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© E.V. Blackburn, 2011 Electrophilic aromatic substitution.

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1 © E.V. Blackburn, 2011 Electrophilic aromatic substitution

2 © E.V. Blackburn, 2011 Substitution? The characteristic reactions of benzene involve substitution in which the resonance stabilized ring system is maintained:

3 © E.V. Blackburn, 2011 Reactivity - an electron source, benzene reacts with electron deficient reagents - electrophilic reagents.

4 © E.V. Blackburn, Nitration ArH + HNO 3 /H 2 SO 4 ArNO 2 + H 2 O 2. Sulfonation ArH + H 2 SO 4 /SO 3 ArSO 3 H + H 2 O 3. Halogenation ArH + X 2 /FeX 3 ArX + HX Electrophilic aromatic substitution

5 © E.V. Blackburn, Friedel - Crafts alkylation ArH + RCl/AlCl 3 ArR + HCl 5. Friedel - Crafts acylation ArH + RCOCl/AlCl 3 ArCOR + HCl Friedel - Crafts reactions

6 © E.V. Blackburn, 2011 Substituent effects Toluene is more reactive than benzene % 63%3%

7 © E.V. Blackburn, 2011 Reactivity Compare the time required for reactions to occur under identical conditions. Compare the severity of reaction conditions. Make a quantitative comparison under identical reaction conditions. How is “reactivity” determined in the lab?

8 © E.V. Blackburn, 2011 Substituent effects It also directs the attacking reagent to the ortho and para positions on the ring. In some way, the methyl group makes the ring more reactive than that of the unsubstituted benzene molecule.

9 © E.V. Blackburn, 2011 Substituent effects Nitrobenzene undergoes substitution at a slower rate than does benzene. It yields mainly the meta isomer. 2% 7%91%

10 © E.V. Blackburn, 2011 Substituent effects A group which makes the ring less reactive than benzene is called a deactivating group. A group which makes the ring more reactive than that of benzene is called an activating group. A group which leads to the predominant formation of ortho and para isomers is called an “ortho - para directing group.” A group which leads to the predominant formation of the meta isomer is called a “meta directing group.”

11 © E.V. Blackburn, 2011 Activating, o,p directors -OH-NH 2 -NHR-NR 2 moderately activating -OR-NHCOR weakly activating -aryl-alkyl strongly activating All activating groups are o,p directors.

12 © E.V. Blackburn, 2011 Deactivating, m directors -NO 2 -SO 3 H -CO 2 H-CO 2 R -CONH 2 -CHO-COR-CN ++ -NH 3 -NR 3 All m directors are deactivating.

13 © E.V. Blackburn, 2011 Deactivating, o, p directors -F, -Cl, -Br, -I

14 © E.V. Blackburn, 2011 Orientation in disubstituted benzenes Here the two directing effects are additive.

15 © E.V. Blackburn, 2011 Orientation in disubstituted benzenes When two substituants exert opposing directional effects, it is not always easy to predict the products which will form. However, certain generalizations can be made....

16 © E.V. Blackburn, 2011 Orientation in disubstituted benzenes Strongly activating groups exercise a far greater influence than weakly activating and all deactivating groups.

17 © E.V. Blackburn, 2011 If there is not a great difference between the directive power of the two groups, a mixture results: 58% 42% Orientation in disubstituted benzenes

18 © E.V. Blackburn, 2011 Usually no substitution occurs between two meta substituents due to steric hindrance: 1% 62% 37%......nitration Orientation in disubstituted benzenes

19 © E.V. Blackburn, 2011 Synthesis of m- bromonitrobenzene In order to plan a synthesis, we must consider the order in which the substituents are introduced If, however, we brominate and then nitrate, the o and p isomers will be formed.

20 © E.V. Blackburn, 2011 Orientation and synthesis Let’s look at converting a methyl group into a carboxylic acid: Now let’s see how we can make the three nitrobenzoic acids: If a synthesis involves the conversion of a substituants into another, we must decide exactly when to do the conversion.

