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Fundamentals of Organic Chemistry CHEM 109 For Students of Health Colleges Credit hrs.: (2+1) King Saud University College of Science, Chemistry Department CHEM 109 CHAPTER 3. AROMATIC HYDROCARBONS Aromatic Hydrocarbons

2 o Originally called aromatic due to fragrant odors, although this definition seems inaccurate as many products posses distinctly non-fragrant smells! o Currently a compound is said to be aromatic if it has benzene-like in its properties. o Their properties hydrocarbons. differ markedly

from those of aliphatic Aromatic hydrocarbons undergo electrophilic substitution whereas aliphatic hydrocarbons undergo ionic addition to double and triple bonds and free radical substitution. The Structure of Benzene Ring

3 o Benzene is the parent hydrocarbon of aromatic compounds, because of their special chemical properties. o Today a compound is said to be aromatic if it is benzene-like in its properties. tructure of Benzene - Molecular formula = C6H6 The carbon-to-hydrogen ratio in benzene, suggests a highly unsaturated structure. - Benzene reacts mainly by substitution. It does not undergo the typical addition reactions of alkenes or alkynes.

The Structure of Benzene Ring 4 o Kekul structure for benzene. He suggested that six carbon atoms are located at the corners of a regular hexagon, with one hydrogen atom attached to each carbon atom. He suggested that single and double bonds alternate around the ring (conjugated system of double bonds). Kekul suggested that the single and double bonds exchange positions around the ring so rapidly that the typical reactions of alkenes cannot take place.

The Structure of Benzene Ring 5 o Resonance Model for Benzene. o Benzene is o planar. All of the carboncarbon bond lengths are identical: 1.39 A, intermediate between typical single (1.54A) and double (1.34 A) bond lengths. o carboncarbon Each carbon is therefore

sp2-hybridized. o Bond angles of 120. Aromatic Character (Aromaticity) 6 To be classified as aromatic, a compound must have: Cyclic structure Cyclic structure contains what looks like a continuous system of alternating double and single bonds Aromatic compounds must be planar Fulfill Huckel rule The number of electrons in the compound =

(4n + 2) Where (n = 0,1, 2, 3, and so on). Aromatic Character (Aromaticity) 7 Structure and name of aromatic 6 1 compound Exampl es

4n + 2 n Nomenclature of Aromatic Compounds 8 o Monosubstituted benzenes that do not have common names accepted by IUPAC are named as derivatives of benzene. Common names are accepted by IUPAC (parent compounds). Nomenclature of Aromatic Compounds

9 o When two substituents are present, three isomeric structures possible. are They are designated by the prefixes; ortho- (o-), meta- (m-) and (p-). paraIf substituent X is attached to carbon 1; o- groups are on carbons 2 and 6, m- groups are on carbons 3 and 5, and pgroups are on carbon 4. o Example s;

Nomenclature of Aromatic Compounds 10 The prefixes; ortho- (o-), meta- (m-) and para- (p-) are used when the two substituents are not identical. o When more than two substituents are present, their positions are designated by numbering the ring. Nomenclature of Aromatic Compounds 11 o Two groups with special names occur frequently in aromatic

compounds; the phenyl group and the benzyl group. o Examples; Reactions of Benzen trophilic Aromatic Substitution Reactions 12 1) Halogenation 2) Nitration 3) Sulfonation

Reactions of Benzen trophilic Aromatic Substitution Reactions 13 4) Alkylation (Friedel-Crafts) 5) Acylation (Friedel-Crafts) The Mechanism of Electrophilic We can Aromatic generalize Substitution this two-step mechanism

electrophilic aromatic substitutions. for all the Reactions of Benzen trophilic Aromatic Substitution Reactions 14 The Mechanism of Electrophilic

HalogenatiAromatic Substitution on Reactions of Benzen trophilic Aromatic Substitution Reactions 15 The Mechanism of Electrophilic Nitratio Aromatic Substitution nIn aromatic nitration reactions, the sulfuric acid catalyst protonates the nitric acid, which then loses water to generate the nitronium ion (NO2+), which contains a

positively charged nitrogen atom. Reactions of Benzen trophilic Aromatic Substitution Reactions 16 The Mechanism of Electrophilic Aromatic Substitution Sulfonati We use either concentrated or fuming sulfuric acid, and the electrophile may be sulfur trioxide, SO3, or protonated on

sulfur trioxide, +SO3H. Reactions of Benzen trophilic Aromatic Substitution Reactions 17 The Mechanism of Electrophilic Aromatic Substitution FriedelCrafts The electrophile is a carbocation, which can be formed either Alkylation by removing a halide ion from an alkyl halide with a Lewis acid catalyst (for example, AlCl3) .

Reactions of Benzen trophilic Aromatic Substitution Reactions 18 The Mechanism of Electrophilic FriedelCrafts Aromatic Substitution Acylation The electrophile is an acyl cation generated from an acid derivative, usually an acyl halide. The reaction provides a useful general route to aromatic ketones.

Reactions of Benzen substituted Benzenes: Orientation 19 o Substituents already present on an aromatic ring determine the position taken by a new substituent. o Example; nitration of toluene gives mainly a mixture of o- and pnitrotoluene. o On the other hand, nitration of nitrobenzene under similar conditions gives mainly the meta isomer. Reactions of Benzen

bstituted Benzenes: Orientation& Reactivity 20 Directing and Activating Effects of Common Functional Groups Substituents that release electrons to the ring will activate the ring toward electrophilic substitution. Substituents that withdraw

electrons from the ring will deactivate the ring toward electrophilic substitution. Reactions of Benzen de-Chain Reactions of Benzene-Derivatives 21 1. Halogenation of an Alkyl Side Chain Reactions of Benzen

de-Chain Reactions of Benzene-Derivatives 22 2. Oxidation of an Alkyl Side Chain o Conversion into a carboxyl group, -COOH, by treatment with hot potassium permanganate. o Regardless the length of the alkyl chain, the product is always the same.

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