GENERAL
`CHEMISTRY
`LINUS PAULING
`Research Professor
`Linus Pauling Institute
`of Science and Medicine
`
`DOVER PUBLICATIONS, INC., New York
`
`NOVARTIS EXHIBIT 2044
`Par v Novartis, IPR 2016-00084
`Page 1 of 7
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`

`
`11"
`
`Copyright © 1947, 1950, 1970 by Linus Pauling.
`All rights reserved under Pan American and International
`Copyright Conventions.
`
`This Dover edition, first published in 1988, is an unabridged,
`slightly altered and corrected republication of the work published
`by W. H. Freeman and Company, San Francisco in 1970. The
`endpaper tables have been moved to the front section of the book
`and the color illustrations have been moved to the inside covers.
`
`Manufactured in the United States of America
`Dover Publications, Inc., 31 East 2nd Street, Mineola, N.Y.
`11501
`
`Library of Congress Cataloging-in-Publication Data
`
`Pauling, Linus, 1901-
`General chemistry / Linus Pauling.
`cm.
`p. (cid:9)
`Reprint. Originally published: 3rd ed. San Francisco : W.H.
`Freeman, 1970.
`Bibliography: p.
`Includes index.
`ISBN 0-486-65622-5 (pbk.)
`1. Chemistry. I. Title.
`QD33.P34 1988
`540—dc19 (cid:9)
`
`87-32165
`CIP
`
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`[7,4] Aromatic Hydrocarbons. Benzene (cid:9)
`
`231
`
`The potential energy barrier restricting rotation about the double bond
`has been found by experiment to be about 200 kJ mole-'.*
`is the first member of a homologous series of
`Acetylene, (cid:9)
`hydrocarbons containing triple bonds. Aside from acetylene, these sub-
`stances (called alkynes) have not found wide use, except for the manu-
`facture of other chemicals.
`Acetylene is a colorless gas (b.p. — 84°C), with a characteristic garliclike
`odor. It is liable to explode when compressed in the pure state, and is
`usually kept in solution under pressure in acetone. It is used as a fuel, in
`the oxyacetylene torch and the acetylene lamp, and is also used as the
`starting material for making other chemicals.
`Acetylene is most easily made from calcium carbide (calcium acetylide,
`CaC2). Calcium carbide is made by heating lime (calcium oxide, CaO) and
`coke in an electric furnace :
`CaO + 3C (cid:9)
`Calcium carbide is a gray solid that reacts vigorously with water to pro-
`duce calcium hydroxide and acetylene:
`CaC2 2H20 Ca(OH)2 C2H2(g)
`The existence of calcium carbide and other carbides with similar for-
`mulas and properties shows that acetylene is an acid, with two replaceable
`hydrogen atoms. It is an extremely weak acid, however, and its solution in
`water does not taste acidic.
`Acetylene and other substances containing a carbon-carbon triple bond
`are very reactive. They readily undergo addition reactions with chlorine
`and other reagents, and they are classed as unsaturated substances.
`
`CaC2 CO(g)
`
`7-4. Aromatic Hydrocarbons. Benzene
`
`An important hydrocarbon is benzene, which has the formula CoH6. It is a
`volatile liquid (m.p. 5.5°C, b.p. 80.1°C, density 0.88 g cm-3). Benzene and
`other hydrocarbons similar to it in structure are called the aromatic hydro-
`carbons. Their derivatives are called aromatic substances—many of them
`have a characteristic aroma (agreeable odor). Benzene itself was dis-
`covered in 1825 by Faraday, who found it in the illuminating gas made by
`heating oils and fats.
`For many years there was discussion about the structure of the benzene
`
`*The height of the barrier can be estimated from the spectroscopic value of the fre-
`quency of the torsional vibration of the molecule (twisting vibration of one end relative
`to the other end), and from the activation energy (Chapter 16) of the cis-trans isomeriza-
`tion reaction.
`
`NOVARTIS EXHIBIT 2044
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`232 (cid:9)
`
`The Nonmetallic Elements and Some of Their Compounds [Chap. 7]
`
`molecule. The German chemist August Kekule (1829-1896) in 1865 pro-
`posed that the six carbon atoms form a regular hexagon in space, the six
`hydrogen atoms being bonded to the carbon atoms, and forming a larger
`hexagon. Kekule suggested that, in order for a carbon atom to show its
`normal quadrivalence, the ring contains three single bonds and three
`double bonds in alternate positions, as shown below. A structure of this
`sort is called a Kekule structure.
