Benzene and Aromatic Compounds

Background Benzene and Aromatic Compounds Benzene (C 6 H 6 ) is the simplest aromatic hydrocarbon (or arene). Benzene has four degrees of unsaturation, making it a highly unsaturated hydrocarbon. Whereas unsaturated hydrocarbons such as alkenes, alkynes and dienes readily undergo addition reactions, benzene does not. 2

Background Benzene and Aromatic Compounds Benzene does react with bromine, but only in the presence of FeBr 3 (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 sixmembered 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 3 bond.

Background Benzene and 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. 4

Benzene and Aromatic Compounds The Structure of Benzene 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 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. 5

Benzene and Aromatic Compounds The Structure of Benzene 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). 6

Benzene and Aromatic Compounds The Structure of Benzene Because each bond has two electrons, benzene has six electrons. 7

Benzene and Aromatic Compounds 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). 8

Molecular Orbital Model of Benzene The concepts of hybridization of atomic orbitals and resonance provided the first adequate structure of benzene. 1.09 Å H H C H sp 2 -sp 2 120 o C C H 120 o C H C C sp 2-1s H 1.39 Å Benzene has a six carbon skeleton in a regular hexagon with C-C- C angles of 120 o. All the carbons are the same length (1.39 Å) as well as the hydrogens (1.09 Å). The hybridization of the C-C bonds is sp 2 -sp 2 whereas the C-H bond is sp 2-1s. 9

Benzene and Aromatic Compounds Nomenclature of Benzene Derivatives 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. 10

Benzene and Aromatic Compounds Nomenclature of Benzene Derivatives 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 11

Benzene and Aromatic Compounds Nomenclature of Benzene Derivatives 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. 12

Benzene and Aromatic Compounds Nomenclature of Benzene Derivatives 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. 13

Benzene and Aromatic Compounds Nomenclature of Benzene Derivatives A benzene substituent is called a phenyl group, and it can be abbreviated in a structure as Ph-. Therefore, benzene can be represented as PhH, and phenol would be PhOH. 14

Benzene and Aromatic Compounds Nomenclature of Benzene Derivatives The benzyl group, another common substituent that contains a benzene ring, differs from a phenyl group. Substituents derived from other substituted aromatic rings are collectively known as aryl groups. 15

Benzene and Aromatic Compounds The Criteria for Aromaticity Hückel s Rule Four structural criteria must be satisfied for a compound to be aromatic. [1] A molecule must be cyclic. To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms. 16

Benzene and Aromatic Compounds The Criteria for Aromaticity Hückel s Rule [2] A molecule must be planar. All adjacent p orbitals must be aligned so that the electron density can be delocalized. Since cyclooctatetraene is non-planar, it is not aromatic, and it undergoes addition reactions just like those of other alkenes. 17

Benzene and Aromatic Compounds The Criteria for Aromaticity Hückel s Rule [3] A molecule must be completely conjugated. Aromatic compounds must have a p orbital on every atom. 19

Benzene and Aromatic Compounds The Criteria for Aromaticity Hückel s Rule [4] A molecule must satisfy Hückel s rule, and contain a particular number of electrons. Hückel's rule: Benzene is aromatic and especially stable because it contains 6 electrons. Cyclobutadiene is antiaromatic and especially unstable because it contains 4 electrons. 20

Benzene and Aromatic Compounds The Criteria for Aromaticity Hückel s Rule Note that Hückel s rule refers to the number of electrons, not the number of atoms in a particular ring. 21

Benzene and Aromatic Compounds The Criteria for Aromaticity Hückel s Rule Considering aromaticity, a compound can be classified in one of three ways: 1. Aromatic A cyclic, planar, completely conjugated compound with 4n + 2 electrons. 2. Antiaromatic A cyclic, planar, completely conjugated compound with 4n electrons. 3. Not aromatic (nonaromatic) A compound that lacks one (or more) of the following requirements for aromaticity: being cyclic, planar, and completely conjugated. 22

Benzene and Aromatic Compounds The Criteria for Aromaticity Hückel s Rule Note the relationship between each compound type and a similar open-chained molecule having the same number of electrons. 23

Benzene and Aromatic Compounds The Criteria for Aromaticity Hückel s Rule 1 H NMR spectroscopy readily indicates whether a compound is aromatic. The protons on sp 2 hybridized carbons in aromatic hydrocarbons are highly deshielded and absorb at 6.5-8 ppm, whereas hydrocarbons that are not aromatic absorb at 4.5-6 ppm. 24

Benzene and Aromatic Compounds Examples of Aromatic Rings Completely conjugated rings larger than benzene are also aromatic if they are planar and have 4n + 2 electrons. Hydrocarbons containing a single ring with alternating double and single bonds are called annulenes. To name an annulene, indicate the number of atoms in the ring in brackets and add the word annulene. 25

