Types of Reactions.pdf

Types Of Reactions Organic chemistry Types of Reactions Three major reaction types we have studied: 1. Addition, 2. Elimination, 3. Substitution. Types of Reactions Reactions of the carbon-carbon double bond: addition The double bond consists of a strong σ bond and a weak π bond; therefore, the reac...

Types Of Reactions Organic chemistry Types of Reactions Three major reaction types we have studied: 1. Addition, 2. Elimination, 3. Substitution. Types of Reactions Reactions of the carbon-carbon double bond: addition The double bond consists of a strong σ bond and a weak π bond; therefore, the reaction would involve the breaking of this weaker bond. Where the π bond is broken and two strong σ bonds are formed in its place. A reaction in which two molecules combine to yield a single molecule of product is called an addition reaction. Types of Reactions The typical reaction of an alkene is electrophilic addition, or, in other words, addition of acidic reagents (deficient in electrons). In many of its reactions the carbon-carbon double bond serves as a source of electrons: that is, it acts as a base. Reagents of another kind, free radicals, seek electrons or, rather, seek an electron. And so we find that alkenes also undergo free-radical addition. Besides the addition reactions characteristic of the carbon-carbon double bond, alkenes may undergo the free-radical substitution characteristic of alkanes. Addition Reactions Addition reactions 1. Addition of hydrogen. catalytic hydrogenation We have already encountered hydrogenation as the most useful method for preparing alkanes. Addition reactions 2. Addition of halogens Alkenes are readily converted by chlorine or bromine into saturated compounds that contain two atoms of halogen attached to adjacent carbons; iodine generally fails to react. Addition reactions 3. Addition of hydrogen halides. Markovnikov's rule An alkene is converted by hydrogen chloride, hydrogen bromide, or hydrogen iodide into the corresponding alkyl halide. Addition reactions 3. Addition of hydrogen halides. Markovnikov's rule Propylene could yield either of two products, the n-propyl halide or the isopropyl halide, depending upon the orientation of addition, that is, depending upon which carbon atoms the hydrogen and halogen become attached to. Actually, it is found that the isopropyl halide greatly predominates. Addition reactions 3. Addition of hydrogen halides. Markovnikov's rule In the same way, isobutylene could yield either of two products, isobutyl halide or tert-butyl halide; here the orientation of addition is such that the tert-butyl halide greatly predominates. Addition reactions 3. Addition of hydrogen halides. Markovnikov's rule In 2-pentene each of the doubly-bonded carbons holds one hydrogen, so that according to the rule we should expect neither product to predominate. Here again the prediction is essentially correct, roughly equal quantities of the two isomers actually being obtained. Markovnikov’s Rule (extended): in an electrophilic addition to an alkene, the electrophile adds in such a way as to generate the most stable intermediate. Addition reactions 3o > 2o > 1o > +CH3 An electrophile adds to a double bond to give the most stable carbonation in the intermediate. Addition reactions The examples have involved the addition of hydrogen iodide; exactly similar results are obtained in the addition of hydrogen chloride and, except for special conditions indicated in the following section, of hydrogen bromide. Addition of hydrogen bromide. Peroxide effect Addition of hydrogen chloride and hydrogen iodide to alkenes follows Markovnikov's rule. Addition reactions Addition of hydrogen bromide to a particular alkene yields a product in agreement with Markovnikov's rule; by others, a product in contradiction to Markovnikov's rule; and by still others, a mixture of both products. The orientation of addition of hydrogen bromide to the carbon-carbon double bond is determined by the presence or absence of peroxides. Organic peroxides are compounds containing the ─O─O─ linkage. Addition reactions These anti-Markovnikov reactions were most likely when the reagents or solvents came from old supplies that had accumulated peroxides from exposure to the air. Peroxides give rise to free radicals that initiate the addition, causing it to occur by a radical mechanism. The oxygen–oxygen bond in peroxides is rather weak, so it can break to give two alkoxy radicals. Alkoxy radicals (R-O.) initiate the anti-Markovnikov addition of HBr. Addition reactions The anti-Markovnikov orientation found in the products of the peroxide-catalyzed reaction. Addition reactions Only the addition of hydrogen bromide shows the peroxide effect. The presence or absence of peroxides has no effect on the orientation of addition of hydrogen chloride, hydrogen iodide, sulfuric acid, water, etc. Addition reactions 4- Addition of sulfuric acid Alkenes react with cold, concentrated sulfuric acid to form compounds of the general formula ROSO3H, known as alkyl hydrogen sulfates. These products are formed by addition of hydrogen ion to one side of the double bond and bisulfate ion to the other. It is important to notice that carbon is bonded to oxygen and not to sulfur. Addition reactions If the sulfuric acid solution of the alkyl hydrogen sulfate is diluted with water and heated, there is obtained an alcohol bearing the same alkyl group as the original alkyl hydrogen sulfate. The alkyl hydrogen sulfate has been cleaved by water to form the alcohol and sulfuric acid, and is said to have been hydrolyzed. Addition reactions All types of alcohol can be prepare by this method?? NO Because the addition of sulfuric acid follows Markovnikov's rule, certain alcohols cannot be obtained by this method. For example, isopropyl alcohol can be made but not npropyl alcohol. Addition reactions 5- Addition of water. Hydration Water adds to the more reactive alkenes in the presence of acids to yield alcohols, this addition follows Markovnikov's rule. Electrophilic Addition Electrophilic Addition Mechanism A wide variety of electrophilic additions involve similar mechanisms. First, a strong electrophile attracts the loosely held electrons from the pi bond of an alkene. The electrophile forms a sigma bond to one of the carbons of the (former) double bond, while the other carbon becomes a carbocation. Second, the carbocation (a strong electrophile) reacts with a nucleophile (often a weak nucleophile) to form another sigma bond. Electrophilic Addition Electrophilic Addition Step (1) is the difficult step, and its rate largely or entirely controls the overall rate of addition. This step involves attack by an acidic, electron-seeking reagent that is, an electrophilic reagent and hence the reaction is called electrophilic addition. Electrophilic Addition NOTE: The electrophile need not necessarily be a Lowry-Bronsted acid transferring a proton, as shown here, but can be almost any kind of electron-deficient molecule (Lewis acid). Electrophilic Addition Electrophilic Addition Rearrangement The mechanism of electrophilic addition is consistent with the occurrence of rearrangements. If carbonium ions are intermediates in electrophilic addition, rearrangements are not only observed, but they occur according, to just the pattern that would be predicted Electrophilic Addition For example, addition of hydrogen chloride to 3,3-dimethyl-l-butene yields not only 3-chloro-2,2dimethylbutane, but also 2-chloro-2,3dimethylbutane: Since a 1,2-shift of a methyl group can convert the initially formed secondary cation into the more stable tertiary cation, such a rearrangement does occur, and much of the product is derived from this new ion. Free Radical Addition Mechanism of the peroxide initiated addition of the HBr: