Aromatic Compounds I

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2302272 Org Chem II Part I Lecture 1 Aromatic Compounds I Instructor: Dr. Tanatorn Khotavivattana E-mail: tanatorn.k@chula.ac.

1 Org Chem II Part I Lecture 1 Aromatic Compounds I Instructor: Dr. Tanatorn Khotavivattana tanatorn.k@chula.ac.th Recommended Textbook: Chapter 16 in Organic Chemistry, 8 th Edition, L. G. Wade, Jr., 2010, Prentice Hall (Pearson Education)

2 Course Outline 1 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

3 Organic Chemistry 2

4 Organic Chemistry 3

5 Organic Chemistry 4

6 Discovery of Benzene 5 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 C 6 H 6. He named it benzin C 6 H 6 Other related compounds with low C:H ratios had a pleasant smell, so they were classified as aromatic

7 Kekulé Structure 6 Proposed in 1866 by Friedrich Kekulé, shortly after multiple bonds were suggested Failed to explain existence of only one isomer of 1,2-dichlorobenzene

8 Resonance Structures of Benzene 7 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

9 Structure of Benzene 8 Each sp 2 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.

10 Unusual Reactivity of Benzene 9 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

11 Unusual Reactivity of Benzene 10 When bromine adds to benzene, a catalyst such as FeBr 3 is needed. The reaction that occurs is the substitution of a hydrogen by bromine (cf. addition of bromine across alkenes) Addition of Br 2 to the double bond is not observed.

12 Resonance Energy 11 Predicted heat of hydrogenation of -359 kj/mol; observed value = -208 kj/mol, a difference of 151 kj (resonance energy)

13 Annulenes 12 Hydrocarbons with alternating single and double bonds All annulenes were proposed to be aromatic (?) However!! cyclobutadiene is so reactive that it dimerizes before it can be isolated Cyclooctatetraene adds Br 2 readily to the double bonds

14 Aromaticity 13 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

Aromaticity 14 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

15 Aromaticity 14 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!

16 Aromaticity Molecular orbital Benzene: The polygon rule 15 Antiaromatic

17 Aromaticity Examples 16 Aromatic Compounds Antiaromatic Compounds Antiaromatic (if planar)

18 Deprotonation of Cyclopentadiene 17 Cyclopentadiene is acidic because deprotonation will convert it to an aromatic ion By deprotonating the sp 3 carbon of cyclopentadiene, the electrons in the p orbitals can be delocalized over all five carbon atoms and the compound would be aromatic

19 Cyclopentadienyl Cation 18 Huckel s rule predicts that the cyclopentadienyl cation, with four pi electrons, is antiaromatic; therefore, the cyclopentadienyl cation is not easily formed

20 Tropylium Ion 19 Cyclooctatetraene Dianion

21 20 Which of the following is an aromatic compound? Non-aromatic Aromatic There is an sp 3 carbon in the ring, delocalization will not be complete. All carbons are sp 2 hybridized and it obeys Huckel s rule.

22 Problem #1 21

23 Problem #2 22

24 Polynuclear Aromatic Hydrocarbons (PAHs) 23 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. 24

25 Polynuclear Aromatic Hydrocarbons (PAHs) 24 As the number of fused aromatic rings increases, the resonance energy per ring continues to decrease and the compounds become more reactive.

26 Larger Polynuclear Aromatic Hydrocarbons 25 Formed in combustion (tobacco smoke). Many are carcinogenic.

27 26 Graphite Planar layered structure. Layer of fused benzene rings, bonds: Å. Only van der Waals forces between layers. Conducts electrical current parallel to layers.

28 27 Some New Allotropes Fullerenes: 5- and 6-membered rings arranged to form a soccer ball structure. Nanotubes: half of a C 60 sphere fused to a cylinder of fused aromatic rings.

29 Classification Organic Compounds 28 Acyclic (Open chain) Cyclic (Closed chain) Carbocyclic Heterocyclic Alicyclic Aromatic Non-aromatic Aromatic

Applications of Heterocyclic Compounds 29

photographic chemicals, and so on, and new

30 Applications of Heterocyclic Compounds 29 Heterocyclic compounds can be synthesized in many ways 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

31 Applications of Heterocyclic Compounds 30 Medicinal chemistry especially is associated intimately with heterocyclic compounds; most of all chemicals used in medicine are based on heterocyclic frameworks

32 Heterocyclic Aromatic Compounds 31 Pyridine Pyridine has six delocalized electrons in its pi system. The two non-bonding electrons on nitrogen are in an sp 2 orbital, and they do not interact with the pi electrons of the ring.

33 Pyridine 32 Pyridine is basic, with a pair non-bonding electrons available to abstract a proton. The protonated pyridine (the pyridinium ion) is still aromatic.

34 Pyrrole 33 Pyrrole is a much weaker base than pyridine This difference is due to the structure of the protonated pyrrole

35 Basic or Nonbasic? 34 N N Pyrimidine has two basic nitrogens. N N H Not basic Imidazole has one basic nitrogen and one nonbasic. Not basic N N N N Only one of purine s nitrogens is not basic. H

36 Other Heterocyclics 35

37 Problem #3 36

38 Nomenclature 37 Common Names of Benzene Derivatives The following compounds are usually called by their historical common names, and almost never by the systematic IUPAC names:

39 Nomenclature - Disubstituted Benzenes 38 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.

40 Nomenclature - Three or More Substituents 39 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.

41 Nomenclature Benzene as Substituents 40 When the benzene ring is named as a substituent on another molecule, it is called a phenyl group. (often abbreviated Ph)

42 Nomenclature Benzene as Substituents 41 When the benzene ring is named as a substituent on another molecule, it is called a phenyl group. (often abbreviated Ph)

43 Problem #4 42

44 Physical Properties of Aromatic Compounds 43 Density: More dense than nonaromatics, less dense than water. Solubility: Generally insoluble in water. Melting points: More symmetrical than corresponding alkane, pack better into crystals, so higher melting points. MP = - 95 o C BP = 69 o C MP = 7 o C BP = 81 o C

45 Physical Properties of Aromatic Compounds 44 Boiling points: Intermolecular force H-bonding (functional groups on aromatic) Dipole-dipole (dipole moment) London (molecular weight)

46 Physical Properties of Aromatic Compounds 45

47 Homework 46

48 47

Organic Chemistry, 7 L. G. Wade, Jr. 2010, Prentice Hall

Organic Chemistry, 7 L. G. Wade, Jr. 2010, Prentice Hall Organic Chemistry, 7 th Edition L. G. Wade, Jr. Chapter 16 Aromatic Compounds 2010, Prentice Hall Discovery of Benzene Isolated in 1825 by Michael Faraday who determined C:H ratio to be 1:1. Synthesized