1. Aromatic Chemistry & Amines
Dr. Fawaz Aldabbagh
NUI, Galway
Recommended Text: Organic Chemistry by Paula Y. Bruice
4th or 5th Edition

2. Benzene First isolated by Michael Faraday in 1825
General Formula: C6H6
Aromatic originally referred to the smell, as opposed to aliphatic
Test for Aromatic: High Carbon content – burn with a smoky flame

3. Structure of Benzene
1. Each Carbon atom is sp2-hybridised : Making three σ-bonds with two adjacent carbons and one hydrogen atom: bond angles = 120º
This is the σ-Framework
2. The p-orbital on each carbon atom overlaps above and below the σ-framework with the p-orbital on the adjacent carbon atom

4. 1856: Friedrich Kekulé Resonance Structures
Resonance arrow indicates structures are electronically equivalent
The problem is that C-C single and double bonds would be different lengths and molecule would be a distorted hexagon
1930s: X-ray crystal structures showed benzene is planar and six carbon-carbon bonds have the same length (C-C 1.39 A), which is shorter than C-C single bond at 1.54 A. But longer than a C=C.
Thus, benzene does not have alternating bonds!

5. Kekulé resonance structures
Neither canonical form is correct structure
Resonant Hybrid is correct structure

6. 37 kcalmol-1 of resonance stabilization
Another Problem with the Kekulé Model:
Extra Stability than 1,3,5-Cyclohexatriene Structure Would Suggest!
Experimental ∆Hº = 50 kcalmol-1
Benzene is more stable than expected, and cannot be cyclohexatriene.
The six π-electrons must be delocalised and not localised
37 kcalmol-1 of resonance stabilization

7. Delocalisation of the six π-electrons
A resonance hybrid is more stable than the predicted stability of its resonance contributor, and the more resonance contributors there are, the greater the stability
p-orbitals must be aligned!

8. When Is A Molecule Aromatic?
For a molecule to be aromatic it must:
Be cyclic
Have a p-orbital on every atom in ring
Be planar
Posses 4n+2 p electrons (n = number of rings)
Erich Hückel

9. Huckel’s Rule
Planar Monocyclic Ring;
(4n + 2) = 2, 6, 10, 14 π electrons are aromatic

10. Aromatic Hydrocarbons Aromatic Molecules?
Are the following structures aromatic?
Cyclic? P-Orbital? Planar? 4n+2 p electrons?

11. Which is Aromatic?
Draw Resonance Structures if so!
Homework: Try Problem 4 : page 645 of Bruice (5th Ed)
or Problem 2 : page 597 of Bruice (5th Ed)

12. Aromatic Hydrocarbons: Nomenclature
Substituent plus benzene
However, there are exceptions

13. Two identical substituents: di- (3: tri- etc.)
Nomenclature
Two identical substituents: di- (3: tri- etc.)
Numbering of C atoms in ring
Kekule structures are the most important for writing mechanisms

14. Electrophilic Aromatic Substitution
Electrophilic attack – Slow Rate Determining Step
Transition State or Wheland Intermediate
Delocalised Cyclohexadienyl cation

15. Fast Step is the loss of a proton ---rapid re-aromatization
E.g. Nitration of benzene
+ H+
Sir Christopher Ingold's ideas (1930s), terminology and nomenclature for reaction mechanisms (e.g. electrophilic, nucleophilic, inductive, mesomeric, SN1, SN2 etc) were generally accepted and employed everywhere.

16. Derive the Lewis Structure of NO2+, which is the electrophile (E+) for nitration
Generating NO2+
Sulfuric acid is a stronger acid than nitric acid

17. The Nitration of Benzene

18. Label the parts of the Electrophilic Aromatic Substitution Profile
Formation of the intermediate is the RDS

19. All reactions will be described in your lectures
Some Reactions of Benzene
You should be able to write mechanisms for all the transformations below.
All reactions will be described in your lectures
Sulfonation is the only reversible electrophilic aromatic substitution

20. Friedel-Crafts
Charles Friedel
James Mason Crafts
Homework: Give a mechanism and rationalise the substitution pattern

21. Mechanism for Acylation
Mechanism for Alkylation
Water is required here

22. Carbonyl Group can complex with Lewis acid (e. g
Carbonyl Group can complex with Lewis acid (e.g. AlCl3), thus more than one equivalent of AlCl3 is required for the reaction & water is added at the end of the reaction to liberate the product from the complex

23. Ortho- and para- directing
Reactions of Substituted Benzenes
toluene
R (alkyl) groups are weakly activating at ortho- & para-
Toluene reacts 25 times faster than benzene in nitration
phenol
aniline
Note – no catalyst
-NH2, -OH, -OMe, MeCONH- are all powerful activating groups
Ortho- and para- directing
+ R, - I

