Ethylbenzene is made commercially from benzene and ethene using phosphoric acid as the catalyst. Thus it is necessary to consider carefully for each substitution reaction what the actual substituting agent may be. The sulfonation with fuming sulfuric acid gives benzenesulfonic acid. In the case of most deactivating groups on the ring, the electrophile will add to the meta position (figure 7b). These electrophilic substitution reactions can result in an inversion of configuration if the electrophilic attack occurs at an angle of 180o to the leaving group (attack from the rear). Missed the LibreFest? Note that the carbocation intermediate formed has a number of resonance forms. The reaction describing the removal of a proton from the sigma complex is given below: Thus, the electrophile replaces the hydrogen atom in the benzene ring. Answer: O O CH 3 Methylbenzoate Toluene HNO 3 H2SO 4 HNO 3 H2SO 4. These reactions can be classified into the following five types. Additionally, the nitrogen in pyridine easily gets a positive charge either by protonation (from nitration or sulfonation) or Lewis acids (such as AlCl3) used to catalyze the reaction. Also, the sulfonyl chloride is a more useful intermediate than the sulfonic acid, but can be converted to the acid by hydrolysis if desired: Sulfonation is important in the commercial production of an important class of detergents - the sodium alkylbenzenesulfonates: The synthesis illustrates several important types of reactions that we have discussed in this and previous chapters. The two primary types of electrophilic substitution reactions undergone by organic compounds are electrophilic aromatic substitution reactions and electrophilic aliphatic substitution reactions. Groups that are electron-withdrawing by resonance decrease the electron density especially at positions 2, 4 and 6, leaving positions 3 and 5 as the ones with comparably higher reactivity, so these types of groups are meta directors (see below). Recall: CC R 4 R 1 R 3 R 2 +EY δ+δ− R 1C R 2 C E R 3 R 4 Y R 1C R 2 C E R 3 R 4 Y In aromatic rings, however, we see substitution of one of the benzene ring hydrogens for an electrophile. Because of this, a convenient synthesis of alkylbenzenes starts with acylation, followed by reduction of the carbonyl group with zinc and hydrochloric acid (Section 16-6). Study Notes. Fluorine is too reactive to be of practical use for the preparation of aromatic fluorine compounds and indirect methods are necessary (see Section 23-10B). We may write the reaction as first involving formation of a propyl cation, which is a primary carbocation: \[\ce{CH_3CH_2CH_2Cl} + \ce{AlCl_3} \rightarrow \ce{CH_3CH_2CH_2^+} + \overset{-}{\ce{Al}} \ce{Cl_4}\]. The products formed in this electrophilic substitution reaction include a proton and a chlorobenzene molecule. Nitration reaction involves the replacement of a hydrogen with a nitro (NO. The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. This makes pyridine N-oxide, which due to the negative oxygen atom, makes the reaction faster than pyridine, and even benzene. The nitration of methylbenzene (toluene) is a typical example of a nitration that proceeds well using nitric acid in a 1:2 mixture with sulfuric acid. It is also important to note that when an electrophilic aromatic substitution reaction is performed on a mono-substituted benzene ring containing an activating group, the new electrophile will add to the ortho and the para positions of the ring (figure 7a). - Section 15 of, 3 - Chemical Quantities and Stoichiometry, 4 - Types of Chemical Reactions and Solution Stoichiometry, 7 - Quantum Mechanical View of the Atom, and Periodicity, 9 - Covalent Bonding and Molecular Orbitals, 10 - Liquids, Solids, and Intermolecular Forces, 15 - Applications of Acid-Base Equilibria, 16 - Spontaneity, Entropy, and Free Energy, 18 - Transition Metals and Coordination Chemistry, 20 - An Introduction to Organic Chemistry, 1 - Structural, Bonding, Molecular Properties, 2 - The Nature of Organic Compounds: Alkanes and Cycloalkanes, 3 - Stereochemistry of Alkanes and Cycloalkanes: 3-D Structures of Molecules, 4 - The Study of Organic Reactions: An Overview, 10 - Substitution (SN2, SN1) and Elimination (E2, E1) Reactions, 11 - Mass Spectrometry and IR Spectroscopy, 13 - Conjugated Systems and UV