CH 15: Benzene and Aromaticity

1. CH 15: Benzene and Aromaticity Renee Y. Becker Valencia Community College CHM 2211C

2. Aromatic Compounds Aromatic was used to described some fragrant compounds in early 19th century Not correct: later they are grouped by chemical behavior (unsaturated compounds that undergo substitution rather than addition) Current: distinguished from aliphatic compounds by electronic configuration Aromatic hydrocarbons are structural relatives of benzene. Arenes: Another name for alkyl substituted benzene.

3. Aromatic Compounds

5. Naming Aromatic Compounds Many common names (toluene = methylbenzene; aniline = aminobenzene) Monosubstituted benzenes systematic names as hydrocarbons with –benzene C6H5Br = bromobenzene C6H5NO2 = nitrobenzene, and C6H5CH2CH2CH3 is propylbenzene

6. The Phenyl Group When a benzene ring is a substituent, the term phenyl is used (for C6H5) You may also see “Ph” or “f” in place of “C6H5” “Benzyl” refers to “C6H5CH2”

7. Disubstituted Benzenes Relative positions on a benzene ring ortho- (o) on adjacent carbons (1,2) meta- (m) separated by one carbon (1,3) para- (p) separated by two carbons (1,4) Describes reaction patterns (“occurs at the para position”)

10. Example 1: Draw the structural formulas for each of the following compounds: • o-Ethylbenzoic acid • m –Chlorostyrene • p-Nitroaniline

11. Naming Benzenes With More Than Two Substituents Choose numbers to get lowest possible values List substituents alphabetically with hyphenated numbers Common names, such as “toluene” can serve as root name (as in TNT)

14. Example 2: Name

15. Example 3: Write the structural formulas for: 1-Phenylethanol 2,4,6-Tribromoanaline p-Diisopropylbenzene

16. Structure and Stability of Benzene Benzene reacts with slowly with Br2 to give bromobenzene (where Br replaces H) This is substitution rather than the rapid addition reaction common to compounds with C=C, suggesting that in benzene there is a higher barrier

18. Structure and Stability of Benzene

19. Structure and Stability of Benzene Consider the hydrogenation of benzene: Need P, Strong Cat. (Rh)

20. Kekulé Benzene Formula of benzene is C6H6 All of the hydrogens of benzene are equivalent Structural theory requires that there are four bonds to each carbon

21. Kekulé Benzene

22. Kekulé’s Flaw:

23. Kekulé’s Solution:

24. Structural features of benzene All of the carbon-carbon bonds are 140 pm. That’s in between the Csp2-Csp2 single bond length of 146 pm and the Csp2-Csp2 double bond length of 134 pm. All carbons are sp2 hybrids bond = 120

25. Drawing Benzene and Its Derivatives The two benzene resonance forms can be represented by a single structure with a circle in the center to indicate the equivalence of the carbon–carbon bonds This does indicate the number of  electrons in the ring but reminds us of the delocalized structure

27. Molecular Orbital Description of Benzene The 6 p-orbitals combine to give Three bonding orbitals with 6  electrons Three unoccupied antibonding orbitals Benezene has a “closed-shell” π electron configuration Completely filled valence shell All molecular orbitals doubly occupied or empty

29. Recall: Key Ideas on Benzene Unusually stable - heat of hydrogenation 150 kJ/mol less negative than a cyclic triene Planar hexagon: bond angles are 120°, carbon–carbon bond lengths 139 pm Undergoes substitution rather than electrophilic addition Resonance hybrid with structure between two line-bond structures One more important factor is the number of electrons in the cyclic orbital

30. Aromaticity How can we tell if a chemical is aromatic? • Has a delocalized conjugated  sysytem • Usually alternating single and double bonds • Coplanar structure • All contributing atoms in the same plane

31. Aromaticity • Atoms arranged in one or more rings • A # of  electrons that is even, but not a multiple of four • 4n + 2 =  electrons • Where n = 0,1,2,3 • If compound has 4n  electrons it is called antiaromatic

32. Example 4: • Let’s take a look at aromatic handout from website