Marco Teórico y Conceptual

CH,, -L ‘Bu

(30)

in a d d itio n to 4 jd ’“ dimethyldiphonyImethane„ The form ation of t h i s X)roduct suggests a tw o-step mechanism involving i n i t i a l a tta c k on tho m ethylated arom atic compound to give an in term ed iate which w ill s u b s titu te on an aromatic rin g to give the diphenylm ethanes,

P o ssib le interm o diato s arc a benzyl carbonium ion or a benzyl chloride» This i s o u tlin ed in Scheme V.

A s a tis f a c to r y mechanism must ex p lain the follow ing observa­ tio n s . The form ation of diphenylmethane is probably a fu n ctio n of the la rg e number of ch lo rin e atoms in tho pentachlorophenate anion since no re a c tio n i s observed w ith £-bromophenate io n s. Loss of c h lo rin e appears to be a s s is te d by tho presence of methylbenzenos. Thus5 under th e same conditions? about 4C m/lOO ni. of ch lo rid e ion

was displaood when sodiuin pentachlorophenate was decomposed in t~hutylhenzene 9 as opposed to ah out l30 m/100 m. in methylhenzene solvents» F in a lly c h lo rin e appears to he lo s t from p o s itio n s ortho and para to the phenate group only. The mechanisms which follow s a tis f y some, hut n ot a l l , of these p o in ts .

A mechanism loading to form ation of a henzyl carhonium ion is o u tlin e d in Scheme VI;

01 0 I / / I 01 0 01 01 01 01 or OIX Scheme VI

Although t h i s scheme ex p lain s the lo s s of ch lo rin e from p^tlio and p a ra p o s itio n s only, i t i s u n sa tisfa c to ry in other ways. I t does

137

not) fo r example., exp lain why the re a c tio n i s not o’bserved w ith o-bromophenate s a l t s ; in f a c t one might expect the oarheno

in term ed iate to he formed more re a d ily from the o-hromophenate anion. Since Kulka suggests a r a d ic a l mechanism fo r the form ation of diphenyleno dioxide., i t i s p o ssib le th a t some kind of r a d ic a l c h lo rin a tio n of the methylbenzene i s involved here. Such a

mechanism i s form ulated below, A ch lo rine atom i s used as i n i t i a t o r in ste p (i) since Kulka suggests th a t t h i s r a d ic a l i s formed in the form ation of diphenyleno d ioxide.

0 1" ■’r HCl

CH₂

( i)

CH^Cl

presonoe of mothyllDenzenes.- "hut i t doos not explain the p r e f e r e n tia l lo3s of c h lo rin e from the orJWio and J)ara p o s itio n s . Although

s to r ic in flu en c es v/ould be expected to in c re ase the l a b i l i t y of the ch lo rin e in sodium pentaohlorophenate, i t i s su rp risin g th a t no a tta c k on solvent i s d etected w ith sodium o-bromophenatoy i f t h i s mechanism i s c o r r e c t. F in a lly the f a c t th a t very l i t t l e ^ i f any?

I l l

bibenzyl i s formed i s s u rp ris in g , although R ussell and Brown, ' fo r example, do n ot re p o rt the form ation of b ibenzyls in the photo­

c h lo rin a tio n of toluene w ith c h lo rin e .

A mechanism involving n u c le o p h ilic a tta c k on ch lo rin e can also be form ulated. 0 Cl 01 01 01 -I- 01 Oh 01 01 r \ V1 CILOl \y

Again t h i s i s n o t wholly s a tis f a c to r y . I t f a i l s to ex p lain , fo r example, th e la ck of a tta c k on mot a c h lo rin e .

130

On th e whole th e form ation of benzyl c h lo rid e r a th e r than a carbonium ion as an in te rm ed ia te seems to provide s l i g h t l y le s s

u n s a tis f a c to ry mechanisms, but more experim ental evidence i s req u ired before a c le a r p ic tu re can be obtained. None of th e evidence

presented gives any reason to suggest the p a r tic ip a tio n o f a t r i p l e t carbene s im ila r to th a t p o stu late d in th e re a c tio n o f bromodurene w ith base.

5» Conclusions

The experim ents5 in vo lv in g 2 ,6 -d im eth y lary l h a lid e s and stro ng bases discussed in t h i s th e s is ,in d i c a te the form ation of a b ifu n c­ tio n a l in term ed iate d if f e r in g from benzyne in term ed iates in th a t i t has a t r i p l e t s tr u c tu re , but formed by a s im ila r se t of r e a c tio n s =

Most o f the evidence obtained from th ese experiments i s

c o n siste n t w ith the suggestion th a t the polymcthylbenzeno i s formed by d ir e c t n u c le o p h ilic a tta c k on the halogen by base. Although th e f a te of the base derived product formed by t h i s a tta c k i s obscure, the f a c t th a t the y ie ld o f polymethylbenzene is dependent only on th e n atu re of th e base and th e halogen appears to be s ig n if ic a n t.

The d iffe re n c e s in the products obtained from the various

2 ,6-dim ethylaryl h a lid e s stu d ied are explained s a t i s f a c t o r i l y by the mechanism which i s o u tlin ed in Scheme I I (page 106).

On the o th e r hand the attem pt to extend t h i s mechanism to the decomposition of o-halophenol s a l t s is not v a lid . Although the o rig in of the products formed from sodium pentachlorophenate i s u n c e rta in , th e re i s no evidence th a t a t r i p l e t carbene in term ediate

analogous to th a t formed by th e 2 ,6-dim ethylaryl h a lid e s i s involved. I t i s c le a r , th e re fo re , th a t th e new re a c tio n s described in t h i s th e s is , although form ally involving r e la tiv e l y simple re a c ta n ts , are not e a s ily r a tio n a lis e d in terms o f c l a s s i c a l r e a c tiv e

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