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Organic Chemistry Jin Hongwei College of Chemical Engineering and Materials Science [email protected] Chapter Four Alkadienes and Conjugated System What are alkadienes? The Classification and nomenclature of Dienes. The Structures of Dienes. Conjugated Dienes and Conjugated System. Chemical Properties of Conjugated Dienes. Synthesis of Conjugated Dienes. What are Alkadienes? Alkadienes are hydrocarbons that contain two carbon-carbon double bonds.二烯烃－－－含有两个碳碳双键的烃。Alkadienes have the general formula CnH2n-2.Alkatriene, alkadiyne, alkenyne. BACK BACK The Classification and Nomenclature of Dienes There are three kinds of dienes according to the position of the two carbon-carbon doubles. For example: Cumulated Diene: Conjugated Diene: Isolated Diene: How many trans-cis isomers are there in 2,4-Hexadiene? The Structure of Alkadienes The Structure of Allene: The Structure of 1,3-Butadiene: BACK BACK The Conjugated System π,π-conjugated system: p, π-conjugated system:σ,π-conjugated system:σ, p –conjugated system: Chemical Properties of Conjugated Dienes(1) Electrophilic Attack on Conjugated Dienes: 1,4-Addition Not only are conjugated dienes somewhat more stable than nonjugated dienes, they also display special behavior when they react with electrophilic reagents. For example, 1,3-butadiene reacts with one molar equivalent of hydrogen chloride to produce two products: Step1 Step2 Chemical Properties of Conjugated Dienes(2) The Diels-Alder Reaction: A 1,4-Cycloaddition Reaction of Dienes In 1928 two German chemists, Otto Diels and Kurt Alder, developed a 1,4-cycloaddition reaction of dienes that has since come to bear their names. The reaction proved to be one of such great versatility and synthetic utility that Diels and Alder were awarded the Nobel Prize in chemistry in 1950. For example: Chemical Properties of Conjugated Dienes(3) Electrocyclic Reactions (热顺光对）An electrocylic reaction is a pericyclic process that involves the cyclization of a conjugated polyene. One πbond is broken, the other πbond change position, a new σbond is formed, and a cyclic compound results. For example: BACK Synthesis of Conjugated Dienes There are many methods to prepare conjugated dienes. For example: Additional problems of chapter four 4.1 Draw the possible products resulting from addition of 1 equivalent of HCl to 1-phenyl-1,3-butadiene. Which would you expect to predominate, and why? 4.2 Predict the products of the following Diels-Alder reactions: (a) (b) 4.3 Show the structure, including stereochemistry, of the product from the following Diels-Alder reaction: