Benzene Chemistry (Heterocyclic Chemistry) PDF

Course: Heterocyclic Chemistry Benzene Chemistry Kekule’s Rule – Terence Habiri From its elemental composition and molecular weight, benzene was known to contain six (6) carbon atom and six (6) hydrogen atoms. Thus, benzene has a...

Course: Heterocyclic Chemistry Benzene Chemistry Kekule’s Rule – Terence Habiri From its elemental composition and molecular weight, benzene was known to contain six (6) carbon atom and six (6) hydrogen atoms. Thus, benzene has a molecular formula of C6H6, the controversial question was, how are this atoms arranged? In 1858, August Kekule, an Organic Chemist, proposed that carbon atoms can join together to from chains. Later in 1865, he offered an answer to benzene as being closed to form rings, which he later described them in 1890. As he was writing a text book, he dozed off to sleep. In his vision, he saw atoms in rows, twisting and turning in snake motion. One of the snake sized itself and form a circle. When he woke up, he gave a thought that benzene ring must be circular. Below are different alignments to describe benzene structures. H H H H H H H CH2 H H H H H H H H H Kekule Formula Dewar formula I II III CH3 CH2 H3C H2C IV VI According to proven experiments, benzene yield only one monosubstitution product, C6H5Y. For example, the one bromobenzene,C6H5Br is obtained when one hydrogen atom is replaced by bromide, similarly, there is only one chorobenzene, C6H5Cl, or one nitrobenzene, C6H5NO2 etc has been made. This fact place a severe limitation on the structure of benzene. That is, each hydrogen must be exactly equivalent to every other hydrogen, since the replacement of any one of them yields the same product. Therefore, structure (IV) is rejected since it yields two isomeric monobromo derivatives, 1-bromo, 2- bromo and all hydrogens are not equal as required above. Similarly, formula (II) and (III) will not be entertained because it does not fit the description. Benzene is also proven to yield only three (3) isomeric distribution product, C6H4Y2 or C6H4YZ. For example, three isomeric dibromobenzene, C6H4Br2 or three isomeric nitrobenzene, C6H4NO2, has ever been made, this (IV) must be rejected. Br H H H H H Br H H H H H H H Br H (I) H (II) H (III) Course: Heterocyclic Chemistry a) The diagram above shows that each Hydrogen must be other when replaced. Br Br Br H Br H H H H H H H Br H H H (I) H (II) Br (III) b)The above diagram shows that three isomeric distribution product, C6 H4Y2 can be obtained. Kekule’s proposed structure holds true when describing the form of benzene structure. Kekule intuitively anticipated some 100 years ago our present concept of delocalized electrons. Kekule’s structure has come to hold true for the alternating single and double bonds. Benzene undergoes substitution than addition, thus, Kekule’s structure is one that we call today, ‘cyclohexatriene.’ Below are some of the examples of substitution reaction that benzene participates. Nitration: N2SO4 C6H6 + NONO 2 C6H5NO 2 + H2O Sulfonation: CH4 C6H6 + HOSO3H C6H5SO3H + H2O Halogenation: Fe C6H6 + Cl 2 C6H5Cl + HCl Friedel – Crafts Alkylation: AlCl3 C6H6 + RCl C6H5R + HCl Friedel – Crafts Acylation: AlCl3 C6H6 + RCOCl C6H5COR + HCl Recently, the concept of resonance is been introduced – thus the structure of benzene differs in the arrangement of electron as shown below. (I) (II) Reference: Thornton, Robert, Organic Chemistry, 6th Edition, 1992 Prontice Hall Inc, New Jersy Course: Heterocyclic Chemistry Electrophilic Aromatic Substitution Reaction and its Mechanism Nitration of Benzene – Emerlyn Ebu Benzene reacts slowly with hot concentrated Nitric Acid. The product that is formed is known as Nitrobenzene and Nitration is the process that forms it. Nitrobenzene looks like this: NO 2 Nitrobenzene To increase the rate of Nitration process, benzene is heated with a mixture of concentrated nitric acid and concentrated Sulphuric acid. The reaction takes place around 50 – 55 °C. The concentrated Sulphuric Acid increase the rate of the reaction by increasing the concentration of the nitronium ion (NO2+) which is the electrophile. This electrophiles than attacks the benzene ring, replacing one of the hydrogen atoms in a reaction. The following is the machenism of the Nitration of Benzene: General: NO 2 + + HNO 3 50-55 + H3 O + HSO4- Steps involved: + H O NO 2 + HOSO3H H O NO 2 + HSO4- 1. H + + + H O NO 2 + H2SO4 NO 2 + H3 O 2. H 3. H H + H + slow NO 2 NO 2 NO 2 + + H NO 2 fast NO 2 + HSO4- + H2SO4 4. Note: In: Step1 – Nitric acid acts as a base and accepts a proton from Sulphuric acid. Step 2 - Protonated Nitric acid dissociates and produces a nitronium ion which react with benzene by attacting the π clouds and form an arenium ion. Step 3 – The arenium ion transfers a proton to HSO4, a base, and becomes nitrobenzene Course: Heterocyclic Chemistry Reference: Solomons Organic Chemistry, 5th Edition, pp 641 EFFECTS OF SUBSTITUENTS ON REACTIVITY AND ORIENTATION Biangau Muyupe Activating groups: Ortho-Para Directors. When substituted benzene undergo electrophilic attack, groups already on the ring affect both the rate of the reaction and the site of attack, we say therefore that substituent group affect both reactivity and orientation in electrophilic substituent. According to their influence on their reactivity of ring, those that cause the ring to be more reactive than benzene itself. We call activating group according to the way they influence orientation substituent in one class that tend to bring about electrophilic substituent primarily at the position of ortho and para to themself are known as ortho-para directors. The methyl group is an activating group and an ortho-para director. Toluene reacts considerably faster than benzene in a electrophilic substitution. CH3 an activating group More reactive than benzene toward electrophilic substitiution When toluene undergoes electrophilic substitution, most of the substitution takes place at its ortho and para position. For example when toluene is nitrated with nitric acid and sulfuric acid, we get mononitrotoluene in the following properties. ortho para meta CH3 CH3 CH3 CH3 NO 2 + + NO 2 o-Nitrotoluene m-Nitrotoluene (4%) NO 2 (59%) p-Nitrotoluene (37%) Of the mononitrotoluene obtained from the reaction 96% (59% + 37%) has the nitro group in an ortho or para position only 4% has nitro group in the meta position. Course: Heterocyclic Chemistry The same behaviour is observed in halogenation, sulphonation and so forth. All alkyl groups are activating groups and they also ortho-para directors. The methoxyl group CH3-, and the acetamide group CH3CONH-, are strong activating groups and both are ortho-para directors. The hydroxyl group and amino groups are powerful activating groups and are also powerful ortho-para directors. Phenol and aniline react with bromine of water to produce product in both the ortho position and para position are substituted. This tribromo products are obtained in nearly quantitative yield. NH2 NH2 Br Br Br2 H2O Br 2,4,6-Tribromophenol (~100%) OH OH Br Br Br2 H2O Br 2,4,6,-Tribromophenol (~100%) Course: Heterocyclic Chemistry Limitations Of Friedel-Crafts Reaction Kurande Wangum Several restrictions limit the usefulness of Friedel-Craft reactions. 1. Multiple Alkylation and Rearrangement. A complication can arise if the product bears more than one alkyl substituent on the benzene nucleus. At room temperature or above, polyalkyl benzenes tend to undergo a different kind of rearrangement in the presence of a halogen acid and aluminum chloride. The alkyl residues can migrate on the benzene ring. For example, 1, 2, 4- trialkyl benzenes rearrange to 1, 3, 5- trialkyl benzenes. That product is favored which is least hindered and forms the most stable sigma complex by protonation; In the first step, proton transfer to the benzene generates a sigma complex. A 1, 2 alkyl shift occurs rapidly and reversibly with this carbocation. A succession of 1, 2 alkyl and 1, 2 hydride shifts leads to the most stable trisubstituted sigma complex with an alkyl group at each ring site that bears positive charge. This stable complex does not further the Substitution in alkylation reaction. Mechanism 1. Multiple Alkylation and Rearrangement