Benzene (Chapter 15)

Ortho- o–

Meta- m–

Para- p–

Phenol

Aniline

Benzaldehyde

Benzoic Acid

Styrene

A six-carbon cyclic compound consisting of six equally overlapping p orbitals and six equal-length C—C bonds. All carbons within the benzene ring are sp2-hybridized.

Benzene is more stable than 1,3,5-Hexatriene due to the “closure” of the six-membered carbon compound. Six-membered cyclic π systems are more stable than acyclic π systems.

A cyclic planar system of sp2-hybridized atoms containing (4n + 2)π electrons. Aromatic systems are highly stabilized.

A cyclic planar system of sp2-hybridized atoms containing (4n)π electrons. Antiaromatic systems are highly destabilized (i.e. less stable than alkenes or dienes).

• Non-planar • Acyclic • sp3-Hybridized Atoms Nonaromatic systems are comparible in stability to alkenes and dienes.

• Highly Unstable (Antiaromatic + Ring Strain) • Ability to Act as Alkene and Diene • Alternating Single + Double Bonds • Two Distinct Isomers (Rapid Isomerization through Symmetrical Transition State) The two isomers of 1,3–Cyclobutadiene are not resonance structures (because atoms are moved during the isomerization process).

• Non-Aromatic (Tub-Shaped) • Non-Conjugated π Bonds • Acts an Simple Alkene

• Benzene is unreactive in electrophilic addition mechanisms. • Benzene is reactive in electrophilic substitution mechanisms.

Cationic/anionic cyclic structures containing (4n +2)π electrons are highly stabilized due to aromatic stabilization. • Anionic cyclic structures containing (4n +2)π electrons have highly acidic conjugate acids. • Cationic cyclic structures containing (4n +2)π electrons serve as effective bases.

• If the PCHC contains (4n +2)π electrons, it is aromatic. • If all individual rings of the PCHC are aromatic, the PCHC is aromatic . • If the PCHC consists of one (or more) aromatic ring and no antiaromatic rings (i.e. solely nonaromatic rings), the PCHC is aromatic.

• If the PCHC consists of one (or more) antiaromatic rings, the PCHC is antiaromatic. • If the PCHC contains (4n)π electrons, it is antiaromatic.

A strong (highly partial-positive) electrophile is required for EAS reactions to occur (due to the low reactivity of the benzene ring). Since the C—E bond formation must be highly thermodynamically favorable (to overcome the thermodynamic instability of the loss of aromaticity) for EAS to occur, the electrophile must be highly electronegative.

Creation of the Arenium Ion Intermediate The creation of the arenium ion intermediate results in the loss of aromaticity, which destabilizes the cyclic compound.

The C—E bond formed during EAS is stronger/shorter (i.e. more stable) than the C—H bond that is broken. E = Electrophile

Since Friedel-Crafts Alkylation substitutes an electron-donating alkyl group into the benzene ring, the resulting phenyl compound is more electron-rich (i.e. more reactive). Overalkylation results in additional alkylations of the phenyl compound at the para and ortho (if steric hindrace is insignificant) positions.

If an EAS reaction results in the substitution of an electron-donating group into the benzene rings, additional substitutions will occur at the para and ortho positions (if steric hindrance is minimal).