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![CARBOCATION
Acyl-cation
Acyl or acetyl cation is as stable as tert-butyl cation.
Q. Which of the following carbocations do not exist? Which one is more stable?
• It is impossible to form carbocation at bridgehead atoms in [2.2.1] systems
since they can not be planar.
• However, larger bridgehead carbocations can exist.
• Cyclopropyl group provides extra stability already discussed in previous
slide.](https://image.slidesharecdn.com/reactiveintermediatesburange-251018110403-d4961c22/85/Reactive-intermediates-and-their-applications-6-320.jpg)
![CARBOCATION
Acyl-cation
Acyl or acetyl cation is as stable as tert-butyl cation.
Q. Which of the following carbocations do not exist? Which one is more stable?
• It is impossible to form carbocation at bridgehead atoms in [2.2.1] systems
since they can not be planar.
• However, larger bridgehead carbocations can exist.
• Cyclopropyl group provides extra stability already discussed in previous
slide.](https://image.slidesharecdn.com/reactiveintermediatesburange-251018110403-d4961c22/75/Reactive-intermediates-and-their-applications-6-2048.jpg)
Uploaded byDr. Anand S. Burange
Reactive intermediates and their applications
Reactive intermediates and their applications by Dr. Anand S. Burange Topics covered: Carbocation history, work of George Olah, carbocation synthesis, Carbene and carbanion, Wagner Meerwein,Friedel Crafts reaction, Simmons Smith Cyclopropanation, Appel reaction, Pinalcol-pinacolone rearrangement, and many more.
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Reactive intermediates and their applications
- 1. By Dr. Anand SBurange Ph.D. (ICT, Mumbai), CSIR-NET, GATE, CSIR-Nehru PDF (NCL, Pune), CSIR-RA Asst. Professor in Chemistry Wilson College, Mumbai 26th August 2021 Reactive intermediates and their applications Suzuki coupling : Burange et al., ChemistrySelect, 2016, 1, 2673 – 2681
- 2. REACTIVE INTERMEDIATES The intermediatesformed during the reactions: short lived, mostly non isolable, quickly converted into more stable molecule. Reactive intermediates Carbocation Carbanion Carbene Nitrene Radical Aryne
- 3. Carbocation REACTIVE INTERMEDIATES:CARBOCATION For manyyears they were called as carbonium ions; in 1902 it was suggested inappropriate to use suffix ‘onium’ ion for lesser covalency which is often used for higher covalency than neutral atom. e.g. ammonium, hydronium ion. Finally accepted by IUPAC in 1987 after Olah’s discovery. SP2 Hybridization CH5 + species: Possible???? George Olah et al., found an intermediate of carbon where actually covalency of carbon is five (higher), i.e. CH5 + Methanonium ion (carbonium ion). Chemtech, 1971, 1, 566; JACS 1972, 94, 808 Stability order of alkyl carbocation 3o>2o >1o > methyl Reason: Hyperconjugation and Field effect Lot of literature available: work by Olah and his co-workers
- 4. CARBOCATION Can we preparecarbocation? YES Alcohols, alkenes, and even alkanes give carbocations when treated with super acid. JOC 1971, 36, 2354; JACS 1967, 89, 3576. Super Acid FSO3H + SbF5 dissolved in SO2 or SO2ClF is known as super acid. Which gas evolves when alkanes are treated with super acid? JACS 1967, 89, 4739 Evidence for stability of carbocation? ➢Both propyl fluorides gave isopropyl cation. ➢ All four butyl fluorides gave tert- butyl cation. ➢All seven pentyl fluorides gave tert- pentyl cation. To date no primary cation survived long enough for detection.
- 5. CARBOCATION Allylic, benzylic andcyclopropyl carbocations Benzylic are more stable than simple allylic cations Cyclopropyl methyl cations are even more stable than benzylic. More the canonical forms; more is the stability. Reason for extra stability of cyclopropyl methyl carbocation Non classical Norbornyl cation 7- Norbornenyl cation
- 6. CARBOCATION Acyl-cation Acyl or acetylcation is as stable as tert-butyl cation. Q. Which of the following carbocations do not exist? Which one is more stable? • It is impossible to form carbocation at bridgehead atoms in [2.2.1] systems since they can not be planar. • However, larger bridgehead carbocations can exist. • Cyclopropyl group provides extra stability already discussed in previous slide.
- 7. CARBOCATION How they exist? ➢More stable ones can be prepared in solution and in some case they even exist as solid salts. ➢ In solution, carbocations may be free i.e. in case of polar solvents they are solvated. ➢ In non-polar solvents they exist as an ion pair i.e. closely associated with negative ion/counter ion. Commercially available carbocation Ph3C+BF4 - Aryl methyl carbocations are further stabilized by electron donating groups at ortho- and para- position (s).
- 8. Applications of carbocationsin Organic reactions 8
- 9. Wagner-Meerwein Rearrangement andFriedel Crafts # Problems #1 How will you synthesize n-butyl benzene? #2 Predict the product The acid catalyzed nucleophilic 1,2 migration of alkyl, aryl or hydride group from an alcohol to alkene is known as Wagner-Meerwein Rearrangement Ans: FC Acylation followed by reduction Ans: Formation of carbocation, followed by –Me migration, and later loss of proton
- 10.
- 11. Pinacol-pinacolone rearrangement 1,2 diol(pinacol:trivial name), undergoes acid catalyzed rearrangement which involve formation of carbocation, alkyl/aryl group migration to yield carbonyl compound. Spirocycles can also be prepared by this reaction. ? ? ?
- 12. Semipinacol rearrangement In presenceof base, less hindered hydroxyl group of pinaocol reacts with TsCl to form tosylate. Formed tosylate further gives carbonyl compound in non-acidic condition (basic/neutral). Less hindered ? ?
- 13. Tiffeneau-Demjanov rearrangement Semipinacol rearrangement ofdiazonium salts derived from 2-amino alcohols sometimes called as Tiffeneau_Demjanov rearrangement. Orbital Approach Anti pariplanar relationship
- 14.
- 15. REACTIVE INTERMEDIATE: CARBENE CarbeneHighly reactive species. Practically all having lifetime considerably under 1 sec. Carbenes have been isolated only by entrapment in matrices at low temperature (77K or less). Ex. :CH2 (methylene); :CCl2 (dichlorocarbene) Six electron species Types of Carbene How to distinguish between singlet and triplet carbene? Method developed by Skell, where different type of carbene on reaction with cis-2-butene give different stereoisomer. H-C-H bond angle 103o SP2 hybridized H-C-H bond angle 136o SP3 hybridized
- 16.
- 17. CARBENE Reaction of singletand triplet carbenes : Skell et al , JACS 1956, 78, 4496; Tetrahedron 1985, 41, 1427
- 18. SIMMONS SMITH CYCLOPROPANATION InSimmons-Smith, zinc carbenoid behaves like singlet carbene and its addition to alkene are stereospecific. ? ?
- 19.
- 20. REACTIVE INTERMEDIATE: CARBANION Carbanion Simplecarbanion of R3C- could assume a pyramidal (SP3) or a planar (SP2) configuration or may be something between depending upon the nature of R. SP3 or SP2 Stability order Why alpha hydrogen of carbonyls are acidic? Stabilized by electron withdrawing group. Methyl> 1o > 2o> 3o
- 21.
- 22. Acetoacetic Ester Synthesis/ClaisenCondensation Why α- hydrogen atoms are acidic? Acetoacetic ester Application Concept of 1,3-dianion How many α- hydrogen atoms are present in acetoacetic ester? Less stable anion more stable anion Which anion is more reactive? Less stable anion is more reactive since stable anion is involved in delocalization with two EWGs.
- 23. # PROBLEMS JACS, 1974,96, 1082 Tet. Lett, 1996, 37, 471
- 24.
- 25.