Reactions of Benzene

11 Key excerpts on "Reactions of Benzene"

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  Reaction Mechanisms in Organic Chemistry

  • Metin Balcı(Author)
  • 2021(Publication Date)
  • Wiley-VCH

    (Publisher)

  The chemical reactivity of benzene contrasts with the reactivity of the alkenes in that substitution reactions occur in preference to addition reactions. Although aromatic compounds have multiple double bonds, these compounds do not undergo cycloaddition reactions. The lack of reactivity toward addition reactions results from the resonance stability of benzene. In this section, we will discuss the following: 1. Electrophilic substitution Reactions of Benzene, nucleophilic substitution reactions, and addition reactions. 2. How the substituents attached to the benzene ring can change the reactivity for additional substitution reactions and the regioselectivity observed in the products? 3. Conversion of the substituents into new functional groups. Benzene undergoes electrophilic aromatic substitution reactions in which an electrophile substitutes one of the hydrogen atoms attached to the benzene ring and the aromaticity of the ring system is preserved as shown below. H H H H H H E E H H H H H + H Electrophilic aromatic substitution A number of substituents can be introduced into the benzene ring through electrophilic substitution reactions. The ben- zene ring can be substituted by halogens, a nitro group, sulfonic acid, an alkyl group, an acyl group, etc. Hydroxylation Alkylation Acylation Nitration Sulfonation Br CH 2 CH 3 OH NO 2 SO 3 H R O Halogenation 334 6 Aromaticity Because of the π electron clouds above and below the benzene ring, benzene is a nucleophile. Therefore, it will react with electrophiles. The electrophilic substitution reaction proceeds in two steps. In the first step, an electrophile approaches the π electrons of benzene and forms a bond to one of the carbon atoms generating a positive charge on the other carbon atom. The formed cation, -complex, is a nonaromatic cyclohexadienyl carbocation also called an arenium ion.

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  Petrochemistry & Oil Refinery Engineering

  • (Author)
  • 2014(Publication Date)
  • Academic Studio

    (Publisher)

  Major commodity chemicals and polymers derived from benzene Reactions The most common reactions that benzene undergoes are substitution reactions. ________________________ WORLD TECHNOLOGIES ________________________ • Electrophilic aromatic substitution is a general method of derivatizing benzene. Benzene is sufficiently nucleophilic that it undergoes substitution by acylium ions or alkyl carbocations to give substituted derivatives. Electrophilic aromatic substitution of benzene • The Friedel-Crafts alkylation involves the alkylation of benzene (and many other aromatic rings) using an alkyl halide in the presence of a strong Lewis acid catalyst. Friedel-Crafts alkylation of benzene with methyl chloride • The Friedel-Crafts acylation is a specific example of electrophilic aro-matic substitution. The reaction involves the acylation of benzene (or many other aromatic rings) with an acyl chloride using a strong Lewis acid catalyst such as aluminium chloride or Iron(III) chloride, which act as a halogen carrier. Friedel-Crafts acylation of benzene by acetyl chloride • Sulfonation. The most common method involves mixing sulfuric acid with sulfate, a mixture called fuming sulfuric acid. The sulfuric acid protonates the sulfate, giving the sulfur atom a permanent, rather than resonance stabilized positive formal charge. This molecule is very electrophillic and Electrophillic Aromatic Substitution then occurs. ________________________ WORLD TECHNOLOGIES ________________________ • Nitration: Benzene undergoes nitration with nitronium ions (NO 2 + ) as the electrophile. Thus, warming benzene at 50–55 °C, with a combination of concentrated sulfuric and nitric acid to produce the electrophile, gives nitrobenzene. • Hydrogenation (reduction): Benzene and derivatives convert to cyclohexane and derivatives when treated with hydrogen at 450 K and 10 atm of pressure with a finely divided nickel catalyst.