R R H H H H H R R H R R + + + HCl + AlCl3 + AlCl4- R + R R R 2. Polyacylation O CH3 CH3 AlCl3 + + H ,H2O + HCl H3C Cl H3C O Course: Heterocyclic Chemistry Heterocyclic Chemistry Rachael Enne ( 4 n + 2 ) π Electron Rule Huckels Rule is named after a German physicist, Erich Huckel. He concerned himself with compounds containing one planar ring in which each atom has a p orbital as in benzene. His calculation shows that planar monocyclic rings containing ( 4 n + 2) π electrons where n =0,1,2,3… ( ie..rings containing 2,6,10,14, etc…electrons) have closed shells of delocalized electrons like benzene and should have substantial resonance energies. In other words planar monocyclic rings with 2, 6, 10, 14 …etc delocalized electrons should be aromatic. A regular polygon corresponding to the ring of the compound being considered so that one corner of the polygon is at the bottom. The points where the corner of the polygon touch the circle corresponds to the energy levels of the π molecule orbitals of the system. Polygon in Circle The π moleculer orbitals were plannar, unlike benzene, this molecule is predicted to have two non bonding orbitals and because it has 8 π electrons it would have not be expected to be aromatic. Course: Heterocyclic Chemistry ALKYL (ARENES) Pricilla Tipitap ARENES are aromatic hydrocarbons that consist of both aliphatic and aromatic groups. ARYL GROUP is derived from an arene by removal of a hydrogen atom and its symbol is Ar- or represented by ArH. Benzoid Arenes Reations are the substitution reactions that occur when they react with electrophile reagents. General type: Ar – H + E – A Ar – E + H – A E + E-A + H-A ELECTROPHILES are either a positive ion (E+ ) or some other electron deficient species with a large partial positive charge. Example: Benzene can be brominated when it reacts with bromine in the presence of FeBr3. Bromine and FeBr3 react to produce positive bromine ions, Br+. These positive bromine ions act as electrophiles and attack benzene ring, replacing one of the hydrogen atoms in a reaction that is called electrophilic aromatic substitution (EAS). Electrophilic Aromatic Substitution allows the direct introduction of wide variety of groups onto an aromatic ring and because of this they provide synthetic routes to many important compounds. ELECTROPHILIC AROMATIC SUBSTITUTION REACTIONS Course: Heterocyclic Chemistry X X2, FeX3 Halogenation (X=Cl,Br) + HX NO 2 HONO 2 H2SO4 + H2O Nitration SO3H Sulfonation SO3 H2SO4 R Frieldel - Crafts Alkylation RCl, AlCl3 (R - can rearrange) + HCl COR Frieldel - Crafts Acylation RCOCl, AlCl 3 + HCl BENZENE is susceptible to electrophilic attack primarily because of its exposed  electrons. - resembles an alkene for alkene reaction with an electrophile site of attack is exposed  bond. - differs from alkene where benzene’s closed shell of six  electrons give it a special stability. So although benzene is susceptible attack, it undergoes substitution reaction rather than addition reactions. Substitution reactions allow the aromaticsexlet of  electrons to be regenerated after attack by the electrophile has occurred. Electrophiles attack the  system of benzene to form a nonaromatic carbocation known as an arenium ion ( complex). Kekule structures used make it easier to keep track of the  electrons. Step 1: + E E E H H H E-A + + - the electrophile takes two electrons of the six-electrons  system to form a  bond to one carbon atom of the benzene ring. Formation of this bond interrupts the cyclic system of  electrons, because in the formation of the arenium ion the carbon that forms a bond to the electrophile becomes sp3 hybridized and therefore fo p orbital available. Now only five carbon atoms of the ring are still sp2 hybridyzed and still have p orbitals. Four  electrons of arenium ion are delocalised through these five p orbitals. Positive charge distributed in the arenium ion ring shown by calculated electrostatic potential map. For arenium ion formed by electrophilic addition of bromine to benzene. Course: Heterocyclic Chemistry Step 2: + E E H + H-A A- - proton is removed from carbon atom of the arenium