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UNIT 5.pdf chemistry department ppt by amare

Chemistry within a few hours so 5-10 can get it out about it and behavior with a bit more of the flask using the above upgrade to be chosen by Abdii and behavior with a new one and

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Chapter 5 Aromatic Chemistry
• Benzene (C6H6) is the simplest aromatic hydrocarbon (or arene). • Benzene has four degrees of unsaturation, making it a highly unsaturated hydrocarbon. • Although unsaturated hydrocarbons such as alkenes, alkynes and dienes readily undergo addition reactions, benzene does not. Introduction to Aromatic Compounds
• Benzene does react with bromine, but only in the presence of FeBr3 (a Lewis acid), and the reaction is a substitution, not an addition. • Proposed structures of benzene must account for its high degree of unsaturation and its lack of reactivity towards electrophilic addition. • August Kekulé proposed that benzene was a rapidly equilibrating mixture of two compounds, each containing a six-membered ring with three alternating  bonds. • In the Kekulé description, the bond between any two carbon atoms is sometimes a single bond and sometimes a double bond. Introduction to Aromatic Compounds
• These structures are known as Kekulé structures. • Although benzene is still drawn as a six-membered ring with alternating  bonds, in reality there is no equilibrium between the two different kinds of benzene molecules. • Current descriptions of benzene are based on resonance and electron delocalization due to orbital overlap. • In the nineteenth century, many other compounds having properties similar to those of benzene were isolated from natural sources. • Since these compounds possessed strong and characteristic odors, they were called aromatic compounds. It should be noted however, that it is their chemical properties, and not their odor, that make them special. Introduction to Aromatic Compounds
Any structure for benzene must account for the following facts: 1. It contains a six-membered ring and three additional degrees of unsaturation. 2. It is planar. 3. All C—C bond lengths are equal. The Structure of Benzene The Kekulé structures satisfy the first two criteria but not the third, because having three alternating  bonds means that benzene should have three short double bonds alternating with three longer single bonds.
• The resonance description of benzene consists of two equivalent Lewis structures, each with three double bonds that alternate with three single bonds. • The true structure of benzene is a resonance hybrid of the two Lewis structures, with the dashed lines of the hybrid indicating the position of the  bonds. • We will use one of the two Lewis structures and not the hybrid in drawing benzene. This will make it easier to keep track of the electron pairs in the  bonds (the  electrons). The Structure of Benzene
• In benzene, the actual bond length (1.39 Å) is intermediate between the carbon—carbon single bond (1.53 Å) and the carbon—carbon double bond (1.34 Å). The Structure of Benzene
Aromaticity The Criteria for Aromaticity (Hückel’s Rule) Points for discussion Discuss the terms aromatic, antiaromatic and nonaromatic. According to this rule any molecule (species) is considered as aromatic if it is planar , cyclic, completely conjugated and contains 4n+2π pi-electrons in the ring. Thus aromatic species must satisfy the following four structural criteria. [1] A molecule must be cyclic. • To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms. 8
[2] A molecule must be completely conjugated  Having a cyclic structure is not a sufficient condition for a molecule to be aromatic. There must be a complete conjugation. To do so, each and every carbon atom in the ring contain a p orbital. Why? 9
[3] Aromatic compound must be planar  This is to enable all adjacent p-orbitals to be aligned parallel to each other that results in delocalization of pi-electron density in the ring. Example  Benzene is planar and its p-orbitals are parallel to each other that result in delocalization of pi-electrons making benzene an aromatic compound whereas this case is different for cyclooctatetraene. Cyclooctatetraene is not planar and delocalization of pi-electrons is not possible. Thus, it is not aromatic Since cyclooctatetraene is non-planar, it is not aromatic, and it undergoes addition reactions just like those of other alkenes. Why? 10
[4] Huckle’s Rule (the 4n+2π Electron rule) • For a molecule to be aromatic, it should satisfy 4n+2π pi-electron rule. Thus, planarity of monocyclic compounds and its conjugation will be aromatic if it contains 4n+2π pi-electrons where n is an integer, n= 0, 1,2, 3, 4, 5, … ∞. Points for Discussion benzene is aromatic and specially stable but cyclobutadiene is antiaromatic and specially unstable. Why? What are the ways to demonstrate this stability differences? 11
 Monocyclic hydrocarbons with alternating single and double bonds are called annulenes.  Cyclobutadiene and cyclooctatetraene are not aromatic, because they have an even number of pi electron pairs  Cyclooctatetraene is a stable compound because it is large enough to form a tub shape, thereby removing degenerate orbitals Aromaticity
Some Aromatic Heterocyclic Compounds  A heterocyclic compound has ring atoms other than carbon  The heteroatom donates either one or two electrons to the  system Heteroatom donates two electrons Heteroatom donates one electron Aromaticity
14  Some Aromatic ions  If they fulfills the criteria of aromaticity, charged species could be considered as aromatic species Fore example: ☺Try Yourself 1. Draw all resonance structure for cyclopropenyl cation, cyclopentadienyl anion and cycloheptatrienyl cation 2. How do you account for the observed acidic characters of cyclopentadiene and ability of cycloheptatriene to form ionic salts?