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  Foundations of Chemistry

  An Introductory Course for Science Students

  • Philippa B. Cranwell, Elizabeth M. Page(Authors)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  15 The chemistry of aromatic compounds At the end of this chapter, students should be able to: • Understand the structure of benzene and other aromatic compounds • Be able to describe and explain electrophilic aromatic substitution, S E Ar • Determine if a ring substituent is activating or deactivating • Compare and contrast the reactivity of benzene, phenol, and aniline with electrophiles 15.1 Benzene 15.1.1 The structure of benzene Benzene , C 6 H 6 , is a molecule that can be described as aromatic . This is because when benzene and benzene-containing compounds were first isolated, they were noted to smell pleasant, so their name was derived from the Greek ‘ aroma ’ , meaning spice, which was then used in Latin to mean ‘ sweet smell ’ . Benzene is a simple aromatic compound that is often studied in undergraduate courses. It is a colourless liquid with a slightly sweet odour and is fully aromatic. Benzene can be represented in a number of ways depending upon what information needs to be depicted (Figure 15.1). Each carbon atom in the benzene molecule is trigonal planar in geometry, with a bond angle of 120 between the other two carbon atoms and the hydro-gen. In terms of the bonding, there are three σ bonds and one π bond per carbon atom. The π bond is formed by an electron in the p orbital of one carbon atom overlapping with an electron in a p orbital on an adjacent carbon atom. This overlap causes a cyclic system of bonding p orbitals (or π bonds) to be formed, where the electrons are free to move around the ring. The movement of electrons in this manner is called delocalisation , and benzene can be said to have a delocalised electron system above and below the ring . Benzene has a delocalised electron system above and below the ring. For a refresher on the shapes of molecules, please see Chapter 2. Foundations of Chemistry: An Introductory Course for Science Students , First Edition. Philippa B. Cranwell and Elizabeth M. Page. © 2021 John Wiley & Sons Ltd.

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  Important Chemical Substances

  • (Author)
  • 2014(Publication Date)
  • Research World

    (Publisher)

  Major commodity chemicals and polymers derived from benzene. Reactions The most common reactions that benzene undergoes are substitution reactions. ________________________ WORLD TECHNOLOGIES ________________________ • Electrophilic aromatic substitution is a general method of derivatizing benzene. Benzene is sufficiently nucleophilic that it undergoes substitution by acylium ions or alkyl carbocations to give substituted derivatives. Electrophilic aromatic substitution of benzene • The Friedel-Crafts alkylation involves the alkylation of benzene (and many other aromatic rings) using an alkyl halide in the presence of a strong Lewis acid catalyst. Friedel-Crafts alkylation of benzene with methyl chloride • The Friedel-Crafts acylation is a specific example of electrophilic aromatic substitution. The reaction involves the acylation of benzene (or many other aromatic rings) with an acyl chloride using a strong Lewis acid catalyst such as aluminium chloride or Iron(III) chloride, which act as a halogen carrier. Friedel-Crafts acylation of benzene by acetyl chloride • Sulfonation. The most common method involves mixing sulfuric acid with sulfate, a mixture called fuming sulfuric acid. The sulfuric acid protonates the sulfate, giving the sulfur atom a permanent, rather than resonance stabilized positive formal charge. This molecule is very electrophillic and Electrophillic Aromatic Substitution then occurs. ________________________ WORLD TECHNOLOGIES ________________________ • Nitration: Benzene undergoes nitration with nitronium ions (NO 2 + ) as the electrophile. Thus, warming benzene at 50–55 °C, with a combination of concentrated sulfuric and nitric acid to produce the electrophile, gives nitrobenzene. • Hydrogenation (reduction): Benzene and derivatives convert to cyclohexane and derivatives when treated with hydrogen at 450 K and 10 atm of pressure with a finely divided nickel catalyst.