21 © E.V. Blackburn, 2011 The nitrobenzoic acids m-nitrobenzoic acid bp 225 o C bp 238 o C

22 © E.V. Blackburn, 2011 The nitrobenzoic acids o-nitrobenzoic acidp-nitrobenzoic acid

23 © E.V. Blackburn, 2011 Nitration HONO 2 + 2H 2 SO 4 H 3 O + + 2HSO NO 2 + nitronium ion - a Lewis acid

24 © E.V. Blackburn, 2011 The structure of the intermediate carbocation The positive charge is not localized on any one carbon atom. It is delocalized over the ring but is particularly strong on the carbons ortho and para to the nitro bearing carbon.

25 © E.V. Blackburn, 2011 Sulfonation

26 © E.V. Blackburn, 2011 Halogenation

27 © E.V. Blackburn, 2011 Friedel - Crafts alkylation

28 © E.V. Blackburn, 2011 An electrophilic carbocation?

29 © E.V. Blackburn, 2011 An electrophilic carbocation?

30 © E.V. Blackburn, 2011 An electrophilic carbocation? ~33%~67%

31 © E.V. Blackburn, 2011 An electrophilic carbocation? When RX is primary, a simple carbocation does not form. The electrophile is a complex:

32 © E.V. Blackburn, 2011 Limitations A polysubstitution is possible - the reaction introduces an activating group! Aromatic compounds bearing -NH 2, -NHR or -NR 2 do not undergo Friedel - Crafts substitution. Why? Aromatic rings less reactive than the halobenzenes do not undergo Friedel - Crafts reactions.

33 © E.V. Blackburn, 2011 Friedel - Crafts acylation - the reaction

34 © E.V. Blackburn, 2011 Friedel - Crafts acylation acylium ion

35 © E.V. Blackburn, 2011 Limitations

36 © E.V. Blackburn, 2011 The mechanism slow, rate determining step fast Evidence - there is no significant deuterium isotope effect.

37 © E.V. Blackburn, 2011 Isotope effects A difference in rate due to a difference in the isotope present in the reaction system is called an isotope effect.

38 © E.V. Blackburn, 2011 Isotope effects If an atom is less strongly bonded in the transition state than in the starting material, the reaction involving the heavier isotope will proceed more slowly. The isotopes of hydrogen have the greatest mass differences. Deuterium has twice and tritium three times the mass of protium. Therefore deuterium and tritium isotope effects are the largest and easiest to determine.

39 © E.V. Blackburn, 2011 Primary isotope effects These effects are due to breaking the bond to the isotope. Thus the reaction with protium is 5 to 8 times faster than the reaction with deuterium.

40 © E.V. Blackburn, 2011 Evidence for the E2 mechanism - a large isotope effect

41 © E.V. Blackburn, 2011 The mechanism slow, rate determining step fast Evidence - there is no significant deuterium isotope effect.

42 © E.V. Blackburn, 2011 The reactivity of aromatic rings The transition state for the rate determining step: Factors which stabilize carbocations by dispersal of the positive charge will stabilize the transition state which resembles a carbocation; it is a nascent carbocation.

43 © E.V. Blackburn, 2011 Carbocation stability electron donation stabilizes the carbocation electron withdrawal destabilizes the carbocation

44 © E.V. Blackburn, 2011 Orientation A deactivating group deactivates all positions on the ring but deactivates the ortho and para positions more than the meta position. Why? Examine the transition state for the rate determining step for ortho, meta and para attack. An activating group activates all positions on the ring but directs the attacking reagent to the ortho and para positions because it makes these positions more reactive than the meta position.

45 © E.V. Blackburn, 2011 CH 3 - an o/p director para attack ortho attack meta attack 3°3° 3°3°

46 © E.V. Blackburn, 2011 NO 2 - a m director para ortho meta

47 © E.V. Blackburn, 2011 NO 2 - a m director para

48 © E.V. Blackburn, 2011 NH 2 - an o/p director??

49 © E.V. Blackburn, 2011 Deactivation results from electron withdrawal: Halogen - a deactivating group

50 © E.V. Blackburn, 2011 Halogen - an o/p directing group o/p directors are electron donating. How can a halogen substituent donate electrons?

51 © E.V. Blackburn, 2011 Halogen - an o/p directing group

52 © E.V. Blackburn, 2011


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