Organic Chemistry PDF

Ass. L. Mervat M. Organic chemistry The branch of chemistry that deals with the study of carbon and its compounds is called organic chemistry. Modern research has revealed that carbon can combine with other elements to create an almost infinite variety of compounds. In health, agriculture, a...

Ass. L. Mervat M. Organic chemistry The branch of chemistry that deals with the study of carbon and its compounds is called organic chemistry. Modern research has revealed that carbon can combine with other elements to create an almost infinite variety of compounds. In health, agriculture, and everyday life, organic chemicals have an impact on us every day. All organic compounds can be classified into two broad classes: 1- Aliphatic compounds: are open-chain compounds and those cyclic compounds that resemble the open-chain compounds like alkanes, alkenes, alkynes, and their cyclic analogs. 2- Aromatic compounds: are benzene and compounds that resemble benzene in chemical behavior. Aromatic properties are those properties of benzene that distinguish it from aliphatic hydrocarbons. Some compounds that possess aromatic properties have structures that seem to differ considerably from the structure of benzene: actually, however, there is a basic similarity in electronic configuration Structure of benzene Benzene has been known since 1825, it’s chemical and physical properties are perhaps better known than those of any other single organic compound. In spite of this, no satisfactory structure for benzene had been advanced until about 1931, and it was ten to fifteen years before this structure was generally used by organic chemists. The difficulty was not the complexity of the benzene molecule, but rather the limitations of the structural theory as it had so far developed. Since an understanding of the structure of benzene is important both in our study of aromatic compounds and in extending our knowledge of the structural theory, we shall examine in some detail the facts upon which this structure of benzene is built Molecular formula. Isomer number. Kekule's structure Benzene has the molecular formula C6H6. From its elemental composition and molecular weight, benzene was known to contain six carbon atoms and six hydrogen atoms. The question was: how are these atoms arranged? In 1858, August Kekule (of the University of Bonn) had proposed that carbon atoms can join to one another to form chains, then, in 1865, he offered an answer to the question of benzene: these carbon chains can sometimes be closed, to form ring. Kekule's structure of benzene was one that we would represent today as I Other structures are, of course, consistent with the formula C6H6 for example: II- V. Of all these, Kekule's structure was accepted as the most nearly satisfactory the evidence was of a kind with which we are already familiar isomer number (isomers are chemical compounds that have identical chemical formulae but differ in properties and the arrangement of atoms in the molecule)  Benzene yields only one mono-substitution product (C6H5Y). Only one Bromobenzene (C6H5Br) is obtained when one hydrogen atom is replaced by bromine, similarly, only one chlorobenzene (C6H5C1) or one nitrobenzene, (C6H5NO2)…. etc., has ever been made. This fact places a severe limitation on the structure of benzene: each hydrogen must be exactly equivalent to every other hydrogen, since the replacement of any one of them yields the same product Structure V, for example, must now be rejected, since it would yield two isomeric mono-bromo derivatives, the 1-bromo and the 2-bromo compounds, all hydrogens are not equivalent in V. Similar reasoning shows us that II and III are likewise unsatisfactory. I and IV, among others, are still possibilities. Benzene yields three isomeric di-substitution products, (C6H4Y2 )or C6 H4YZ. Three and only three isomeric di-bromo-benzenes, C6H4Br2,three chloronitrobenzenes, C6H4C1NO2, etc., have ever been made. This fact further limits our choice of a structure; for example, IV must now be rejected. At first glance, structure I seems to be consistent with this new fact; that is, we can expect three isomeric dibromo derivatives, the 1,2- the 1,3-, and the 1,4-dibromo compounds Closer examination of structure I shows, however, that two 1,2-dibromo isomers (VI and VII), 'differing in the positions of bromine relative to the double bonds, should be possible But Kekule visualized the benzene molecule as a dynamic thing:"... the form whirled mockingly before my eyes. “. V He described it in terms of two structures, VIII and IX, between which the benzene molecule alternates. As a consequence, the two 1,2-dibromobenzenes (VI and VID would be in rapid equilibrium and hence could not be separated. Later, when the idea of tautomerism became defined, it was assumed that