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  Keynotes in Organic Chemistry

  • Andrew F. Parsons(Author)
  • 2013(Publication Date)
  • Wiley

    (Publisher)

  7 Benzenes

  Key point . Benzene is an aromatic compound because the six π electrons are delocalised over the planar 6-membered ring. The delocalisation of electrons results in an increase in stability and benzene is therefore less reactive than alkenes or alkynes. Benzene generally undergoes electrophilic substitution reactions , in which a hydrogen atom is substituted for an electrophile. The electron-rich benzene ring attacks an electrophile to form a carbocation, which rapidly loses a proton so as to regenerate the aromatic ring. Electron-donating (+I, +M) substituents (on the benzene ring) make the ring more reactive towards further electrophilic substitution and direct the electrophile to the ortho -/para - positions. In contrast, electron-withdrawing (−I, −M) substituents (on the benzene ring) make the ring less reactive towards further electrophilic substitution and direct the electrophile to the meta - position.

  7.1 Structure

  • Benzene (C6 H6 ) has six sp2 carbon atoms and is cyclic, conjugated and planar. It is symmetrical and all C–C–C bond angles are 120°. The six C–C bonds are all 1.39 Å long, which is in between the normal values for a C–C and C=C bond.

  Hybridisation is discussed in Section 1.5

  • Benzene has six π electrons, which are delocalised around the ring and a circle in the centre of a 6-membered ring can represent this. However, it is generally shown as a ring with three C=C bonds, because it is easier to draw reaction mechanisms using this representation.

  Drawing a benzene ring is introduced in Section 2.5

  As benzene has six π-electrons, it obeys Huckel's rule and is aromatic . Huckel's rule states that only cyclic planar molecules with 4n + 2 π-electrons can be aromatic: for benzene n = 1. (Systems with 4n π-electrons are described as anti-aromatic .)

  Naming derivatives of benzene is discussed in Section 2.4

  Aromatic compounds can be monocyclic or polycyclic, neutral or charged. Atoms other than carbon can also be part of the ring and for pyridine, the lone pair of electrons on nitrogen is not part of the π-electron system (Section 1.7.5).

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  Biochemistry

  An Organic Chemistry Approach

  • Michael B. Smith(Author)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  8 This step is the first in the biosynthesis pathway of the ergot alkaloids. Reported work suggests that the enzyme catalyzes a stepwise reaction via carbocation intermediates.

  8 Gebler, J.C.; Woodside, A.B.; Poulter, C.D. Journal of the American Chemical Society 1992 , 11 4, 7354–7360.

  9.6 Reduction of Aromatic Compounds

  There are several Reactions of Benzene derivatives that involve the benzene ring itself. Reduction reactions are particularly important for the preparation of many useful derivatives. When benzene is treated with one molar equivalent of hydrogen gas the product is expected to be cyclohexa-1,3-diene, which reacts with hydrogen faster than benzene. It is, therefore, often difficult to isolate cyclohexa-1,3-diene in good yield, and a mixture of products is common (cyclohexadiene, cyclohexene, and cyclohexane). Control of the amount of hydrogen gas, the catalyst, and the reaction temperature allows isolation of cyclohexene product. With an excess of hydrogen gas (three or more molar equivalents) benzene is cleanly converted to cyclohexane . When benzene is heated with three molar equivalents of hydrogen gas in the presence of a Raney nickel catalyst, abbreviated Ni(R), reduction yields cyclohexane as the product. Raney nickel refers to nickel prepared by a specified procedure. Hydrogenation of benzene is also possible using a palladium (Pd) or a rhodium (Rh) catalyst.

  An alternative method for the reduction of benzene rings uses alkali metals (group 1 or 2) such as sodium or lithium in liquid ammonia, often in the presence of ethanol. This method is used for the reduction of alkynes to (E )- alkenes, and when benzene reacts with sodium and ethanol in liquid ammonia, the product is cyclohexa-1,4-diene as shown. This reaction is known as the Birch reduction .

