Reactions of Aromatic Compounds

Transcription

1 Reactions of Aromatic Compounds Aromatic compounds are stabilized by this aromatic stabilization energy Due to this stabilization, normal S N 2 reactions observed with alkanes do not occur with aromatic compounds (S N 2 reactions never occur on sp 2 hybridized carbons!) In addition, the double bonds of the aromatic group do not behave similar to alkene reactions

2 Aromatic Substitution While aromatic compounds do not react through addition reactions seen earlier Br Br 2 Br 2 Br FeBr 3 Br With an appropriate catalyst, benzene will react with bromine The product is a substitution, not an addition (the bromine has substituted for a hydrogen) The product is still aromatic

3 Electrophilic Aromatic Substitution Aromatic compounds react through a unique substitution type reaction Initially an electrophile reacts with the aromatic compound to generate an arenium ion (also called sigma complex) The arenium ion has lost aromatic stabilization (one of the carbons of the ring no longer has a conjugated p orbital)

4 Electrophilic Aromatic Substitution In a second step, the arenium ion loses a proton to regenerate the aromatic stabilization The product is thus a substitution (the electrophile has substituted for a hydrogen) and is called an Electrophilic Aromatic Substitution

5 Energy Profile Transition states Transition states Potential energy Intermediate E H Starting material Products E Reaction Coordinate The rate-limiting step is therefore the formation of the arenium ion

6 The properties of this arenium ion therefore control electrophilic aromatic substitutions (just like any reaction consider the stability of the intermediate formed in the rate limiting step) 1) The rate will be faster for anything that stabilizes the arenium ion 2) The regiochemistry will be controlled by the stability of the arenium ion The properties of the arenium ion will predict the outcome of electrophilic aromatic substitution chemistry

7 Bromination To brominate an aromatic ring need to generate an electrophilic source of bromine In practice typically add a Lewis acid (e.g. FeBr 3 ) to bromine In presence of aromatic ring this electrophilic bromine source will react Br 2 FeBr 3 Br H Br

8 With unsubstituted benzene the position of reaction is arbitrary With a monosubstituted aromatic ring, however, can obtain three possible products How to predict which is favored? Controlled by stability of arenium ion intermediate

9 The stability is different depending upon placement of carbocation

10 Alkyl groups are electron donating Therefore toluene will favor electrophilic substitution at ortho/para positions (only ortho/para substitution places carbocation adjacent to alkyl group on ring) Often obtain more para than ortho due to sterics

11 In addition to orientational control, substituents affect reactivity CH 3 NO 2 As the aromatic ring acquires more electron density, the arenium ion will be more stable

12 Toluene therefore reacts faster in an electrophilic aromatic substitution than benzene The alkyl group is called an activating group (it activates the ring for a faster rate) Any substituent that increases the rate for an electrophilic aromatic substitution is called an activating substituent Substituents that lower the rate for an electrophilic aromatic substitution are called deactivating subsituents (nitro is one example of a deactivating group) These are groups that lower the electron density of the aromatic ring

13 Factors that affect Activators/Deactivators There are in general two mechanisms that can affect a substituent 1) Inductive Substituents that are more electronegative than carbon will inductively pull electron density out of the ring 2) Resonance Substituents that have a lone pair of electrons adjacent to the ring can donate electron density into the ring through resonance O CH3 O CH3 O CH3 O CH3

14 Many substituents will have both inductive and resonance effects Need to balance the effects Electronegative atoms with lone pair of electrons have opposing effects R Z Z Alkyl substituents inductively donate, no resonance effects, therefore activators Inductively withdrawing therefore deactivating Resonance donating, therefore activating when a neutral O or N is directly bonded to a benzene ring, the resonance effect dominates and the net effect is activating when a halogen is bonded to a benzene ring, the inductive effect dominates and the net effect is deactivating

15 Other Deactivating Groups There are two other main classes of deactivating groups 1) A conjugated system where both inductive and resonance effects pull electron density from the ring Z Y O N O O N Whenever Z is more electronegative than Y as seen in structure 2) A formal positive charge is placed directly adjacent to ring H 3 C CH 3 N CH3

16 Activating vs. Deactivating Ability can be compared The substituents can be compared relatively

17 Orientational Control Can Be Predicted 1) All activating groups favor ortho/para substitution 2) Deactivating groups with a lone pair of electrons adjacent to the ring favor ortho/para substitution (halogens are in this category) 3) Other deactivating groups favor meta substitution

18 With deactivating groups besides halogens, the favored substitution is meta Can be predicted by stability of arenium ion Only the meta substitution does not place a carbocation adjacent to EWG

19 All positions with a deactivating group are slower than benzene * The meta position is the LEAST disfavored Consider reaction coordinate for the first, rate-determining step

20 Multiple Substituents How to determine orientation of electrophilic aromatic substitution if there is more than one substituent? A few rules to consider: 1) The effects are cumulative 2) The stronger substituent according to the relative effects will be correspondingly more important for directing effects 3) When given a choice, a new substituent typically will not go ortho to two other substituents

21 Consider some examples: ortho/para director 3 CN Br 2 meta director FeBr 3 CH 3 CN ortho/para director ortho/para director H 3 C CN Br 2 meta director FeBr 3 H 3 C OCH 3 ortho/para director Br 2 FeBr 3 H 3 C H 3 C Br CN Br Stronger director wins Br OCH 3 Stronger director wins

22 Reactivity of aromatic ring can affect amount of reaction In reactions with strongly activated rings, polyhalogenation occurs With either phenol or aniline, the reaction will proceed until all ortho/para positions are reacted when catalyst is used

23 With these highly activated ring systems, catalyst is not necessary Reaction will only proceed with strongly activated ring systems, still need catalyst for less activated aromatic rings, but with phenol or aniline reaction of one halogen can occur with no catalyst present

24 Other Electrophiles Beside Bromine Chlorination reaction is similar to bromination Often use Lewis acid with chlorine substituents to avoid cross contamination (e.g. often use AlCl 3 for chlorination but FeBr 3 for bromination) Also with strongly activated rings obtain polychlorination products with catalyst

25 Iodination requires a stronger oxidizing reagent The reagents are a method to generate iodonium ion in situ The iodonium can then react as the electrophile

26 Fluorine is much harder to add in an electrophilic addition There is not an easy method to generate F+ Typically to add fluorine to an aromatic ring a diazonium route is used This chemistry will be dealt in more detail in chapter 19

27 Other Common Electrophiles Besides Halogens Nitration Adding a nitro group to an aromatic ring is a convenient and useful reaction Need to generate a source of nitronium ion

28 An advantage of nitration is the nitro group can be reduced to an amine Nitro Deactivating/Meta Director Amine Activating/Ortho-Para Director Allows the introduction of an amine group to the aromatic ring (almost all compounds that contain a nitrogen attached to aromatic ring occurred through a nitration) This conversion changes the electronic properties of the ring

29 Sulfonation Allows the introduction of a sulfur group (many biological/medicinal uses) The reverse step can also occur

30 The desulfonation step allows introduction of isotopes Occurs through arenium ion where compound is sulfonated, then add D+ and desulfonate to eliminate sulfur and add deuterium

31 Friedel-Crafts Alkylation In general carbon-carbon bonds can be made by reacting an aromatic ring with a carbocation This reaction is called a Friedel-Crafts alkylation

32 The key is formation of electrophilic carbon The alkyl halide reacts with the Lewis acid With tertiary and secondary alkyl halides this generates a discrete carbocation With primary alkyl halides the full carbocation is not formed

33 Any method that generates a carbocation can be used in a Friedel-Crafts alkylation Two other common methods: 1) From alkenes Often use HF as acid source since fluoride is weak nucleophile, therefore higher lifetime of carbocation to react 2) From alcohols

