Electrophilic aromatic substitution is an organic reaction in which an atom that is attached to an aromatic system (usually hydrogen) is replaced by an electrophile.Some of the most important electrophilic aromatic substitutions are aromatic nitration, aromatic halogenation, aromatic sulfonation, and alkylation and acylation Friedel–Crafts reaction. The number of resonance structures is determined by the formula n + 1, {\rm{n}} + {\rm{1}}, n + 1, where n {{n}} n is the number of benzene rings fused. Naphthalene undergoes a number of usual electrophilic substitution reactions, such as nitration, halogenation, sulphonation, Friedel-Crafts alkylation, Friedel-Crafts acylation, etc. Samacheer Kalvi 11th Chemistry Hydrocarbons Additional Questions Solved Orientation and Reactivity. Flourobenzene is less reactive than benzene. It is less sterically crowded and accounts for a stable carbocation. 99, … Corannulene was found to be more reactive than benzene, naphthalene ... benzene 1 to give naphthalene 2 or form more complex poly-cycles such as phenanthrene 3, the tethered napthyl system ... Whilst the order of reactivity towards electrophilic aromatic substitution is relatively consistent across the reactions con- Furans, Pyrroles, Thiophenes –Electrophilic Substitution α-Substitution favoured over . 1226 CHAPTER 25 • THE CHEMISTRY OF THE AROMATIC HETEROCYCLES B. Acidity of Pyrrole and Indole Pyrrole and indole are weak acids. Reactivity Groups already present on the benzene ring may activate the ring (activating groups), making it more reactive towards electrophilic substitution than benzene, e.g. The greater acidity of pyrroles and indoles is a consequence of the resonance … An electrophilic substitution reaction generally involves three steps: 1. Electrophilic Aromatic Substitution: Aromatic compounds such as benzene and its derivatives do not undergo addition reactions like alkenes do even though they have carbon-carbon double bonds. In aldehydes, the relatively small hydrogen atom is attached to one side of the carbonyl group, while a larger R group is affixed to the other side. (We say toluene is activated toward electrophilic aromatic substitution, and that the methyl group is an activating group). a. Explanation: In the electrophilic substitution, position 1 in naphthalene is more reactive that the position 2 because the carbocation formed by the attack of electrophile at position 1 is more stable than position 2 because of the resonance since it has 4 contributing structures. The 5-membered ring heterocycles (furan, pyrrole, thiophene) are π-electron rich aromatics (6π electrons over 5 atoms) This makes them more reactive than benzene (since the aromatics the nucleophilic component in these electrophilic substitution reactions) Reactions . Naphthalene was nitrated with a variety of nitrating agents. Benzene and Phenols, IIT JEE Super Course in Chemistry: Organic Chemistry - Trishna Knowledge Systems | All the textbook answers and step-by-step explanations Of the compounds listed, which would be the most reactive toward chlorination? The intermediate carbocation is more resonance stabilized. Benzene derivatives in a general sense react in the same way that benzene does, although there are some interesting differences. Naphthalene can undergo Electrophilic Aromatic Substitution at... C1 and C2 When Naphthalene is undergoing Electrophilic Aromatic Substitution, Carbon ______ is way more reactive than the other. Ch12: Electrophilic aromatic substitution questions. Am. β-substitution, because of more resonance forms for intermediate and so the charge is less localized (also applies to the transition state) Some . (2 Marks) (ii) Furan is more reactive than thiophene towards electrophilic substitution(3 Marks) (iii) Sulphonation of naphthalene yields 1-substituted product at low temperatures and 2-substituted product at high temperatures; the 2-substituted product is more stable than the 1- substituted product. Examples of Electrophilic Aromatic Substitution of Other Aromatic Compounds. The rate of electrophilic substitution reaction is directly proportional to nucleophilicity of Benzene ring. Step 3 Loss of a proton from the carbocation to give a new aromatic compound. The nitrogen atom in pyridine is electron-attracting and makes pyridine more reactive at carbon towards nucleophiles than benzene, but less reactive at carbon towards electrophiles than benzene. 