Aromaticity and Reactions of Benzene

Transcription

1 Aromaticity and eactions of Benzene ark College Benzene is a unique molecule it is highly unsaturated with 6 carbons and 6 hydrogens, it is planar, and has a high degree of symmetry. These features explain and account for the special reactivity and stability contained within the molecule. Unlike regular alkenes, or even conjugated dienes, benzene (and its derivatives) do not undergo addition reactions; instead they undergo substitution reactions. Before we look at the reaction, we will consider some characteristics of benzene, and what makes it aromatic. Bonding and esonance Benzene exists as the amalgam of two resonance structures, shifting the position of the three pi bonds. As a result, each C-C bond is not an isolated double or single bond. The bonds all have a bond order of 1.5, as evidenced by the C-C bond length of 139 pm, which is between the bond lengths of C-C (154 pm) and C=C (133 pm). This helps explain why benzene does not react in the same way as regular alkenes. But why is benzene so stable? Let s consider some bond energies. 2 cat.! = kcal/mol 2 cat.! = kcal/mol 2 cat.! = kcal/mol 2 cat.! = -101 kcal/mol The energy released upon hydrogenating cyclohexene is 28.6 kcal/mol, which gives an indication of the relative instability of the starting alkene. From this, we should expect that 1,3,5-cyclohexatriene would release three times as much energy (3 x 28.6 kcal/mol, or 85.6 kcal/mol), or be three times as unstable. Complete hydrogenation of benzene releases only 49.8 kcal/mol, due to the increased stability of the starting material. This increased stability is referred to as the resonance energy, and describes the stability gained by having a cyclic system of alternating double and single bonds. This same resonance energy is not seen in cyclooctatetraene, which is also a cyclic system of alternating double and single bonds. Therefore, not only is the cyclic nature of the molecule necessary, but the number of pi bonds is also significant. ückel ules for Aromaticity 1. The molecule must be cyclic. 2. The molecule must be planar. 3. There must an unhybridized p orbital on each atom of the ring. 4. There must be an odd number of electron pairs; the number of pi electrons must be equal to 4n + 2, where n is an integer. Following these rules, we can see that benzene fits all rules, but cyclooctatetraene (CT) has an even number of pi bonds. As a result/consequence of not being aromatic, CT is also not planar; it takes on a boat shape. ther heterocycles can also fit within the definitions of aromaticity, if we change the hybridization. Let s consider a few different heterocycles: Chemistry of Aromatic Systems Page 1 of 10

2 ark College N N Pyridine Pyrrole Furan 1,4-Dioxin Pyridine is aromatic each atom, including the Nitrogen, is sp 2 -hybrized and there are three pi bonds. Upon initial inspection, neither pyrrole nor furan are aromatic, since the N and are sp 3 -hybridized. owever, if the N and are re-hybridized, a lone pair sits in the new unhybridized p orbital, which puts another pair of electrons into the pi system, satisfying all four of ückel s rules. If both oxygens in 1,4- dioxin are rehybridized, the resulting molecule would have 8 pi electrons, which does not satisfy the last (4n+2) rule. Aromatic molecules can also be formed from anions or cations, recognizing that a carbanion or carbocation rehybridizes to sp 2. This is the situation for the cyclopentadienyl anion, or the cyclopropyl cycloheptatienyl cations. Nomenclature of Benzene ings Because it is a common structural motif, benzene rings are typically named with benzene as the parent name. owever, the names of some common derivatives have been adopted as well. C 3 N 2 C 3 Toluene Phenol Aniline Anisole When the benzene is the predominant functional group, molecules are named by listing the substituent group as a side group, and benzene (or one of the above) as the parent name. When the benzene is a side group on a longer or more complicated linear molecule, the benzene becomes a phenyl side group (-Ph). A benzene ring attached to a longer chain through a C 2 group is often referred to as a benzyl group ( Bn or Bz). A phenyl group 2 C A benzyl group When two groups are attached to a benzene ring, they can be arranged in one of three patterns: 1,2 or ortho 1,3 or meta 1,4 or para When naming disubstituted rings, use either the numbering scheme or the ortho- (o-), meta- (m-), or para- (p-). The designations are listed in front of the names. Br o-bromochlorobenzene Chemistry of Aromatic Systems Page 2 of 10 N 2 m-nitrotoluene If more that two substituent groups are attached to the ring, then they are numbered, making sure that the lowest set of numbers are used.

