Reaction mechanism ppt for advance organic chemistry.pptx

Reaction mechanism is a part of oraganic chemistry M Pharm (Pharmaceutical Chemistry)

1. REACTION MECHANISM AND METHODS OF DETERMINING THEM

2. The reactions of organic compounds can be classified into four main types. 1. Addition reactions 2. Substitution reactions 3. Elimination reactions 4. Rearrangement reactions Types of organic reactions

3. Addition reactions are those in which atoms or groups of atoms are simply added to a double or triple bond without the elimination of any atom or other molecules. In these reactions, at least one л bond is lost while two new o bonds are formed. Double bonds become saturated and triple bonds are converted into double bonds or may become saturated by further addition. For example: 1. Addition reactions

4. Electrophilic addition reactions : The mechanism of the above reaction involves the following steps: Step 1: Hydrogen bromide gives a proton and bromide ion. Step 2: The electrophile attacks the л bond of ethylene to give a carbonium ion. Step 3: The nucleophile attacks the carbonium ion to give the addition product Mechanisms of addition reactions

5. When an unsymmetrical reagent adds to an unsymmetrical double bond, the positive part of the reagent becomes attached to the double bonded carbon atom which bears the greatest number of hydrogen atoms. Anti-Markovnikov rule In the presence of a peroxide initiator, hydrogen halide adds to alkene via free-radical mechanism. Such reactions are said to be anti-Markovnikov, since the positive part adds to the less substituted carbon, exactly the opposite of Markovnikov reaction. This process was first explained by Kharasch Markovnikov's rule

6. When an addition reaction involves the initial attack by a nucleophile, the reaction is referred to as nucleophilic addition. Aldehydes and ketones which contain carbon-oxygen double bonds undergo such reactions. The carbonyl group is highly polar in character. This is because of higher electronegativity of oxygen as compared to carbon. The carbonyl group may be represented as shown below: The addition of HCN to acetone is an example of nucleophilic addition. Nucleophilic addition reactions

7. The mechanism of the above reaction involves the following steps : Step 1: Hydrogen cyanide gives a proton and a cyanide ion. Step 2: The nucleophile attacks the positively charged carbonyl carbon to give the corresponding anion. Step 3: The electrophile combines with the anion to form the addition product.

8. Substitution reactions are those reactions in which an atom or group of atoms directly attached to a carbon in the substrate molecule is replaced by another atom or group of atoms. For example, The chlorination of methane in the presence of ultraviolet light, as follows: Types of substation reaction: 1. Electrophilic substitution reaction 2. Nucleophilic substitution reaction 3. Free radical substitution reaction 2. Substitution reactions

9. Electrophilic substitution reactions: When a substitution reaction involves the attack by an electrophile, the reaction is referred to as electrophilic substitution. e.g. bromination of benzene in the presence of FeBr3. The mechanism of the above reaction involves the following steps: Step 1: Formation of the electrophile.

10. Step 2: The electrophile (Br+) attacks the л electron system of the benzene ring to form a resonance stabilized carbonium ion. Step 3: Elimination of proton to give the substituted product.

11. Nitration Sulfonation Friedel-Craft Alkylation

12. Friedal craft acylation

13. 2. Nucleophilic substitution reactions : When a substitution reaction involves the attack by a nucleophile, the reaction is referred to as S N . The hydrolysis of alkyl halides by aqueous NaOH is an example of nucleophilic substitution. The nucleophilic reactions are divided into two classes: SN2 mechanism The terminology SN2 stands for "substitution nucleophilic bimolecular". Rate of SN2 reaction depends on the concentration of both the substrate and the nucleophile, the reaction is of second-order Rate = [Substrate] [Nucleophile] Two reactants take part in the transition state of the slow or rate-determining step of a reaction and is therefore bimolecular. The reaction consists of single step

14. For example: hydrolysis of methyl bromide by aqueous NaOH. The reaction and transition state are represented in the following figure. The alkyl halide substrate contains a polarized carbon halogen bond. The SN2 mechanism begins when hydroxide ion approaches the substrate carbon from the opposite side of the bromine ion. This is because both hydroxide ion and bromine atom are electron rich. In transition state, both OH and Br are partially bonded to the substrate carbon. Carbon in the resulting complex is trigonal bipyramidal in shape. With the loss of the leaving group, the carbon atom again assumes a pyramidal shape and its configuration is inverted.

15. SN1 mechanism The terminology SN1 stands for "substitution nucleophilic unimolecular". Rate of S1 reaction depends only on the concentration of the alkyl halide, the reaction is of first- order Rate = [Substrate] Activated complex contains only one species, alkyl carbocation and is therefore unimolecular The reaction consists of two steps This mechanism proceeds via two steps i ) The first step (the slow step) involves the breakdown of the alkyl halide into an alkyl carbocation and a leaving group anion. This is the rate determining step. ii) The second step (the fast step), the nucleophile can attack the planar carbonium ion from either side to give the product.

16. Elimination reactions are those which involve the removal of atoms or groups of atoms from two adjacent atoms in the substrate molecule to form a multiple bond. Elimination reactions may be regarded as reverse of addition reactions. In these reactions, two a bonds are lost and a new л bond is formed. Saturated compound become unsaturated. For example: 3. Elimination reactions

17. These reactions are also divided into two classes: E2 Reaction E2 stands for elimination bimolecular. The reaction rate, influenced by both the alkyl halide and the base and of second order. E2 typically uses a strong base, it needs a chemical strong enough to pull off a weakly acidic hydrogen. A good leaving group is required because it is involved in the rate determining step. E2 is the one step process with a transition state. Typically undergone by primary substituted alkyl halides but is possible with some secondary alkyl halides. Because E2 mechanism results in formation of a pi bond, the two leaving groups (often a hydrogen and a halogen) need to be anti-periplanar (or 180°). That's why eliminations often favor the trans- product over the cis-product (stereoselectivity).

18. E1 Reaction E1 stands for elimination unimolecular. The reaction rate is influenced only by the concentration of the alkyl halide and is of first-order. A strong base not required, since it is not involved in the rate-determining step. A good leaving group is required, since it is involved in the rate-determining step. E1 typically takes place with tertiary alkyl halides, but is possible with some secondary alkyl halides. E1 reactions are two step processes:

19. Step 1: The alkyl halide ionizes to give the carbonium ion. Step 2: A proton is abstracted by the base from the adjacent ẞ-carbon atom to give the alkene.

20. Zaitsev's or Saytzeff's rule It states that although more than one product can be formed during alkene synthesis, the more substituted alkene is the major product. This infers that the hydrogen on the most substituted carbon is the most probable to be deprotonated, thus allowing for the most substituted alkene to be formed

21. 4. Rearrangement reactions Rearrangement reactions involve the migration of an atom or group of atoms from one site to another within the same molecule. The product is always the structural isomer of the original compound. For example: Fries Rearrangement The reaction of an aryl ester with a Lewis acid (AICI,) catalyst followed by an aqueous acid to give phenols is known as Fries rearrangement.

22. Identification of product Stereo chemical evidences Cross over experiments Isotopic labelling technique Identification of reaction intermediate Methods adopted for determining the mechanism of a reaction: