aromatic compounds ichemistry3.chemistry.sc.chula.ac.th/images/download/bsac/coursemat/...

2302272 – Org Chem II – Part I

Lecture 1

Aromatic Compounds I

Instructor: Dr. Tanatorn Khotavivattana

E-mail: tanatorn.k@chula.ac.th

Recommended Textbook:

Chapter 16 in Organic Chemistry, 8th Edition, L. G. Wade, Jr., 2010,

Prentice Hall (Pearson Education)

Course Outline

Quiz : from 9:00(or earlier) – 9:30

Open book, discussion allowed

10 marks out of 100 marks total

Homework : Hand in at the beginning of next class

A4 paper

5 marks out of 100 marks total

Final Exam : 35 marks out of 100 marks total

Short note allowed

1

Organic Chemistry2

Organic Chemistry3

Organic Chemistry4

Discovery of Benzene

• Isolated in 1825 by Michael Faraday

who determined C:H ratio to be 1:1

• Synthesized in 1834 by Eilhard

Mitscherlich who determined molecular

formula to be C6H6. He named it

benzin

• Other related compounds with low C:H

ratios had a pleasant smell, so they

were classified as aromatic

C6H6

5

Kekulé Structure

• Proposed in 1866 by Friedrich Kekulé,

shortly after multiple bonds were

suggested

• Failed to explain existence of only one

isomer of 1,2-dichlorobenzene

6

Resonance Structures of Benzene

• Benzene is actually a resonance hybrid between the two

Kekulé structures

• The C—C bond lengths in benzene are shorter than typical

single-bond lengths, yet longer than typical double-bond

lengths (bond order 1.5)

• Benzene's resonance can be represented by drawing a circle

inside the six-membered ring as a combined representation

7

Structure of Benzene

• Each sp2 hybridized C in the ring has an unhybridized p

orbital perpendicular to the ring which overlaps around the

ring.

• The six pi electrons are delocalized over the six carbons.

8

• Benzene is actually much more stable than we would

expect; For example, an alkene decolorizes potassium

permanganate by reacting to form a glycol. When

permanganate is added to benzene, however, no reaction

occurs

Unusual Reactivity of Benzene9

Unusual Reactivity of Benzene

• When bromine adds to benzene, a catalyst such as FeBr3 is

needed.

• The reaction that occurs is the substitution of a hydrogen by

bromine (cf. addition of bromine across alkenes)

• Addition of Br2 to the double bond is not observed.

10

Resonance Energy

• Predicted heat of hydrogenation of -359 kJ/mol; observed

value = -208 kJ/mol, a difference of 151 kJ (resonance

energy)

11

Annulenes

• Hydrocarbons with alternating single and double bonds

• All annulenes were proposed to bearomatic (?)

• However!! cyclobutadiene is soreactive that it dimerizes before itcan be isolated

• Cyclooctatetraene adds Br2 readilyto the double bonds

12

Aromaticity

• Aromatic structures are more stable

than their open-chain counterparts

• Antiaromatic structures are less stable than

their open-chain counterparts (delocalization

of pi electrons increases the electronic

energy!)

• A cyclic compound that does not have a continuous, overlapping ring of p orbitals

cannot be aromatic or antiaromatic. It is said to be nonaromatic, or aliphatic. Its

electronic energy is similar to that of its open-chain counterpart

13

14

Aromaticity

Criteria:

• The molecule must be cyclic

• This cycle must be fully conjugated

• The cycle must be planar

• The electrons must be able to “circulate”

Hückel’s Rule: If the number of pi electrons in the cyclic system is:

• (4N + 2) = the system is aromatic

• (4N) = the system is antiaromatic

Cyclooctatetraene would be antiaromatic if

Hückel’s rule applied (4N; N = 2). Cyclooctatetraene

adopts a nonplanar “tub” conformation that avoids

most of the overlap between adjacent pi bonds;

becomes nonaromatic instead!

15

Aromaticity – Molecular orbital

• Benzene:

The polygon ruleA

ntiaro

matic

16

Aromaticity – Examples

Aromatic Compounds Antiaromatic Compounds

Antiaromatic (if planar)

17

Deprotonation of Cyclopentadiene

• Cyclopentadiene is acidic because deprotonation will convert it to an aromatic

ion

• By deprotonating the sp3 carbon of cyclopentadiene, the electrons in the p orbitals

can be delocalized over all five carbon atoms and the compound would be

aromatic

18

Cyclopentadienyl Cation

• Huckel’s rule predicts that the cyclopentadienyl cation, with four pi electrons, is

antiaromatic; therefore, the cyclopentadienyl cation is not easily formed

19

Tropylium Ion

Cyclooctatetraene Dianion

Which of the following is an aromatic compound?