Kekule's "alternation" essentially amounted to tautomerism. Stability of the benzene ring. Reactions of benzene Benzene undergoes substitution rather than addition. Kekule's structure of benzene is one that we would call "cyclohexatriene" We would expect this cyclohexatriene, like the very similar compounds, cyclohexadiene and cyclohexene, to undergo readily the addition reactions characteristic of the alkene structure. As the examples in below table show, this is not the case; under conditions that cause an alkene to undergo rapid addition, benzene reacts either not at all or very slowly. Cyclohexene vs. Benzene Reagent Cyclohexene Benzene KMNO4 (cold, dilute, Rapid oxidation No reaction aqueous) Br2\CCl4 (in the dark) Rapid addition No reaction HI Rapid addition No reaction H2+Ni Rapid hydrogenation at Slow hydrogenation at 25o , 20 Ib ̸ in2 100-200o , 1500 Ib ̸ in2 In place of addition reactions, benzene readily undergoes a new set of reactions, all involving substitution. Reactions of benzene 1. Nitration 2. Sulfonation 3. Halogenation 4. Friedel-Crafts alkylation 5. Friedel- Crafts acylation In each of these reactions an atom or group has been substituted for one of the hydrogen atoms of benzene. The product can itself undergo further substitution of the same kind, the fact that it has retained the characteristic properties of benzene indicates that it has retained the characteristic structure of benzene. It would appear that benzene resists addition, in which the benzene ring system would be destroyed, whereas it readily undergoes substitution, in which the ring system is preserved. Stability of the benzene ring. Heats of hydrogenation and combustion Heats of hydrogenation and combustion of benzene are lower than expected, (heat of hydrogenation is the quantity of heat evolved when one mole of an unsaturated compound is hydrogenated). In most cases the value is about (28-30) kcal for each double bond the compound contains. The cyclohexene has a heat of hydrogenation of 28.6 kcal and cyclohexadiene has one about twice that (55.4 kcal.) We might reasonably expect cyclohexatriene to have a heat of hydrogenation about three times as large as cyclohexene, that is, about 85.8 kcal. for benzene (49.8 kcal) is 36 kcal less than this expected amount. The fact that benzene evolves 36 kcal less energy than predicted can only mean that benzene contains 36 kcal less energy than predicted. In other words, benzene is more stable by 36 kcal than we would have expected cyclohexatriene to be. Carbon-carbon bond lengths in benzene All carbon- carbon bonds in benzene are equal and are intermediate in length between single and double bonds. Carbon-carbon double bonds in a wide variety of compounds are found to be together about 1.34 A. In benzene actually possessed three single and three double bonds, as in a Kekule structure, we would expect to find three short bonds (1.34 A) and three long bonds (1.48 A, probably, as in 1,3-butadiene). Actually, x-ray diffraction studies show that the six carbon-carbon bonds in benzene are equal and have a length of 1.39 A, and are thus intermediate between single and double bonds. Resonance structure of benzene The currently accepted structure is the result of an extension or modification of the structural theory, this extension is the concept of resonance (structures that differ only in the arrangement of electrons) The Kekule structures I and II, we now immediately recognize, meet the conditions for resonance. When it is realized that all carbon-carbon bonds in benzene are equivalent, there is no longer any difficulty in accounting for the number of isomeric di-substitution products. It is clear that there should be just three, in agreement with experiment Finally, the "unusual" stability of benzene is not unusual at all: it is what one would expect of a hybrid of equivalent structures. The 36 kcal of energy that Benzene does not contain compared with cyclohexatriene is resonance energy. It is the 36 kcal of resonance energy that is responsible for the new set of properties we call aromatic properties. Addition reactions convert an alkene into a more stable saturated compound. Hydrogenation of cyclohexene, for example, is accompanied by the evolution of 28.6 kcal, the product lies 28.6 kcal lower than the reactants on the energy scale. But addition would convert benzene into a less stable product by destroying the resonance-stabilized benzene ring system; for example, the first stage of hydrogenation of benzene requires 5.6 kcal to convert benzene into the less stable cyclohexadiene. As a consequence, it is easier for reactions of benzene to take an entirely different course, one in which the ring system is retained substitution. Orbital picture of benzene In benzene each carbon is bonded to three other atoms, it uses sp 