E1 & E2 reaction tkkk.pptx

Elimination reactions , Hoffman rule , Savtseff rule

1. ELIMINATION REACTION [E1 & E2 ; HOFFMAN & SAYTZEFF’S RULE] DEPARTMENT OF PHARMACEUTICAL SCIENCES, RTM NAGPUR UNIVERSITY, NAGPUR Presented by Tahmina khan

2. CONTENT Introduction Basic Requirements of Elimination Reactions E2 reaction E1 reaction Zitsev rule Hoffman rule Referance

3. Elimination Reaction- An elimination reaction is a type of reaction in wich two substituents are removed from a molecule in either one or two step mechanism. The one-step mechanism is known as the E2 reaction , and the two-step mechanism is known as the E1 reaction The numbers refer not to the number of steps in the mechanism, but rather to the kinetics of the reaction: E2 is bimolecular (second-order) while E1 is unimolecular (first-order).

4. Basic Requirements of Elimination Reactions- 1) Substrate – A. For 1,2 elimination reaction the constituents required are substrate which undergo and contains a good leaving group.

B.The best choice for leaving group containing substrate is alkyl halide. C. In substrate there should be necessary to present beta hydrogens more than 1.

5. 2) Nucleophile Or Base – In the reaction, base is required to abstract proton.

3) Solvent – Also at a time solvent like water or alcohol required as a solvent .

6. E2 Reaction- E2 is a single step elimination, with a single transition state. The product of an E2 elimination reaction has one more degree of unsaturation than the starting material For instance, the base induced elimination of “HX” of an alkyl haLide rise to an alkene Ex

7. Mechanism The most common mechanism for dehydrohalogenation is the E2 mechanism. It exhibits second-order kinetics, and both the alkyl halide and the base appear in the rate equation rate = k[(CH),CBr||HO|| The reaction is concerted all bonds are broken and formed in a single step. E2 reactions are regioselective and favor the formation of Zaitsev products.

8. Energy profile For an E2 reaction-

9. Reactivity of E2 reaction 1. Reactivity of E2 reactions depends on alkyl halide.alkyl halide containing beta hydrogen must be needed. 2. The order of reactivity in E2 reaction is – Tertiary RX > Secondary RX > Primary RX 3. 3. Tertiary alkyl halide is more active because it contains more no.of beta hydrogens i.e 9 beta hydrogens

10. 4.Secondary alkyl halide conatins 6 and primary ones contains 3 beta hydrogens. 5. If we start reaction with tertiary alkyl halide then we get more substituated product so more is substituated product more is stability hence tertiary alkyl halide most reactive.

11. Orientation of E2 Reaction In orientation of E2 reaction saytzeff rule is used to show it.
Saytzeff rule is says that hydrogen is lost from that carbon which have lesser number of hydrogens.

12. E1 Reaction- It is a two-step process of elimination: the first step is the loss of the leaving group, which leave in a very slow step resulting in a formation of carbocation is called “Ionization”. In the second step the base attack the neighbouring hydrogen, forcing the electron form The hydrogen carbon bond to make the double bond the process is called “Deprotonation” Ex

13. Mechanism- The E1 reaction proceeds via a two-step mechanism: the bond to the leaving group breaks first before the a bond is formed. The slow step is unimolecular, involving only the alkyl halide. It exhibits first-order kinetics, rate = k[(CH3),CCI] E1 reactions also are regioselective and follow Zaitsev rule

14. Energy profile For E1 Reaction-

15. Reactivity of E1 reaction In reactivity of E1 reaction reactivity decided by
formation of carbocation
1. The order of reactivity in E1 reaction is- Tertiary carbocation > Secondary carbocation > Primary carbocation
2. Tertiary carbocation is more active because it contains more no.of electron rich group i.e methyl group which donate electrons to carbocations and nullify its charge and stabilize it

16. Orientation of E1 Reaction Orientation of E1 reaction is also followed by saytzeff rule. Saytzeff rule is says that hydrogen is lost from that carbon which have lesser number of hydrogens. Strong base prefer E2 reaction and weak base prefer E1 reaction.

17. The Zaitsev (Savtseff) Rule- When alkyl halides have two or more different ẞ carbons, more than one alkene product is formed. In such cases, the major product is the more stable product-the one with the more substituted double bond. This phenomenon is called the “Zaitsev rule”. The Zaitsev product or the more substituted alkene product is more stable than the less substituted product. The stability of the more substituted alkene is a result of number of different contributing factors, including hyperconjugation.

18. Hoffman rule According to this rule, major product is always least substituted alkene i.e., major product is formed from ẞ-carbon which has maximum number of hydrogen. Ex

19. Referance- 1. Advanced Organic Chemistry Book by Arun Bahl and B.S Bahl.

2. Textbook of Organic chemistry by Morrison & Boyd 7 th Edition.

3. Alkenes Elimination reactions by BCCCoaching Classes.