Substituents activation or deactivation

Why Substituents Activate or Deactivate a Benzene Ring Figure 18.6 Energy diagrams comparing the rate of electrophilic aromatic substitution of substituted benzenes... 

Since the positions of the Is,min are the locations, on the average, of the most easily removed electrons, these should also be the sites that are most reactive toward electrophilic attack. This has been fully confirmed for a group of monosubstituted benzene derivatives [10,21]. The Is,min correctly predict the ortho/para- or meta- directing tendencies of the substituents, even the rather unusual NH3+, which is a metal para director [22]. Furthermore, the magnitudes of the Is,min relative to that of unsubstituted benzene correctly indicate whether each substituent activates or deactivates the aromatic ring toward electrophiles. These analyses have been extended to other aromatic systems, including azines and azine N-oxides [18,23,24]. 

In compounds bearing several different groups there wiU be a complex interaction of activation by the azine-nitrogen with activation or deactivation by the substituents (Section II, E). The complexity of the interaction is emphasized by the realization that the effects of two identical substituents in an azine (e.g., in 2,4-dichloropyrimidine) are not the same on each other (Section II,B,2,a). 

Evaluating relative reactivity at different positions by competitive displacement in polychloroazines requires the unjustified assumption that the chloro groups have a negligible or equal effect on each other. In a polychloro compound bearing a different kind of substituent, the activation or deactivation of the chloro groups is generally neither equal nor negligible, as already demonstrated. 

Inductive and resonance effects account for the directing effects of substituents as well as for their activating or deactivating effects. Take alkyl groups, for instance, which have an electron-donating inductive effect and are ortho and para directors. The results of toluene nitration are shown in Figure 16.13. 

The pKa of p-(tiifluoromethyl)benzoic acid is 3.6. Is the trifluoromethyl substituent an activating or deactivating group in electrophilic aromatic substitution ... 

The trend in relative effectiveness of RAFT agents with varying Z is rationalized in terms of interaction of Z with the C=S double bond to activate or deactivate that group towards free radical addition. Substituents that facilitate addition generally retard fragmentation. O-Alkyl xanthates (Z=0-alkyl, Table... 

Interesting situations develop when multiple (>2) substituents are placed in fairly close proximity. Depending on the donor or acceptor nature of the substituents and that of the carbon, center in question, and the number of bonds separating them, there will be an activating or deactivating effect. Typical scenarios are summarized in the following diagrams. 

The effects summarized in Table II show, with few exceptions, that para groups, whether activating or deactivating, exert larger effects than meta substituents. Ortho groups demonstrate both electronic and steric effects and almost without exception are deactivating. Substituents situated ortho to a reactive site are more deactivating than those which are meta or para. 

Aromatic fluorine bonds can be activated or deactivated by substituents on the ring. Experimental results show that the aromatic C-F bond is activated for reduction if there is an electron-withdrawing group (e. g., ester and carbonitrile groups) or electron-releasing groups (c.g., hydroxy and amino groups) ortho or para to the fluorine atom. Substituents in meta positions usually decrease the reactivity. 

This is so because the methyl substituent can affect the rate and the position of further substitution. A substituent can either activate or deactivate the aromatic ring towards electrophilic substitution and does so through inductive or resonance effects. A substituent can also direct the next substitution so that it goes mainly ortho/para or mainly meta. 

A full account of the scope and limitations of the amination chemistry of these ligands has recently appeared [158]. With ligand 15, a number of aminations were conducted at room temperature. In the absence of an ortho substituent, couplings of primary amines with unactivated aryl chlorides at room temperature required 5 mol % catalyst. However, a variety of secondary amines, both cyclic and acyclic, reacted with activated or deactivated aryl chlorides at room temperature. Thirteen examples were demonstrated. The scope of this process was broader, however, when reactions were conducted at 80 or 110 °C. Under these conditions, unactivated aryl chlorides reacted with a variety of amines in high yields. In favorable cases, such as reactions of aryl chlorides bearing one ortho methyl group, reactions of N-methyl... 

The substitutent activation (or deactivation factors Ar[79JCS(P2)381] for 8.8 and 8.9 are 30 and 20, respectively. The observed partial rate factors then follow since a larger value of Af predicts a larger substituent effect. 

Substituents either activate or deactivate a benzene ring towards electrophiles, and direct selective substitution at specific sites on the ring. All substituents can be divided into three generai types. 

---