Vinyl carbon.pptx

Vinyl carbon

1. S n i mechanism & Nucleophilic substitution at a vinyl carbon

2. S n i mechanism : It occurs with retention of configuration and there is no possibility of a neighboring group effect. In the S n i mechanism (substitution nucleophilic internal) part of the leaving group must be able to attack the substrate, detaching itself from the rest of the leaving group in the process.

3. First step: The reaction of alcohols with thionylcholoride to give alkyl halides the chlorosulphinate is formed with retention. Since R-O bond is not broken during this reaction. These alkyl cholorosulphinate can be isolated.

4. Second step: (part of the leaving group attacks)

5. Evidence for this mechanism is as follows: The addition of pyridine to the mixture of alcohol and thionyl chloride results in the formation of alkyl halide with inversion of configuration. The pyridine coordinates with the Hcl produced during the formation of intermediate chlorosulphite from ROH and SOcl2 to form pyridine hydro chloride and the cl - is an effective nucleophile.

6. The displacement of the chlorosulphinate ester by cl via S N 2 mechanism gives product with complete inversion of configuration.

7. Factor affecting S n i mechanism: Effect of changing the solvent: At 75 C various by a factor of 10,000 in changing from dioxin to nitro benzene. Which is in consistent with unpaired, because a covalent process would show only moderate response to solvent polar. However, nucleophilic solvent has to be avoided to get S n i mechanism or retention, last there should be any attack by solvent.

8. 2. Effect of substituent : Eg : α -phenyl ethyl chloroformate show a large negative value -3.86. Which is explains a tight ion pair with positive charge development in the transition state. So this reaction is facilitated by substituents those increasing electron density at the Centre carbon.

9. 3. Effect of chloride ion: When excess chloride is added or pyridine is added the external nucleophile or the cl - formed during the reaction and ionization Attacks from the rear side leading to inversion. (Generally energies are added to remove Hcl formed.)

10. Nucleophilic substitution at a vinyl carbon : While allyl carbocation is stabilized by resonance for vinyl substrates the molecule is stabilized. Hence vinylic substrates are somewhat non-reactive towards nucleophilic reagents just as phenyl .

11. There are 3 different mechanism: Tetra hedral mechanism Addition – elimination mechanism Elimination- addition mechanism

12. Tetrahedral mechanism : It takes place with much less facility than with carbonyl groups, since negative charge of the intermediate must be born by the less electronegative carbon. (Then O, S, and N) hence also the addition is followed by elimination . Intermediate can be stabilized by combining with a positive species and that is addition to C=C bond here the addition and substitution often complexes .

13. Evidence for tetrahedral intermediate mechanism : When the leaving group is changed from Br or cl to F. the rate increase although F is a poor leaving group, because of superior electron with drawing character of F makes C-F bonds carbon more positive and more susceptible to nucleophilic attack. This atoms that C-X bond does not break in rate determinate step (as it would in S N 1 and S N 2) however in more cases second step is rate determining step

14. 2. Addition – elimination mechanism : Nucleophilic addition of ArSH to 1, 1-dichloro ethylene catalyzed by Eto -. The product was not 1, 1-di thiophenoxy compound but the rearranged 1, 2-dithiophenoxy compound.

15. 3. Elimination – addition mechanism : The reaction does not proceed without Eto - . No simple substitution Rate α [ Eto ] but not on [ ArSH ]. Both lead to retention. Since both are anti addition and elimination.

16. Characteristics of nucleophilic substitution at vinyl carbon: 1. Groups : ZCH=CHX ; when Z is electron withdrawing group. Such as HCO, RCO, - COOEt , ArSO2, CN and F carbanion stability increased by spreading of negative charge. Rentetion observed here is attributed to hyper conjugation involving the carbanion electron pair and the substitution on adjacent carbon stereo conversance 1:1 mixture of E or Z observed, when the carbanionic carbon has two electron with drawing group

17. 2. α - Statidisation α - Aryl vinyl halide ( ArCBr =CR2) or α - cyclo propyl, α - vinyl and an adjacent double bond make vinyl substrate to undergo sn1 reaction. 3. Good leaving group: Even without α - stabilization good leaving groups such as (-O SO2 CF3) triflurate make the vinyl substrate to undergo nucleophilic substitution.

18. 4. Stereo chemical outcome: Stereo chemical outcome of S N 1 mechanism of vinyl substrate is often randomization. Cis or trans substrate gives 1:1 mixture of cis and trans products indicating vinyl cation is linear ( sp ). But when R 1 and R 2 are present the entry of the nucleophile is influenced by the relative size of R 1 and R 2 as the empty p orbital lies in the plane of the double bond.