Polycyclic Aromatic Compounds  Aromatic compounds can have rings that share a set of carbon atoms (fused rings)  Compounds from fused benzene or aromatic heterocycle rings are themselves aromatic 15
16 Class Activity Determine whether the following compounds are aromatic, anti- aromatic or non-aromatic. Explain your answers.
• To name a benzene ring with one substituent, name the substituent and add the word benzene. • Many monosubstituted benzenes have common names which you must also learn. Naming Aromatic Compounds
• There are three different ways that two groups can be attached to a benzene ring, so a prefix—ortho, meta, or para—can be used to designate the relative position of the two substituents. ortho-dibromobenzene or o-dibromobenzene or 1,2-dibromobenzene meta-dibromobenzene or m-dibromobenzene or 1,3-dibromobenzene para-dibromobenzene or p-dibromobenzene or 1,4-dibromobenzene Cont…
• If the two groups on the benzene ring are different, alphabetize the names of the substituents preceding the word benzene. • If one substituent is part of a common root, name the molecule as a derivative of that monosubstituted benzene. Cont…
For three or more substituents on a benzene ring: 1. Number to give the lowest possible numbers around the ring. 2. Alphabetize the substituent names. 3. When substituents are part of common roots, name the molecule as a derivative of that monosubstituted benzene. The substituent that comprises the common root is located at C1. Cont…
• A benzene substituent is called a phenyl group, and it can be abbreviated in a structure as ―Ph-‖. • The benzyl group, another common substituent that contains a benzene ring, differs from a phenyl group. Cont…
 At the beginning of this Chapter it was stated that the principal reaction of benzene and its derivatives is substitution rather than addition.  Indeed, electrophilic substitution in aromatic systems is one of the most important reactions in chemistry and has many commercial applications.  The П-electron cloud above and below the plane of the benzene ring is a source of electron density and confers nucleophilic properties on the system.  Thus, reagents that are deficient in electron density, electrophiles are likely to attack, whilst electron-rich nucleophiles should be repelled and therefore be unlikely to react. 22 AROMATIC SUBSTITUTION REACTIONS
 Therefore, electrophilic aromatic substitution reaction is energetically more favorable and nucleophilic aromatic substitution reaction is not common, but it does occur in certain circumstances.  Furthermore, in electrophilic substitution the leaving group is a proton, H+, but in nucleophilic substitution it is a hydride ion, H-.  In simple terms, electrophilic aromatic substitution proceeds in two steps.  Initially, the π-bond electrons in benzenes attack a strong electrophile and lose its aromaticity to give a resonance stabilized carbocation, called a sigma complex 23 Electrophilic Aromatic substitution reactions (EAS)
 In the second step loss of the proton on the tetrahedral carbon atom of the sigma complex helps to regain the aromaticity.  The overall reaction is the electrophilic aromatic substitution reaction  General Mechanism of EAS reaction 24 H E E H E + H-base :base
1. Application of Electrophilic Aromatic Substitution reaction  There are five common electrophilic aromatic substitution reactions A. Halogenation B. Nitration C. Sulfonation D. Friedel–Crafts alkylation E. Friedel–Crafts acylation
A. Halogenations of Benzene Bromination of Benzene  follows the same general mechanism for the electrophilic aromatic substitution reaction.  Bromine itself is not electrophilic enough to react with benzene.  But the addition of a strong Lewis acid (electron pair acceptor), such as FeBr3, catalyses the reaction, and leads to the substitution product. 26 Cont….
