Addition rxns of c c multiple bond
Download as PPTX, PDF0 likes171 views
1. The document discusses addition reactions of C-C multiple bonds, specifically alkenes and alkynes. It describes various reagents that add across the double or triple bonds, such as hydrogen halides, water, and halogens. 2. Markovnikov's rule is explained, stating that hydrogen adds to the carbon with more hydrogen substituents in alkene additions. Anti-Markovnikov additions are also possible using peroxides. 3. Methods to form alcohols from alkenes like acid-catalyzed hydration and oxymercuration-demercuration are described.
Addition rxns of c c multiple bond
- 1. Department of Chemistry University of Kashmir Topic:-Addition Reactions of C-C Multiple Bond Aadil Ali WAni Roll no: 16062120001 Teacher Incharge Prof. Syed Wajahat Amin Shah
- 2. Addition Reactions of C-C Multiple bond A chemical reaction of two or more reacting molecular entities, resulting in a single reaction product containing all atoms of all components, with formation of two chemical bonds and a net reduction in bond multiplicity in at least one of the reactants. The reverse process is called an elimination reaction. The addition may occur at only one site (α-addition, 1/1/addition), at two adjacent sites (1/2/addition) or at two non-adjacent sites (1/3/- or 1/4/addition, etc.). For example:
- 3. Addition Reaction of Alkene with HBr
- 4. Addition of HX to Alkenes: Markovnikov’sRule Markovnikov'srule:in the addition of HX to an alkene, the hydrogen atom adds to the carbon atom of the double bond that already has the greater number of hydrogen atoms.
- 5. Addition of HX to Alkenes: Markovnikov’sRule
- 6. Addition of HX to Alkenes: Markovnikov’sRule Modern statement of Markovnikov'srule:In the ionic addition of an unsymmetrical reagent to a double bond, the positive portion of the adding reagent attaches itself to a carbon atom of the double bond so as to yield the more stable carbocation as an intermediate.
- 7. Stereochemistry of the Addition of HX to Alkenes
- 9. Reason • H-Cl bond is stronger than H-Br Bond. H-Cl is not decomposed by peroxide-free radical. • Iodine free radical formed as H-I bond is weaker but iodine-free radicals readily combine with each other to form iodine molecules rather than to attack the double bond. • In H-Br both the chain propagation steps are exothermic and hence peroxide effect is observed, while in HCl (or HF) the second step is Endothermic and in HI the first step is endothermic therefore , peroxide effect is not observed in HCl or HI. X ̇ +CH₂=CH₂ CH₂̇ ̶ CH ̶ X (I) X ΔH(KJ/mole) Cl -67.0 Br -25.1 I +46.o Cl +12.6 H ̶ X + X ̶ CH₂ ̶ CH₂̇ X ̶ CH₂ ̶ CH₂ ̶ H +X ̇ (II) Br - 50.2
- 10. Addition of Sulfuric Acid to Alkenes
- 11. Addition of Water to Alkenes: Acid-Catalyzed Hydration
- 12. Addition of Water to Alkenes: Rearrangement
- 13. Oxymercuration-Demercuration This is another alternative for converting alkenes to alcohols with Markovnikov orientation. This method has the advantage of not involving free carbocationic species, and thus removes the possibility of rearrangements. The reagent is called mercuric acetate, and is usually abbreviated to Hg(OAc)2. In solution it ionizes into acetate ion and a positively charged mercury species which is very electrophilic Oxymercuration is the electrophilic attack of this species on a double bond, giving a 3 membered ring compound called a mercurinium ion. Oxymercuration-demercuration also gives Markovnikov orientation of the alcohol.
- 14. Hydroboration of Alkenes We have studied three ways of hydrating alkenes to give Markovnikov orientated alcohols. There is also a way to obtain anti-Markovnikov oriented alcohols: hydroboration. Borane adds to alkenes with anti-Markovnikov orientation, and these alkylboranes can then be oxidized to alcohols. Borane (BH3) itself is unstable, and exists either as a dimer or as a complex with THF.
- 15. Halohydrin Formation When the halogenation reaction takes place in the presence of a nucleophilic solvent, a nucleophile different from the halide can open the halonium ring. If the reaction is performed in the presence of water, Halohydrins are produced. The stereochemistry of the intermediate bromonium ring determines that the final product must be of anti addition, since ring opening must occur via backside attack. Orientation of Halohydrin Formation When propene reacts with chlorine water, the major product has the Cl bound to the less highly substituted carbon, and hydroxyl to the most highly substituted carbon
- 16. Addition Reactions of Alkynes Many of the reactions of alkynes are similar to those of alkenes since they both type of compound involve π bonds. Catalytic Hydrogenation to Alkanes Under normal conditions, the reaction cannot be stopped, producing the alkane.
- 17. Hydrogenation to cis-Alkenes However, It is possible to add only one equivalent of hydrogen to the triple bond and produce cis alkenes. The catalyst used to perform this reaction is known as the Lindlar’s catalyst.
- 18. The mechanism is the same as for hydrogenation of alkenes, but the catalyst is not active enough (we say that the catalyst is poisoned) to carry out the hydrogenation of the resulting alkene. Gives cis-alkenes since hydrogenation is syn reaction (same side of triple bond)
- 19. Reduction to trans-Alkenes It is also possible to form trans-alkenes from alkynes. In this case hydrogen is not involved, and the reduction is carried out with sodium metal (or lithium) in liquid ammonia by a radical mechanism)
- 20. Addition of Halogens You might expect the mechanism to be similar to the halogenation of alkenes, yet stereochemical evidence suggests otherwise. ● The halogenation of an alkene undergoes anti addition ONLY. ● The mechanism for alkyne halogenation is not fully elucidated.
- 21. Addition of Hydrogen Halides Similar to the reaction we have seen for alkenes, When the reaction is performed with a terminal alkyne, the orientation follows Markovnikov (i.e. most substituted carbocation is formed). • Peroxides can be used in the hydrohalogenation of alkynes to promote anti‐Markovnikov addition just like with alkenes.
- 22. Addition Reactions ‐ hydration of alkynes Internal alkynes undergo hydration with concentrated acid to form ketones. • Terminal alkynes require the presence of an additional Hg2+ catalyst (usually HgSO4) to yield methyl ketones by Markovnikov addition of water.
- 23. Oxidation The same reagents that oxidized alkenes can oxidize alkynes in a similar way. Using KMnO4 under neutral conditions, a vicinal diketones are obtained with internal alkynes.