Lecture 9. Chemistry! Chemistry 328N. February 20, 2018

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1 Lecture 9 Chemistry! February 20, 2018

2 Some istory Michael Faraday British physicist and chemist, best known for his discoveries of electromagnetic induction and of the laws of electrolysis. e also discovered benzene!

3 What in the World is Benzene?? C 6 6 discovered by Michael Faraday in 1823 Empirical formula is C Synthesized in 1834 from benzoic acid Remarkable chemical stability Unsaturation number is very high but. Does not add Bromine Substitution with Br 2 / FeBr 3 Not oxidized by Permanganate or ozone No reaction with strong Br (aq) No reaction with ydrogen on Pd..??????

4 Friedrich August Kekulé ( )

5 Midnight Ride of Kekule Again the atoms were gamboling before my eyes. This time the smaller groups kept modestly to the background. My mental eye, rendered more acute by repeated vision of this kind, could now distinguish larger structures, of manifold conformation; long rows, sometimes more closely fitted together; all twining and twisting in snakelike motion. But look! What was that?!!! One of the snakes seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lighting I awoke... Let us learn to dream, gentlemen. Arthur Koestler (in "The Act of Creation") called this incident "probably the most important dream in history since Joseph's seven fat and seven lean cows.

6 The man and his snakes August Kekule

7 Then Pharaoh said to Joseph: "In my dream, I was standing on the bank of the Nile, when up from the Nile came seven cows, fat and well-formed; they grazed in the reed grass. Behind them came seven other cows, scrawny, most ill-formed and gaunt. Never have I seen such ugly specimens as these in all the land of Egypt! The gaunt, ugly cows ate up the first seven fat cows. But when they had consumed them, no one could tell that they had done so, because they looked as ugly as before. Then I woke up.

8 Benzene per Kekulé August Kekulé proposed a structure for benzene in 1872 This structure, however, did not really account for the unusual chemical reactivity of benzene

9 Kekule explains why there are only 3 isomers of dibromobenzene X X X X X X

10 Kekule s proposal is an equilibrium between two structures Br Br Br Br Pauling s Resonance Theory describes resonance structures generated by electron movement (only!) that are not real, they are constructs the weighted sum of which describes the real molecule, which is presented as the resonance hybrid Br Br Br Br Br Resonance hybrid Br

11 Benzene - Resonance We can represent benzene as the hybrid of two equivalent Kekulé structures each makes an equal contribution to the hybrid, and thus the C-C bonds are neither double nor single, but something in between

12 All C C bond distances = 140 pm (1.4 Ǻ) 146 pm 134 pm 140 pm is the average between the C - C single bond length and the C=C double bond length in 1,3-butadiene.

13 Unusual Stability of Benzene Benzene is the best and most familiar example of a substance that possesses "special stability" or "aromaticity" Aromatic molecules have stability that is substantially greater for a molecule than would be expected on the basis of any of the Lewis structures written for it

14 Phenols The functional group of a phenol is an -O group bonded to a benzene ring Phenol

15 Acidity of Phenols Phenols are much more acidic than aliphatic alcohols that also contain the -O group Phenol: pk a = 9.95 O+ 2O O - + 3O + Ethanol: pk a = 15.9 C 3 C 2 O + 2 O C 3 C 2 O - + 3O + delocalization of the negative charge by resonance stabilizes the phenoxide ion relative to the alkoxide ion

16 Lets move electrons together!! PLEASE FOLLOW ME STEP BY STEP

17 Acidity of Phenols O + O - O O pka 9.95 Strong acid Strong Weak + + base base pka 15.7 Weak acid Strong acids have weak conjugate bases Stabilization of anions leads to weak/stable conjugate bases!

18 Acidities of Phenols Part of the acid-strengthening effect of -NO 2 is due to its electron-withdrawing inductive effect In addition, -NO 2 substituents in the ortho and para positions help to delocalize the negative charge by Resonance O O O + + N N O O O delocalization of negative charge onto oxygen

19 Acidities of Phenols Nitro groups increase the acidity of phenols by both an electron-withdrawing inductive effect and a resonance effect O O O Phenol pk a 9.95 NO 2 m-nitrophenol pk a 8.28 NO 2 p-nitrophenol pk a 7.15

20 Acidity of Phenols Alkyl and halogen substituents effect acidities by inductive effects alkyl groups are electron-releasing by induction halogens are electron-withdrawing by induction O O O O O C 3 C 3 Phenol m-cresol p-cresol pk a 9.95 pk a pk a Cl m-chloro phenol pk a 8.85 Cl p-chlorophenol pk a 9.18

21 Finally.Chemistry!! A + B C

22 Synthesis: Alkyl-Aryl Ethers Alkyl-aryl ethers can be prepared by the Williamson ether synthesis but only using phenoxide salts and alkyl halides aryl halides are unreactive to S N 2 reactions + X RO - Na + X no reaction

23 Alkyl-Aryl Ethers O - Cl O Cl - S n 2 reactions are accelerated by: polar aprotic solvents phase transfer catalysis crown ethers Remember: methyl > 1 0 >2 0 and 3 0 is a no go! {Review chapter 8??}

24 Reactions at Benzyl Carbons C 3

25 Benzylic Reactions Benzylic radicals ( and cations) are easily formed because of the resonance stabilization of these intermediates the benzyl radical is a hybrid of five contributing structures. C

26 Benzylic Bromination Bromination proceeds by a radical mechanism Br NBS CCl 4 A Regioselective reaction!! O Slow + Br N O O N O Br NBS = N-bromosuccinimide

27 Flash Card Tricks front?? Br Back NBS CCl 4???

28 Benzylic Oxidation Benzene is unaffected by strong oxidizing agents such as 2 CrO 4 and KMnO 4 halogen and nitro substituents are also unaffected by these reagents alkyl groups with at least one hydrogen on the benzylic carbon are oxidized to a carboxyl group O 2 N Cl K 2 CrO 7 O 2 N Cl C 3 2 SO 4 CO 2 2-Chloro-4-nitrotoluene 2-Chloro-4-nitrobenzoic acid

29 Benzylic Oxidation if there is more than one alkyl group on the benzene ring, each is oxidized to a -CO 2 group O C 3 K 2 Cr 2 O 7 C O C 2 C 3 2 SO 4 C O O O K 2 Cr 2 O 7 O 2 SO 4 3 C C3