Spectroscopy, 15 - Electrophilic Aromatic Substitution (E.A.S. Examples of activated aromatic rings are toluene, aniline and phenol. The final result of the electrophilic aromatic substitution might thus be hard to predict, and it is usually only established by doing the reaction and determining the ratio of ortho versus para substitution. As an example, the alkylation of benzene with 1-chloropropane leads to a mixture of propylbenzene and isopropylbenzene. This section surveys some of the most common electrophilic aromatic substitution reactions and their mechanisms. To learn more about this topic and other related topics, such as aromatic hydrocarbons, register with BYJU’S and download the mobile application on your smartphone. The carbocation intermediate that is formed is attacked by a base that results in the loss of a proton. Their catalytic activity may be attributed to their ability to polarize the halogen-halogen bond in the following way: \[\overset{\delta \oplus}{\ce{Br}} \cdots \overset{\delta \ominus}{\ce{Br}} \cdots \ce{FeBr_3}\]. In principle it could react by either mode 1 or 2, but the energetic advantage of reforming an aromatic ring leads to exclusive reaction by mode 2 (ie. The C=C is … Related carbocation rearrangements are discussed in Sections 8-9B and 15-5E. This reasoning is consistent with low yields of meta-substituted product. These reagents provide a source of \(\ce{NO_2^+}\) and have some advantage over \(\ce{HNO_3} \cdot \ce{H_2SO_4}\) mixtures in that they are soluble in organic solvents such as ethanenitrile or nitromethane. 1. The catalyst is considered to function by assisting the generation of the active electrophile, \(\ce{HgX}^\oplus\). Therefore, these reactions can be used to obtain aryl halides from aromatic rings and iodine, bromine, or chlorine. The reaction is also much slower (a relative reaction rate of 6×10−8 compared to benzene) because the ring is less nucleophilic. Your email address will not be published. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. The term 'ipso-substitution' is not used, since it is synonymous with substitution. In the first, slow or rate-determining, step the electrophile forms a sigma-bond to the benzene ring, generating a positively charged benzenonium intermediate. This is because the Lewis acids form a complex with the chlorine molecule, liberating a highly electrophilic Cl+ species. Answer: O O CH 3 Methylbenzoate Toluene HNO 3 H2SO 4 HNO 3 H2SO 4. 2. Approximately 24,700,000 tons were produced in 1999. Such reagents are hazardous materials and must be handles with care. Substitution of the sulfonic acid \(\left( \ce{-SO_3H} \right)\) group for a hydrogen of an aromatic hydrocarbon can be carried out by heating the hydrocarbon with a slight excess of concentrated or fuming sulfuric acid: The actual sulfonating agent normally is the \(\ce{SO_3}\) molecule, which, although neutral, has a powerfully electrophilic sulfur atom: Sulfonation is reversible and the \(\ce{-SO_3H}\) group can be removed by hydrolysis at \(180^\text{o}\): A useful alternative preparation of sulfonyl derivatives is possible with chlorosulfonic acid: This procedure has an advantage over direct sulfonation in that sulfonyl chlorides usually are soluble in organic solvents and may be easily separated from the reaction mixture. The two primary types of electrophilic substitutions include electrophilic aliphatic substitution and electrophilic aromatic substitution. The cation may bond to a nucleophile to give a substitution or addition product. The aromatic ring is regenerated from this cationic intermediate by loss of a proton from the \(sp^3\)-hybridized carbon. Some main factors that are used to determine the attack of the electrophile are resonance, probability, and steric hindrance. Required fields are marked *, Electrophilic Substitution Reaction Mechanism. Sulfonation occurs when hydrogen is replaced with the help of sulfonic acid (SO, This reaction is also quite similar to nitration where an electrophile is generated by protonation of SO. The positive charge is thereby transferred from \(\ce{C_1}\) to \(\ce{C_2}\): Alkylation of benzene with the isopropyl cation then produces isopropylbenzene: \[\ce{C_6H_6} + \ce{CH_3} \overset{\oplus}{\ce{C}} \ce{HCH_3} \rightarrow \ce{C_6H_5CH(CH_3)_2} + \ce{H}^\oplus\].