12,32 (b) Predict the products of the following reactions: i) Oxidation of propylbenzene. Qu 1. Answer: So naphthalene is more reactive compared to single ringed benzene . Chlorobenzene is less reactive than benzene towards electrophilic substitution reaction. This result was not unexpected as the enhanced reactivity of naphthalene 2 compared to benzene 1 towards electrophilic aromatic substitution, due to the lower decrease in the aromatic stabilisation energy on reaction, is well noted in the literature. Electrophilic substitution in naphthalene proceeds more readily than in the case of benzene. ... towards electrophilic aroma tic substitution, ... was found to be more rea ctive than benzene 1, naphthalene 2. Aromatic Substitution, Electrophilic 11.B. • As we will see, there are many reactions, depending upon the particular electrophile, they all use the same Phenol is more reactive than benzene towards electrophilic substitution reaction. Exercise 22-59 Phenanthrene is oxidized more easily than benzene or naphthalene. Proton affinities are calculated at all reactive positions for the normal benzenoid hydrocarbons, benzene, naphthalene, phenanthrene and anthracene, a strained benzenoid hydrocarbon, biphenylene, and a nonalternant hydrocarbon, fluoranthene, and the results are compared to experimental protodetritiation rates. Nitration is the usual way that nitro groups are introduced into aromatic rings. 23.5D). Chapter 11. The reason behind it is the more number of resonating intermediate structure are possible to accommodate the positive charge when electrophile … Aromatic compounds like phenol, aniline, naphthalene, pyrrole, furan, thiophene, pyridine, indole, and imidazole can undergo electrophilic substitution reaction. Show your reasoning. Generation of electrophile: Due to the presence of Lewis acid, generation of electrophile takes place. (Notice that either of the oxygens can accept the electron pair.) β-substitution usually observed – … the compound toward electrophilic substitution, as noted by the general relative rate, „ , of less than 1.0. Therefore in toluene, ortho and para positions are the most reactive towards an electrophile, thus promoting electrophilic substitution reaction. As halogen atoms make the aromatic ring less reactive towards the upcoming electrophile, they are regarded as deactivating groups but they act as ortho para deactivators due to the development of partial negative charge on ortho and para postion. Comparison of data with Perrin's electrochemical nitration [Perrin, C. L. (1977) J. With p K a values of about 17.5, pyrrole and indole are about as acidic as alcohols and about 15–17 pK a units more acidic than primary and secondary amines (Sec. The following diagram shows three oxidation and reduction reactions that illustrate this feature. Predict whether anthracene is more likely to undergo electrophilic substitution at the 1,2 or 9 position. The methyl group hence makes it around 25 times more reactive than benzene. Orientation and Reactivity in Monosubstituted Benzene Rings 34. Chromic acid oxidation of phenanthrene forms a substance known as phenanthraquinone. You should already been given the information either in lecture or in notes. Electrophilic Substitution of Naphthalene: Polynuclear hydrocarbons are more reactive towards electrophilic attack than benzene. 1) Toluene reacts about 25 times faster than benzene under identical conditions. 3. Benzenes : Page 4 The General Mechanism for Electrophilic Aromatic Substitution: • The rate determining step (R.D.S.) Write reasonable mechanistic steps that could be involved in this reaction: C 6 H 6 + C H 2 O + H C l Z n C l 2 C 6 H 5 C H 2 C l + C H 2 O. b. Chem. The group which increase the electron density in benzene ring i.e. This contrasts with the structure of benzene, in which all the C–C bonds have a common length, 1.39 Å. Naphthalene is more reactive than benzene, both in substitution and addition reactions, and these reactions tend to proceed in a manner that maintains one intact benzene ring. ii) Friedel-Crafts acylation of pyrrole. Problem 4-59 Phenanthrene is oxidized more easily than benzene or naphthalene. When an electrophilic substitution reaction is performed on a monosubstituted benzene, the new group may be directed primarily to the ortho, meta, or para position and the substitution may be slower or faster than with benzene itself. The first result to note is the complete selectivity under the reaction conditions with naphthalene 2 reacting whilst benzene 1 did not. Thus, it can be said that the resonance energy per ring is more in benzene than in naphthalene. Which compound is most reactive towards electrophilic aromatic substitution? Therefore it undergoes nitration easily than benzene. Predict whether anthracene is more likely to undergo