Which of the following compounds are Aromatic, anti Aromatic and non aromatic Compounds i) ii) iii) iv) v) O vi) vii vii) ix) x) N Aromatic Compound Aromatic Heterocyclic Compound 26

Benzene and Aromatic Compounds Examples of Aromatic Rings [10]-Annulene has 10 electrons, which satisfies Hückel's rule, but a planar molecule would place the two H atoms inside the ring too close to each other. Thus, the ring puckers to relieve this strain. Since [10]-annulene is not planar, the 10 electrons can t delocalize over the entire ring and it is not aromatic. 28

Background Electrophilic Aromatic Substitution The characteristic reaction of benzene is electrophilic aromatic substitution a hydrogen atom is replaced by an electrophile. 29

Benzene does not undergo addition reactions like other unsaturated hydrocarbons, because addition would yield a product that is not aromatic. Substitution of a hydrogen keeps the aromatic ring intact. There are five main examples of electrophilic aromatic substitution. 30

Regardless of the electrophile used, all electrophilic aromatic substitution reactions occur by the same two-step mechanism addition of the electrophile E + to form a resonance-stabilized carbocation, followed by deprotonation with base, as shown below: σ sigma complex * The intermediate cyclohexadienyl cation thus formed is, in general called arenium ion (or some times a σ sigma complex 32

The first step in electrophilic aromatic substitution forms a carbocation, for which three resonance structures can be drawn. To help keep track of the location of the positive charge: 33

The energy changes in electrophilic aromatic substitution are shown below: 34

Halogenation In halogenation, benzene reacts with Cl 2 or Br 2 in the presence of a Lewis acid catalyst, such as FeCl 3 or FeBr 3, to give the aryl halides chlorobenzene or bromobenzene respectively. Analogous reactions with I 2 and F 2 are not synthetically useful because I 2 is too unreactive and F 2 reacts too violently. 35

Chlorination proceeds by a similar mechanism. 36

Nitration and Sulfonation Nitration and sulfonation introduce two different functional groups into the aromatic ring. Nitration is especially useful because the nitro group can be reduced to an NH 2 group. 37

Generation of the electrophile in nitration requires strong acid. 38

Generation of the electrophile in sulfonation requires strong acid. 39

Friedel-Crafts Alkylation and Friedel-Crafts Acylation In Friedel-Crafts alkylation, treatment of benzene with an alkyl halide and a Lewis acid (AlCl 3 ) forms an alkyl benzene. 40

In Friedel-Crafts acylation, a benzene ring is treated with an acid chloride (RCOCl) and AlCl 3 to form a ketone. Because the new group bonded to the benzene ring is called an acyl group, the transfer of an acyl group from one atom to another is an acylation. 41

Friedel-Crafts Alkylation and Friedel-Crafts Acylation 42

In Friedel-Crafts acylation, the Lewis acid AlCl 3 ionizes the carbon-halogen bond of the acid chloride, thus forming a positively charged carbon electrophile called an acylium ion, which is resonance stabilized. The positively charged carbon atom of the acylium ion then goes on to react with benzene in the two step mechanism of electrophilic aromatic substitution. 44

Three additional facts about Friedel-Crafts alkylation should be kept in mind. [1] Vinyl halides and aryl halides do not react in Friedel- Crafts alkylation. 45

[2] Rearrangements can occur. These results can be explained by carbocation rearrangements. 46

Rearrangements can occur even when no free carbocation is formed initially. 48

[3] Other functional groups that form carbocations can also be used as starting materials. 49

Each carbocation can then go on to react with benzene to form a product of electrophilic aromatic substitution. For example: 50

Starting materials that contain both a benzene ring and an electrophile are capable of intramolecular Friedel-Crafts reactions. 51

Formylation It is the substitution of formyl group -CHO in the benzen ring. Formylation can be brought about by Gattermann Koch Reaction Formyl Fuoride Gattermann Koch Reaction When carbon monoxide is passed through a solution of Benzene containing anhydrous AlCl3, CuCl and HCl O formylation CO + HCl Cl H O O H Cl + AlCl 3 H AlCl 4 O + H O H 52

Formylation Using Formyl Fluoride Formylation of benzene occurs when formyl fluorde reacts with it in the presence of BF3 in ether. The reaction is carried out at low temperature O + H O F BF 3 H HBF 4 + 53