ion that bears the electrophile. The two electrons that bonded this proton to carbon become a part of the  system. The carbon atom that bears the electrophile becomes sp2 hybridized again, and a benzene derivative with six fully delocalised  electrons is formed – represented by any one of the resonance structures for the arenium ion. Mechanism shown using Modern Formula for Benzene: Step 1. E + E-A + H Step 2. E E + H + H-A + A Experimental evidence show that arenium ion is a true intermediate in electrophilic substitution reactions – not a transition state and therefore lies in an energy valley between two transition states. Free energy of activation , ∆ G‡ (1), for the reaction leading from benzene and the electrophile, E+ , to the arenium ion has shown to be much greater than the free energy of activation, ∆ G‡ (2), leading from arenium to the final product. The reaction leading from benzene and an electrophile to the arenium ion is highly endothermic, because benzene ring loses its resonance energy. Reaction leading from arenium ion to the substituted benzene, by contrast, is highly exothermic because in it the benzene ring regains its resonance energy. Free Energy H - A E- A E Step1. Course: Heterocyclic Chemistry + E + A- Slow rate determining H E Step 2. CH3 + + H-A fast E H E A- ARENES Alkybenzenes , some examples Toluene, ethylbenzene and isopropylbenzene. CH3 C2CH 3 CH(CH 3)2 CH=CH 2 Methylbenzene Ethylbenzene Isopropylbenzene Phenylethene (cumune) (styrene) (toluene) Alkenylbenzene- phenylethene(styrene). The aliphatic portion of these compounds is commonly called the side chain. Benzyl radical – hydrogen abstraction from the methyl group of methyl – benzene (toluene) produces a radical. H3C H2C H3C Benzylic Carbonl Toluene Benzyl Radical Course: Heterocyclic Chemistry - applies to all radicals that have an unpaired electron on the side chain carbon atom that is directly attached to the benzene ring. Benzylic hydrogen atoms – hydrogen atoms of the carbon atom directly attached to the benzene ring. Benzylic cation – produced by departure of a leaving group (LG) from benzylic position. H H CH3 + -LG- A Benzylic cation HALOGENATION OF BENZENE George Nori’s Edit Benzene reacts with halogens only in the presence of LEWIS acids. EXAMPLE 1 Cl + FeCl3 Cl2 * + HCl 25 C EXAMPLE 2 Br FeBr2 + Br2 heat + HBr Bromobenzene (75%) The most commonly used LEWIS acids to facilitate chlorination and bromination are; ▪ FeCl3 ▪ FeBr3 ▪ AlCl3 Course: Heterocyclic Chemistry All of these LEWIS acids exist in anhydrous form. In the reaction, Ferric chloride and Ferric bromide is generated by adding iron to it. Hence, Iron reacts with halogen producing ferric halide. 2Fe + 3X2 → 2FeX2 A MECHANISM FOR AROMATIC BROMATION Step 1; Bromine combines with FeBr3 to form a complex that dissociates forming a positive bromine ion and FeBr-4 Br Br + FeBr3 Br Br FeBr3 Br + Br FeBr3 Step 2; the positive bromine ion attacks benzene forming an arenium ion H H H Br Br Br slow + Br Step 3; A proton is removed from the arenium ion to finally become Bromobenzene Br FeBr H Br Br H Br FeBr3 SUMMARY ▪ In step 1, the function of the Lewis acid is portrayed. The ferric bromide reacts with bromine to produce a positive bromine ion, Br+ ( and FeBr4- ) ▪ In step 2, this Br+ ion attacks the benzene ring to produce an arenium ion. Course: Heterocyclic Chemistry ▪ In step 3, a proton is removed from the arenium ion by FeBr4-. This results in the formation of Bromobenzene and hydrogen bromide, the products of the reaction and simultaneously regenerating the catalyst, FeBr3. ▪ TERMS Lewis acid: – a molecule or an atom that accepts a pair of electrons. Arenium ion: - a nonaromatic carbocation that results from the electrophillic attack of the pi system of the benzene ring. Sulfonation of Benzene Benzene reacts with fuming sulfuric acid at room temperature to produce benzenesulfonic acid. This process or reaction is better known as Sulfonation of Benzene. Sulfonation also takes place in concentrated sulfuric acid alone, but it is slow. Fuming sulfuric acid in which benzene reacts with is the sulfuric acid that contains added sulphur trioxide (SO3). The reaction can be simply illustrated as:............ O 25o + S Conc. H2SO4.. O O O S O O Sulfur trioxide H Benzenesulfonic acid (56%) The electrophile appears to be sulphur trioxide which means it is an electron-pair acceptor, or an electron seeking reagent. A mechanism for the reaction as it occurs step by step is illustrated as: Step 1: 2H2SO4 SO3 + H3O+ + HSO4- This equilibrium produces SO3 in concentrated H2SO4 Step 2: Course: Heterocyclic Chemistry.. O.. O.....- + S CH3 S Other resonance O O O O structures SO3 is the actual electrophile that reacts with benzene to form an arenium ion. Step 3:......... O.... O. Fast... +.... - - HSO4 - + S O S O H2SO4 O O A proton is removed from the arenium ion to form the benzenesulfonate ion. Step 4:... O.... O.......... + Fast O H S - + H - S + H2O O O H O O The benzenesulfonate ion accepts a proton to become benzensulfonate acid. All of the steps are equilibra. This means that the overall reaction is an equilibrium as well. In concentrated sulfuric acid , the overall equilibrium is the sum of steps 1-4. Since all of the steps are equilibria.the position of equilibrium can be influenced by the conditions we employ. If we want to sulfonate benzene we use concentrated sulfuric acid or fuming sulfuric. Under these conditions the positon of equilibrium lies to the right, therefore we obtain benzenesulfonic acid. On the other hand , we may want to remove a sulfonic acid group from a benzene ring. This is done by employing sulfuric acid (dilute) and pass steam through the mixture. Under this conditions, with a high concentration of water, the equilibrium lies to the left and desulfonation occurs. Course: Heterocyclic Chemistry PHENOLS A phenol is any of a class of organic compounds with a hydroxyl group attached to a carbon atom in a ring of an aromatic compound. The simplest of the class is phenol (C6H5OH). OH phenol Physical properties of Phenols Phenols have melting points of (182°C) and boiling point of 43°C. Phenols are similar to alcohols but form stronger hydrogen bonds with water, so they dissolve more readily in water and boil at higher temperatures. They may be colorless liquids or white solids; many have a sharp, spicy odor. Some are found in essential oils. Phenol is a member of a group of compounds that are considered as derivatives of water in which hydrogen atoms are replaced by hydrocarbon groups. In phenol, one hydrogen atom is replaced by an aryl group (an aryl group is any aromatic group). The table below shows the basic physical properties of phenols: Phenol is soluble in organic solvents and slightly soluble in water at room temperature, but infinitely soluble above 66° C (150.8° F). Course: Heterocyclic Chemistry There are several members of this group. They come under certain classifications; In many compounds phenol is the base name: Cl Br NO 2 OH OH OH 4-chlorophenol 2-nitrophenol 3-bromophenol (p-chlorophenol) (o-chlorophenol) (m-chlorophenol) The methyl phenols commonly called cresols: CH3 CH3 CH3 OH OH OH o-cresol m-cresol p-cresol (2-Methylphenol) (3-Methylphenol) (4-Methylphenol) The benzenediols also have common names: OH OH OH OH OH OH pyrocatechol resorcinol hydroquinone (1,2-Benzenediol) (1,3-Benzenediol) 1,4-Bensenediol) There are also naturally occurring phenols. For example tyrosine, an amino acid in proteins, methyl salicylate found in oil of wintergreen, eugenol found in oil of cloves, and thymol found in thyme are all naturally occurring phenols. Course: Heterocyclic Chemistry The functional group of phenols is the hydroxyl group and is attached to an aromatic ring. Phenols, like alcohols have relatively high boiling points for their molecular weight, as a consequence of intermolecular hydrogen bonding between the hydroxyl groups. For example, phenol (bp 182°C) has a boiling point more than 70°C higher than toluene (bp 110.6°C), even though the two compounds have almost the same molecular weight. Phenols are much stronger acids than the alcohols, mainly as a consequence of charge delocalization in phenoxide ions as a consequence of resonance. Phenols can