbitals

molecular orbitals

Anti-aromatic

The inscribed polygon method

• Draw the polygon in question inside a circle with its vertices touching the circle and one of the vertices pointing down. Mark the points at which the polygon intersects the circle.

• Draw a line horizontally through the center of the circle and label MOs as bonding, nonbonding, or antibonding.

• Add the electrons, beginning with the lowest energy MO.

Label the cation, radical, and anion as aromatic, antiaromatic, or

nonaromatic.

Heterocyclic aromatic compounds

Nucleic acids

Thymine

Aromatic Question

Thiamin, or vitamin B1, contains a five-membered, nitrogen-sulfur heterocycle called a thiazalium ring. Explain why the thiazolium ring is aromatic.

Cycloheptatrienyl cation, radical and

anion problem