Entering groups

The ranking of the leaving and the entering groups in the reactions of Figures 3-22... 

The methyl groups direct the entering group primarily to the ortho and para positions (Table 3). The preferred site of attack by an electrophile on pseudocumene and hemimellitene is shown as follows (13) however, steric hindrance can cause a shift such as a / fX butylation of hemimellitene predominandy at the 5-position. Mesitylene, all three tetramethylbenzenes, and pentamethylbenzene can only form one mono- and one disubstituted isomer (except pentamethylbenzene). Hence, high purity derivatives are possible. 

During certain substitution reactions, the carboxyl group is often replaced by the entering group. An example is fuming nitric acid, which results in the formation of trinitrophenol. Another is the bromination of saUcyhc acid in aqueous solution to yield 2,4,6-tribromophenol [25376-38-9] (eq. 6). 

Steric effects play a major role in determining the ortho para ratio in Friedel-Crafts alkylations. The amount of ortho substitution of toluene decreases as the size of the entering alkyl group increases along the series methyl, ethyl, /-propyl. No ortho product is found when the entering group is /-butyl. ... 

The position of substitution in disubstituted thiophenes can, in most cases, easily be deduced from the directing effect of each substituent. Thus with a - -M-substituent in the 2-position and a —M-substituent in the 5-position, both substituents direct the entering group to the 3-position as is exemplified by the nitration of methyl 2-bromo-5-thiophenecarboxylate to methyl 2-bromo-3-nitro-5-thio-phenecarboxylate (109) or in the chlororaethylation of methyl 2-methyl-5-thiophenecarboxylate to methyl 2-methyl-3-chloromethyl-5-thiophenecarboxylate (110). °... 

Parker has suggested that the presence of a lone-pair on the entering group in the intermediate complex (234) increases the rate by assisting in breaking the bond to the leaving group (Le) as indicated. 

Dediazoniation refers to all those reactions of diazo and diazonium compounds in which an N2 molecule is one of the products. The designation of the entering group precedes the term dediazoniation, e. g., azido-de-diazoniation for the substitution of the diazonio group by an azido group, or aryl-de-diazoniation for a Gomberg-Bachmann reaction. The IUPAC system says nothing about the mechanism of a reaction (see Sec. 1.2). For example, the first of the two dediazoniations mentioned is a heterolytic substitution, whereas the second is a homolytic substitution. 

The calculations so far have been concerned with the permanent charge distribution which the molecule would have when isolated, and which would be observed in its dipole moment. The factor that actually determines the orientation of the entering group, however, is the charge distribution at the instant of attack, which differs from the permanent charge distribution in consequence of polarization by the entering group. In most cases it is not necessary to take this latter effect into account explicitly, since it represents only a small correction. However, in certain cases it is apparently of decisive importance.7... 

Substitutions. A name consists of the entering group, the syllable de , and the leaving group. If the leaving group is hydrogen, it may be omitted (in all examples, the substrate is written on the left). 

The concept of the transition state is fundamental to understanding the chemical and thermodynamic basis of catalysis. Equation (7) depicts a displacement reaction in which an entering group E displaces a leaving group L, attached initially to R. 

In a ligand substitution reaction, two groups must always receive attention. There is a bond to the leaving group to be broken and a bond to the entering group to be formed. The relative importance of these two processes provides a basic dichotomy for the classification of substitutions. If a reaction rate is sensitive to... 

Fig. 8. Relative reactivities of different entering groups Y with PtdienBr in aqueous solution at 25 °C. Data from ref. 74.

In the case of the hexacarbonyls, the rate-expression contains not only the same type of first-order term but in addition one second-order overall. For good entering groups (but not CO, for example) the rate expression contains a term strictly first-order in both the complex and the entering nucleophile. The first-order rates of CO exchange are practically identical with the rates of substitution in hydrocarbon solvents, but there is nevertheless some acceleration in ether (THF, dioxan) solutions. This solvent-dependence is not so well-marked ° as in the case of nickel tetracarbonyl. The second-order rate of substitution very strongly depends upon the basicity of the entering nucleophile... 

This is the best known of the reactions of sulphonyl nitrenes and a large number of examples were studied by Curtius and his coworkers. The first attempt to establish semi-quantitative correlations between the nature of the substituent and orientation of the entering group was made by... 

EFFECT OF ENTERING AND LEAVING GROUPS 4.5.1 The entering group... 

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