Chemistry Elimination E1_and_Elimination E2.pptx

Presentation about the Elimination E1 and E2

1. ELIMINATION PRESENTATI ON

2. 2 Elimination Reactions General Features of Elimination: Elimination reactions involve the loss of elements from the starting material to form a new  bond in the product. Alkyl halide + base →

3. 3 Alkyl Halides and Elimination Reactions General Features of Elimination: Removal of the elements HX is called dehydrohalogenation . Dehydrohalogenation is an example of  elimination. The curved arrow formalism shown below illustrates how four bonds are broken or formed in the process.

4. 4 Alkene: The Products of Elimination Recall that the double bond of an alkene consists of a  bond and a  bond. Alkyl Halides and Elimination Reactions

5. 5 The two group always will be anti position and dihedral angle will be 180 deg.Bulky grp also away from each other .Its stereoselective rexn. In view of Stereochemistry Elimination Reactions

6. 6 Alkyl Halides and Elimination Reactions Stereochemistry of the E2 Reaction: Two possible geometries for the E2 reaction Exception of E2

7. 7 The higher the percent s -character, the more readily an atom accepts electron density. Thus, sp 2 carbons are more able to accept electron density and sp 3 carbons are more able to donate electron density. Consequently, increasing the number of electron donating groups on an sp 2 carbon atom makes the alkene more stable. Rate of rexn depends upon C-X bond.K.isotopic effect 3-8. Alkyl Halides and Elimination Reactions Alkene: The Products of Elimination The stability of an alkene increases as the number of R groups bonded to the double bond carbons increases.

8. 8 Alkyl Halides and Elimination Reactions Alkene: The Products of Elimination trans -2-Butene (a disubstituted alkene) is more stable than cis -2-butene (another disubstituted alkene), but both are more stable than 1-butene (a monosubstituted alkene).

9. 9 Alkyl Halides and Elimination Reactions Mechanisms of Elimination: E2 An energy diagram for an E2 reaction:

10. For E2 elimination, the C-Cl bond must be anti periplanar to the C—H bond on a  carbon, and this occurs only when the H and Cl atoms are both in the axial position. The requirement for t rans diax i al geomet r y means t hat e l i m i na t i on m u 10 st occur f rom the less stable conformer, B. Alkyl Halides and Elimination Reactions Stereochemistry of the E2 Reaction: The stereochemical requirement of an anti periplanar geometry in an E2 reaction has important consequences for compounds containing six-membered rings. Consider chlorocyclohexane which exists as two chair conformations. Conformation A is preferred since the bulkier Cl group is in the equatorial position.

11. 11 Alkyl Halides and Elimination Reactions Mechanisms of Elimination: E2

12. 12 Alkyl Halides and Elimination Reactions Mechanisms of Elimination: E2 The S N 2 and E2 mechanisms differ in how the R group affects the reaction rate.

13. 13 Alkyl Halides and Elimination Reactions Mechanisms of Elimination: E2 Characteristics of the E2 mechanism.

14. 14 Elimination Reactions Mecha n isms o f E liminati on : E 1

15. 15 Elimination Reactions Mecha n isms o f E liminati on : E 1 Energy Diagram for an E1 reac tion:

16. 16 The strength of the base usually determines whether a reaction follows the E1 or E2 mechanism. Strong bases like ¯ OH and ¯ OR favor E2 reactions, whereas weaker bases like H 2 O and ROH favor E1 reactions. No kinetic isotopic effect and Regioselective To know major product we have to see the stability of carbocation. Elimination Reactions Mechanisms of Elimination: E1 The rate of an E1 reaction increases as the number of R groups on the carbon with the leaving group increases.

17. 17 Elimination Reactions Mechanisms of Elimination: E1 Table 8.3 summarizes the characteristics of the E1 mechanism.

18. Comparing E1 vs E2 mechanisms: Base strength not important Needs a strong base 3 o > 2 o (1 o C+ is unstable) 3 o > 2 o > 1 o Base: E1 E2 Substrate: E1 E2 Solvent: E1 E2 Enhanced by polar protic solvent Solvent may vary, aprotic preferred Leaving Group: E1 E2 Good LG important to form C + Good LG enhances reaction 18 Elimination Reactions

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