ORGANIC CHEMISTRY I CHEM 2323

ARENES

INTRODUCTION

Aromatic-aliphatic compounds

The benzene is viewed as a substituent group, not as a functional group.

Arenes have both the aromatic and aliphatic groups.

Reaction may occur on either the aliphatic portion or the aromatic portion depending on the reaction conditions.

STRUCTURE and NOMENCLATURE

A compound containing a very complicated side chain might be named as a phenyl alkane.

The simplest alkenyl benzenes are called styrenes.

Aryl halides are compounds in which the halogen is attached directly to the ring.

Aralkyl halides are compounds in which the halogens are attached to the alkyl portion, not to the benzene ring.

Benzyl is an aralkyl halide with the following structure.

PHYSICAL PROPERTIES

• low polarity
• almost always less dense than water.
• The BP increases as molecular mass increases.
• For larger compounds the BP or MP increases approximately 20 - 30 ^oC / carbon.
• Melting Point - para compounds normally have higher melting points than their respective ortho, meta structures.
• Solubility - para is normally less soluble than ortho or meta.

INDUSTRIAL SOURCE OF ALKYL BENZENES

Petroleum is the chief source through catalytic reforming.

An alternate source is coal.

When coal is heated in the absence of air, coke, coal gas and coal tar are formed.

Coal tar is distillated giving alkyl benzenes.

PREPARATION OF ALKYL BENZENES

1. Friedel - Craft Alkylation

The alkyl group (R) may rearrange.

BF₃, HF, etc (Lewis acid) may be substituted for AlCl₃.

2. Conversion of side chain: Clemmensen or Wolff-Kishner reduction

MECHANISM OF FRIEDEL-CRAFT ALKYLATION

LIMITATIONS OF FRIEDEL-CRAFT

• danger of poly substitution (use excess benzene to reduce poly substitution)
• rearrangement
• aryl halides will not undergo Friedel-Craft substitution
• aromatic rings less reactive than aryl halides will not undergo Friedel-Craft substitution
• aromatic rings containing -NH₂ -NHR -NR₂ will not undergo Friedel-Craft substitution (they are strongly basic and tie up the AlCl₄- in the mechanism)

REARRANGEMENT

2^o and 3^o are more stable than 1^o

REACTIONS OF ALKYL BENZENES

1. Hydrogenation

2. Oxidation

3. Substitution on the ring

4. Substitution in the side chain (free radical halogenation)

PREPARATION OF ALKENYL BENZENES

Industrial Preparation

Laboratory Preparation