Nonclassical Carbocation

A slide containing brief information about Non-classical carbocations.

1. NONCLASSICAL CARBOCATIONS CENTRAL UNIVERSITY OF GUJARAT, GANDHINAGAR

2. PRE REQUISITE EXAMPLES

3. Nonclassical Carbocations Nonclassical carbocations  are stabilized by charge delocalization from contributions of neighbouring C=C , C–C or C–H bonds. The bridged carbocations, involve delocalization of electrons and formation of three-center, two-electron bonds, and are sometimes called nonclassical ions . Most common example : Norbornyl carbocation

4. MAIN DIFFERENCE BETWEEN CLASSICAL AND NONCLASSICAL CARBOCATIONS

5. EVIDENCES OF THEIR EXISTENCE C=C as a neighbouring group A B When acetolysis of A and B is done, it is observed that it is 10 11 times faster for A than that for B and the reaction proceeds with a retention of configuration. The rate data is not sufficient to prove it but it is believed that the nonclassical intermediate C is responsible for such behavior. In any case it is clear that the C=C assists in the departure of the OTs. NMR spectrum of the stable norbornadienyl cation ( D ) shows that 2 and 3 protons are not equivalent with 5 and 6 protons which indicates an interaction between the charged carbon and one of the double bond. This proves the existence of C . C D

6. EVIDENCES OF THEIR EXISTENCE The Carbon–Carbon Single Bond as a Neighboring Group Winstein and Trifan found that acetolysis of both exo-2 norbornyl brosylate and endo-2-norbornyl brosylate produce exclusively exo-2-norbornyl brosylate . The exo isomer was found to solvolyze 350 times faster than endo isomer Enantiomerically pure exo- brosylate gives a complete racemic exo-acetate and endo- brosylate gives acetate that is atleast 93% racemic . exo endo

7. EVIDENCES OF THEIR EXISTENCE The racemic exo isomers and high exo/endo rate were interpreted by Winstein and Trifan as indicating that 1,6 bond assists in the departure of the leaving group and that a nonclassical intermediate is involved. Solvolysis of the endo isomer is not assisted by the 1,6 bond because it is not in a favorable position for backside attack explaining the normal rate. Hence the faster rate for exo must be caused by anchimeric assistance.

8. Stereo chemistry of the product is explained by using the nonclassical intermediate. The 1 and 2 positions are equivalent and would be attacked by the nucleophile with equal facility but only from the exo direction in either case. Solvolysis in the case of the endo isomer is not completely racemic , contains 2 more than 1. 1 2

9. A vast amount of research and work has been done on 2-norbornyl system and almost (not all) all chemists accept the the Winstein’s interpretation.

10. References Advanced Organic Chemistry Part A: Structure and Mechanisms, Francis A. Carey and Richard J. Sundberg , Fifth edition, Springer MARCH’S ADVANCED ORGANIC CHEMISTRY REACTIONS, MECHANISMS, AND STRUCTURE,Michael B. Smith, Jerry March, SIXTH EDITION,WILEY www.unacademy.com https://chem.libretexts.org/