electrophilic substitution at the 1,2 or 9 position. On the other hand the nitrogen atom in pyrrole is electron donating and this makes pyrrole much more reactive towards electrophiles at carbon than benzene. That makes the ring much more reactive than it is in benzene. ... Naphthalene is thus more reactive towards electrophilic aromatic substitution than benzene because it loses less resonance energy during sigma complex formation. Methods used include PM3 and Hartree-Fock calculations at … A good example is naphthalene compared to benzene, the former has a resonance energy of 61 kcal/mol, and the latter only 36 kcal/mol. Borazine is more reactive than benzene towards electrophilic aromatic substitution reaction. Ethyl benzene has the maximum hyperconjugable hydrogen. In an electrophilic aromatic substitution reaction, existing substituent groups on the aromatic ring influence the overall reaction rate or have a directing effect on positional isomer of the products that are formed. Pyrrole, thiophene, and furan gives electrophilic aromatic substitution reaction. Cl OH CN A B C A) A B) B C) C Ans: B 48. This is clearly observed in phenol and aniline, which are more reacting than benzene towards electrophilic substitution reactions. The donation of the oxygen's lone pair into the ring system increases the electron density around the ring. Substitution of a chloromethyl group, − C H 2 C l, on an aromatic ring is chloromethylation and is accomplished using methanal, H C l, and a metal-halide catalyst ( Z n C l 2). NO 2 OCH 3 NH 2 A B C CH 3 D A) A B) B C) C D) D Ans: C 49. (4) In Borazine molecule nitrogen is more electro negative than the boron. iii) Friedel-Crafts alkylation of pyridine 11. (a) First a Friedel-Crafts alkylation reaction using EtCl to add an Et group to the benzene followed by a radical bromination at the 2 o benzylic position (since it is the most stable radical), to give 1-bromo-1-phenylethane. Their mechanism is similar to benzene [4-8]. These compounds are more reactive compared to benzene. However they also release electrons by delocalization of lone pairs (+R effect). Naphthalene has two aromatic rings, but only 10 pi electrons (rather than the twelve electrons that it would prefer). (i) 1-Position in naphthalene is more reactive than the 2-position towards electrophilic substitution. 11.B.i. Corannulene was found to be mor e reactive than benzene, naphthalene. (ii) Pyrrole is more basic than pyridine. Soc. Since naphthalene has two benzene rings, it has three resonance structures. is the reaction between the benzene and the electrophile, the benzene is the Lewis Base/Nucleophile in the R.D.S. Thus it is strongest in the This means that naphthalene has less aromatic stability than two isolated benzene … Chromic acid oxidation of phenanthrene forms a substance known as phenanthraquinone. pyridine is less reactive than benzene. Ethyl benzene will be the most reactive towards electrophilic and aromatic substitution. The reason for the difference in reactivity could be the ability of the heteroatom to activate the aromatic ring towards electrophilic aromatic substitution reactions. Show your reasoning. Electrophiles majorly attack on 2 nd position rather than 3 rd position in these heterocyclic compounds. Nitrogen acquires partial negative charge and boron acquires partial positive charge and back bonding take place between boron and nitrogen. The Mechanism for Electrophilic Substitution of Benzene. 16.4 ELECTROPHILIC AROMATIC SUBSTITUTION REACTIONS OF BENZENE 755 Step 2 Reaction of the benzene p electrons with the electrophile to form a carbocation inter- mediate. The presence of the heteroatom influences the reactivity compared to benzene. Since fluorine is the most electronegative element, this effect should be strongest in fluorine among the halogens, and it decreases in effect as one goes further away from the position bearing the fluorine. Of the compounds listed, which would be the most reactive toward nitration? Naphthalene is more reactive than benzene, both in substitution and addition reactions, and these reactions tend to proceed in a manner that maintains one intact benzene ring. Feedback: The most reactive site of naphthalene itself towards an electrophile is C1 and remember that hydroxy is a strongly activating, o,p-directing group in electrophilic substitution of arenes .Consequently, the initial bromination occurs predominantly at C1. Since the resonance effect is more stronger than inductive effect the net result is electron releasing to rest of the molecule.

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