`H (cid:9)
`
`H
`
`HC HHC H
`\/ \
`c
`C
`(cid:9) \/ \
`,/ \/ C\
`H
`C (cid:9) H H C (cid:9)
`H (cid:9)
`
`CH3
`
`11
`Other hydrocarbons, derivatives of benzene, can be obtained by re-
`placing the hydrogen atoms by methyl groups or similar groups. Coal tar
`and petroleum contain substances of this sort, such as toluene, C7118, and
`the three xylenes, C81-110. These formulas are usually written C6H5CH3
`and C6H4(CH3)2, to indicate the structural formulas:
`CH3
`CH3
`CH3
`C113 HH
`H (cid:9)
`H
`H (cid:9)
`CH3
`H (cid:9)
`H
`H (cid:9) HH
`H
`Toluene
`
`Meta-xylene
`(m-xylene)
`
`CH3
`Para-xylene
`(p-xylene)
`
`H
`Ortho-xylene
`(o-xylene)
`In these formulas the benzene ring of six carbon atoms is shown simply
`as a hexagon. This convention is used by organic chemists, who often also
`do not show the hydrogen atoms, but only other groups attached to the
`ring.
`It is to be noted that we can draw two Kekule structures for benzene
`and its derivatives. For example, for ortho-xylene the two Kekule struc-
`tures are
`
`CH3 (cid:9)
`CH3
`
`CH3
`
`[
`
`In the first structure there is a double bond between the carbon atoms to
`which methyl groups are attached, and in the second there is a single bond
`in this position. The organic chemists of a century ago found it impossible,
`however, to separate two isomers corresponding to these formulas. To
`
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`[7-4] Aromatic Hydrocarbons. Benzene (cid:9)
`
`233
`
`explain this impossibility of separation Kekule suggested that the molecule
`does not retain one Kekule structure, but rather slips easily from one to
`the other. In the modern theory of molecular structure the ortho-xylene
`molecule is described as a hybrid of these two structures, with each bond
`between two carbon atoms in the ring intermediate in character between
`a single bond and a double bond. Even though this resonance structure is
`accepted for benzene and related compounds, it is often convenient simply
`to draw one of the Kekule structures, or just a hexagon, to represent a
`benzene molecule.
`The structure of the benzene molecule was determined by the electron
`diffraction method in 1929 and the following years. It is a planar hexagon
`with carbon-carbon bond length 1.40 A (C—H bond length 1.06 A). This
`value for a bond with 50% double-bond character is reasonable in com-
`parison with the values 1.54 A for C—C, 1.33 A for C—C, and 1.42 A for
`34% double-bond character (graphite). The planar configuration is re-
`quired by the properties of the double bond (Section 6-4).
`Benzene and its derivatives are extremely important substances. They
`are used in the manufacture of drugs, explosives, photographic developers,
`plastics, synthetic dyes, and many other substances. For example, the sub-
`stance trinitrotoluene, C61-12(CH3)(NO2)3, is an important explosive (TNT).
`The structure of this substance is
`CH3
`02N NO2
`HH
`NO2
`
`In addition to benzene and its derivatives, there exist many other aro-
`matic hydrocarbons, containing two or more rings of carbon atoms.
`Naphthalene, C10 -18, is a solid substance with a characteristic odor; it is
`used as a constituent of moth balls and in the manufacture of dyes and
`other organic compounds. Anthracene, C14H10, and phenanthrene,
`are isomeric substances containing three rings fused together. These
`substances are also used in making dyes, and derivatives of them are im-
`portant biological substances (cholesterol, sex hormones; see Chapter 24).
`For naphthalene, anthracene, and phenanthrene we may write the follow-
`ing structural formulas.:
`H H (cid:9)
`
`H H
`
`H
`
`H
`H
`
`H H H
`
`„ (cid:9)
`
`H H (cid:9)
`
`H H H
`
`Naphthalene (cid:9)
`
`Anthracene
`
`H H
`H
`Phenanthrene
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`(cid:9)
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`(cid:9)
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`234 (cid:9)
`
`The Nonmetallic Elements and Some of Their Compounds [Chap. 7]
`
`These molecules also have hybrid structures: the structural formulas
`shown above do not represent the molecules completely, but are analogous
`to one Kekul6 structure for benzene.