Carboxylation It is the substation of carboxylic group (-COOH) in the benzene ring. For example carboxylation of benzene takes place on treatment with Phosgene gas in the presence of O AlCl3 Mechanism + Cl O Cl AlCl 3 H 2 O OH O O Cl Cl + AlCl 3 Cl AlCl 4 Cl O BF 3 H O Cl O Cl O O Cl H 2 O OH 54

1) Why is benzene less reactive than an alkene? The pi electrons of benzene are delocalized over 6 atoms, thus making benzene more stable and less available for electron donation. While an alkene s electrons are localized between two atoms, thus making it more nucleophillc and more reactive toward electrophiles. 56

2) Show how the other two resonance structures can be deprotonated in step two of electrophillic aromatic substitution. H E B E H B E E 57

H B E E 58

3) Draw a detailed mechanism of the chlorination of benzene. Formation of Electrophile Cl Cl FeCl 3 Cl Cl FeCl 3 Electrophillic Additon H H H Cl Cl FeCl 3 Cl Cl + FeCl 4 H Cl 59

Deprotonation H Cl FeCl 4 Cl + HCl + FeCl 3 60

4) Draw stepwise mechanism for the sulfonation of A. a SO 3 H 2 SO 4 b Formation of Electrophile SO 3 H O O S O H OSO 3 H SO 3 H + HSO 4 61

Electrophillic Addition R R R H SO 3 H H H SO 3 H SO 3 H R Deprotonation H SO 3 H R HSO 4 R H SO 3 H SO 3 H + H 2 SO 4 62

5) What product is formed when benzene is reacted with each of the following alkyl halides? a) + (CH 3 ) 2 CHCl AlCl 3 CH(CH 3 ) 2 b) + Cl AlCl 3 63

c) O O + H 3 CH 2 C AlCl 3 CH 2 CH 3 Cl 64

6) What acid chloride is necessary to produce each product from benzene using a Friedal-Crafts acylation? a) O O CH 2 CH 2 CH(CH 3 ) 2 Cl CH 2 CH 2 CH(CH 3 ) 2 b) O O Cl 65

c) O O Cl 66

7) Draw a stepwise mechanism for the following friedal-crafts alkylation? CH 2 CH 3 + CH 3 CH 2 Cl AlCl 3 Formation of Electrophile CH 3 CH 2 Cl AlCl 3 H 3 CH 2 C Cl AlCl 3 67

Electrophillic Additoon H H CH 2 CH 3 H H 3 CH 2 C Cl AlCl 3 CH 2 CH 3 Protonation H CH 2 CH 3 Cl AlCl 3 H CH 2 CH 3 CH 2 CH 3 + HCl + AlCl 3 68

8) Which of these halides are reactive in a Friedal-Crafts alkylation? Br Br Br A B C D Br Br B D Br Look at the carbon to which the halogen is attached and determine its hybridization. If sp 2 its unreactive, while sp 3 is reactive. 69

9) Draw a stepwise mechanism for the following reaction. C(CH 3 ) 3 + (CH e ) 2 CHCH 2 Cl AlCl 3 + HCl + AlCl 3 Formation of Electrophile CH 3 CH 3 H 3 C H 2 C Cl AlCl 3 H 3 C H 2 C Cl AlCl 3 H H 1,2 H shift CH 3 H 3 C CH 3 + AlCl 4 70

Electrophillic Additon CH 3 H 3 C CH 3 H H C(CH 3 ) 3 H C(CH 3 ) 3 H C(CH 3 ) 3 Deprotonation AlCl 4 H C(CH 3 ) 3 C(CH 3 ) 3 + AlCl 3 + HCl 71

10) Draw the product of each reaction a) + H 2 SO 4 b) + (H 3 C) 2 C CH 2 H 2 SO 4 C(CH 3 ) 3 72

c) + OH H 2 SO 4 d) OH + H 2 SO 4 73

Substituted Benzenes Many substituted benzene rings undergo electrophilic aromatic substitution. Each substituent either increases or decreases the electron density in the benzene ring, and this affects the course of electrophilic aromatic substitution. 74

Considering inductive effects only, the NH 2 group withdraws electron density and CH 3 donates electron density. 75

Resonance effects are only observed with substituents containing lone pairs or bonds. An electron-donating resonance effect is observed whenever an atom Z having a lone pair of electrons is directly bonded to a benzene ring. 76

An electron-withdrawing resonance effect is observed in substituted benzenes having the general structure C 6 H 5 -Y=Z, where Z is more electronegative than Y. Seven resonance structures can be drawn for benzaldehyde (C 6 H 5 CHO). Because three of them place a positive charge on a carbon atom of the benzene ring, the CHO group withdraws electrons from the benzene ring by a resonance effect. 77

