Hammett p a relationships

Streitwieser pointed out that the eorrelation whieh exists between relative rates of reaetion in deuterodeprotonation, nitration, and ehlorination, and equilibrium eonstants for protonation in hydrofluorie aeid amongst polynuelear hydroearbons (ef. 6.2.3) constitutes a relationship of the Hammett type. The standard reaetion is here the protonation equilibrium (for whieh p is unity by definition). For eon-venience he seleeted the i-position of naphthalene, rather than a position in benzene as the referenee position (for whieh o is zero by definition), and by this means was able to evaluate /) -values for the substitutions mentioned, and cr -values for positions in a number of hydroearbons. The p -values (for protonation equilibria, i for deuterodeprotonation, 0-47 for nitration, 0-26 and for ehlorination, 0-64) are taken to indieate how elosely the transition states of these reaetions resemble a cr-eomplex. 

It is perhaps interesting, in view of the very considerable success of Hammett plots, to say a word finally about the thermodynamic implications of linear free energy relationships in general. We have already mentioned (p. 359) the relationship between free energy change, AG, and log k or log K and each AG term is, of course, made up of an enthalpy, AH, and an entropy, AS, component ... 

A linear relationship between the standard enthalpies and entropies of a series of structurally related molecular entities undergoing the same reaction thus, AH° -I3AS° = constant or AAH° = (3AS°. When P > 0, this relationship is referred to as an isoequilibrium relationship. When the absolute temperature equals the factor P (often referred to as the isoequilibrium temperature), then all substituent effects on the reaction disappear (i e., AAG° = 0). In other words, a reaction studied at T = p will exhibit no substituent effects. This would suggest that, when one studies substituent effects on a reaction rate, the reaction should be studied at more than one temperature. Note also that the p factor in the Hammett equation changes sign at the isoequilibrium temperature. See Isokinetic Relationship... 

Reactivity-selectivity relationships are obtainable in carbene chemistry provided an independent selectivity parameter is found. Under such circumstances the relative rate data then serve as a measure of relative reactivity. This has been done with a number of Hammett studies. Thus the addition of dichlorocarbene to compounds [ 1 ] — [3] (listed in Table 15) indicates that a reactivity-selectivity relationship is obtained using p as a measure of selectivity. A more reactant-like transition state is obtained for the more reactive substrate in accordance with the Leffler- Hammond postulate. 

The rate constants for the solvent-recombination process of the carbocations [3C (X,Y,Z)] were determined by the use of the azide clock method (Richard etal., 1984 Richard and Jencks, 1984a,b,c McClelland et al., 1991) and the rate constant of the forward reaction was derived using (38b) as /Ch = /CwXr+ (McClelland et al., 1989,1991). While ordinary Hammett-type relationships were found to be inapplicable to the substituent effects on solvent recombination, there is a rate-equilibrium correlation for all available data on triarylmethyl cations, shown as the linear log/c , vs. p/Cr<- plot, in Fig. 34 with a slope of 0.64. Such a relationship was earlier suggested by Arnett and Hofelich (1983) and Ritchie (1986). The correlation of ky, with the cr scale was... 

Azo coupling reaction rates of substituted benzenediazonium ions with a given nucleophile under standard conditions (solvent, temperature, pH, etc.) are an ideal case for the application of the Hammett relationship in its classical form (71) kj, and k refer to the rate constants of the benzenediazonium ion and its m- or p-substi-tuted derivatives, respectively, with the given coupling component, p and a are Hammett s reaction and substituent constants, respectively. 

The Russian workers claim that substituents on the azido group which increase the asymmetry, presumably by changing the equivalence of the bonding, shift the bands in question to higher frequency. They also suggest that this asymmetric stretching band for aromatic sulphonyl azides can be correlated by a Hammett Opp relationship where p has a small value. (They find the ultraviolet spectral parameters responding in similar fashion.)... 

The analysis of equation 64 offers important insight into the nature of p and a in the Hammett equation. Although the correct general dependence of p on temperature, as predicted from the isokinetic relationship between the enthalpy and the entropy of a reaction is (1, 79, 80)... 

Reference to Chapter 3 (Scheme 3) shows that the alkaline hydrolysis of aryl carbamate esters has a large Hammett p coefficient (2.54) whereas that for the alkaline hydrolysis of aryl acetate esters is only small (0.67) (Scheme 2). The small p value for alkaline hydrolysis of the acetate case is consistent with little bond fission in the transition structure and the large p for the carbamate case (see Chapter 3 for mechanism) indicates substantial bond fission and a gross difference in mechanism between the two reactions. The simple Hammett relationship only correlates the alkaline hydrolysis rates of the carbamate when meta substituents are employed. Para substituents, which withdraw negative charge by resonance (4-NO2, 4-CN, etcf require values to obtain a good correlation. This is interpreted to mean that the transition structure has sufficient aryl oxide ion character for resonance to occur. In... 

Polymer Tacticity. Our initial results on the polymerization of several different p-substituted-a-methylstyrene monomers indicated that there was some relationship between polymer stereoregularity and both the type of initiator and substituent in these monomers ( ). However, our recent investigations with a much wider variety of monomers, catalysts and cocatalysts revealed that the classical approach to analyzing substituent effects in organic reactions, the use of the Hammett pa relationship, gave no simple and self-consistent relationship between tacticity and the a (or a ) constant for the para-substituent. These results are summarized in the data in Table I for the cationic polymerization of a-methylstyrene and a series of five p-substituted-a-methylstyrene monomers initiated with two different Friedel-Crafts catalysts, TiCl and SnCl, either alone or with a cocatalyst benzyl chloride (BC) or t-butyl chloride (TBC), in methylene chloride at -78°C. Where a cocatalyst was used, the initiator was presumably a carbonium ion formed by the following reaction ... 

In the bromination of substituted styrenes, a pa plot is noticeably curved. If the extremes of the curve are taken to represent straight lines, the curve can be resolved into two Hammett relationships with p = —2.8 for EWG substituents and p = —4.4 for ERG substituents. The corresponding (3-methylstyrenes give a similarly curved plot. The stereoselectivity of the reaction of the (3-methylstyrenes is also dependent on the substituents. The reaction is stere-ospecifically anti for strong EWGs, but is only weakly stereoselective, e.g., 63% antv31% syn, for methoxy. Discuss a possible mechanistic basis for the curved Hammett plots and the relationship to the observed stereochemistry. 

The Hammett p-o relationship (or p-a -) (Figure 5) shows that there is no significant effect of substituent X on the aromatic ring of benzoyl chlorides on the initial reaction rate. 

A relationship between the electronic situation of the reacting olefin and the calculated regioselectivity could be predicted. Figure 10 shows the linear free-energy relationship (Hammett plot) between the a- and p-(3-l-2) additions of five electronically different olefins (R = —H, -CH3, -CH2CH3, -CHO, -CN) to OSO3NCNNH3. 

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