  In this reaction, two hydrogen atoms are incorporated into the benzene ring with a net reduction of one C=C unit. Note that the two hydrogen atoms are incorporated from the solvent (ethanol), and that the remaining C=C units are not conjugated. The mechanism shown in Figure 9.9

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  Oxidation of C-H Bonds

  • Wenjun Lu, Lihong Zhou(Authors)
  • 2017(Publication Date)
  • Wiley

    (Publisher)

  9 Oxidation of Benzene

  9.1 Introduction

  Benzene (C6 H6 ) is the basic aromatic compound having six identical aryl sp2 C─H bonds, that is, phenyl sp2 C─H bonds. Currently benzene is mainly produced from petroleum oils as raw materials through refining, catalytic reforming, or pyrolysis of gasoline [1] (Scheme 9.1 ). Benzene is used to synthesize ethylbenzene, followed by dehydrogenation to styrene monomer exclusively, leading ultimately to various materials including polystyrene, styrene–acrylonitrile copolymer (SAN), acrylonitrile–butadiene–styrene terpolymers (ABS), etc. Besides, some benzene is used to produce cumene, which is converted into phenol and acetone through cumene oxidation process and is hydrogenated to cyclohexane, followed by oxidation to adipic acid, caprolactam, hexamethylenediamine, etc. in the manufacture of nylon. Other major applications of benzene include production of nitrobenzene for aniline synthesis and linear alkylbenzenes (LABs) used as detergents, chlorobenzene, maleic anhydride (MAH), etc. However, although the commercial utilization of benzene to supply bulk chemicals such as styrene, phenol, aniline, etc. has been conducted for decades, these processes are often indirect and need multisteps. Thus, direct oxidation of phenyl sp2 C─H bond to produce important chemicals in one step is still of commercial interest currently. On the other hand, due to the prevalence of benzene core in natural products, pharmaceuticals, and functional materials, etc., efficient methods of converting benzene into universal synthetic building blocks are also highly desired.

  Scheme 9.1

  Production and application of benzene.

  Different from cleavage of methyl or alkyl sp3 C─H bonds described in previous chapters, generally there are two ways, namely, direct and indirect methods to cleave a phenyl sp2 C─H bond on the benzene ring. In the direct way, a phenyl sp2 C─H bond may be cleaved either homolytically or heterolytically. However, the phenyl sp2 C─H bond, with a high BDE value of 472 kJ/mol, is even more stable than the methyl sp3 C─H bond (BDE = 439 kJ/mol). Thus, it is more reluctant for phenyl sp2 C─H bond to break homolytically. Moreover, the pKa value of phenyl sp2 C─H bond is 43, slightly more acidic than methyl sp3 C─H bond (pKa  = 48), but it is still not acidic enough for phenyl sp2 C─H bond to be deprotonated readily for further utilization in organic synthesis. Fortunately, some electrophilic transition‐metal complexes could attack on the phenyl sp2 C─H bonds to form phenyl sp2 C─M species via C─H activation process under mild conditions. For example, in 1965, Van Helden and Verberg reported formation of biphenyl through oxidative homocoupling of benzene mediated by palladium complex to cleave phenyl sp2 C─H bonds [2]. A cross‐coupling of benzene and alkene to produce styrene derivatives was established in 1969 by Fujiwara, Moritani, et al. using catalytic palladium to cleave phenyl C─H bond and applying copper or silver salt combined with O2 as oxidant to regenerate palladium to fulfill the catalytic cycle [3]. In these direct oxidation cases, inert phenyl sp2 C─H bond is cleaved by palladium complex to form phenyl–palladium species, and further oxidative functionalization of this phenyl–palladium species completes the oxidation of phenyl sp2 C─H bond (Scheme 9.2

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  Organic Chemistry

  An Acid-Base Approach, Third Edition

  • Michael B. Smith(Author)
  • 2022(Publication Date)
  • CRC Press

    (Publisher)

  19 Aromatic Compounds and Benzene Derivatives

  DOI: 10.1201/9781003174929-19

  The video clips for this chapter are available at: https://routledgetextbooks.com/textbooks/9780367768706/chapter-19.php

  The scientist photographs are also available at: https://routledgetextbooks.com/textbooks/9780367768706/image-gallery.php

  Benzene was identified as a special type of hydrocarbon in Section 5.9. Benzene and derivatives are aromatic hydrocarbons with one ring or several rings fused together. The aromatic character of benzene and derivatives have special stability, which imparts a unique chemical profile.