2 orbitals These lie in the same plane, that of the carbon nucleus, and are directed toward the corners of an equilateral triangle. If we arrange the six carbons and six hydrogens of benzene to permit maximum overlap of these orbitals to obtained below structure Benzene is a flat molecule, with every carbon and every hydrogen lying in the same plane. It is a very symmetrical molecule, too, with each carbon atom lying at the angle of a regular hexagon; every bond angle is 120o. The molecule is not yet complete, however. There are still six electrons to be accounted for. In addition to the three orbitals already used, each carbon atom has a fourth orbital, a p orbital. As we know, this p orbital consists of two equals lobes, one lying above and the other lying below the plane of the other three orbitals, that is, above and below the plane of the ring; it is occupied by a single electron. As in the case of ethylene, the orbital of one carbon can overlap the p orbital of an adjacent carbon atom, permitting the electrons to pair and an additional π bond to be formed But the overlap here is not limited to a pair of p orbitals as it was in ethylene; the p orbital of any one carbon atom overlaps equally well the p orbitals of both carbon atoms to which it is bonded. The result is two continuous doughnut-shaped electron clouds, one lying above and the other below the plane of the atoms. the delocalization of the π electrons and their participation in several bonds that makes the molecule more stable. the chemical properties of benzene are just what we would expect of this structure. Despite delocalization, the π electrons are nevertheless more loosely held than the σ electrons. The π electrons are thus particularly available to a reagent that is seeking electrons: the typical reactions of the benzene ring are those in which it serves as a source of electrons for electrophilic (acidic) reagents. Because of the resonance stabilization of the benzene ring, these reactions lead to substitution, in which the aromatic character of the benzene ring is preserved. Representation of the benzene ring the benzene ring by a regular hexagon containing a circle, it is understood that a hydrogen atom is attached to each angle of the hexagon unless another atom or group is indicated. Aromatic character. The Hückel 4n + 2 Aromatic compounds are compounds which are resistant to the addition reactions but rather than undergo electrophilic substitution reactions. Aromatic Compounds are defined as those which meet the following criteria: 1. The structure must be cyclic, and contain some number of conjugated π bonds. 2. The unhybridized p orbitals must overlap to form a continuous ring of parallel orbitals. This is usually achieved through a planar (or almost planar) arrangement, allowing for the most efficient overlap. Delocalization of the π electrons over the ring must result in a lowering of the electronic energy. 3. Hückel's Rule states that if the number of π electrons in the cyclic system is equal to (4n+2), where n is a whole number integer, then the system is aromatic. Thus, systems with 2, 6, 10, 14, … π electrons are aromatic. Q\ which of the following compounds is aromatic?  1. Cycle 2. Delocalization of the π electrons over the ring 3. 4n+2=6 n=1 (integer no.) (aromatic) 1. Cycle 2. Delocalization of the π electrons over the ring 3. 4n+2=4 n=1 ̸ 2 (not integer no.) (anti-aromatic) 1. Cycle 2. 4n+2=2 n=0 ( integer no.) 3. no resonance (not aromatic) 1. 4n+2=6 n=1 ( integer no.) 2. Delocalization of the π electrons 3. non-cycle (not aromatic) Nomenclature of benzene derivatives For many of benzene derivatives we simply prefix the name of the substituent group to the word -benzene, as, for example, in chlorobenzene, bromobenzene, iodobenzene, or nitrobenzene. Other derivatives have special names for example, methylbenzene is always known as toluene, aminobenzene as aniline, hydroxybenzene as phenol, and so on. The most important of these special derivatives are: If several groups are attached to the benzene ring, we must not only tell what they are, but also indicate their relative positions. The three possible isomers of a disubstituted benzene are differentiated by the use of the names ortho, meta, and para. For example: If the two groups are different, and neither is a group that gives a special name to the molecule, we simply name the two groups successively and end the word with -benzene, as, for example, chloronitrobenzene, bromoiodobenzene, etc. If one of the two groups is the kind that gives a special name to the molecule, then the compound is named as a derivative of that special compound, as, for example, nitrotoluene, bromophenol, etc. If more than two groups are attached to the benzene ring, numbers are used to indicate their relative positions. For example: