Please use this identifier to cite or link to this item: http://localhost:8081/xmlui/handle/123456789/1240
Authors: Kumar, Anurag
Issue Date: 1990
Abstract: Mechanism of aromatic nucleophilic substitution reactions involving amines as nucleophile is not yet firmly established in nonpolar and dipolar organic solvents. There fore, in order to support either of the existing mechanisms or to propose a new mechanism I have studied aminolysis of four oxime ethers viz. 0-(2,4-dinitrophenyl) substituted cyclohexanone oxime (DNPCHOX), acetophenone oxime (DNPAPOX), benzophenone oxime (DNPBPOX) and 4'-bromobenzophenone oxime (DNPBrBPOX) in a number of nonpolar aprotic, dipolar aprotic and protic solvents and mixtures of nonpolar aprotic and dipolar protic solvents. SNAr ractions of DNPCHOX, DNPAPOX, DNPBPOX and DNPBrBPOX with pyrrolidine, a secondary alkylamine and several primary alkylamines viz. n-propyl-, n-butyl-, n-pentyl- and n-hexylamine were studied in benzene ;spectrophotometrically under pseudofirst- order conditions at the Amax of corresponding aminolysis product which was identified by tic and spectral techniques. Reactions have been found to be wholly base catalysed. Plots of second-order rate coefficient (k ) versus Ppyr rolidine"! gave a curvature concave towards rate coefficient axis passing through the origin for all substrates. Similar plots were observed for the reaction of DNPCHOX w.ith all four primary alkylamines and of DNPAPOX with n-pentyl- and n-hexylamine. These plots suggests the involvement of more than two amine molecules in the reaction. Plots of k versus [amine ] for (ii; the reaction of DNPAPOX with n-propyl- and n-butylamine and of DNPBPOX and DNPBrBPOX with all four primary alkyl amines were found to be straight lines passing through the origin indicating the involvement of only two amine molecules in the reaction. For pyrrolidinolysis reactions where order w.r.t. amine is three rate constants decreased with increase in temperature. These results have been explained in terms of electrophilic catalysis by homoconjugates of conjugate acid of nucleophile with nucleophile. In order to have more data to support above mechanism, pyrrolidinolysis and n-butylaminolysis of DNPCHOX and DNPBPOX was further studied in chlorobenzene where a behaviour parallel to that in benzene was observed. n-Butylaminolysis of above two substrates in 1,2-dichloroethane showed the involvement of two amine molecules in the reaction. Pyrrolidinolysis and n-butylaminolysis of DNPCHOX and DNPBPOX was further investigated in a number of dipolar aprotic solvents of different hydrogen bond acceptor ability viz. MeCN,DMF and DMSO. Pyrrolidinolysis in dipolar aprotic solvents showed a behaviour different from that observed in benzene and chlorobenzene. The difference observed was that in these solvents, only two molecules of amine were involved in the reaction. An unusual observation noted was that the rate constant decreased with increase in temperature. This observation has been explained by assuming that the electrophilic catalysis occurs through heteroconjugates (iii) of conjugate acid of nucleophile with hydrogen bond acceptor solvents. n-Butylaminolysis of both substrates showed a behaviour significantly different from that of pyrrolidino lysis. It exhibited an usual positive temperature effect on rates and an uncatalytic route also which has been explained on the basis of availability of non hydrogen bonded ammonium proton in the intermediate state. To further support the above mechanism of electro philic catalysis by homoconjugates and heteroconjugates, reactons of DNPCHOX with pyrrolidine were studied in benzenemethanol mixtures at several [pyrrolidine] . Addition of methanol to benzene results in an initial decrease in rate (upto 10% additon of methanol) followed by gradual increase. Overall order in amine is also reduced from three to two in 15% methanol and remains so upto 100% methanol. These results are also satisfactorily accommodated by adopting the concept of electrophilic catalysis by homo- and hetero con jugates . To see the effect of size of nonconjugated oxime moiety, aminolysis of 0-(2,4-dinitrophenyl) cyclopentanone oxime was carried out with one secondary and five primary alkylamines in 1:1 water-acetonitrile. Reactions were insensi tive to base catalysis. Results obtained were then compared with those on aminolysis of DNPCHOX in the same medium (pre viously studied). It was observed that just a mere reduction (one-CH^ unit) in the size of nonconjugated oxime moiety resulted in retardation of rate about eight times.
Other Identifiers: Ph.D
Research Supervisor/ Guide: Jain, A. K.
Gupta, V. K.
metadata.dc.type: Doctoral Thesis
Appears in Collections:DOCTORAL THESES (chemistry)

Files in This Item:
File Description SizeFormat 

Items in DSpace are protected by copyright, with all rights reserved, unless otherwise indicated.