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In organic chemistry, an addition reaction is an organic reaction in which two or more molecules combine to form a larger molecule called the adduct. [1] [2] An addition reaction is limited to chemical compounds that have multiple bonds. Examples include a molecule with a carbon–carbon double bond (an alkene) or a triple bond (an alkyne).
In organic chemistry, syn-and anti-addition are different ways in which substituent molecules can be added to an alkene (R 2 C=CR 2) or alkyne (RC≡CR).The concepts of syn and anti addition are used to characterize the different reactions of organic chemistry by reflecting the stereochemistry of the products in a reaction.
Some authors have broadened the definition of the Michael addition to essentially refer to any 1,4-addition reaction of α,β-unsaturated carbonyl compounds. Others, however, insist that such a usage is an abuse of terminology, and limit the Michael addition to the formation of carbon–carbon bonds through the addition of carbon nucleophiles.
The reaction equation for hydration of ethylene is: H 2 C=CH 2 + H 2 O→ H 3 C-CH 2 OH Example of hydrohalogenation: addition of HBr to an alkene. Hydrohalogenation involves addition of H−X to unsaturated hydrocarbons. This reaction results in new C−H and C−X σ bonds.
Adding the hydrogen ion to one carbon atom in the alkene creates a positive charge on the other carbon, forming a carbocation intermediate. The more substituted the carbocation, the more stable it is, due to induction and hyperconjugation. The major product of the addition reaction will be the one formed from the more stable intermediate.
Cyclopropanation is also stereospecific as the addition of carbene and carbenoids to alkenes is a form of a cheletropic reaction, with the addition taking place in a syn manner. For example, dibromocarbene and cis-2-butene yield cis-2,3-dimethyl-1,1-dibromocyclopropane, whereas the trans isomer exclusively yields the trans cyclopropane. [16]
The periselectivity of a particular reaction depends on the structure of both the ketene and the substrate. Although the reaction is predominantly used to form four-membered rings, a limited number of substrates undergo [3+2] or [4+2] reactions with ketenes. Examples of all three modes of cycloaddition are discussed in this section.
Separately, unsubsituted compounds with a relative stable radical can dissociate from hydrogen. In general, these reactions risk polymerized byproducts (see § Side reactions). For example, in the thiol-ene reaction, thiols, [5]: 165–166 disulfides, [5]: 207 and hydrogen sulfide [5]: 191 add across a double bond. But if the unsaturated ...