24. +R activate through π-bonds
Alkyl groups (e.g. CH3) inductively donate electrons (+I)
(through σ-bonds)
+ I
+ R
+R activate through π-bonds
Activates o & p towards E+ attack
+ R, - I
The strongest activator wins out
NHCOCH3 directs para, but makes benzene less reactive compared to aniline

25. Strong –R Deactivators: NO2, CHO, COOH, CO2R, CN
Deactivates o & p towards E+ attack; Thus, strong meta-directors
Undergoes nitration 10,000 times slower than benzene
- R, -I

26. Two Reasons For Substituent Directing Effects
Activation by electron with-drawl or donation by Substituent
Stability of the Wheland Intermediate Carbocation
Favoured
Not Favoured
Favoured

27. directors reinforce each other
Halogens are weak deactivators – directing ortho- and para-
- I, +R
The halo groups are the only o- and p-directors that are deactivating groups
o-bromochlorobenzene
p-bromochlorobenzene

28. Tutorial Questions
Write a mechanism for the generation of the nitronium ion (NO2+) from concentrated nitric and sulfuric acid.
Write a mechanism for the nitration of benzene.
Draw dinitrobenzenes A-C.
Predict the ratio of isomeric products A-C from the nitration of nitrobenzene, and comment on the rate of nitration of benzene compared to nitrobenzene. Rationalize your answers with resonance structures.
Draw the structure of an aromatic that undergoes electrophilic aromatic substitution faster than benzene. Briefly give reasons for your answer.

29. Tutorial Questions What are the four rules of aromaticity?
Draw one aromatic molecule, which does not contain a benzene ring.
Draw the following molecules;
ortho-dibromobenzene, para-nitroaniline, phenol, meta-nitroanisole and 2,4,6-trinitrotoluene (TNT)
Predict the major product for the following reaction, and name the product.
5. Provide a brief synthesis for TNT starting from toluene
6. Provide a mechanism for the following reaction;

30. Synthesis of Substituted Benzenes Using
Arenediazonium Salts
Benzenediazonium
chloride
0 ºC
Nitrous acid made on ice

32. Arenediazonium Ion as an Electrophile
Aryl diazonium salts undergo coupling reactions with activated aromatic rings, such as phenols and anilines to yield brightly coloured azo compounds (Ar-N=N-Ar)
Diazonium coupling is a typical electrophilic aromatic substitution, where the phenol or aniline is the electron-rich aromatic and the diazonium salt is the electrophile.
Electrophilic aromatic substitution occurs at the para-position to the electron-rich aromatic
William Perkin

33. Examples of Azo-Dyes
Yellow of margarine
Orange solid
The mechanism for formation of an azo-dye will be described in your lectures

34. Tutorial Questions
1. Draw the chemical structure of A, and give a full reaction mechanism for the formation of yellow azo-compound B (Scheme 1).
Scheme 1
Scheme 2
2. Draw structures A, C and E, and give reagents & brief conditions X and Y.
3. Give a mechanism for the formation of C (Scheme 2).

35. Amines Amines are derivatives of Ammonia
Ammonia & amines are bases by virtue of their lone pair of electrons
R = alkyl groups
Ammonia
NH3
Methylamine
NH2Me
1º amine
Dimethylamine
NHMe2
2º amine
Triethylamine
NMe3
3º amine

36. Amines are bases because of the lone pair on the nitrogen atom - red litmus paper to blue

37. Aniline is a weaker base than aliphatic amines due to the resonance delocalisation of the lone pair of electrons in the free amine
This delocalisation of the lone pair of electrons makes it less available to H+
The delocalisation is lost upon formation of the salt

38. Reactions of Amines with Nitrous Acid - Revisited
1º aliphatic amines form very unstable diazonium salts
1º aromatic amines form stable diazonium salts at 0 ºC of great synthetic utility
Secondary Amines with nitrous acid
N-Nitrosoamines
N-Nitrosodimethylamine (yellow oil)
N-Nitoso-N-methylaniline (yellow oil)
Tertiary Amines with nitrous acid
Nitrous acid reacts to give salts with 3º aliphatic amines (not useful),
with 3º aromatic amines NO+ becomes an electrophile in electrophilic aromatic substitution
p-Nitoso-N,N-dimethylaniline
(exclusive product)

39. Tutorial Questions
Draw the structure of a primary, a secondary and a tertiary aliphatic amine, and name all three amines.
2. Give a mechanism for the reaction of triethylamine with HCl.
Explain the origin of amine basicity and how does the basicity of aniline compare with that of a primary aliphatic amine. Draw resonance structures to explain your answer.
4. Describe the reactions of nitrous acid (HNO2) with primary, secondary and tertiary aromatic amines.