  To begin this chapter, you should know the following points:

  • Resonance and resonance-stability (Sections 2.6 and 6.3.1).
  • Nomenclature and structure of alkenes, alkyl halides, alcohols, amines, aldehydes, and ketones (Sections 5.1, 5.2 5.85, and 5.6).
  • Carboxylic acids and carboxylic acid derivatives (Chapter 18 ).
  • The structure and nature of π-bonds (Section 5.1).
  • Reactivity of alkenes (Sections 10.1–10.6).
  • E2 elimination reactions of alkyl halides (Sections 12.1–12.3).
  • E1 elimination reactions (Section 12.4).
  • The CIP rules (Section 9.2).
  • Electron-releasing and withdrawing substituents (Sections 3.8, 6.3.2).
  • Brønsted-Lowry acid-base reactions of alkenes (Sections 10.5–10.7).
  • Carbocation stability (Sections 7.2.1, 10.1 and 10.3).
  • Leaving groups (Sections 11.1–11.3 and 18.4).
  • Lewis acids and Lewis bases (Sections 2.7 and 6.8).
  • Rate of reaction (Section 7.11).
  • Reduction of functional groups (Sections 17.2–17.5).

  19.1 Benzene and Aromaticity

  Structure of Benzene

  Benzene is a liquid first isolated from an oily condensate deposited from compressed illuminating gas by Michael Faraday (England; 1791–1867) in 1825. Benzene is a hydrocarbon and the parent of a large class of compounds known as aromatic hydrocarbons. In 1834, Eilhard Mitscherlich (Germany; 1794–1863) established the formula to be C6 H6 and named the material benzin. Justus Liebig (Germany; 1803–1873) changed the name to benzol. In 1837, August Laurent (France; 1807–1853) proposed the name pheno (Greek; I bear light) since it was isolated from illuminating gas. The name was not adopted, but has given rise to the term phenyl for a benzene ring used as a substituent C6 H5

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  Organic Chemistry

  • David R. Klein(Author)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  REVIEW OF REACTIONS Reactions at the Benzylic Position Reduction Oxidation OH O Na 2 Cr 2 O 7 H 2 SO 4 , H 2 O 1) KMnO 4 , H 2 O, heat 2) H 3 O + Substitution Reactions OH Br HBr S N 1 H 2 O + Br NaBr S N 2 NaOH OH + Free-Radical Bromination Br Heat NBS Elimination Reactions OH H 2 O E1 Conc. H 2 SO 4 + Br EtOH NaBr E2 NaOEt + + Catalytic Hydrogenation 3 H 2 Ni 100 atm 150°C + Birch Reduction R Na, CH 3 OH NH 3 R O O Na, CH 3 OH NH 3 820 CHAPTER 17 Aromatic Compounds REVIEW OF CONCEPTS AND VOCABULARY SECTION 17.1 • Derivatives of benzene are called aromatic compounds, regardless of whether they are fragrant or odorless. SECTION 17.2 • Monosubstituted derivatives of benzene are named systemat- ically using benzene as the parent and listing the substituent as a prefix. • IUPAC also accepts many common names for monosubsti- tuted benzenes. • When a benzene ring is a substituent, it is called a phenyl group. • Disubstituted derivatives of benzene can be differentiated by the use of the descriptors ortho, meta, and para, or by the use of locants. • Polysubstituted derivatives of benzene are named using locants. Common names can be used as parents. SECTION 17.3 • Benzene is comprised of a ring of six identical CC bonds, each of which has a bond order of 1.5. • No single Lewis structure adequately describes the structure of benzene. Resonance structures are required. SECTION 17.4 • Benzene exhibits unusual stability. It does not react with bro- mine in an addition reaction. • The stabilization energy of benzene can be measured by comparing heats of hydrogenation. • The stability of benzene can be explained with MO theory. The six π electrons all occupy bonding MOs. • The presence of a fully conjugated ring of π electrons is not the sole requirement for aromaticity. The requirement for an odd number of electron pairs is called Hückel’s rule. • Cyclobutadiene is antiaromatic; cyclooctatetraene adopts a tub-shaped conformation and is nonaromatic.

