Essais de Chloration Dirigée: XIII. La Photochloration Directe des Hydrocarbures Arylaliphatiques [1]

In the photochlorination of arylaliphatic hydrocarbons (C6H5R; R = CzH5 ; n. C3H7 ; i. C3H7 ; n. C4Hg ; sec. C4H9 ; n. CsH I substitution only takes place in the side chain, and preferentially in the a position, as was showed in the previous paper. Now, in order to make clear which effect is operative-either the polar effect of the substituent or a complexation effect of the aromatic nucleus of the reagent, photochlorination of the title compounds was examinated in various solvents (CCI4, aromatics and CSz). Firstly, diluting by CCI4, it was observed that relative reactivity, r.r. (alb), of an hydrogen a versus another b, varies with the concentration and can be expressed as a linear function :including complexation constant K, , specific rate k,,, and relative reactivity koO/kbo at infinite dilution (whose numerical values can be calculated by means of the former formula), if R is CzH, ; n.C3H7; i.C3H7 ; sec.C4H9, and with a more sophisticated one:for the other terms, R = n. C4H9, n. C5H ,.With benzene, selectivity is enhanced but with chlorobenzene it is depressed. Finally, the best results concerning the selectivity are obtained by photochlorination in the presence of C S z . We mention here the most interesting result of cumene (R = i. C3H7), giving up to 98% of a-chloroisomer in the presence instead of 86% of the same in the absence of CS2.En examinant les resultats relatifs A la photochloration directe des hydrocarbures arylaliphatiques, que nous avons present& dans le m h o i r e precedent, nous avons constate, entre autres, la proportion preponderante d'isomeres a-chlores.A titre d'exemple, epinglons le cas du cumene oh les repartitions experimentales (R. E. 85,5 % a-C1 et 14,5 P-Cl) sont quasi inverskes vis-a-vis des rbpartitions statistiques (R. S. 14,3 Yo a-CI et 85,7 % P-Cl) et m&me tres Bloignkes de celles calculQs au moyen des coefficients de HASS (R.T. 44 % cr-CI et 56 % P-Cl). A notre avis, la difference considkable existant entre ces repartitions peut Qtre attribuQ A deux actions distinctes au moins : d'une part, a I'activation par le groupe phenyle mettant en jeu son effet polaire, et d'autre part a I'augmentation de selectivite resultant de la complexation du chlore atomique par le substrat lui-mhe, agissant par effet de milieu.Rappelons succinctement, a ce propos, I'action des solvants.

Essais de Chloration Dirigée: XIV. Effets de Solvant sur la Photochloration D'Hydrocarbures Arylaliphatiques(¹)

Corbiau

Bruylants

1970

Essais de Chloration Dirigée: XV. Corrélation Structure‐Réactivité Dans la Photochloration des Alkylbenzènes [1,2]

Corbiau

Soumillion

Bruylants

1970

Self Cite

A quantitative discussion of the experimental results previously obtained in the photochlorination of the alkylbenzenes (C6H5‐R: R = C2H5‐, n.C3H7‐, i.C3H7‐, n.C4H9‐, n.C5H11) is presented. The correlation used is as following: log r. r.(a/b) = ρ*(Σσa* – Σσb*) + Ψa – Ψb and accounts for the inductive (σ*) and mesomeric effects (Ψ) of the substituents upon the relative reactivity [r. r. (a/b)] of the ≥ C‐H bond; the last one is only acting upon the α position. The absolute value of both parameters (ρ and Ψ) increases with the concentration of the aromatic substrate. The relative magnitude of their actions is commented.

Essais De Chloration Dirigée: XVI. Effet Mésomère et Réactivité Dans Les Réactions De Chloration Radicalaire

Soumillion

Gouverneur

Burton

et al. 1971

Self Cite

From their results concerning mainly radical chlorinations of substituted alkanes and from other literature data dealing with this subject, the authors discuss about mesomeric effects of substituents in relation with medium and reactants modifications. With the aid of a properly modified structure-reactivity correlation, it can be demonstrated that changes in reactivity of chlorinating agents or in structure of aliphatic substrates result in quantitative variation in the importance of mesomeric effects of substituents.Dans la chloration photochimique de derives aliphatiques, I'espece active, soit le chlore atomique, est tres reactionnelle, et engendre des lors, un melange de tous les isomeres inonochlores possibles dont la distribution est en relation directe avec la structure de la molCcule de depart. Les reactivites relatives des differentes positions d'une chaine aliphatique portant un groupement fonctionnel sont a la base d'une etude possible des effets de substituants.Parmi ceux-ci, I'effet inducteur joue un rble tres important en creant, dans la molecule, un gradient de densit6 Clectronique, entrainant le chlore radicalaire, denature electrophile, a se fixer aux endroits les plus riches en electrons, c'est-a-dire au voisinage d'un substituant Clectrodonneur, mais le plus loin possible d'un electrocapteur.Les travaux de notre laboratoire ont demontre quantitativement le phenombne, grtice a un traitement des resultats experimentaux par la correlation structure-reactivite (l), adaptee aux photochlorations sous la forme [l a 51 :