To predict whether a substituted benzene is more or less electron rich than benzene itself, we must consider the net balance of both the inductive and resonance effects. For example, alkyl groups donate electrons by an inductive effect, but they have no resonance effect because they lack nonbonded electron pairs or bonds. Thus, any alkyl-substituted benzene is more electron rich than benzene itself. 78

The inductive and resonance effects in compounds having the general structure C 6 H 5 -Y=Z (with Z more electronegative than Y) are both electron withdrawing. 79

These compounds represent examples of the general structural features in electron-donating and electron withdrawing substituents. 80

Electrophilic Aromatic Substitution and Substituted Benzenes. Electrophilic aromatic substitution is a general reaction of all aromatic compounds, including polycyclic aromatic hydrocarbons, heterocycles, and substituted benzene derivatives. A substituent affects two aspects of the electrophilic aromatic substitution reaction: 1. The rate of the reaction A substituted benzene reacts faster or slower than benzene itself. 2. The orientation The new group is located either ortho, meta, or para to the existing substituent. The identity of the first substituent determines the position of the second incoming substituent. 81

Consider toluene Toluene reacts faster than benzene in all substitution reactions. The electron-donating CH 3 group activates the benzene ring to electrophilic attack. Ortho and para products predominate. The CH 3 group is called an ortho, para director. 82

Consider nitrobenzene It reacts more slowly than benzene in all substitution reactions. The electron-withdrawing NO 2 group deactivates the benzene ring to electrophilic attack. The meta product predominates. The NO 2 group is called a meta director. 83

All substituents can be divided into three general types: 84

Keep in mind that halogens are in a class by themselves. Also note that: 86

To understand how substituents activate or deactivate the ring, we must consider the first step in electrophilic aromatic substitution. The first step involves addition of the electrophile (E + ) to form a resonance stabilized carbocation. The Hammond postulate makes it possible to predict the relative rate of the reaction by looking at the stability of the carbocation intermediate. 87

The principles of inductive effects and resonance effects can now be used to predict carbocation stability. 88

Orientation Effects in Substituted Benzenes There are two general types of ortho, para directors and one general type of meta director. All ortho, para directors are R groups or have a nonbonded electron pair on the atom bonded to the benzene ring. All meta directors have a full or partial positive charge on the atom bonded to the benzene ring. 90

To evaluate the effects of a given substituent, we can use the following stepwise procedure: 91

A CH 3 group directs electrophilic attack ortho and para to itself because an electron-donating inductive effect stabilizes the carbocation intermediate. 92

An NH 2 group directs electrophilic attack ortho and para to itself because the carbocation intermediate has additional resonance stabilization. 93

With the NO 2 group (and all meta directors) meta attack occurs because attack at the ortho and para position gives a destabilized carbocation intermediate. 94

Figure 18.7 The reactivity and directing effects of common substituted benezenes 95

12)Identify each group as having an electron donating or electron withdrawing inductive effect. a) CH 3 CH 2 CH 2 CH 2 - b) Br- Electron donating Electron withdrawing c) CH 3 CH 2 O- Electron withdrawing 96

14)Identify as electron donating or electron withdrawing. a) OCH 3 Lone pair on oxygen, electron donating b) I Halogen, electron withdrawing c) C(CH 3 ) 3 Alkyl group, electron donating 97

15)Predict the products. a) OCH 3 CH 3 CH 2 Cl OCH 3 OCH 3 + AlCl 3 CH 2 CH 3 H 3 CH 2 C b) Br HNO 3 Br Br H 2 SO 4 + NO 2 O 2 N 98

c) NO 2 Cl2 NO 2 FeCl 3 Cl 99

16)Predict the products when reacted with HNO3 and H2SO4. Also state whether the reactant is more or less reactive than benzene. a) O O Less NO 2 b) N O 2 N N Less 100

c) OH OH OH + NO 2 O 2 N More d) Cl Cl Cl + NO 2 O 2 N Less 101

d) CH 2 CH 3 CH 2 CH 3 CH 2 CH 3 + NO 2 O 2 N More 102

17)Label each compound as less or more reactive than benzene. a) C(CH 3 ) 3 more b) OH more OH 103

c) O OCH 2 CH 3 less d) N(CH 3 ) 3 less 104

18) Rank each group in order of increasing reactivity. a) Cl OCH 3 b) 2 3 1 NO 2 CH 3 2 3 1 105

Disubstituted Benzenes 1. When the directing effects of two groups reinforce, the new substituent is located on the position directed by both groups. 106

2. If the directing effects of two groups oppose each other, the more powerful activator wins out. 107

Synthesis of Benzene Derivatives In a disubstituted benzene, the directing effects indicate which substituent must be added to the ring first. Let us consider the consequences of bromination first followed by nitration, and nitration first, followed by bromination. 108

109