be converted to phenoxides not only by reactive metals such as sodium, but also by reactions with aqueous bases such as sodium or potassium hydroxide. Phenols can be separated from alcohols by making use of their difference in acidity. Synthesis of Phenols 1. Laboratory synthesis Phenols are synthesized by hydrolysis of arendiazonium salts and are carried out in mild conditions. The general reaction for this is; + Cu2O Ar NH2 HONO Ar N2 Ar OH 2+ Cu , H2O Example; NH2 OH Br Br (1) NaNO2, H2SO4 (0 -5oC) (2) Cu2O, Cu2+, H2O CH3 CH3 2-Bromo-4-methylphenol (80-92%) 2. Industrial synthesis Phenol is an important chemical in industry in the production of commercial products ranging from aspirin to a variety of plastics. ▪ Hydrolysis of Chlorobenzene (Dow process) Course: Heterocyclic Chemistry Cl ONa o 350 C + 2NaOH (high pressure) + NaCl + H2O Sodium phenoxide ONa OH o 350 C + HCl (acidification) + NaCl Sodium phenoxide ▪ Alkali fusion of Sodium Benzenesulfonate SO3Na ONa + 2NaOH + Na 2SO3 + H2O Sodium Benzenesulfonate ONa OH HCl + NaCl 3. From Cumen hydro peroxide This process involves the conversion of two inexpensive organic compounds- benzene and propene- into two more valuable ones- phenol and acetone. Reaction 1: H3C CH3 250o + H2C CHCH 3 H3PO4 pressure Cumen Course: Heterocyclic Chemistry Reaction 2: CH3 CH3 95-135oC H5C6 CH3 + O2 H5C6 O OH CH3 CH3 Reaction 3: H3C CH3 H3O+ H5C6 O OH 0 C6H5OH + O 50-90 C CH3 H3C phenol acetone Reactions of Phenols Phenols readily undergo aromatic electrophilic substitution reactions. The hydroxyl group is ring- activating and ortho - para directing. Phenols with large groups ortho to the hydroxyl group are used as antioxidants, a consequence of their ability to ‘trap’ peroxy radicals. Examples; Aromatic substitution in phenols. OH OH Br Br H2O + 3Br2 Br Oxidation of phenols. CH3 O Cr6+ CH3 O p-xylene benzo-1,4-quinone Strength of Phenols as acids. Phenols are stronger acids compared to alcohols having pKa values smaller than 11. Example: Course: Heterocyclic Chemistry OH OH Cyclohexanol (pKa = 18) Phenol (pKa = 9.89) Some other examples are given below showing only the pKa values of phenols. Phenols are acidic because the benzene ring of phenol withdraws electrons from OH, and makes O positive, which makes a characteristic of acids. Other reactions Phenols react with carboxylic acid anhydrides and acid chlorides to form esters. Resonance structure for phenols Distinguishing and separating phenols from alcohols and carboxylic acids: ▪ Phenol dissolves in NaOH, whereas most 6-carbon alcohols do not. ▪ Most phenols are insoluble in aqueous NaHCO3, but carboxylic acids are soluble. Thus, aqueous NaHCO3 provides a method for distinguishing and separating most phenols from carboxylic acids. Course: Heterocyclic Chemistry THE STABILITY OF BENEZENE Jonathan Wambo Benzene shows unusual reactions by undergoing substitutions and when on the basis of Kekule structure it is expected to undergo addition.Kekule suggested that carbon atom of Benzene are in the ring and that they are bonded to each other by alternating a single bond and a double bond and one hydrogen atom id bond id attached to each carbon atom.However,Benzene atom is more stable than Kekule structure if considered to follow thermochemical results. Cyclohexane, a six member ring containing double bond can be hydrogenated easily to cyclohexane when Δ° for this reactions is measured and found to -120kjmol-1 like that of and similarly substituted alkene. + Δ°=-120kjmol-1 Cyclohexane cyclohexane The hydrogenation of 1,3-cyclohexadiene would be twice as much as the heat thus have a Δ° equal to about -240kjmol-1.In which that was the experiment done and ;the result was -232kjmol-1.This results is quite similar to what has been already done and the difference is taken into account that the compound containing conjugate double bond are usually more stable than those that contain isolated double bond. A chemical alkadienes are chemically more stable than the isomeric isolated alkadienes.Two examples of this extra stability of conjugate dienes can be in a analysis of the heat of hydrogenation