5_Ring structures, aromatic compounds.pdf

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