ORGANIC - CLUTCH CH AROMATICITY.

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2 CONCEPT: AROMATICTY INTRODUCTION Aromatic compounds display an unusual stability for their high level of electron density. Their high level of unsaturation should make them extremely reactive, however they are difficult to react with. EXAMPLE: Three typical addition reactions with cyclohexene vs. benzene What is responsible for this crazy level of stability? Categories of Aromatics: : These compounds possess an unusually level of stability : These compounds do not possess any unique level of stability or instability : These compounds possess an unusually level of stability. Very reactive! EXAMPLE: Differing aromaticity of conjugated trienes Page 2

3 CONCEPT: FOUR TESTS OF AROMATICTY For a compound to qualify as aromatic, it must meet 4 distinct tests. These are called Huckel s Rule compounds. 1. Cyclic: 2. Fully Conjugated: 3. Planar: 4. Huckel s Rule: (4n + 2) number of π electrons Any compound that one or more of these tests is considered Any compound that meets all these conditions, but only has (4n) π electrons is These compounds are said to follow Breslow s Rule Page 3

4 CONCEPT: COUNTING PI ELECTRONS When counting π-electrons, we are trying to identify the number of electrons that are freely available to circulate through conjugated p-orbitals. Double Bond/Anion = Radical = Cation = EXAMPLE: Count the number of π-electrons present in all of these molecules: Page 4

5 CONCEPT: AROMATICITY OF HYDROCARBONS We can use our knowledge of the Four Tests of Aromaticity to confirm aromaticity Huckel s Rule = Aromatic (4n + 2) π electron numbers:,,,, etc. Breslow s Rule = Anti-aromatic (4n) π electron numbers:,,,, etc. Non-aromatic = FAILS one or more test (including odd number of π electrons) EXAMPLE: Determine the aromaticity of the following molecules Page 5

6 CONCEPT: AROMATICTY OF ANNULENES An annulene, sometimes referred to as a polyolefin, is the name given to a fully conjugated monocyclic hydrocarbon. Due to their simple structure, rings of different sizes can be named as [n]annulenes, where n = number of carbons As annulenes get bigger, the challenge becomes predicting planarity. Predicting Annulene Planarity: Pertaining to All-cis annulenes, EXAMPLE: Cyclooctatetrene vs. Cyclooctatetraene dianion If 4n + 2 π electrons 10+ = Non-aromatic 9 or less = Aromatic If 4n π electrons 8+ = Non-aromatic 7 or less = Antiaromatic EXAMPLE: Determine if the following annulenes display any form of aromaticity. Page 6

7 CONCEPT: AROMATICITY OF HETEROCYCLES Heterocycles are cyclic structures that contain a within the ring. Heteroatoms can choose to donate up to one lone pair each only if: 1. They are already sp 3 hybridized 2. It will help to create aromaticity EXAMPLE: Determine the aromaticity of the following heterocycles. Will any lone pairs be donated to the ring? Page 7

8 CONCEPT: INSCRIBED POLYGON METHOD Also known as the polygon-in-circle method, or Frost Circle, this helps us visualize the identities of π electrons and molecular orbitals in a ring. EXAMPLE: Use the polygon-in-circle method to predict stability of the following molecules. Step 1: Draw polygon with one corner facing down. Step 2: Draw molecular orbitals on all corners of ring Step 3: Draw a line that splits the polygon down the middle Step 4: Insert π electrons into orbitals starting from lowest energy and working up (Aufbau Principle). Filled molecular orbitals contribute to unique stability (aromaticity) Partially filled molecular orbitals contribute to unique instability (antiaromaticity) Page 8

9 PRACTICE: Apply the polygon circle method to the following compound. Does it show any special stability? If yes, why? Tropyllium cation Page 9

10 CONCEPT: BENZENE NOMENCLATURE Benzene was one of the first organic molecules to be identified (1825), so common names predominate. Common Benzene Derivatives: Monosubstituted Benzene: Disubstituted Benzene: Multisubstituted Benzene: No location necessary No numerical locations Numerical locations necessary 1,2 = (o-) Do not use -o, -m, -p 1,3 = (m-) 1,4 = (p-) Page 10

11 EXAMPLE: Correctly name the following benzene derivatives Page 11

12 CONCEPT: ACIDITY OF AROMATIC HYDROCARBONS Aromatic hydrocarbons are not naturally acidic. In fact, the pka of benzene is If a hydrocarbon can become aromatic by donating a proton, it will be uniquely acidic. i.e. cyclopentadiene If a hydrocarbon becomes anitaromatic by donating a proton, it will be uniquely non-acidic. i.e. cycloheptatriene EXAMPLE: Would the following hydrocarbon be expected to display unusual acidity? Explain your reasoning. EXAMPLE: Would the following two hydrocarbons be expected to have similar acidities? Explain your reasoning. Page 12

13 CONCEPT: BASICITY OF AROMATIC HETEROCYCLES Heterocycles often have multiple lone pairs available to react with acids. The question is which lone pair do we react? EXAMPLE: Draw the product of the following acid/base reaction with imidazole. Acids can only react with lone pairs that are not necessary for aromaticity. sp 2 -hybridized lone pairs are basic. EXAMPLE: Draw the product of the following acid/base reaction. Page 13

14 CONCEPT: IONIZATION OF AROMATICS Double bonds can be viewed as a loose pair of electrons that can undergo resonance movement and ionization if that helps to create an aromatic compound. Resonance of Fulvalenes: Fulvalenes are hydrocarbons composed of two fully conjugated rings joined by an exocyclic double bond. Which atom would you expect to most readily react with an electrophile (E + )? Does this molecule possess a net dipole? If so, indicate the direction. Resonance of Azulene: Azulene is a polycyclic aromatic molecule with a distinctive blue color.. Which atom would you expect to most readily react with an electrophile (E + )? Does this molecule possess a net dipole? If so, indicate the direction. Page 14

15 PRACTICE: Which carbon in the following compound is most likely to react with an electrophile? CH 2 (heptafulvene) Page 15