Aromatic compounds

Unit 2 chemistry

aromatic compounds

Sources: Understanding Chemistry for Advanced Level by Lister and Renshaw, Chemistry for CAPE Unit 2

SECTION 01

Structure of Benzene

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benzene

• Flat, planar hexagonal ring structure with a ring of electron density above and below the plane of the ring.

• sp2 hybridization is responsible for the planar hexagonal structure and the overlap of the 'spare' p orbitals into an extended delocalised pi system all the way round the hexagon is responsible for the rings of electron density

• exhibits resonance where one or more canonical forms of the compound can be drawn but the actual structure is a hybrid of the two structures

Benzene memes

VIdeo

The carbon-carbon bond lengths between the 6 carbon atoms in benzene are found to be halfway between those expected for a carbon-carbon single bond and a carbon-carbon-carbon double bond

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SECTION 02

Reaction mechanism of benzene

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Electrophilic substitution

• The high electron density of the delocalized system of the aromatic ring attracts ELECTROPHILES.

• Substitution reactions occur in benzene as they preserve the stability of the aromatic ring whereas addition reactions would destroy it

• The electrophile forms a bond with one of the carbon atoms using electrons from the pi cloud, an intermediate is formed and the aromatic system is destroyed. To regain stability, the carbon ejects a hydrogen ion.

• Refer to Chapter 4.1 Some reactions of benzene: Electrophilic substitution in benzene.

vIdeos on the electrophilic substitution mechanism of benzene

Video 01

Video 02

SECTION 03

reactions of benzene: bromination and nitration

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All reactions of benzene will follow the electrophilic substitution mechanism. In bromination, the electrophile is Br+ while in nitration the electrophile is NO2+. Both of these electrophiles are created during the reactions as the first step.

Refer to CAPE Unit 2 Guide Chapter 4.1. under the headings Bromination of benzene and nitraton of benzene

Bromination of benzene

The bromine molecule is not polar so an electrophile must be generated to attack the high electron density of the benzene ring. Unlike alkenes, however, there is no "double bond" so the delocalized pi system CANNOT POLARIZE the bromine molecule.

"A catalyst also called a Friedel-Craft catalyst or halogen carrier is added to help polarize the bromine molecule and create the Br+ electrophile. In bromination, the catalyst is iron (III) bromide or iron filings and bromine

VIdeo

Bromination of benzene using FeBr3 catalyst at room tempeature

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nitration of benzene

The electrophile for this reaction is called the nitryl cation or the nitronium ion NO2+. It is generated between the reaction of concentrated sulphuric acid and concentrated nitric acid. Together they are called the nitrating mixture. Please see the preferred equation for the formation of the nitryl cation/nitronium ion on the next page

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VIdeo

Nitration of benzene with concentrated sulphuric acid and concentrated nitric acid at 50-550C STEP 1: Generation of the nitronium ion (the electrophile)HNO3+ H2SO4---> NO2+ + 2HSO4- + H3O+

SECTION 04

Reactions of methyl benzene

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Methylbenzene

The common name for methyl benzene is toluene. It is a derivative of benzene meaning that it is a benzene ring with a methyl substiutent attached to the ring

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Bromination of methylbenzene

This reaction is similar to that of the bromination of benzene. It requires a halogen carrier/catalyst and occurs at room temperatuare. Please note that HBr is also formed in the reaction

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Can you figure out this meme?

Remember ortho is positon #2 from the main substutuent, para is position #4 and meta is position #3

Halogenation of methyl benzene

Reaction mechanism

overall equation

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halogenation of methyl benzene

Thisi is the same reaction as on the previous page with the electrophile being used as Br2 instead of Cl2

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Halogenation of the MEthyl group

If methyl benzene is treated with bromine in uv light INSTEAD OF WITH the halogen carrier, SUBSTITUTION WILL OCCUR ON THE METHYL GROUP INSTEAD OF THE BENZENE RING

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nitration of methyl benzene

This reaction mechanism is the same as with the nitration of regular benzene. The first step involves the generation of the NO2+ electrophile.The methyl group is an activator and will direct the NO2+ electrophile in either the -2- or -4- positions.

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SECTION 05

reduction of nitrobenzene

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Reaction

SECTION 06

phenol

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phenol

Molecular formula C6H5 OH Sparingly soluble in water becaue the large aryl group is hydrophobic and minimizes hydrogen bonding with water molecule. Click info for more.

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SECTION 07

diazotization and coupling reactions

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DIAZOTIZATION REACTIONPhenylamine can react with nitrous acid HNO2 which is made in situ by the reaction of sodium nitrite in the presence of hydrochloric acid to form compounds called diazonium salts at temperatures below 100C

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COUPLING

The diazonium salts formed are very reactive as electrophiles. Under alkaline conditions they can react with phenols or other aromatic amines in a coupling reaction where the two compounds become joined. The resulting products are coloured compounds called azo compounds. HCl is also formed in this reaction.

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summary

azo compounds

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