Steric effect aliphatic

The solubility of a compound is thus affected by many factors the state of the solute, the relative aromatic and aliphatic degree of the molecules, the size and shape of the molecules, the polarity of the molecule, steric effects, and the ability of some groups to participate in hydrogen bonding. In order to predict solubility accurately, all these factors correlated with solubility should be represented numerically by descriptors derived from the structure of the molecule or from experimental observations. 

Taft began the LFER attack on steric effects as part of his separation of electronic and steric effects in aliphatic compounds, which is discussed in Section 7.3. For our present purposes we abstract from that treatment the portion relevant to aromatic substrates. Hammett p values for alkaline ester hydrolysis are in the range +2.2 to +2.8, whereas for acid ester hydrolysis p is close to zero (see Table 7-2). Taft, therefore, concluded that electronic effects of substituents are much greater in the alkaline than in the acid series and. in fact, that they are negligible in the acid series. This left the steric effect alone controlling relative reactivity in the acid series. A steric substituent constant was defined [by analogy with the definition of cr in Eq. (7-22)] by Eq. (7-43), where k is the rate constant for acid-catalyzed hydrolysis of an orr/to-substituted benzoate ester and k is the corresponding rate constant for the on/to-methyl ester note that CH3, not H, is the reference substituent. ... 

A very interesting steric effect is shown by the data in Table 7-12 on the rate of acid-catalyzed esterification of aliphatic carboxylic acids. The dissociation constants of these acids are all of the order 1(T, the small variations presumably being caused by minor differences in polar effects. The variations in esterification rates for these acids are quite large, however, so that polar effects are not responsible. Steric effects are, therefore, implicated indeed, this argument and these data were used to obtain the Es steric constants. Newman has drawn attention to the conformational role of the acyl group in limiting access to the carboxyl carbon. He represents maximum steric hindrance to attack as arising from a coiled conformation, shown for M-butyric acid in 5. 

It is not necessary to look very far to find aliphatic reactions that do not yield straight line plots with [10], however and, as with previous deviations from linearity (p. 375), these departures are commonly much more informative about the details of reaction pathways than are neat straight lines. Where such departures from linear (polar effects only) plots are observed, suggesting the operation of significant—and changing—steric effects, it is possible to incorporate a steric substituent parameter, Eg, whose evaluation is based on an earlier observation. 

In the second edition of his book33 Stewart proposed a parallel between the rate of esterification of 2-substituted benzoic acids and the molecular weights of the substituents, the nitro group strongly deviating from this relationship. The first actual attempt to define a set of steric parameters is due to Kindler39. It was unsuccessful these parameters were later shown to be a function of electrical effects. The first successful parametrization of the steric effect is due to Taft40, who defined the steric parameter Es for aliphatic systems by the expression ... 

Taft steric Steric effects in aliphatic and ortho-substituted aromatic systems... 

Aliphatic compounds containing terminal gem-dinitro functionality form adducts with Michael acceptors.Of particular interest is the reaction of a,a,o),a)-tetranitroalkanes with Michael acceptors. ° Most a, o, y, y-tetranitroalkanes will react with two equivalents of Michael acceptor to form bis-adducts, like in the case of 1,1,4,4-tetranitrobutane, which reacts with two equivalents of methyl vinyl ketone, methyl acrylate, acrylonitrile etc. ° The influence of steric effects becomes apparent with a,a,y,Y-tetranitroalkanes, like 1,1,3,3-tetranitropropane, which can form either mono-adducts or bis-adducts depending on the Michael acceptor used 1,1,3,3-tetranitropropane will only react with one equivalent of methyl acrylate and the sole product of this reaction is methyl 4,4,6,6-tetranitrohexanoate. °... 

Ordinarily, alkyl nitrate esters will not nitrate amines under neutral conditions. However, Schmitt, Bedford and Bottaro have reported the use of some novel electron-deficient nitrate esters for the direct At-nitration of secondary amines. The most useful of these is 2-(trifluoromethyl)-2-propyl nitrate, which nitrates a range of aliphatic secondary amines to the corresponding nitramines in good to excellent yields. Nitrosamine formation is insignificant in these reactions. 2-(Trifluoromethyl)-2-propyl nitrate cannot be used for the nitration of primary amines, or secondary amines containing ethylenediamine functionality like that in piperazine. Its use is limited with highly hindered amines or amines of diminished nucleophilicity due to inductive or steric effects. 

The variation in the add-catalyzed hydrolysis of aliphatic carboxylic esters RCOOEt is mostly subject to the steric effect intramolecularly exerted by the substituent R on the formation of the tetrahedral reaction intermediate. Taft4) defined the steric parameter E, as Eq. 3,... 

Since the total steric effect of component substituents R1, R2 and R3 on the reaction center in the transition state of ester hydrolysis is similar, if not identical, to that of three N-substituents of aliphatic amines on a certain electron acceptor or electrophile, we attempted to separate electronic and steric effects of N-substituents of amines NR3R2R3 for various reactions by Eq. 24 where is either the rate or equilibrium constant, Scr is the sum of a values for R1, R2 and R3 and E (R ) E (R2) Si E (R3), and showed that Eq. 24 is generally applicable 22). 

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