in the table given below. HEAT OF HYDROGENATION OF ALKENES AND ALKADIENES: COMPOUND H2 (MOL) Δ° (KJMOL-1) 1-Butene 1 -127 1-Pentene 1 -126 Trans-2-Pentene 1 -115 1,3,Buitadiene 2 -239 Trans-1,3,Pentadiene 2 -226 Course: Heterocyclic Chemistry 1,4,-Pentadiene 2 -254 1,5,hexadiene 2 -226 Pt + 2H2 1,3-cyclohexadiene cyclohexane calculated Δ°=(2x- 120)=240kjmol-1 observed) Δ°=-232kjmol-1 Figure 14.2:shows the relative stability of cyclohexane,1,3,cyclohexandiene,1,3,5-cyclohexatriene (hypothetical) and Benzene. Resonance (stabilization) energy =152kjmol-1 +3H2 +2H2 Benzene +3H2 +H2 Δ°=-120kjmol-1 Δ°=-232kjmol-1 Δ°=-360kjmol-1 Δ°=-208kjmol-1 Cyclohexane However, if the idea is widely extended, Benzene is simply 1,3,5-cyclohexatriene which would be predicted that benzene is up is up to approximately 360kjmol-1(3x-120)when it is hydrogenated. The reactions is exothermic but only 208mol-1. Course: Heterocyclic Chemistry + 3H2 Pt Benzene cyclohexane According to figurre.14.1,it is obvious that benzene is more stable was calculated. Indeed it is more stable than the hypothetical 1,3,5-cyclohaxetriene by 152kjmol-1.thde difference can be seen in the amount of heat that is actually release which is calculated on Kekule structure which is called resonance energy of the compound. Calculation: Δ°=(3x-120) =-360kjmol-1 Observed Δ°=-208kjmol-1 Difference =152kjmol-1 Course: Heterocyclic Chemistry ALKENYLBENZENES James Popon Stability of conjugated Alkenylbenzenes Alkenylbenzenes that have their side- chain double bond conjugated with the benzene ring are more stable than those that do not. C C C C C C more stable than Conjugated system Nonconjugated system Part of the evidence for this comes from acid-catalyzed alcohol dehydrations, which are known to yield the most stable alkene. For example, dehydration of an alcohol such as the one that follows yields exclusively the conjugated system. H C H H C C HA,heat C C C (-H2O) OH Because conjugation always lowers the energy of an unsaturated system by allowing the П (pi) electrons to be delocalized, this behavior is just what we would expect. Additions to the Double Bond of Alkenylbenzenes In the presence of peroxides, hydrogen bromide adds to the double bond of 1-phenylpropene to give 2- bromo-1-phenylpropane as the major product. CH HBr CHCH3 CHCH2CH3 peroxides Br 1-Phenylpropene 2-Bromo-1-phenylpropane Course: Heterocyclic Chemistry In the absence of peroxides, HBr adds in just the opposite way. CH HBr CHCH2CH3 CHCH3 (no peroxides) Br 1-phenylpropene 1-Bromo-1-phenylpropane The addition of hydrogen bromide to 1-phenylpropene proceeds through a benzylic radical in the presence of peroxides and through a benzylic cation in their absence. Oxidation of the Side Chain Strong oxidizing agents oxidize toluene to benzoic acid. The oxidation can be carried out by the action of hot alkaline potassium permanganate. This method gives benzoic acid in almost quantitative yield. O CH3 COH - (1) KMnO4,OH ,heat (2)H3O+ Benzoic acid (~100%) An important feature of side-chain oxidations is that oxidation takes place initially at the benzylic carbon; Alkenylbenzenes with alkyl groups longer than methyl are ultimately degraded to benzoic acids. Course: Heterocyclic Chemistry O CH2CH2CH2R C OH (1)KMnO4,OH- heat (2)H3O+ An alkylbenzene Benzoic acid Side-chain oxidations are similar to benzylic halogenations, because in the first step the oxidizing agent abstracts benzylic hydrogen. Once oxidation is begun at the benzylic carbon, it continues at that site. Ultimately, the oxidizing agent oxidizes the benzylic carbon to a carboxyl group, and, in the process, it cleaves off the remaining carbon atoms of the side chain. Side-chain oxidation is not restricted to alkyl groups. Alkenyl, alkynyl, and acyl groups are oxidized by hot alkaline potassium permanganate in the same way. C6H5CH CHCH3 or O C6H5CH CCH3 (1) KMnO4,OH-,heat C6H5COH (2) H3O or O C6H5CCH2CH3 Oxidation of the Benzene Ring The benzene ring of an alkylbenzene can be converted to a carboxyl group by ozonolysis, followed by treatment with hydrogen peroxide: O (1) KMnO4,OH-,heat R C6H5 R COH (2) H2O2