Organic Chemistry

1. Organic Chemistry Aromatic compounds

2. Aromatic Compounds Are Unusually Stable Benzene is an aromatic compound

3. Benzene is unusually stable because of electron delocalization Compounds with unusually large resonance energies, like benzene, are called aromatic compounds

4. Criteria for Aromaticity 1. A compound must have an uninterrupted cyclic cloud of p electrons above and below the plane of the molecule 2. The p cloud must contain an odd number of pairs of p electrons

5. Hückel’s Rule For a planar, cyclic compound to be aromatic, its uninterrupted p cloud must contain (4n + 2) p electrons, where n is any whole number

6. Monocyclic hydrocarbons with alternating single and double bonds are called annulenes Cyclobutadiene and cyclooctatetraene are not aromatic because they have an even number of p electron pairs Furthermore, cyclooctatetraene is nonplanar

7. not aromatic not aromatic aromatic Cyclopentadiene does not have an uninterrupted ring of p orbital-bearing atoms Cyclopentadienyl cation has an even number of p electron pairs Cyclopentadienyl anion has an uninterrupted ring of p orbital-bearing atoms and an odd number p electron pairs

8. The resonance hybrid shows that all the carbons in the cyclopentadienyl anion are equivalent Each carbon has exactly one-fifth of the negative charge associated with the anion

9. These compounds are aromatic

10. Aromatic Heterocyclic Compounds A heterocyclic compound is a cyclic compound in which one or more of the ring atoms is an atom other than carbon

11. Pyridine Is Aromatic

12. Pyrrole Is Aromatic The lone-pair electrons on the nitrogen atom of pyrrole are p electrons

13. Furan and thiophene are aromatic compounds like pyrrole

14. Examples of Heterocyclic Aromatic Compounds

15. The Effect of Aromaticity on the pKa Values of Some Compounds

16. Why is the pKa of cyclopentadiene so much lower than that of ethane? The conjugate base is aromatic

17. Aromaticity influences chemical reactivity The cycloheptatrienyl cation is aromatic

18. Antiaromaticity A compound is antiaromatic if it is a planar, cyclic compound with an uninterrupted ring of p orbital-bearing atoms, and the p cloud must contain an even number of pairs of p electrons Antiaromatic compounds are highly unstable

19. A Molecular Orbital Description of Aromaticity and Antiaromaticity Aromatic compounds are stable because they have filled bonding p molecular orbitals

20. Nomenclature of Monosubstituted Benzenes Some are named by attaching “benzene” after the name of the substituent

21. Some have to be memorized