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Free Radical Addition Reactions of Conjugated Dienes, Allylic Rearrangement

In general, conjugated dienes react similarly to 1,3-butadiene.
In general, conjugated dienes react similarly to 1,3-butadiene. In addition to its usual chemical reactions, it undergoes radical additions and catalytic hydrogenation. However, since the compound has no isolated double bonds, it undergoes these reactions more readily than most alkenes. It is also common to find 1,2 and 1,4 addition products:

1,2- and 1,4-additions can be produced by other electrophiles, such as hydrogen halides. As shown in Equation, the mechanism of hydrogen chloride's reaction with 1,3-butadiene is described. As with alkenes, a carbocation is first formed. In contrast, if you add a proton to C1C1 (but not C2C2), the charge will be distributed over two carbons, resulting in a cation with substantial delocalization energy. When Cl- attacks one or both of the positive carbon atoms, the 1,2- or 1,4- addition product is formed:

It is important to remember that in reactions in which 1,2 and 1,4 additions occur simultaneously, the ratio of the products will also depend on the temperature, the solvent, and the length of time of the reaction. Due to the reversible nature of carbocation formation, the ratio of products at equilibrium does not have to match the ratio of attacks by Cl⊖ at C1C1 and C3C3 of the carbocation. An example of the difference between kinetic and equilibrium control can be seen in the product ratios.

Because 1,2 forms more rapidly at low temperatures, whereas k-1 or k-3 are slower back reactions. It is not because 1,4 is formed more rapidly at equilibrium 1, but because it is stronger and more stable. Radical-chain mechanisms are also used in conjugated dienes to create addition reactions. A 1,4 adduct is almost always the addition product. Hydrogen bromide radical addition produces l-Bromo-2-butene.

Allylic rearrangement

When a carbon atom moves between an allyl double bond and the next carbon atom in an organic compound, this is called a rearrangement or shift of the double bond. Nucleophilic substitution is the mechanism for these reactions. Carbocations with several resonance structures are the intermediates in SN1 reactions that are favorable for the reaction. Recombination with nucleophile Y results in the spread of products (or product distribution). SN1 substitutions are a process similar to this. In a process known as SN2 substitution, a nucleophile may attack directly at the allylic position and displace the leaving group in a single step. As a consequence, free allyl compounds are likely to form when a strong nucleophile is used. As a result, the product will resemble that of SN1'. Hence, a two-butene aldehyde and a three-butene aldehyde mixture result from the reaction of 1-chloro-2-butene with sodium hydroxide:

Even so, the OH group on the primary atom only contributes a minor amount to the product. When 1- chloro-3-methyl-2-butene is substituted with 2-methyl-3-butene-2-ol, the secondary product has an 85% yield, while the primary product has a 15% yield. One reaction mechanism involves the nucleophile attacking the double bond through a conjugate addition instead of directly attacking the electrophilic site:

Normally, this phenomenon occurs in allylic compounds with bulky leaving groups under SN2 conditions, or bulky non-leaving substituents that give rise to significant steric hindrance, thus increasing conjugate substitution. SN1' or SN2' reactions follow SN1-like mechanisms, while SN2-like mechanisms follow SN2-like mechanisms. SNi has an analog, SNi', applicable to reactions between allylic compounds and reagents such as SOCl2, similar to how SN1' and SN2' exist for SN1, respectively.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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