Chapter 7 - I Chapter 7 AROMATIC COMPOUNDS IN THIS CHAPTER...

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Unformatted text preview: I . Chapter 7 , AROMATIC : COMPOUNDS IN THIS CHAPTER: V lntroduction V Electronic Structure V Aromaticity and HL'ickel’s Rule V Antiaromaticity V Nomenclature V Chemical Reactions V Order of introducing Groups V Solved Problem Introduction Benzene, (CfiHé) is the prototype of aromatic compounds, which are unsaturated compounds showing a low degree of reactivity. Benzene exists as a resonance hybrid of two structures shown below. As a result, all of the C—C bonds and all the hydrogens in benzene are equivalent. 57 58 ORGANlC CHEMISTRY 'l T T “\Tl/ \T/H “\T/C\lc/” “\TI/x°~\..lT/H c c c c c:-. .a'c H/ \C/ \H H/ \C/ \H H/ \-c-/ \H l. l 1. There are three disubstituted benzenes—the 1,2—, 1.3—. and 1,4-position isomers—designated as ortho, meta, and para. respectively. T“ T’“ T“3 H\T‘/C\T/CH3 H\T/C\|C/H H\T/C\|C/H C C C C C C H/ \c/ \H H/ \G/ \CHa H/ \C/ \H l l it 1,2- or ortho (0-) 1,3- or meta (171-) 1,4— or para (13-) Electronic Structure Each carbon atom in benzene is Sp2 hybridized. The 6 bonds comprise the skeleton of the molecule. Each carbon also has a p orbital at right angles to the plane of the ring, forming a cyclic, conjugated 7t system containing 6 electrons. This 7: system is parallel to and above and below the plane of the ring. The six Tl: electrons in the It system are associ— ated with all six C‘s. They are therefore more delocalized, accounting for the great stability of benzene and other aromatic rings. Aromaticity and HUckel’s Rule Hiickel’s rule states that if the number of 7: electrons in a planar, cyclic. conjugated structure is equal to 4n + 2. where :1 equals zero or a whole CHAPTER 7: Aromatic Compounds 59 number, the species is aromatic. This rule applies to heterocycles (rings containing a non—carbon atom such as nitrogen or sulfur) as well as to carbocycles (like benzene). benzene pyridine pyrrole (lone pair is part of the 1! system) Polycyclic Aromatic Compounds. The prototype of this class of com— pounds is naphthalene, CmHS, although there are many others. Although the Hiickel 4n + 2 rule is rigorously derived for monocyclic systems, it can also be applied to other compounds. Naphthalene Anthrncene Antiaromaticity Planar cyclic conjugated species with 4:2 It electrons are called antiaro- matic and are quite unstable. 1,3—Cyclobutadiene (n = l ), for which two equivalent contributing resonance structures can be writtten, is an extremely unstable antiaromatic molecule. Nomenclature Some benzene derivatives are usually referred to by their common, n0n~ systematic names. such as toluene (CGHSCH3), xylene (C6H4(CH3)2). phenol (CGHSOH), and aniline (CfiHSNHZ). Derived names combine the name of the substituent as a prefix with the word benzene. Examples 60 ORGANIC CHEMISTRY include nitrobenzene (Cfil—ISNOE) and ethylbenzene (CGHSCHQCH3) Some common aromatic groups that are substituents (Ar—) are Cfil-ls— (phenyl). Cij—C6H4— (biphenyl), and p—CH3C6H4(p-tolyl). Another common group is CfiHSCH3—, known as benzyl. For disubstituted benzenes with a group‘gjving the ring a common name, 0-, p-, or m— is used to designate the position: of the second group. Otherwise, positions of groups are-desig- nated by the lowest combina- tion of numbers. ’ ' Chemical Reactions The unusual stability of the benzene ring dominates the chemical reac- tions of benzene and naphthalene. Both compounds resist addition reaC» tions which lead to destruction of the aromatic ring. Rather. they under— go substitution reactions. in which a group or atom replaces an H from the ring. thereby preserving the stable aromatic ring. Atoms or groups other than H may also be replaced. Reduction. Benzene is resistant to catalytic hydrogenation (high tem4 peratures and high pressures of H2 are needed) and to reduction with Na in alcohol. Reduction with lithium in liquid ammonia (known as the Birch reduction) produces 1,4-cyclohexadiene. Li —> NH3 (liq) Oxidation. Benzene is very stable to oxidation except under very vig- orous conditions. In fact, when an alkylbenzene is oxidized, the alkyl CHAPTER 7: Aromatic Compounds 61 group is oxidized to 3 COOH group, while the benzene ring remains intact. For this reaction to proceed, there must be at least one H atom on the C attached to the ring. 0 H CH3 C KMnO4 \OH Electrophilic Aromatic Substitution. Aromatic rings undergo substi— tution reactions [replacement of a hydrogen with another group) with strong electrophiles. The mechanism for these reactions involve attack of the electrophile on the it electrons of the ring. followed by loss of a proton to reestablish aromaticity. These reactions typically require Lewis acid catalysts to help generate the electrophiles. The first step in this reaction is reminiscent of electrophilic addition to an alkene. Aromatic substitution differs in that the intermediate carbocation loses a cation (most often H") to give the substitution product, rather than adding a nucleophile to give the addition product. H H We E E —> H FeBrJ, 5035 Br: H1504 Br CI No2 SOSH The introduction of acyl groups is accomplished by treating benzene with an acid chloride and AlClS. This reaction is known as the Friedel- Crafts Acylation. 62 ORGANIC CHEMISTRY O O O \ I + AICIJ 0 CH3/ \CI CH3 The 5 ring H’s of monosubstituted benzenes are not equally reactive. The ring substituents determine the orientation 01’ E (meta, or a mixture of arr/10 and para), and the reactivity of the ring toward substitution. Electron—donating groups (—OCH3, —NR2, alkyl) make the ring more reactive, and direct electrophilic attack to the arr/10 and para positions. These groups are known as ortho, para directors. Electron withdraw- ing groups (—NOQ, acyl) are meta directors and deactivate the ring toward electrophilic attack. Halogens are ortho, para directors, but weak deactivators. A YduNe'ed'to ' - :Aromatiicity, antlaromaticity, and ‘ ~;fl—;ltick’elfs Rule ,7; - eElectroprhilic aromatic Véubstitu’e ' frtion golp—‘and m4directors Order of Introducing Groups In the preparation 01‘ highly substituted aromatic compounds, it is essen- tial to introduce the substituents in the proper order. For example, if benzene is first treated with CHSCI in the presence of A103, then with HNO3/HISO4 the para product will be formed. Reversing the order of these steps yields the meta product. CHAPTER 7: Aromatic Compounds 63 CH3 CH3 I. CH3CI,A]C13 —--—-> NOT 2. HN03, H2504 N02 N02 Reactivity of the Benzylic Positions Benzylic carbons are adjacent to an aromatic ring. The chemistry of benzylic and of allylic positions are very similar. Intermediate carboca- tions, free radicals, and carbanions formed at these positions are stabilized by delocalization with the adjacent II system! the benzene ring in the case of the benzylic position. Benzylic halides can be prepared in good yield through free-radical halogenation, as shown below. Br Brz _, hv Benzylic halides are highly reactive. even reacting with nucleophiles as weak as water. Alkyl halides do not undergo nucleOphilic substitutions with such weak nucleophiles. 64 ORGANIC CHEMISTRY Solved Problem Problem 7.1 Use PhH, PhMe, and any aliphatic or inorganic reagents to prepare the following compounds in reasonable yields. (a) m-bromobenzenesulfonic acid (b) 3—nitro-4—bromobenzoic acid (a) SO3H sogH SO} B r: -——> —> H3804 FeBr3 Br (In—director is added first) ('3) CH3 CH3 Bl‘z —-> FeBrg COQH COZH HN03 <— Bl' H2504 Br N02 Nitration of p—BrCflHJCl-l3 would have given about a 50-50 mixture of two products; 2—nitro—4-bromotoluene would be unwanted. When oxi— dation precedes nitration, an excellent yield of the desired product is obtained. ...
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Chapter 7 - I Chapter 7 AROMATIC COMPOUNDS IN THIS CHAPTER...

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