Frost Circles a Great Trick

Transcription

1 Aromatics

2 Frost Circles a Great Trick Inscribe a polygon of the same number of sides as the ring to be examined such that one of the vertices is at the bottom of the ring The relative energies of the MOs in the ring are given by where the vertices touch the circle The MOs below the horizontal line through the center of the ring are bonding MOs on the horizontal line are nonbonding MOs above the horizontal line are antibonding MOs

3

4 π-mos of Cyclobutadiene (square planar) Antibonding Cyclo- butadiene Bonding 4 π electrons; bonding orbital is filled; other 2 π electrons singly occupy two nonbonding orbitals

5 π-mos of Cyclooctatetraene (square planar) Antibonding Cyclo- octatetraene Bonding 8 π electrons; 3 bonding orbitals are filled; 2 nonbonding orbitals are each half-filled filled

6 Requirements for Aromaticity cyclic conjugation is necessary, but not sufficient not aromatic aromatic not aromatic

7 ückel's Rule the additional factor that influences aromaticity is the number of π electrons

8 ückel's Rule for Aromaticity To be Aromatic a compound must : 1. be Cyclic 2. have one P orbital on each atom in the ring 3. be planar or nearly so to give orbital overlap 4. have a closed loop of 4n+2 pi electrons in the cyclic arrangement of p orbitals

9 π-mos of Benzene Antibonding Benzene 6 π electrons fill all of the bonding orbitals all π antibonding orbitals are empty Bonding

10 π-mos of Cyclobutadiene (square planar) Antibonding Cyclo- butadiene Bonding 4 π electrons; bonding orbital is filled; other 2 π electrons singly occupy two nonbonding orbitals

11 π-mos of Cyclooctatetraene (square planar) Antibonding Cyclo- octatetraene Bonding 8 π electrons; 3 bonding orbitals are filled; 2 nonbonding orbitals are each half-filled filled

12 π-electron Requirement for Aromaticity 4 π electrons 6 π electrons 8 π electrons not aromatic aromatic not aromatic

13 [18]Annulene The protons resonate at -3.0 and at 9.3δ!!! 18 π electrons satisfies ückel's rule resonance energy = 418 kj/mol bond distances range between pm

14 What about??

15 eterocyclic Aromatic Compounds

16 eterocyclic Aromatic Compounds N N O S Pyridine Pyrrole Furan Thiophene

17 eterocyclic Aromatic Compounds and ückel's Rule N Pyridine 6 π electrons in ring lone pair on nitrogen is in an sp 2 hybridized orbital; not part of π system of ring

18 uckel and Pyridine

19 N Pyrrole lone pair on nitrogen must be part of ring π system if ring is to have 6 π electrons lone pair must be in a p orbital in order to overlap with ring π system

20 uckel and Pyrrole

21 Furan O two lone pairs on oxygen one pair is in a p orbital and is part of ring π system; other is in an sp 2 hybridized orbital and is not part of ring π system

22 uckel and Furan

23 omework Supplement..Please name the heterocycle and show Which electrons on the heteroatoms are included in the pi system. Where are the unshared pairs and how are they oriented relative to the plane of the ring?? S N N S

24 Aromatic Ions Radical Anion π e s Radical Cation Cation Anion

25 Cyclopentadiene Let s fill these for each case, radical, anion and cation π e s Radical Cation Anion

26 Cyclopentadienide Anion 6 π electrons delocalized over 5 carbons negative charge dispersed over 5 carbons stabilized anion

27 Acidity of Cyclopentadiene + + pk a = 16 K a = Cyclopentadiene is unusually acidic for a hydrocarbon. Increased acidity is due to stability of cyclopentadienide anion.

28 Cyclopentadienide Anion

29 Let s Move Electrons once

30 π e s Radical Cation Anion Which is stable??

31 Compare Acidities of Cyclopentadiene and Cycloheptatriene pk a = 16 K a = pk a = 36 K a =

32 Cyclopropenyl Cation + also written as + n = 0 4n +2 = 2 π electrons!!

33 n = 0 (4n+2 = 2) fills a bonding MO

34 Cyclooctatetraene Dianion also written as 2 n = 2 4n +2 = 10 π electrons

35 Electrophilic Aromatic Substitution Electrophilic aromatic substitution: a reaction in which a hydrogen atom of an aromatic ring is replaced by an electrophile We study several common types of electrophiles, how they are generated, and the mechanism by which they replace hydrogen is the same for all

36 Electrophilic Aromatic Substiution

37 Electrophilic Aromatic Substitution E + + E E + + Please be sure that you can do this and that it makes sense to you!! + E + E

38 The Energetics + E + E + E + +

39 Nitration 2 SO 4 NO 2 NO 3 O NO 2 + O SO 3 Nitric acid + N O + O O 2 + O NO 2 + SO 4 + O=N=O Nitronium ion

40 Chlorination Chlorination requires requires a Lewis acid catalyst, such as AlCl 3 or FeCl 3 Cl Step 1: formation of a chloronium ion Cl + Fe Cl chloronium Cl ion Cl Cl Cl Fe Cl Cl FeCl 4 Cl Cl

