[ii] Free radical addition reaction
Mechanism
[iii] Cyclic reaction
(a) Electrocyclic reaction – By the effect of temperature when conjugated polyene forms cyclic isomerised product known as electrocyclic reaction.
These are reversible chemical reaction.
(b) Cyclo addition reaction – When conjugated diene system reacts with some 2p electron system it means alkene, alkyne or its derivative to form six membered cyclic compound. This reaction is known as Diels - alder reaction.
[iv] The Diels-Alder reaction
The Diels-Alder reaction is the conjugate addition of an alkene to a diene, Using 1,3-butadiene as a typical diene, the Diels-Alder reaction may be represented by the general equation following general reaction where polarity occurs and followed by addition.
The alkene that adds to the diene is called the dienophile. Because the Diels-Alder reaction leads to the formation of a ring, it is termed a cycloaddition reaction. The product contain a cyclohexene ring as a structural unit.
The Diels-Alder cycloaddition is one example of a class called pericyclic reactions. A pericyclic reaction is a one-step reaction that proceeds through a cyclic transition state. Bond formation occurs at both ends of the diene system, and the Diels. Alder transition state involves a cycle formation of six carbons and six p electrons. The diene must adopt the cis conformation in the transition state.
The most common and simplest of all Diels-alder reactions, cycloaddition of ethene to 1, 3-butadiene, does not proceed readily. It has a high activation energy and a low rate of reaction. However, substituents such as C = O or C º N, when directly attached to the double bond of the dienophile, increase its reactivity, and compounds of this type give high yields of Diels-Alder products at very normal temperature.
The product of a Diels-Alder cycloaddition always contains one more ring than already present in the reactants. The dienophile maleic anhydride contains one ring, so the product of its addition to a diene contains two.
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