34 Limtations of Friedel-Crafts Alkylation 1) Reaction does not work with strongly deactivated aromatic rings Friedel-Crafts alkylations do not work with nitro, sulfonic, or acyl substituents 2) Carbocation rearrangements occur Because a carbocation is formed during this reaction, similar to any reaction involving carbocations the carbocation can rearrange to a more stable carbocation Can therefore never obtain n-alkyl substituents longer than 2 carbons

35 3) Polyalkylation often occurs with Friedel-Crafts alkylation Because an alkyl group is an activating group, the product is more reactive than the starting material Causes the product to react further To prevent polyreaction the starting material (benzene is this example) is used in excess

36 Another Option: Friedel-Crafts Acylation Instead of adding an alkyl group this reaction adds an acyl substituent First need to generate an acid chloride Thus any carboxylic acid can be converted into an acid chloride

37 The acid chloride can then be reacted in a Friedel-Crafts acylation First form an acylium ion by reacting with Lewis acid The acylium ion then reacts with aromatic ring in a typical electrophilic aromatic substitution

38 Advantages of Friedel-Crafts Acylation 1) The acyl substituent is a deactivating group Therefore this reaction can be stopped easily at one addition (no polyacylation occurs) 2) No rearrangements occur Since an isolated carbocation is not formed there is no rearrangement Due to these two advantages, the Friedel-Crafts acylation is a much more convenient reaction than the Friedel-Crafts alkylation -still will not react with strongly deactivated aromatic rings

39 High para substitution preference Due to the steric bulk of the Friedel-Crafts acylation reagent, often see a high preference for the para substitution with an ortho/para directing group *often see para preference with bulky electrophiles

40 Clemmensen Reduction The ability to reduce a carbonyl to a methylene further enhances usefullness of Friedel-Crafts acylation In a Clemmensen reduction the conversion occurs under acidic conditions Overall these two steps, Friedel-Crafts acylation followed by Clemmensen, allows the introduction of an n-alkyl substituent which would not be possible with a Friedel-Crafts alkylation

41 Wolf-Kishner Reduction Another method to reduce a carbonyl to methylene is a Wolf-Kishner reduction Main difference is that the Wolf-Kishner occurs under basic conditions Both Clemmensen and Wolf-Kishner require strong conditions, Clemmensen uses acidic while Wolf-Kishner uses basic conditions

42 Benzaldehyde Adding a formyl group requires stronger conditions than an acyl group addition Cannot isolate formyl chloride Therefore normal Friedel-Crafts acylation conditions cannot be used Need to generate in situ

43 Birch Reduction In addition to adding electrophiles to aromatic ring, the aromatic ring can be reduced by adding electrons to the system (in essence a nucleophilic addition) The electrons need to be generated in situ This electron is called a solvated electron

44 In the presence of an aromatic ring this electron will react e Addition of one electron thus generates a radical anion This strongly basic anion will abstract a proton from alcohol solution

45 The radical will then undergo the same operation a second time The final product has thus been reduced from benzene to a 1,4-cyclohexadiene The aromatic stabilization has been lost

46 Orientation of Birch Reduction What happens if there is a substituent on the aromatic ring before reduction? Which regioisomer will be obtained? Similar to every other reaction studied need to ask yourself, What is the stability of the intermediate structure? The preferred product is a result of the more stable intermediate

47 The intermediate in a Birch reduction is the radical anion formed after addition of electron With electron withdrawing substituent: O NH 3 (l), Na CH 3 OH O O O Placing negative charge adjacent to carbonyl allows resonance With electron donating substituent: OCH 3 NH 3 (l), Na CH 3 OH OCH 3 OCH 3 Want negative charge as far removed from donating group as possible

48 Reactions of Alkyl Substituents to Aromatic Ring Once alkyl groups are attached to aromatic rings they can undergo subsequent reactions Permanganate Oxidation Any carbon adjacent to an aromatic ring that contains at least one hydrogen will be oxidized with permanganate to the carboxylic acid stage