41 Sulfonation + SO 3 2 SO 4 SO 3 Benzenesulfonic acid O O S O + O S O O - Sulfonation can be reversed by eating in 2 O

42 The Friedel-Crafts Reaction.. Circa 1877 Charles Friedel James Craft Making C-C C C bonds is.

43 Friedel-Crafts Alkylation C 3 C 2 Cl C 2 C3 AlCl 3 Cl R Cl + Al Cl Cl R + Cl - Cl Al Cl Cl + - R AlCl 4 An ion pair containing a carbocation

44 A word about the Friedel-Crafts Alkylation 1. Carbocation rearrangements are common C 3 AlCl 3 + C 3 CC 2 Cl C(C 3 ) 3 + Cl Benzene Isobutyl chloride tert-butylbenzene

45 Role of AlCl 3 Acts as a Lewis acid to promote ionization of the alkyl halideb (C 3 ) 3 C Cl + AlCl 3 (C 3 ) 3 C + Cl AlCl 3 + (C 3 ) 3 C + Cl AlCl 3

46 Friedel-Crafts Alkylation 2. They are tough to stop! Product is more reactive than the starting material C 3 C2Cl AlCl etc.

47 Friedel-Crafts Alkylation 2. alkylation fails on benzene rings bearing one or more strongly electron-withdrawing groups O C O O O O CR CO COR CN 2 SO 3 C N NO 2 CF 3 CCl 3 NR 3 +

48 Friedel-Crafts Acylation + O RCX AlX 3 O CR + An acylbenzene X O R-C Cl + Cl Al-Cl Cl O R-C + Cl Cl - Al Cl Cl O - R-C + AlCl 4 An ion pair containing an acylium ion

49 Friedel-Crafts Acylation An acylium ion is a resonance hybrid of two major contributing structures RC + O RC O + F-C acylations are free of ONE major limitation of F-C alkylations; acylium ions do not rearrange They still do not work on deactivated Rings They stop after one substitution

50 ydrogenolysis O C3 COCl AlCl 3 2 Pd/C There are two nice tricks hidden here Please be sure to remember this reaction!!!

51 Di- and Polysubstitution Existing groups on a benzene ring influence further substitution in both orientation and rate Orientation: certain substituents direct preferentially to ortho & para positions; others direct preferentially to meta positions substituents are classified as either ortho-para directing or meta directing

52 Di- and Polysubstitution Rate: certain substituents cause the rate of a second substitution to be greater than that for benzene itself; others cause the rate to be lower substituents are classified as activating toward further substitution, or deactivating

53 Di- and Polysubstitution -OC 3 is ortho-para directing and activating OC 3 OC 3 OC 3 Br Br C 3 CO 2 Anisole o-bromoanisole (4%) Br p-bromoanisole (96%) Br

54 Di- and Polysubstitution -NO 2 is meta directing and deactivating! NO 2 Nitrobenzene + NO 3 2 SO 4 NO 2 + NO 2 NO 2 NO 2 + m-dinitrobenzene (93%) NO 2 NO 2 o-dinitrobenzene p-dinitrobenzene Less than 7% combined

55 Methyl Group C 3 Toluene undergoes nitration 1000 times faster than benzene. A methyl group is an activating substituent.

56 Trifluoromethyl Group CF 3 Trifluoromethylbenzene undergoes nitration 40,000 times more slowly than benzene. The trifluoromethyl group is a deactivating substituent..

57 Relative rates of Nitration Cl O NO x10-8 Reactivity

58 Real Fast Pretty fast Kinda slow Pretty slow Slow Real Slow

59 Effect on Regioselectivity Ortho-para directors direct an incoming electrophile to positions ortho and/or para to themselves. Meta directors direct an incoming electrophile to positions meta to themselves. All meta directors are deactivating All ortho-para directors are activating except halogen

60 Theory of Directing Effects So what s going on here???? The rate of EAS is limited by the slowest step in the mechanism duh For EAS, the rate-limiting step is attack of E + on the aromatic ring to form a resonance-stabilized cation intermediate The more stable this cation intermediate, the faster the ratelimiting step and the faster the overall reaction

61 Di- and Polysubstitution Alkyl groups, phenyl groups, and all groups in which the atom bonded to the ring has an unshared pair of electrons are ortho-para directing. All other groups are meta directing. All ortho-para directing groups except the halogens are activating toward further substitution. The halogens are weakly deactivating

62 Di- and Polysubstitution alkyl groups, phenyl groups, and all groups in which the atom bonded to the ring has an unshared pair of electrons are ortho-para directing. All other groups are meta directing. All ortho-para directing groups except the halogens are activating toward further substitution. The halogens are weakly deactivating

63 Adding a Second Substiuent Methoxy is is therefore an o-p p director

64 Adding a Second Substiuent Nitro is therefore a meta director

65 Di- and Polysubstitution C 3 CO 2 NO 3 2 SO 4 K 2 Cr 2 O 7 2 SO 4 C 3 NO2 NO 2 p-nitrobenzoic acid CO 2 CO 2 K 2 Cr 2 O 7 2 SO 4 NO 3 2 SO 4 NO 2 m-nitrobenzoic acid