27 Mechanism of Bromination of Benzene
Bromination of benzene 28 Cont….
B. Nitration of Benzene 29  Nitration of benzene using a mixture of HNO3 and H2SO4 gives the target product rapidly at lower temperatures Benzene Nitrobenzene  Sulfuric acid protonates the hydroxyl group of nitric acid, allowing it to leave as water and form a nitronium ion (+NO2), a powerful electrophile.  The nitronium ion reacts with benzene to form a sigma complex. Loss of a proton from the sigma complex gives nitrobenzene
Mechanism of Nitration
C. Sulfonation of Benzene  Sulphonation is a reaction that involves substitution of one or more H atom of aromatic compounds by sulphonic group (-SO3H) to form benzene sulfonic acid (aryl sulfonic acid).  Aryl sulfonic acid can be easily synthesized by an electrophilic aromatic substitution using sulfur trioxide (SO3) as the electrophile  The reaction starts with the protonation of one molecule of sulfuric acid by another and the loss of a molecule of water.  Step 1:Generation of sulfur trioxide  Step 2:Attack of the electrophile 31
32 Step 3: Loss of proton regenerates aromaticity Step 4: Protonation of sulfonate group gives the product
D. Friedal-Crafts Alkylation of benzene In the presence of Lewis acid catalysts such as aluminum chloride (AlCl3) or ferric chloride (FeCl3), alkyl halides react with benzene to give alkyl benzenes . This reaction is called the Friedel-Crafts alkylation. Example  Step 1: Formation of carbocation.  Step 2: Attack of the electrophile 33
34 Step 3: Loss of proton gives the alkylated product The Lewis acid catalyst is regenerated in the last step. For secondary and tertiary halides, the reactive species probably is the free carbocation. Whereas for primary alkyl halides (which cannot form stable carbocations) the electrophilic species is a complex of the Lewis acid and the alkyl halide. In this complex, the C-X bond is weakened (dashed line), and there is considerable positive charge on the carbon (but not a free carbocation).
Unexpected Product? CH3CH2CH2CH2Cl, AlCl3 CH2CH2CH2CH3 CHCH2CH3 CH3 minor product major product CH3CH2CH2CH2 Cl AlCl3 CH3CH2CHCH2 H CH3CH2CHCH3 hydride shift 2o 1 o Carbocations Rearrange…
1o RX Typically Undergoes Shift
E. Friedal-Crafts Acylation of benzene  In the presence of aluminum chloride, an acyl chloride reacts with benzene to give acyl benzene.  The Friedel-Crafts acylation is analogous to the Friedel-Crafts alkylation, except that the reagent is acyl chloride instead of an alkyl halide and the product is acyl benzene instead of alkyl benzene  An acyl group is a substituent which contains an alkyl group bonded to a carbonyl group. C O R acyl R C O Cl AlCl3 C O R + HCl 37
Formaton of Electrophile C O R Cl AlCl3   + - R C O R C O AlCl4 acylium ion Mechanism Cont….
 Substituted benzenes undergo the electrophilic aromatic substitution reactions such as halogenation, nitration, sulfonation, alkylation and acylation.  Some substituents make the ring more reactive and some make it less reactive than benzene toward electrophilic aromatic substitution.  The rate determining step of an electrophilic aromatic substitution reaction is the formation of a carbocation intermediate.  So substituents that are capable of donating electrons into the benzene ring can stabilize the carbocation intermediate, thereby increasing the rate of electrophilic aromatic substitution. • In contrast, substituents that withdraw electrons from the benzene ring will destabilize the carbocation intermediate, thereby decreasing the rate of electrophilic aromatic substitution. 39 Directing Effects of Substituents How do derivatives of benzene behave towards electrophilic attack?
40 ˃ ˃ Electron donating group activates the ring Electron withdrawing group deactivates the ring  Substituents can donate electrons into a benzene ring or can withdraw from benzene ring either by inductive effect or resonance effect. Alkyl substituents that are bonded to a benzene ring can donate electrons inductively. Donation of electrons through a σ-bond is called inductive electron donation. Withdrawal of electrons through a σ-bond is called inductive electron withdrawal. For example methyl group is an electron donating group because of hyperconjugation and NH3+ group is an electron withdrawing group because it is more electronegative than a hydrogen.