49 Benzylic Halide As seen in chapter 4, halogenation reactions can occur with either chlorine or bromine under photolytic conditions Reaction proceeds through a radical intermediate The benzylic radical is more stable due to resonance with aromatic ring CH 2

50 Remember that chlorination was more reactive, bromination though occurred selectively Cl 2, h! Cl Cl Br 2, h! Br Realize reaction does not occur on aromatic ring, do not obatin radical from sp 2 hybridized carbon

51 Nucleophilic Aromatic Substitution Another type of reaction with aromatic rings is a nucleophilic addition Instead of adding an electrophile to form an arenium ion, a nucleophile replaces a leaving group This is NOT a S N 2 reaction Initially a carbanion is formed which subsequently loses the leaving group, unlike a S N 2 reaction which is a one step reaction

52 Mechanism NO 2 Cl NaCN NO 2 Cl CN NO 2 Cl CN NO 2 Cl CN O 2 N O 2 N O 2 N O 2 N The anion is stabilized by electron withdrawing groups ortho/para to leaving group NO 2 Cl CN NO 2 CN O 2 N O 2 N To regain aromatic stabilization, the chloride leaves to give the substituted product

53 Unique factors for Nucleophilic Aromatic Substitution 1) Must have EWG s ortho/para to leaving group -the more EWG s present the faster the reaction rate (intermediate is stabilized) 2) The leaving group ability does not parallel S N 2 reactions -bond to leaving group is not broken in rate-determining step (fluorine for example is a good leaving group for nucleophilic aromatic substitution but is a horrible leaving group for S N 2 reaction) More electronegative atom has a faster rate F > Cl > Br > I (polarizability is not a factor)

54 Using Nucleophilic Aromatic Substitution for Peptide Determination React 2,4-dinitrobenzene with peptide O 2 N NO 2 F H 2 N R O N H (peptide chain) O 2 N NO 2 H N R O N H (peptide chain) Sanger reagent cleave O 2 N NO 2 H N R O OH The Sanger reagent can react with the N-terminal amine from the peptide The resultant nucleophilic aromatic substitution product can be cleaved and thus the terminal amino acid structure determined

55 Benzyne Mechanism A second nucleophilic aromatic substitution reaction is a benzyne mechanism Benzyne is an extremely unstable intermediate which will react with any nucleophile present NH 2 H Br NaNH 2, NH 3 NH2 benzyne Need strong base at moderate temperatures, but do not need EWG s on ring

56 Oxidation of Phenols Many reactions of phenols follow the same reactivity seen earlier -they are highly activating groups (ortho/para directors) for electrophilic aromatic substitution Phenols can also be oxidized to create quinones

57 Coenzyme Q All oxygen-consuming organisms use this process

58 Aromatic Substitution using Organometallic Reagents Extremely useful in forming carbon-carbon bonds with aromatic rings Allows a much wider diversity of products than available with Friedel-Crafts reactions Tremendous progress has been made using transition metal compounds to catalyze reactions To understand these reactions one key is knowing what metal is needed to catalyze the reaction and also what functional groups are needed for each specific reaction to occur

59 Organocuprates Formed by reacting organolithium compounds with cuprous iodide (CuI) These lithium dialkyl cuprate reagents (organocuprates) are also called Gilman reagents These Gilman reagents can react with alkyl halides to form new carbon-carbon bonds similar to S N 2 reactions

60 Reactions at sp 2 Hybridized Carbons Unlike S N 2 reactions, however, these organocuprates can react with sp 2 hybridized aryl or vinyl halides

61 Heck Reaction Can also accomplish reaction at alkene using a palladium catalyzed reaction (called Heck reaction) -the halide can be either aryl or vinyl -the halogen can be either bromine or iodine

62 Suzuki Coupling Couples aryl or vinyl halide with boronic acid with palladium catalyst and base Can use boronic acid [R-B(OH) 2 ] or ester [R-B(OR) 2 ] that is alkyl, vinyl or aryl