Organic Chemistry 5: Ring Structures - Aromatic Compounds PDF

Summary

These lecture notes cover organic chemistry, focusing on ring structures and aromatic compounds. It includes topics like the structure of benzene, its properties, and various reactions related to benzene and aromatic compounds. The document also contains naming conventions and examples of related compounds.

Full Transcript

Organic Chemistry 5: Ring Structures Aromatic compounds 1 Ground Rules for this Class: •NO talking •NO sleeping •NO mobile phones •DO NOT come late •DO NOT walk out early •DO NOT play with your computer/phone/iPad •DO NOT capture the attendance for anyone else 2 Chapter Outline 1 Aromatic Hy...

Organic Chemistry 5: Ring Structures Aromatic compounds 1 Ground Rules for this Class: •NO talking •NO sleeping •NO mobile phones •DO NOT come late •DO NOT walk out early •DO NOT play with your computer/phone/iPad •DO NOT capture the attendance for anyone else 2 Chapter Outline 1 Aromatic Hydrocarbons: Structure 2 Naming Aromatic Hydrocarbons 3 Polycyclic Aromatic Hydrocarbons 4 Sources and Physical Properties of Aromatic Hydrocarbons 5 Chemical Properties of Aromatic Hydrocarbons 3 Benzene • Benzene and all substances with structures and chemical properties that resemble benzene are classified as aromatic compounds. Representations of Benzene 4 5 Bonding in Benzene • The electrons are not attached to particular carbon atoms, but are delocalized and associated with the entire molecule. • This electronic structure imparts unusual stability to benzene and is responsible for many of the characteristic properties of aromatic compounds. 6 7 Naming Aromatic Compounds 8 Naming Substituted Benzene Compounds • A substituted benzene is derived by replacing one or more hydrogen atoms of benzene by another atom or group of atoms. • Monosubstituted benzene has the formula C6H5G, where G is the group replacing a hydrogen atom. 9 Monosubstituted Benzenes • Some monosubstituted benzenes are named by adding the name of the substituent group as a prefix to the word benzene. Br CH2CH3 Cl O2N nitrobenzene ethylbenzene chlorobenzene bromobenzene 10 • Certain monosubstituted benzenes have special names. OH CH=CH2 CH3 H2N styrene phenol aniline O O C C H OH benzoic acid toluene benzaldehyde 11 Phenyl Group • The C6H5- group is known as the phenyl group, and the name phenyl is used to name compounds that cannot easily be named as benzene derivatives. 1 CH3 2 CH 3 4 5 CHCH2CH3 H2 C Cl 3-chloro-2-phenylpentane diphenylmethane 12 Disubstituted Benzenes • The prefixes ortho-, meta-, and para(abbreviated o-, m-, and p-) are used to name disubstituted benzenes. G ortho ortho meta meta para 13 Dichlorobenzenes, C6H4Cl2 • The three isomers of dichlorobenzene have different physical properties. meta-dichlorobenzene Cl Cl Cl Cl ortho-dichlorobenzene Cl Cl para-dichlorobenzene 14 Disubstituted Benzenes • When the two substituents are different and neither is part of a compound with a special name, the names of the two substituents are given in alphabetical order, followed by the word benzene. CH2CH3 Cl Br ortho-bromochlorobenzene NO2 para-ethylnitrobenzene 15 Dimethyl Benzenes • The dimethylbenzenes have the special name xylene. CH3 CH3 CH3 H3C ortho-xylene H3C meta-xylene CH3 para-xylene 16 Disubstituted Benzenes • When one of the substituents corresponds to a monosubstituted benzene that has a special name, the disubstituted compound is named as a derivative of that parent compound. NH3 NO2 CH3 HO Br ortho-nitrophenol meta-bromoaniline NO2 para-nitrotoluene 17 Polysubstituted Benzenes • When there are more than two substituents on a benzene ring, the carbon atoms in the ring are numbered starting at one of the substituted groups. • Numbering must be done in the direction that gives the lowest possible numbers to the substituent groups. 18 Polysubstituted Benzenes CH3 O2N OH 1 2 6 1 NO2 5 3 5 4 NO2 2 6 Br Cl 3 4 5-bromo-2-chlorophenol 2,4,6-trinitrotoluene (TNT) 19 Polycyclic Aromatic Compounds 20 Polycyclic Aromatic Hydrocarbons phenanthrene naphthalene anthracene 21 Sources and Physical Properties of Aromatic Hydrocarbons 22 Sources of Aromatic Hydrocarbons • The aromatic hydrocarbons, such as benzene, toluene, xylene, naphthalene, and anthracene, were first obtained in significant quantities from coal tar.  • Coal → Coke + Coal gas + Coal tar • Because of the great demand for aromatic hydrocarbons, processes were devised to obtain them from petroleum. 23 Properties of Aromatic Hydrocarbons • Aromatic hydrocarbons are essentially nonpolar substances, insoluble in water but soluble in many organic solvents. • They are liquids or solids and usually have densities less than that of water. • Aromatic hydrocarbons burn readily, usually with smoky yellow flames as a result of incomplete carbon combustion. 24 Chemical Properties of Aromatic Hydrocarbons 25 Substitution Reactions of Aromatic Hydrocarbons • Halogenation – net addition of -Br or -Cl • Nitration – net addition of –NO2 • Alkylation – net addition of –R (alkyl group) 26 Halogenation of Benzene • When benzene reacts with chlorine or bromine in the presence of a catalyst such as iron (III) chloride or iron (III) bromide, a Cl or Br atom replaces a H atom to form the products. X FeX3 + X2 bromine or chlorine benzene + HX bromobenzene or chlorobenzene 27 Nitration of Benzene • When benzene reacts with a mixture of concentrated nitric acid and concentrated sulfuric acid at about 50C, nitrobenzene is formed. NO2 + HO-NO2 nitric acid benzene H2SO4 + H2O nitrobenzene 28 Alkylation of Benzene • Alkylation of benzene is known as the Friedel-Crafts reaction. • The alkyl group from an alkyl halide (RX), in the presence of AlCl3 catalyst, substitutes for an H atom on the benzene ring. CH2CH3 + CH3CH2Cl chloroethane benzene AlCl 3 + HCl ethylbenzene 29 Side-Chain Oxidation • Carbon chains attached to an aromatic ring are fairly easy to oxidize. CH2CH3 COOH K2Cr2O7/H2SO4 + CO2 heat ethylbenzene benzoic acid 30 Quiz 31 Quiz Can carbon atoms in benzene undergo addition reactions as do carbon atoms in alkenes ? 32 33

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