LM03

1. LM03 CH 9, 10, 11

3. Chapter 10 and 11 - Spectroscopy Modern techniques for structure determination of organic compounds include: • Mass spectrometry • Size and formula of the compound • Infrared spectroscopy • Functional groups present in the compound • Ultraviolet spectroscopy • Conjugated p electron system present in the compound • Nuclear magnetic resonance spectroscopy • Carbon-hydrogen framework of the compound

4. Early Days of Organic Chemistry Aromatic Compounds • Formerly used to describe fragrant substances such as benzaldehyde (from cherries, peaches, and almonds), toluene (from Tolu balsam), and benzene (from coal distillate) • Now used to refer to the class of compounds that contain six-membered benzene-like rings with three double bonds

5. Present Days of Organic Chemistry Aromatic Compounds • Many naturally occurring compounds are aromatic in part • Steroidal hormone estrone • Analgesic morphine • Many synthetic drugs are aromatic in part • Antidepressant fluoxetine (Prozac) • Benzene • Found to cause bone marrow depression • Leads to leukopenia, or lowered white blood cell count, on prolonged exposure

6. 9.1 Naming Aromatic Compounds Aromatic substances have acquired nonsystematic names • Nonsystematic names are discouraged but allowed by IUPAC • Common name for methylbenzene is toluene • Common name for hydroxybenzeneis phenol • Common name for aminobenzene is aniline

7. Naming Aromatic Compounds

8. Naming Aromatic Compounds Monosubstituted Benzenes • Systematically named in same manner as other hydrocarbons • – benzene used as parent name • C6H5Br is bromobenzene • C6H5NO2 is nitrobenzene • C6H5CH2CH2CH3 is propylbenzene

9. Naming Aromatic Compounds Arenes • Alkyl-substituted benzenes • Named depending on the size of the alkyl group • Alkyl substituent smaller than the ring (6 or fewer carbons), named as an alkyl substituted benzene • Alkyl substituent larger than the ring (7 or more carbons), named as a phenyl-substituted alkane Phenyl • Derived from the Greek pheno (“I bear light”) • Michael Faraday discovered benzene in 1825 from the oily residue left by illuminating gas used in London street lamps • Used for the –C6H5 unit when the benzene ring is considered as a substituent • Abbreviated as Ph or F (Greek phi)

10. Naming Aromatic Compounds Benzyl • Used for the C6H5CH2– group

11. Naming Aromatic Compounds Disubstituted benzenes • Named using one of the prefixes • ortho- (o-) • Ortho-disubstituted benzene has two substituents in a 1,2 relationship • meta- (m-) • Meta-disubstituted benzene has its substituents in a 1,3 relationship • para- (p-) • Para-disubstituted benzene has its substituents in a 1,4 relationship

12. Naming Aromatic Compounds Benzenes with more than two substituents • Named by numbering the position of each so that the lowest possible numbers are used • The substituents are listed alphabetically when writing the name Any of the monosubstituted aromatic compounds in Table 8.1 can serve as a parent name, with the principal substituent (-OH in phenol or –CH3 in toluene) attached to C1 on the ring

13. 9.2 Structure and Stability of Benzene Benzene • Benzene is unsaturated • Benzene is much less reactive than typical alkene and fails to undergo the usual alkene reactions • Cyclohexene reacts rapidly with Br2 and gives the addition product 1,2-dibromocyclohexane • Benzene reacts only slowly with Br2 and gives the substitution product C6H5Br

14. Structure and Stability of Benzene A quantitative idea of benzene’s stability is obtained from heats of hydrogenation • Benzene is 150 kJ/mol (36 kcal/mol) more stable than might be expected for “cyclohexatriene”

15. Structure and Stability of Benzene Carbon-carbon bond lengths and angles in benzene • All carbon-carbon bonds are 139 pm in length • Intermediate between typical C-C single bond (154 pm) and typical double bond (134 pm) • Electrostatic potential map shows that the electron density in all six carbon-carbon bonds is identical • Benzene is planar • All C-C-C bond angles are 120° • All six carbon atoms are sp2-hybridized with p orbital perpendicular to the plane of the ring

16. Structure and Stability of Benzene All six carbon atoms and all six p orbitals in benzene are equivalent • Each p orbital overlaps equally well with both neighboring p orbitals, leading to a picture of benzene in which the six p electrons are completely delocalized around the ring • Benzene is a hybrid of two equivalent resonance forms • Neither form is correct by itself • The true structure of benzene is somewhere in between the two resonance forms

17. Structure and Stability of Benzene Six p atomic orbitals combine in a cyclic manner, six benzene p molecular orbitals result The six p electrons occupy the three bonding molecular orbitals and are delocalized over the entire conjugated system

18. 9.3 Aromaticity and the Hückel 4n + 2 Rule Benzene and other benzene-like aromatic molecules share similar characteristics: • Benzene is cyclic and conjugated • Benzene is unusually stable, it is 150 kJ/mol (36 kcal/mol) more stable than might be expected • Benzene is planar and has the shape of a regular hexagon. All bond angles are 120º, all carbon atoms are sp2-hybridized, and all carbon-carbon bond lengths are 139 pm • Benzene undergoes substitution reactions that retain the cyclic conjugation rather than electrophilic addition reactions that would destroy the conjugation

19. Aromaticity and the Hückel 4n + 2 Rule The Hückel 4n + 2 rule • Theory devised in 1931 by the German physicist Erich Hückel • A molecule is aromatic only if it has a planar, monocyclic system of conjugation and contains a total of 4n + 2 p electrons, where n is an integer (n = 0, 1, 2, 3,…) • Only molecules with 2, 6, 10, 14, 18,… p electrons can be aromatic • Molecules with 4n p electrons (4, 8, 12, 16,…) can not be aromatic, said to be antiaromatic because delocalization of their p electrons would lead to their destabilization

