El comercio de bienes entre Québec y Chile

11. G. W. Kenner, B. Lythgoe, A. R. Todd and A. Topham, J. Chem. Soc., 1943, 388.

Su^.

1

12. D. J. Brown, "The Pyrimidines",jwiley Interscience, 19^s0. 13. A. Maggiolo and P. B. Russel, J. Chem. Soc., 1951, 3297.

14. C. Reichardt and K. Halbritter, Angew. Chem. Int. Ed., 1975, JL4,

86

16. A. Holy and Z. Arnold, Coll. Czech. Chem. Comm., 1973, 38^, 1371. 17. J. Kucera and Z. Arnold, Ibid., 1967, J2, 1704.

18. R. M. Silverstein, E. R. Ryskiewicz, C. Willard and R. C. Koehler, J. Org. Chem., 1955, 20,

668

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26. J. A. Kampmeier and E. Hoffmeister, J. Amer. Chem. Soc., 1962, 84, 8787.

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Soc., 1965.

x.

29. W. Wolf and N. Kharasch, J. Org. Chem. , 1961, _2_6, 283 and 1965,

30, 2493. -

30. N. Kharasch, R. K. Sharma and P. Friedman, unpublished results. 31. R. B. Ingalls, W. Wolf and N. Kharasch, "Abstracts 146th Meeting

Amer. Chem. Soc., 1964.

32. R. D. Youssefyeh and L. Lichtenberg, J. Chem. Soc. Perkin I, 1974, 2649.

33. H. S. Ryang and H. Sakurai, Chem. Comm., 1972, 594.

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24, 225.

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1974, 11, 51.

Chapter 2

Nucleophilic Substitution of

2

2:1 Aromatic Nucleophilic Substitution

(a) Introduction

Nucleophilic substitution at an aromatic carbon resembles other nucleophilic substitution reactions at carbon in that a bond to the carbon at the reaction site is formed by a reagent Y and a group X is correspondingly displaced with its bonding electrons (Scheme A).

Scheme A

In simple compounds such as halobenzenes, nucleophilic substitution requires very vigorous conditions, whereas the nucleophilic substitution reactions of haloalkanes are relatively facile. Nevertheless aromatic

industry. For example, both phenol and aniline are prepared industrially by nucleophilic substitution reactions. In the laboratory much use is

displacement reactions of aromatic diazonium salts is a well established process for the introduction of a range groups into an aromatic nucleus.

Several mechanisms are known to exist for aromatic nucleophilic reactions. Among these are the unimolecular mechanism (S^l), the

Yi , Ar - X --- > Y - Ar + X

nucleophilic substitution reactions are of prime importance in the chemical

made of aromatic nucleophilic substitution; for example the nuc ie.ophilic

bimolecular mechanism (S^2), the benzyne (or elimination - addition)

mechanism and the nucleophilic addition - ring opening - ring closing

(ANROC) mechanism. •

(b) Unimolecular Mechanism

A large number of mechanisms have been proposed to account for

experimental findings obtained from studies of the nucleophilic replacement of aromatic diazonium groups but even now the situation has not been

completely resolved.

Early results were interpreted as showing the involvement of an aryl cation (Scheme B). This belief was based on kinetic studies which showed independence of reaction rate with various anions^-, independence

2 3

on acidity over a wide range , and a low solvent sensitivity . The effect of substituents on the rate of displacement was also consistent with a unimolecular nitrogen loss mechanism^.

SLOW

+

| P ^ l _{+ N, —}

FflST

*

Scheme B

Lewis and co-workers showed that this simple explanation did not

5-7

fit all the available experimental results , but were unable to produce

8 9

a plausible alternative mechanism. More recently Swain et al * , have shown that under special conditions (the absence of a strong base or reducing agent or light) nucleophilic displacements on benzene diazonium

loft

ions proceed by the rate determining product|of aryl cations.

(c) Bimolecular Mechanism

The majority of aromatic nucleophilic substitutions proceed via a bimolecular mechanism. The general belief is that the reaction proceeds

FR

EE

EN

ER

GY

via a negatively charged intermediate. This is represented as shown

(Scheme C):- ' . •

Scheme C

Theoretically two possible reaction profiles could exist for this process (Figure la and lb). If the bond formation step is rate

G ^ G

determining A B Fig. la, whilst if the bond breaking step is

G G

rate determining B A Fig. lb.

^ RERCTIOM