3 eactions of Toluene or at the Benzylic Position ark College The benzylic position of an alkyl substituted benzene ring is a special one the carbon-hydrogen bond is particularly weak due to stabilization of a carbocation or radical with resonance from the ring. As such, the position is easy to halogenate or oxidize. The benzylic position can be brominated through a radical mechanism, using NBS and an organic peroxide. The bromine is now a leaving group, so a host of substitution and elimination reaction can occur. A selection of reactions is shown below. nly one benzylic hydrogen is needed - NBS,! - Br 1) N 3 2) - N 2 NaCN CN 2, Pt C 2 N 2 A benzylic hydrogen is also susceptible for oxidation with inorganic reagents (or slowly with just air!). Phenols nly one benzylic hydrogen is needed 1) KMn 4,! 2) + -or- Na 2 Cr 2 7, + The rest of the chain gets chopped off! Phenol, or phenyl alcohol, is considerably more acidic than typical alcohols, due to the ability to delocalize the negative charge into the ring. As a result, pk a values for phenolic compounds are in the 9-10 range, rather than the typical for an aliphatic alcohol. Because of its more acidic nature, mild bases such as sodium hydroxide are all that are required to deprotonate a phenol. The result is a stronger oxygen nucleophile that can be used in an S N 2 reaction to make a phenolic ether. Na Br DMS Not only can the oxygen be used as a nucleophile! nce deprotonated, the anion can be envisioned as an enolate ion, and the carbon can be used as a nucleophile as well. This reaction is used in an industrial setting to make salicylic acid, which is a precursor to aspirin (acetylsalicylic acid), alphahydroxy acids used in cosmetics, and methyl salicylate, or oil of wintergreen (Wint--Green Lifesavers!). This reaction is the Kolbe carboxylation. Chemistry of Aromatic Systems Page 3 of 10

4 The Kolbe Carboxylation ark College Na C + keto Net eaction: 1) Na 2) C 2 3) 3 + enol NM of Aromatic ings The cyclic nature of the delocalized electrons in an aromatic ring sets up a ring current, which creates a sizeable magnetic field. This results in a downfield shift of aromatic protons that often appear in the ppm range. The delocalized nature of the electrons also sets up long-range second and third order coupling between the protons on an aromatic ring, making the splitting patterns difficult to interpret. Alkyl groups adjacent to the ring, such as the methyl group on toluene, appear around ppm. The three isomers of disubstituted benzene rings can often be differentiated in the NM. All will have an integration of 4 in the aromatic region, but will have different and distinctive splitting pattern based on the symmetry of the groups. A B A B C D A B eactions of Benzene rtho substituted rings will often display a large cluster of peaks, with no salient feature. The cluster should integrate to 4. A B C D ften, the meta substituted rings will give two peaks, with one peak, usually a singlet, that integrates to 1. The para substitution pattern is the easiest to distinguish, as the left-right symmetry results in a "doublet of doublets". Because benzene is not a pure alkene, the addition reaction studied for alkenes do not work. Instead, benzene rings undergo an addition-elimination, two-step reaction process, resulting in a substitution of a hydrogen on the ring for an electrophile. The general mechanism is shown below: + Add B lim Chemistry of Aromatic Systems Page 4 of 10

5 ark College The intermediate in the reaction is a resonance-stabilized carbocation, however since aromaticity is broken in this intermediate it is a fairly high-energy intermediate. Deprotonation at the addition site regenerates aromaticity. The net reaction is energetically favorable if the substituted electrophile is more electronegative than. We will consider 5 reactions of benzene, each involving an initial step using a Lewis acid catalyst to generate the electrophile, which then adds to the ring. ften, the base used to remove the proton in the final step also regenerates the Lewis Acid catalyst. alogenation 2 Fe 3 =, Br only I 2 N 3 alogenation reactions set up the ring as a precursor for nucleophilic Grignard reactions. The halogenated ring can be turned into a Grignard nucleophile by reaction with magnesium in ether. The full mechanism is included here, including the formation of the nucleophile and regeneration of the Lewis acid catalyst. I Fe 3 3 Fe 3 Fe Nitration Sulfonation N 3 2 S 4 2 S 4 N 2 S 3 This reaction proceeds by creating an N 2 +, or nitronium, electrophile. Sulfonated aromatic rings are often water-soluble, as the pka of the sulfonic acid group is Sulfonated molecules are often isolated as sodium salts. Friedel-Crafts Acylation 1) Al 3 2) 2 The reaction proceeds through the generation of an acylium-ion electrophile, which is a positively charged carbonyl carbon. Water is required as a work-up step to remove the aluminum chloride from the carbonyl oxygen, which is also a Lewis base. Friedel-Crafts Alkylation Al 3 The electrophile in a Friedel-Crafts alkylation is a carbocation, which is susceptible to rearrangement. This reaction is best for small alkyl chlorides, such as chloromethane or chloroethane, or for 2 or 3 chlorides; any alkyl halide where rearrangement does not occur. Chemistry of Aromatic Systems Page 5 of 10

6 ark College Also, because the addition of an alkyl group activates the ring by inductively adding electron density into the ring, polyalkylation often occurs. If the desired product is a benzene with a linear chain extending off the ring, the Friedel-Crafts alkylation will not yield the desired product- the carbocation will rearrange. To obtain this product, we can proceed via the acylation, and reduce the carbonyl group to a C 2 group. Al 3 not N 2 4,! - Wolff-Kischner eduction Al 3 Zn(g),! emmenson eduction Polysubstituted ings When one group is already on the ring, where does the second group go? What considerations must be made to determine this? The identity of the initial group on the ring, or the heteroatom in an aromatic heterocycle, has ultimate control over the position of a second group. To investigate this further, we need to consider two effects resonance and inductive effects. lectrophilic Aromatic Substitution occurs via a two-step mechanism, with a carbocation intermediate. The ability to stabilize or destabilize this intermediate is key to directing the location of the second substituent. esonance ffects Stabilizing the carbocation through resonance delocalization has a profound directing effect. Substituents that have lone pairs on them can add additional resonance structures to the carbocation intermediate. ther groups that have partial positive charge next to the ring destabilize the carbocation intermediate. Let s consider electrophilic addition to two different rings: aniline and nitrobenzene. Chemistry of Aromatic Systems Page 6 of 10

7 ark College N 2 N 2 N2 N B 2 N 2 ortho + meta N 2 N 2 N 2 N 2 N 2 N 2 N 2 para When the amine is the first substituent on the ring, the lone pair on the nitrogen can be utilized for a fourth resonance structure when the second substituent is added to the ortho or para positions. Therefore, all subsituents with a lone pair right next to the ring direct the second group to the ortho and para positions. Substitution at either position can occur, with the para position preferred when either group (the original or the second) is sterically bulky. N N 2 N2 N B 2 + ortho meta para N 2 N 2 N 2 N 2 N 2 N 2 The structures in boxes are not stabletwo positive charges are next to each other. When a nitro group is on the ring, the ortho and para positions are destabilized by resonance structures that place the ring carbocation next to the positive charge on the nitro group. The meta substituted ring avoids these structures and is therefore preferred. All substituents that have full or partial positive charge next to the ring are meta directors because they avoid unstable resonance structures. Chemistry of Aromatic Systems Page 7 of 10

8 Inductive ffects ark College Inductive effects occur when a side group either donates or withdraws electron density through bonds, by bond dipoles. The carbons in the benzene ring are sp 2 -hybridized and the electronegativity of the carbons are slightly elevated because of this increased electronegativity. Therefore, other carbon groups attached to the ring are slightly electron donating, and cause a second group to add in the ortho and para positions. ther electron-withdrawing groups, such as carbonyls and sulfonates, cause a second group to add to the meta position, as the dipoles pull electron density from the ring, further destabilizing the carbocation. Activation of the ing Activating the benzene ring sets the ring up for further addition. Since this addition occurs via the carbocation intermediate (which is also the slow step of the reaction), groups that stabilize that carbocation through resonance and/or induction have a greater activating effect than groups that withdraw electron density from the ring. The ramifications of these activating effects come into play when a third group is added to the ring. If there are competing directing effects between substituents on the ring, groups that are more activating (typically electron-donating o/p directors) influence the directing effects and the only products reflect the direction of the more stongly-activating group. rtho/para Directors Strongly Activating: Alcohols (-), ethers (-) and amines (-N 1 2 ) Meta Directors Moderately Deactivating: Aldehydes and ketones, Moderately Activating: sters and Amides (bonded thru, N) N Weakly Activating: Aliphatic groups (alkanes) Weakly Deactivating: alogens (-) acid derivatives (bonded thru carbonyls) N Strongly Deactivating: Nitro groups (-N 2 ), nitriles (-CN), trifluoromethyl groups (-CF 3 ) Synthesis Flexibility and the Sandmeyer eaction The discussions of directing and activating effects provide ammunition for flexibility in synthesis and specifying a particular substituent pattern. Nitrogen substituted benzene rings illustrate the ability to control placement beautifully, which we will investigate further in this section. A benzene ring can be directly nitrated with a mixture of nitric and sulfuric acids. The addition of a nitro group places a meta director on the ring. This nitro group can then be reduced to an amine by hydrogenation with a metal catalyst, resulting in an ortho/para directing group. The addition of a second or third group can occur at any step, controlling the regiochemistry of the synthesis. N 2 N 2 N 3 2 S 4 2 /Ni meta director ortho/para director Chemistry of Aromatic Systems Page 8 of 10

9 Although this route gives some flexibility, it doesn t allow for much choice in the side group. Fortunately, the amino group can be converted in a variety of things through the formation of a diazonium salt. This new collection of reactions adds additional flexibility for synthesis. Cu =, Br CuCN CN ark College N 2 N 2 BF 4,! F NaN 2, 0 C KI 3 + I +, 3 P 2 A few example syntheses are given to show the capabilities of this synthetic tool. Prepare m-chlorophenol from benzene. Both groups are o/p directors, so we can't just directly add the groups to the ring, as we will not get the correct subsitution pattern. owever, the alcohol is made via the diazonium salt, which starts as the meta directing nitro group. N 2 N 2 N 2 N 3 2 S 4 2 Fe 3 2 /Ni 1) NaN 2, 0 C 2) 3 + Chemistry of Aromatic Systems Page 9 of 10

10 S 3 Prepare this compound. Both side groups are meta directors, and neither is made from the diazonium salt. owever, we can still use the directing powers of the nitro and amine groups, and then convert the amine back to a hydrogen. ark College N 2 N 2 N 2 N 3 2 S 4 2 S 4 2 /Ni S 3 S 3 1) NaN 2, 0 C N 2 1) Al 3, 2) 2 2) 3 P 2 S 3 S 3 Chemistry of Aromatic Systems Page 10 of 10