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  Introduction to Organic Chemistry

  • William H. Brown, Thomas Poon(Authors)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  • Polynuclear aromatic hydrocarbons contain two or more fused benzene rings. 9.4 What Is the Benzylic Position, and How Does It Contribute to Benzene Reactivity? • The benzylic position is the carbon of an alkyl substituent immediately bonded to the benzene ring. • The benzylic position of a benzene ring can be oxidized by chromic acid without affecting any of the benzene ring atoms. 9.5 What Is Electrophilic Aromatic Substitution? • A characteristic reaction of aromatic compounds is electro- philic aromatic substitution, which involves the substitu- tion of one of the ring hydrogens of benzene for an electrophilic reagent. • The five types of electrophilic aromatic substitution dis- cussed here are nitration, halogenation, sulfonation, Friedel–Crafts alkylation, and Friedel–Crafts acylation. 9.6 What Is the Mechanism of Electrophilic Aromatic Substitution? • The mechanism of electrophilic aromatic substitution can be broken down into three common steps: (1) generation of the electrophile, (2) attack of the electrophile on the aromatic ring to give a resonance‐stabilized cation intermediate, and (3) proton transfer to a base to regenerate the aromatic ring. • The five electrophilic aromatic substitution reactions stud- ied here differ in their mechanism of formation of the elec- trophile (Step 1) and the specific base used to effect the proton transfer to regenerate the aromatic ring (Step 3). 9.7 How Do Existing Substituents on Benzene Affect Electrophilic Aromatic Substitution? • Substituents on an aromatic ring influence both the rate and site of further substitution. • Substituent groups that direct an incoming group preferen- tially to the ortho and para positions are called ortho–para directors. Those that direct an incoming group preferen- tially to the meta positions are called meta directors. • Activating groups cause the rate of further substitution to be faster than that for benzene; deactivating groups cause it to be slower than that for benzene.

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  Brown's Introduction to Organic Chemistry

  • William H. Brown, Thomas Poon(Authors)
  • 2017(Publication Date)
  • Wiley

    (Publisher)

  • A heterocyclic aromatic compound contains one or more atoms other than carbon in an aromatic ring. 9.3 How Are Benzene Compounds Named, and What Are Their Physical Properties? • Aromatic compounds are named by the IUPAC system. The common names toluene, xylene, styrene, phenol, aniline, benzaldehyde, and benzoic acid are retained. • The C 6 H 5 group is named phenyl, and the C 6 H 5 CH 2 group is named benzyl. • To locate two substituents on a benzene ring, either num- ber the atoms of the ring or use the locators ortho (o), meta (m), and para (p). • Polynuclear aromatic hydrocarbons contain two or more fused benzene rings. 9.4 What Is the Benzylic Position, and How Does It Contribute to Benzene Reactivity? • The benzylic position is the carbon of an alkyl substituent immediately bonded to the benzene ring. • The benzylic position of a benzene ring can be oxidized by chromic acid without affecting any of the benzene ring atoms. 9.5 What Is Electrophilic Aromatic Substitution? • A characteristic reaction of aromatic compounds is electro- philic aromatic substitution, which involves the substitu- tion of one of the ring hydrogens of benzene for an electrophilic reagent. • The five types of electrophilic aromatic substitution dis- cussed here are nitration, halogenation, sulfonation, Friedel–Crafts alkylation, and Friedel–Crafts acylation. 9.6 What Is the Mechanism of Electrophilic Aromatic Substitution? • The mechanism of electrophilic aromatic substitution can be broken down into three common steps: (1) generation of the electrophile, (2) attack of the electrophile on the aromatic ring to give a resonance‐stabilized cation intermediate, and (3) proton transfer to a base to regenerate the aromatic ring. • The five electrophilic aromatic substitution reactions stud- ied here differ in their mechanism of formation of the elec- trophile (Step 1) and the specific base used to effect the proton transfer to regenerate the aromatic ring (Step 3).

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