`
`Resonance Energy
`The heat evolved when a molecule of hydrogen is added to a double
`bond is about 120 kJ mole--'. For cyclohexene, for example, the value
`determined by experiment is 119.6 kJ mole-':
`CH2
`CH (cid:9)
`/ \
`CH (cid:9)
`H2C
`I (cid:9)
`1 (cid:9)
`CH2 (cid:9)
`H2C (cid:9)
`/ (cid:9)
`\
`CH2
`
`+ H2 -
`
`H2C/
`1 (cid:9)
`112C (cid:9)
`\
`
`CH2
`1 (cid:9)
`CH2
`/
`CH2
`
`+ 119.6 kJ mole-1
`
`we might well
`
`If the benzene molecule had one Kekule structure,
`expect that the heat of hydrogenation of its three double bonds would be
`approximately three times the heat of hydrogenation of the one double
`bond in cyclohexene, 3 X 119.6 = 358.8 kJ mole--':
`CH2
`CH (cid:9)
`/ \
`/ \
`H2C
`CH (cid:9)
`HC (cid:9)
`I (cid:9)
`Ii (cid:9)
`CH (cid:9)
`HC (cid:9)
`
`CH2
`
`+ 358.8 kJ mole-'
`(incorrect)
`
`+ 3H2 (cid:9)
`
`H2C (cid:9)
`\
`
`CH2 (cid:9)
`/
`CH2
`,CH/ (cid:9)
`The experimental value of the heat of hydrogenation is, however, 150 kJ
`mole-4 smaller:
`3H2(g) -->- C61112(0 -I- 208.4 kJ mole-'
`C81-16(0
`We conclude that the benzene molecule is 150 kJ mole-' more stable
`than it would be if it were represented by a single KekuM structure, with
`each of the three double bonds similar to the double bond in cyclohexene.
`This extra stabilizing energy of 150 kJ mole-1 is called the resonance energy
`of benzene. It is attributed to the fact that the benzene molecule is not
`satisfactorily represented by a single Kekule structure, but instead can be
`reasonably well described as a hybrid of the two Kekule structures. *
`
`*It would, of course, be surprising if the benzene molecule in its normal state were
`actually less stable than the hypothetical molecule with a single Kekule structure; we
`would then ask why the molecule was prevented from assuming this more stable structure.
`The theory of resonance is based upon a theorem in quantum mechanics that the normal
`state of an atom or molecule is the most stable of all possible states.
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`[7.5] Ammonia and Its Compounds (cid:9)
`
`235
`
`The resonance energy of benzene makes the substance far less reactive
`chemically than alkenes or other unsaturated substances. For example, the
`reaction of addition of one hydrogen molecule to benzene to form cyclo-
`hexadiene,
`
`CH
`\
`CH2
`
`HC (cid:9)
`
`CH2
`/
`
`CH
`is endothermic, not exothermic. The properties of benzene and other
`aromatic substances reflect the stability conferred upon them by the reso-
`nance energy.
`
`7-5. Ammonia and Its Compounds
`
`Ammonia, NH3, is an easily condensable gas (b.p. 33.4°C; m.p. — 77.7°C),
`readily soluble in water. The gas is colorless and has a pungent odor, often
`detected around stables and manure piles, where ammonia is produced
`by decomposition of organic matter. The solution of ammonia in water,
`called ammonium hydroxide solution (or sometimes aqua ammonia), con-
`tains the molecular species NH3, NH4OH (ammonium hydroxide), NH4+,
`and OH—. Ammonium hydroxide is a weak base, and is only slightly
`ionized to ammonium ion, NH4+, and hydroxide ion OH—:
`NH3 + H2O .<—=>- NH4OH ± NH4÷ + OH—
`The ammonium ion has the configuration of a regular tetrahedron. The
`NH4+ ion can be described as having electron pairs in four tetrahedral
`spa orbitals. In the ammonium hydroxide molecule the ammonium ion and
`the hydroxide ion are held together by a hydrogen bond (Section 12-4).
`
`The Preparation of Ammonia
`
`Ammonia is easily made in the laboratory by heating an ammonium
`salt, such as ammonium chloride, NH4C1, with a strong alkali, such as
`sodium hydroxide or calcium hydroxide:
`2NH4C1 Ca(OH)2 CaCl2 2H20 2NH3(g)
`The gas may also be made by warming concentrated ammonium hy-
`droxide.
`The principal commercial method of production of ammonia is the
`Haber process, the direct combination of nitrogen and hydrogen under
`high pressure (several hundred atmospheres) in the presence of a catalyst
`(usually iron, containing molybdenum or other substances to increase the
`
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