 Substituents such as OH, OR and Cl have a lone pair on the atom that is directly attached to the benzene ring. This lone pair can be delocalized into the ring. 41 Substituents such as C=O, C≡N and NO2 withdraw electrons by resonance. These substituents also withdraw electrons inductively because the atom attached to the benzene ring is more electronegative than a hydrogen.
42 Substituents that make the benzene ring more reactive toward electrophilic substitution, by donating electrons into the benzene ring, are called the activating groups. In contrast, substituents that make the benzene ring less reactive toward electrophilic substitution, by withdrawing electrons from the benzene ring, are called the deactivating groups. EDG EWG electron donating groups electron withdrawing groups activate ring deactivate ring Deactivating group Activating group
43 Substituted benzene undergoes an electrophilic substitution reaction to give an ortho-isomer, a meta-isomer, a para-isomer or mixture of these isomers. The substituent already attached to the benzene ring determines the location of the new substituent.  All activating substituents and weakly deactivating halogens are ortho-para directors, and all substituents that are more deactivating are meta directors.  When substituted benzene undergoes an electrophilic substitution reaction, an ortho-substituted carbocation, a meta-substituted carbocation, and a para-substituted carbocation can be formed.  The relative stabilities of the three carbocations determine the preferred pathway of the reaction.
44  The methoxy substituent (an activating group), for example, donates electron into the ring and stabilize the ortho- and para- substituted carbocations as shown.  Therefore, the most stable carbocation is obtained by directing the incoming group to the ortho and para positions. Thus, any substituent that donates electrons is an ortho-para director. OCH3 OCH3 OCH3 OCH3 NO2 NO2 NO2 ortho (45%) meta (<0.1%) para (55%) HNO3 H2SO4 Fore example
45 Specially stable Specially stable
46  In contrast, nitro substituent (a deactivating group), for example, withdraws electron from the ring and destabilize the ortho- and para-substituted carbocations as shown below.  Therefore, the most stable carbocation is obtained by directing the incoming group to the meta position.  Thus, any substituent that withdraws electrons is a meta director. NO2 NO2 NO2 NO2 NO2 NO2 ortho (6%) meta (93%) para (0.7%) HNO3, 100o C H2SO4 NO2 Fore example
47
 Halogens are deactivating because of the inductive withdrawal of electron density from the ring,  yet are ortho /para directors since they can use resonance donation to stabilize adjacent carbocations. Summary of (De) Activators and Directors 48
2. Nucleophilic aromatic substitution reaction  Normally electrophilic aromatic substitution is the type of reaction mechanism we associate most commonly with benzene derivatives. However, it is also possible for nucleophiles to displace halides ions (i.e. good leaving groups) from aryl halides if there are strong electron withdrawing groups bound to the ring (and especially if they are located ortho and para to the halide)  Since a nucleophile substitutes for the leaving group on the benzene ring, this is called Nucleophilic aromatic substitution. For example 2, 4- dinitrochclorobenzene Cl NO2 NO2 OH NO2 NO2 NH2 NO2 NO2 Heat 2NH3 Heat 2NaOH 49
 Nucleophilic aromatic substitution reaction cannot proceed by the SN2 because the aryl halide cannot provide a suitable geometry for back side attack of the nucleophile  Yet the SN1 mechanism also cannot operate since the reaction is not found to be unimolecular, and strong nucleophiles are required  There are two different possible reaction mechanisms for NAS.  Addition Elimination Mechanism  Elimination Addition Mechanism (The Benzyne mechanism) 50 Cont….
The Addition Elimination Mechanism 51
The Benzyne Mechanism (Elimination Addition Mechanism)  under forcing conditions, unactivated halobenzenes can react with strong bases.  For example, phenol is produced commercially via the reaction of sodium hydroxide with chlorobenzene.  Analogously, aniline is produced via reaction of chlorobenzene with sodium amide. Cl OH 350o C 2NaOH Cl NH2 NaNH2 NH3, -33o C 52 Cont….
 A clue to the mechanism of this type of reaction was provided by the below reaction:  The products were found to be a 50:50 mixture of meta and para substituted compounds.  These two isomers can be explained as coming from the same intermediate, a Benzyne. 53 Cl NH2 NaNH2 NH3, -33o C CH3 CH3 NH2 CH3 Cont….
 Then the NH2 - anion attacks either side of the benzyne: 54 Cont….