20. Aromaticity and the Hückel 4n + 2 Rule Examples of the Hückel 4n + 2 rule • Cyclobutadiene • Contains four p electrons localized into two double bonds rather than delocalized around the ring • Antiaromatic • Highly reactive • Shows none of the properties associated with aromaticity • Not prepared until 1965

21. Aromaticity and the Hückel 4n + 2 Rule • Benzene • Contains six p electrons (4n + 2 = 6 when n = 1) • Aromatic

22. Aromaticity and the Hückel 4n + 2 Rule • Cyclooctatetraene • Contains eight p electrons • The p electrons are localized onto four double bonds rather than delocalized around the ring • Not aromatic • The molecule is tub-shaped rather than planar • It has no cyclic conjugation because neighboring p orbitals do not have the necessary parallel alignment for overlap • Resembles an open-chain polyene in its reactivity

23. Energy Levels of Cyclic Conjugated Molecules (4n + 2 Electrons) There is always a single lowest-lying MO, above which the MOs come in degenerate pairs Aromaticity and the Hückel 4n + 2 Rule

24. Ions and heterocyclic compounds can also be aromatic 9.4 Aromatic Ions and Aromatic Heterocycles

25. Aromatic Ions There are three ways in which the hydrogen might be removed from cyclopenta-1,3-diene and cyclohepta-1,3,5-triene The hydrogen can be removed with both electrons (H:-) leaving a carbocation The hydrogen can be removed with one electron (H.) leaving a carbon radical The hydrogen can be removed with no electrons (H+) leaving a carbon anion, or carbanion Aromatic Ions and Aromatic Heterocycles

26. 4n + 2 rule predicts cyclopentadienyl anion and cycloheptatrienyl cation to be aromatic Aromatic Ions and Aromatic Heterocycles

27. Aromatic Ions and Aromatic Heterocycles • Aromatic cyclopentadienyl anion, showing the cyclic conjugation and six p electrons in five p orbitals • Aromatic cycloheptatrienyl cation, showing the cyclic conjugation and six p electrons in seven p orbitals

28. Aromatic Heterocycles A cyclic compound that contains atoms of two or more different elements in its ring, usually carbon along with nitrogen, oxygen, or sulfur Pyridine is much like benzene in its p electron structure A six-membered heterocycle with nitrogen in its ring Each of the five sp2-hybridized carbons has a p orbital perpendicular to the plane of the ring and each p orbital contains one p electron Aromatic Ions and Aromatic Heterocycles

29. Aromatic Ions and Aromatic Heterocycles • The nitrogen atom is also sp2-hybridized and has one electron in a p orbital, bringing the total to six p electrons • The nitrogen lone pair electrons are in an sp2 orbital in the plane of the ring and are not involved with the aromatic p system

30. Aromatic Ions and Aromatic Heterocycles • Pyrimidine is much like benzene in its p electron structure • Has two nitrogen atoms in a six-membered, unsaturated ring • Both nitrogens are sp2-hybridized, and each contributes one electron to the aromatic p system

31. Aromatic Ions and Aromatic Heterocycles • Pyrrole is a five membered heterocycle with six p electrons • Aromatic • Each of the sp2-hybridized carbons contributes one p electron • The sp2-hybridized nitrogen atom contributes the two electrons from its lone pair, which occupies a p orbital

32. Aromatic Ions and Aromatic Heterocycles • Imidazole is an analog of pyrrole that has two nitrogen atoms in a five-membered, unsaturated ring • Both nitrogens are sp2-hybridized • One nitrogen is in a double bond and contributes only one electron to the aromatic p system • The other nitrogen is not in a double bond and contributes two from its lone pair

33. Aromatic Ions and Aromatic Heterocycles Nitrogen atoms have different roles depending on the structure of the molecule • In pyridine and pyrimidine, the nitrogen atoms are both in double bonds and contribute only one p electron to the aromatic sextet, like a carbon atom in benzene does • In pyrrole, the nitrogen atom is not in a double bond and contributes two p electrons (the lone pair) to the aromatic sextet • In imidazole, both a double-bonded “pyridine-like” nitrogen that contributes one p electron and a “pyrrole-like” nitrogen that contributes two p electrons are present in the same molecule

34. Aromatic Ions and Aromatic Heterocycles Pyrimidine and imidazole rings are important in biological chemistry • Pyrimidine is the parent ring system in cytosine, thymine, and uracil, three of the five heterocycle amine bases found in nucleic acids • An aromatic imidazole ring is present in histidine, one of the twenty amino acids found in proteins

35. Worked Example 9.1Accounting for the Aromaticity of a Heterocycle Thiophene, a sulfur-containing heterocycle, undergoes typical aromatic substitution reaction rather than addition reactions. Why is thiophene aromatic?

36. Worked Example 9.1Accounting for the Aromaticity of a Heterocycle Strategy • Recall the requirements for aromaticity • A planar, cyclic, conjugated molecule with 4n + 2 p electrons • See how requirements for aromaticity apply to thiophene

37. Worked Example 9.1Accounting for the Aromaticity of a Heterocycle Solution • Thiophene is the sulfur analog of pyrrole • The sulfur atom is sp2-hybridized and has a lone pair of electrons in a p orbital perpendicular to the plane of the ring • Sulfur also has a second lone pair of electrons in the ring plane