Non-aromatic Aromatic

There is an sp3 carbon in

the ring, delocalization will

not be complete.

All carbons are sp2

hybridized and it obeys

Huckel’s rule.

20

Problem #121

Problem #222

24

Polynuclear Aromatic Hydrocarbons (PAHs)

• Composed of two or more fused benzene rings. Fused rings share two carbon

atoms and the bond between them.

• Naphthalene is the simplest fused aromatic hydrocarbon.

23

Polynuclear Aromatic Hydrocarbons (PAHs)

• As the number of fused aromatic rings increases, the resonance energy per

ring continues to decrease and the compounds become more reactive.

24

Larger Polynuclear Aromatic Hydrocarbons

• Formed in combustion (tobacco smoke).

• Many are carcinogenic.

25

Graphite

• Planar layered structure.

• Layer of fused benzene rings, bonds: 1.415 Å.

• Only van der Waals forces between layers.

• Conducts electrical current parallel to layers.

26

Some New Allotropes

• Fullerenes: 5- and 6-membered rings arranged to form a “soccer ball” structure.

• Nanotubes: half of a C60 sphere fused to a cylinder of fused aromatic rings.

27

Classification28

Organic

Compounds

Acyclic

(Open chain)

Cyclic

(Closed chain)

Carbocyclic Heterocyclic

Alicyclic Aromatic Non-aromatic Aromatic

29

Applications of Heterocyclic Compounds

• Many synthetic (as well as natural) heterocyclic compounds are of extreme value

as medicinals, agrochemicals, plastics precursors, dyes, photographic

chemicals, and so on, and new structures are constantly being sought in

research in these areas

• Heterocyclic compounds can be synthesized in many ways

30

Applications of Heterocyclic Compounds

Medicinal chemistry especially is associated intimately with heterocyclic

compounds; most of all chemicals used in medicine are based on heterocyclic

frameworks

31

Pyridine

• Pyridine has six delocalized electrons in its pi system.

• The two non-bonding electrons on nitrogen are in an sp2 orbital, andthey do not interact with the pi electrons of the ring.

Heterocyclic Aromatic Compounds

32

Pyridine

• Pyridine is basic, with a pair non-bonding electrons available to abstract a proton.

• The protonated pyridine (the pyridinium ion) is still aromatic.

33

Pyrrole

• Pyrrole is a much weaker base than pyridine

• This difference is due to the structure of the protonated pyrrole

34

Basic or Nonbasic?

Pyrimidine has two basic

nitrogens.

Imidazole has one basic

nitrogen and one nonbasic.

Only one of purine’s nitrogens

is not basic.N

N

N

N

H

N N H

NN

Not basic

Not basic

Other Heterocyclics35

Problem #336

Common Names of Benzene Derivatives

Nomenclature

The following compounds are usually called by their historical common names,

and almost never by the systematic IUPAC names:

37

Nomenclature - Disubstituted Benzenes

• named using the prefixes ortho-, meta-, and para- to specify the substitution

patterns (abbreviated o-, m-, and p-).

• Numbers can also be used for the IUPAC name.

38

Nomenclature - Three or More Substituents

• Numbers are used to indicate their positions.

• Assign the numbers to give the lowest possible numbers to the substituents.

• The carbon atom bearing the functional group that defines the base name

(as in phenol or benzoic acid) is assumed to be C1.

39

Nomenclature – Benzene as Substituents

• When the benzene ring is named as a substituent on another molecule, it is

called a phenyl group. (often abbreviated Ph)

40

Nomenclature – Benzene as Substituents

• When the benzene ring is named as a substituent on another molecule, it is

called a phenyl group. (often abbreviated Ph)

41

Problem #442

Physical Properties of Aromatic Compounds

• Melting points: More symmetrical than corresponding alkane,pack better into crystals, so higher melting points.

MP = - 95 oC

BP = 69 oC

MP = 7 oC

BP = 81 oC

• Density: More dense than nonaromatics, less dense than water.

• Solubility: Generally insoluble in water.

43

Physical Properties of Aromatic Compounds

• Boiling points: Intermolecular force• H-bonding (functional groups on aromatic)

• Dipole-dipole (dipole moment)

• London (molecular weight)

44

Physical Properties of Aromatic Compounds45

Homework46

47