Aromatic hydrocarbons contain benzene rings and have delocalized pi electrons. Benzene's stability is due to the delocalization of its pi electrons over the ring, giving it resonance energy. This document discusses the structures and properties of aromatic hydrocarbons like benzene, including their stability, aromatic character, reactions, and preparation methods.

1. Organic Chemistry
Aromatic Hydrocarbons-1
(Arenes)

2. AROMATIC HYDROCARBONS OR ARENES
Aromatic hydrocarbons or arenes are the compounds of carbon and
hydrogen which contain at least one benzene type ring their molecules.
Polynuclear arenes are carcinogenic in nature and hence are toxic.

3. KEKULE STRUCTURE OF BENZENE
STRUCTURE OF BENZENE

4. Dewar (1867) Claus (1867) Baeyer (1892)
STRUCTURE OF BENZENE
Some alternative structures which have been proposed by other scientists
are given below :

5. STABILITY AND STRUCTURE OF BENZENE

6. STABILITY AND STRUCTURE OF BENZENE

7. Enthalpy of hydrogenation is the amount of enthalpy change when
one mole of an unsaturated hydrocarbon is hydrogenated.
Stability of benzene and enthalpy of hydrogenation

9. Resonance Energy
The actual molecule of benzene is more stable than hypothetical
cyclohexatriene molecule by a amount equal to :
358.5 - 208.1 = 150.4 kJ mol–1
This is called resonance energy. Thus, resonance energy for benzene is
150.4 kJ mol –1. This can also be confirmed by enthalpy of combustion
data.

10. Benzene

11. Benzene

12. Benzene
Spreading of  electrons in the form of ring of -electrons above and below
the plane of carbon atoms is called delocalisation of -electrons. This
delocalisation of -electrons, results, in the decrease in energy and hence,
accounts for the stability of benzene molecule.

13. AROMATICITY OR AROMATIC CHARACTER
Criteria for Aromaticity
1. Delocalisation.
2. Planarity.
3. (4n + 2)  electrons. For aromaticity, the molecule must be planar,
cyclic system having delocalized (4n+2) electrons.
This is known as Huckel rule.

14. AROMATICITY OR AROMATIC CHARACTER
Criteria for Aromaticity
1. Delocalisation.
2. Planarity.
3. (4n + 2)  electrons. For aromaticity, the molecule must be planar,
cyclic system having delocalized (4n+2) electrons.
This is known as Huckel rule.

15. AROMATICITY OR AROMATIC CHARACTER

16. AROMATICITY OR AROMATIC CHARACTER
All the above species have a system of 6n electrons and are aromatic.

17. Can you Crack it !!
1. Predict which of the following systems would be aromatic and why?

18. Can you Crack it !!
1. Predict which of the following systems would be aromatic and why?

19. Can you Crack it !!
2. Predict which of the following systems would be aromatic ?

20. Can you Crack it !!
2. Predict which of the following systems would be aromatic ?

21. Can you Crack it !!
Although benzene is highly unsaturated it does not undergo addition
reactions. The explanation of this can be suggested as
(a) -electrons of benzene ring are delocalized
(b) since -electrons are present inside the ring, addition cannot take
place
(c) cyclic structures do not show addition reactions
(d) benzene is not a reactive compound.

22. Can you Crack it !!
Although benzene is highly unsaturated it does not undergo addition
reactions. The explanation of this can be suggested as
(a) -electrons of benzene ring are delocalized
(b) since -electrons are present inside the ring, addition cannot take
place
(c) cyclic structures do not show addition reactions
(d) benzene is not a reactive compound.

23. (a) (b)
(c) (d)
Can you Crack it !!
Which of the following species is aromatic.

24. (a) (b)
(c) (d)
Can you Crack it !!
Which of the following species is aromatic.

25. ISOMERISM IN ARENES

26. (b) 2
(d) 4
Can you Crack it !!
Number of acidic hydrogen atoms in but-1-yne is
(a) 1
(c) 3

27. (a) 1
(c) 3
(b) 2
(d) 4
Can you Crack it !!
Number of acidic hydrogen atoms in but-1-yne is

28. (b) n-Pentane
(d) 2,2-dimethyl propene
(a) n-Hexane
(c) 2-Methylbutane
Can you Crack it !!
Which of the following has least boiling point ?

29. (a) n-Hexane
(c) 2-Methylbutane
(b) n-Pentane
(d) 2,2-dimethyl propene
Can you Crack it !!
Which of the following has least boiling point ?

30. In the reaction
NaOH NaOH, CaO
Heat
Can you Crack it !!
(b) CH4
(d) CH3COCH3
CH3COOH >X >Y; Y is :
(a) CH3CHO
(c) C2H6

31. In the reaction
NaOH NaOH, CaO
CH3COOH >X >Y; Y is :
Heat
(a) CH3CHO
(c) C2H6
(b) CH4
(d) CH3COCH3
Can you Crack it !!

32. PREPARATION OF BENZENE AND ITS HOMOLOGUES
1. From alkynes.
2. Decarboxylation of aromatic acids.
3. Reduction of Phenol.
4. From diazonium salts.

33. PREPARATION OF BENZENE AND ITS HOMOLOGUES
1. From alkynes.

34. PREPARATION OF BENZENE AND ITS HOMOLOGUES
1. From alkynes.

35. PREPARATION OF BENZENE AND ITS HOMOLOGUES
2. Decarboxylation of aromatic acids.

36. PREPARATION OF BENZENE AND ITS HOMOLOGUES
3. Reduction of Phenol.