Beckmann Rearrangement with mechanism.pptx

study on the mechanism of beckmann rearrangement

1. Beckmann Rearrangement

2. Introduction – What is Organic Chemistry Organic chemistry is one of the major branches of chemistry and includes the study of: All biological molecules – from simple sugars to complex nucleic acids All fossil fuels – including oil, coal, and natural gas Nearly all synthetic materials – such as nylon, Lycra, and Gore-Tex Many domestic and industrial products – such as paints, detergents, and refrigerants 2

3. Introduction ( Contd …) The word ‘organic’ is now used in several different contexts beyond the chemistry of carbon compounds. Examples include organic food, organic growth, and organic degeneration. To what extent does this common vocabulary represent a common origin in meaning? Does it confuse or enhance communication? 3

4. Types of reactants Saturated Compounds which contain only single bonds Example: Alkanes Unsaturated Compounds which contain double or triple bonds Example: Alkenes, arenes Aliphatics Compounds which do not contain a benzene ring, can be saturated or unsaturated Example: Alkanes, alkenes 4

5. Types of reactants ( Contd …) Arenes Compounds which contain a benzene ring, they are all unsaturated compounds Example: Benzene, phenol Electrophile An electron-deficient species which is therefore attracted to parts of molecules that are electron rich Electrophiles are positive ions and have a partial positive charge Example: NO2+, H+ 5

6. Types of reactants ( Contd …) Nucleophile An electron-rich species which is therefore attracted to parts of molecules that are electron deficient Nucleophiles have a lone pair of electrons and many also have a negative charge Example: Cl-, NH3 6

7. Types of reaction Addition Occurs when two reactants combine to form a single product Characteristic of unsaturated compounds Example: Substitution Occurs when one atom or group of atoms in a compound is replaced by a different atom or group Characteristic of saturated compounds and aromatic compounds Example:   7

8. Types of reaction ( Contd …) Addition – Elimination Occurs when two reactants join together (addition) and in the process a small molecule such as H 2 O, HCl, or NH 3 is lost (elimination) Reaction occurs between a functional group in each reactant Also called a condensation reaction Example: ROH + R’COOH → R’COOR + H 2 O Alcohol Acid Ester 8

9. Types of Bond Breaking (Fission) Homolytic Fission Is when a covalent bond breaks by splitting the shared pair of electrons between the two products Produces two free radicals, each with an unpaired electron X:X → X ■ + X ■ Heterolytic Fission Is when a covalent bond breaks with both the shared electrons going to one of the products Produces two oppositely charged ions X:X → X - + X + 9

10. Types of Bonds - Electrovalent or ionic bond Two elements can achieve stable configuration (i.e., inert gas configuration) by transfer of electrons from one element to the other. This results in the formation of oppositely charged atoms (ions) which are bound together by electrostatic attraction. This type of bond is called electrovalent or ionic bond 10

11. Covalent bond Since it is increasingly difficult to extract a number of electrons from an element due to increasing development of positive charge on it, in general the charge on a simple cation is limited to +3 even when the inert gas configuration is not attained. The reverse is similarly true. Hence, the elements in the middle of a row can neither gain nor lose electrons to achieve inert gas configurations. Also, the transfer of electrons between two electronegative or between two electropositive elements cannot confer inert gas configurations to both the elements. 11

12. Covalent Bond ( Contd …) In such cases, both the elements can acquire the desired inert gas configurations by mutually sharing pairs of electrons—each element contributing an electron to the shared pair. The shared electron pair then belongs to both the elements. The shared electron pair binds the two nuclei, and the bond so formed is called a covalent bond. 12

13. Factors influencing reactivity Inductive Effect Electromeric Effect Resonance Mesomeric Effect 13

14. Nucleophile and Electrophile A nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. 14

15. Beckmann Rearrangement 15 The acid-catalyzed conversion of ketoximes to N-substituted amides is known as Beckmann rearrangement. The reaction is catalysed by acidic reagents such as, H 2 SO 4 , SOCl 2 , SO 3 , P 2 O 5 , PCl 5 , C 6 H 5 SO 2 Cl , etc. The reaction involves the migration of a group from carbon to electron-deficient nitrogen

16. Beckmann Rearrangement (Contd…) 16 Some aldoximes undergo the rearrangement in the presence of polyphosphoric acid (PPA) but the reaction is not a general one. The migration of the group depends not on the migrational aptitude but upon the orientation of the group in relation to the OH group. It is found that the migrating group is always anti (i.e., trans) to the hydroxyl group. Thus, the reaction is stereospecific. That it is always the antigroup which migrates has been confirmed by the rearrangements of the two isomeric oximes of 2-bromo-5-nitroacetophenone. The structures of the two isomeric oximes were first determined by an elegant method as given below.

17. Beckmann Rearrangement ( Contd …) 17 On treatment with cold NaOH solution, one isomer (I) was cyclized to 3-methyl-5-nitrophenyl isooxazole (III) while the other isomer (II) remained unaffected even under drastic conditions. Obviously, the OH and Br groups in isomer (I) are close enough for reaction and cyclization.

18. Beckmann Rearrangement ( Contd …) Hence, the Me and OH groups are anti (i.e., trans) to each other. In isomer (II), the OH and Br groups are far apart for reaction, i.e., the Me and OH groups are syn (i.e., cis) to each other. Thus, the structures of isomers (I) and (II) are confirmed. 18

19. Beckmann Rearrangement ( Contd …) Oxime esters and ethers also undergo Beckmann rearrangement. The acidic reagents convert the OH group to a better leaving group—acids convert OH to HO 2 , other reagents convert OH to an ester-leaving group, e.g., OPCl 4 from PCl 5 , OSO 2 C 6 H 5 from C 6 H 5 SO 2 Cl , etc. The reaction is facilitated by heat, polar solvents or an increase in the acid strength. 19

20. Mechanism 20