1

III Aromatic Compounds

Aromatic Compounds: Compounds that resemble benzene in structure and

chemical behavior (terms comes from fragrant odors)

Benzene: - Cyclic Compound

- Six-Membered Ring

- ONLY Six Hydrogens

- Six Carbons

- Three Double Bonds

Representations:

Natural occurring aromatic systems:

2

3.1 Structure and Bonding of Benzene

Unexpected Stability:

- Very stable

- Very resistant to chemical changes

Example Halogination:

Example Hydrogenation:

- Shorter than expected carbon-carbon bond length

3

Bonding:

Benzene is based on 6 Carbons and 6 Hydrogens

In 1865 Kekulé suggested 2 resonance structures for Benzene:

We are thinking of Benzene as a Resonance Hybrid:

Also suggested structures for Benzene:

Dewar Benzene Ladenburg Benzene

Remember Rules for drawing resonance structures:

1) ONLY electrons move

2) The ONLY electrons that can move are electrons and non-bonding electrons

3) The total number of electrons in the molecules does not change

Electrons can move in the following way:

1) Move π electrons toward a positive charge or a π bond

2) Move a non-bonding electron pair towards a π bond

3) Move a single non-bonding electron toward a π bond

4

Properties:

- Clear liquid

- Planar

- Bond angle 120º

- All carbons are sp2 hybridized

- Delocalized electrons

What is the reason for the stability and chemical inertness of Benzene?

Aromaticity:

Criteria for aromaticity:

1) The molecule MUST by CYCLIC and PLANAR

2) The ring MUST contain ONLY sp2 hybridized carbons that can form a

delocalized system

3) The number of π electrons MUST be 4n+2 (Hückel Rule)

⇒ Ring systems with 6, 10, 14 etc number of electrons are aromatic

Examples:

5

Examples of systems that are NOT aromatic:

3.2 Polycyclic and Heterocyclic Aromatic Compounds

Polycyclic Aromatic Compounds: Compounds based on more than one ring

systems that are fused

Examples:

Heterocyclic Aromatic Compounds: Compounds that contain a heteroatom

Examples:

6

Q1: Why are pyridine and furan aromatic?

Examples of heterocyclic aromatic systems:

7

3.3 Nomenclature

Problem: Aromatic compounds have OFTEN trivial names and do not follow

IUPAC nomenclature

3.3.1 Monosubstituted Aromatics

Toluene:

Phenol:

Aniline:

Benzoic Acid:

Benzaldehyde:

Chlorobenzene:

Nitrobenzene:

Styrene:

Anisole:

8

3.3.2 Disubstituted Benzenes

The ortho, meta, and para nomenclature:

ortho-Xylene:

meta-Xylene:

para-Xylene:

para-Chlorotoluene

meta-Nitrobenzoic acid

9

3.3.3 Polysubstituted Benzenes

2,4,6-Trinitrotoluene (TNT):

1,2,4-Trichlorobenzene:

3-ethyl-2-methylanisole:

2,4,6-Trinitrophenol:

1-Bromo-2-chloro-4-iodobenzene:

3.3.4 Aromatic Compounds Designated by Prefixes

Phenyl:

Benzyl:

Example 2-Methyl-5-phenyl-2-pentene:

10

3.4 Electrophilic Aromatic Substitutions

Electrophilic aromatic substitutions are the characteristic reactions of aromatic

compounds

Aromatic compounds are electron rich (π-cloud)

⇒ Electron deficient species (electrophiles) are attracted to aromatic compounds

In an electrophilic aromatic substitution reaction, and electrophile is put

on the ring and an H+ comes off the ring

Examples:

11

3.4.1 Mechanism

General Mechanism:

Important: Resonance structures

Energy Diagram of electrophilic aromatic substitutions versus additions:

12

3.4.2 Halogenations

Brominations, Chlorinations, and Iodinations

Generation of the electrophile:

Substitutions:

3.4.3 Nitrations

Mechanism:

13

3.4.4 Sulfonations

Mechanism:

3.4.5 Friedel Crafts Acylations

an Acyl Group an Alkyl Group

Example of a Friedel Crafts Acylation

14

Mechanism:

3.4.6 Friedel Crafts Alkylations

Example:

Mechanism:

15

3.5 Structural Effects on Substitutions

Rate: Substituents can influence the rate of reaction

Example Nitration:

Activating Group: Group that increases the reactivity of an aromatic

compound to electrophilic substitution

Deactivating Group: Group that decreases the reactivity of an aromatic

compound to electrophilic substitution

Orientation: Substituents can influence the orientation of a reaction

Example:

Compounds that are ortho-para directors:

Compounds that are meta directors

16

- All Activating Substituents are Ortho/Para Directors

- All Weakly Deactivating Substituents are Ortho/Para Directors

- All Deactivating Substituents (except the Halogens) are Meta Directors

⇒ in all these cases the reason is stability of the Carbocation Intermediate

→ The more stable the Carbocation, the better

17

3.5.1 Interpretation

Substituents can be categorized into Electron Donating Substituents (Groups)

and Electron Withdrawing Substituents (Groups)

Electron Donating Substituents (Groups) increase the reactivity of the benzene

ring toward electrophilic aromatic substitution

Electron Withdrawing Substituents (Groups) decrease the reactivity of the

benzene toward electrophilic aromatic substitution

There are two ways how a substituent can donate (or withdrawal) electrons into

(from) an aromatic ring system

(A) Resonance

(B) Inductive Effect

3.5.1.1 Resonance Electron Donation and Withdrawal (M-Effect)

Example of Donation:

18

Example of Withdrawal:

3.5.1.2 Inductive Electron Donation and Withdrawal (I-Effect)

If a substituent bonded to a benzene ring is LESS electron withdrawing than a

hydrogen, the electrons in the σ bond that attaches the substituent to the ring will

move towards the ring ⇒ They INDUCTIVELY donate electrons to the ring

Inductive Electron Withdrawal other way around

Examples:

I and M effects can work against each other

Example Halogens:

Q2. Which one wins?

19

Putting everything together:

- The more resonance structures the intermediate has, the more stable it is

- The orientation is based on the stability of the Carbocation

→ Inductive donation = ortho/para directors

→ Resonance donation = ortho/para directors (one more resonance form)

→ Positive or partial positive charge on substituent = meta directors

All substituents that donate electrons into the ring either inductively or

by resonance are ortho/para directors

All substituents that cannot donate electrons into the ring either

inductively or by resonance are meta directors

20

3.6 Other Reactions on Aromatic Compounds

Benzyl carbocation:

Oxidation:

Diazotization:

Sandmeyer Reaction:

21

3.7 Designing a Synthesis: Mono and Disubstituted Benzenes

Examples:

22

Summary of Chapter 3:

Aromatic Compounds and Benzene

Aromaticity

4n+2

Planar

only sp2 and sp

Poly and Hetero Aromatic Compounds

Electrophilic Aromatic Substitutions

Mechanisms

Examples

Inductive Effect

Resonance

Activating Groups

Deactivating Groups

EWG

EDG

meta Directors

ortho/para directors

Reactions on Side-Chains

Designing of a Synthesis