Delocalization and Conjugation in molecules with > one C = C double bonds

School of Pharmacy

Bandung Institutes of Technology

FA2202

2

.This lecture is about

the properties of molecules

that have several double bonds and that depend on the joining up or conjugation of the

electrons in these double bonds

And see how a large pi framework spread over many atoms and

how this dominates the chemistry of such compounds.  

how this pi framework is responsible for the otherwise unexpected stability of certain cyclic polyunsaturated compounds, including benzene and other aromatic compounds.

how this framework gives rise to the many colours in our world.

 

3

As you look around you, you will be aware of many different colours— from the greens and browns outside to the bright blues and reds of the clothes you are wearing

All these colours result from

the interaction of light with the pigments in these different things—

some frequencies of light are absorbed, others scattered.

4

.

A large pi framework gives rise to the many colours in the world

5

.

Chlorophyll: highly conjugated compound the ring shown in green is fully conjugated

6

Unconjugated bonds

7

Conjugated bonds

8

sp3 hybridized orbitals of C

(a) Electron configuration of carbon in its most stable state.

(b) An electron is “promoted” from the 2s orbital to the vacant 2p orbital.

(c) The 2s orbital and the three 2p orbitals are combined to give a set of four equal-energy sp3- hybridized orbitals, each of which contains one electron.

9

Four sp3 hybridized orbitals of C

10

sp2 hybridized orbitals of C

(a) Electron configuration of carbon in its most stable state. (b) An electron is “promoted” from the 2s orbital to the vacant 2p orbital. (c) The 2s orbital and two of the three 2p orbitals are combined to give a set of three

equal-energy sp2-hybridized orbitals. One of the 2p orbitals remains unchanged.

11

ethylene the simplest unsaturated

compound 

 

 

• The structure of ethene (ethylene) is planar (all atoms are in the same plane) • The carbon atoms are roughly trigonal • the C= C bond distance is shorter than that of a C–C sigma bond.

12

combination of two sp2 orbitals of C atoms in formation of acethylene

13

No rotation is possible about a double bond

• The π bond has electron density both above and below the σ bond as the parallel p orbitals overlap locking the bond rigid.

• - π bond must be broken before rotation can occur.

14

Trans-hexatriene

15

Cis- and trans- characteristic

.

16

sp hybridized orbitals of C

(a) Electron configuration of carbon in its most stable state. (b) An electron is “promoted” from the 2s orbital to the vacant 2p orbital. (c) The 2s orbital and one of the three 2p orbitals are combined to give a set of two equal-energy sp-hybridized orbitals. Two of the 2p orbitals remain unchanged.

17

π & π* ORBITAL

18

CIS – TRANS CONVERSION

19

σ & π bond

20

Conjugated double bond system

21

Conjugated double bond

‘conjugated’ : ‘joined together, especially in pairs’

‘acting or operating as if joined’.

the properties of a conjugated system are often different from those of the component parts.

We are using the word

conjugation to describe bonds and

delocalization to describe electrons.

22

Addition and substitution reactions

23

Distinct characteristic of aromatic double bonds

Benzene does react with Br2 in the presence of iron(III) bromide as a catalyst, but even then addition isn’t observed.

Benzene and its derivatives reflect a special stability

Substitution occurs instead!

24

25

Different length of single and double bonds

26

Delocalized double bonds od benzene

27

Drawing reaction mechanism for benzene structure

it does not matter which way round you draw the benzene ring – both drawings give the same result but one uses more arrows to get there

28

Rotation about single bond

29

Overlapping all p orbitals in a planar molecule

30

Energy for rotation

31

Conjugation

32

Nonplanarity of Unconjugated double bonds

33

Nonplanar molecule of allene

34

35

Bonding & nonbonding MO

36

Allyl cation

37

Allyl anion

38

Delocalization of pi bond in Allyl anion

39

Resonance in carboxylate ion

40

Resonance at Nitro group

41

Resonance at amide system

42

Special amide group:Peptide unit

43

44

Butadiene systems

45

46

47

48

Approximate wavelengths for different colours

49

Structure of colored molecule

50

.

51

52

Nonaromatic structure of double bond system

53

54

.

55

Margarine manufacture

56

57

.

58

Drug having conjugated bonds

59