Aromatic compounds steric effects

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. ... 

In arriving at a picture of the influence of various substituents in aromatic heterocyclic compounds on quatemization, it is very difficult to separate the functions of electronic and steric effects. 

Vogtle, F., and Hohner, G. Stereochemistry of Multibridged, Multilayered, and Multistepped Aromatic Compounds. Transanular Steric and Electronic Effects. 74, 1-29 (1978). 

In its original form the Hammett equation was appropriate for use with para and meta substituted compounds where the reaction site is separated from the aromatic group by a nonconjugating side chain. Although there have been several extensions and modifications that permit the use of the Hammett equation beyond these limitations, it is not appropriate for use with ortho substituted compounds, since steric effects are likely to be significant with such species. The results obtained using free radical reactions are often poor, and the correlation is more appropriate for use with ionic reactions. For a detailed discussion of the Hammett equation and its extensions, consult the texts by Hammett (37), Amis and Hinton (12), and Johnson (47). 

Iridium nanopartides also catalyze the hydrogenation of benzyhnethylketone, with high selectivity in reduction of the aromatic ring (92% selectivity in saturated ketone, 8% in saturated alcohol at 97% benzylmethylketone conversion). This preferential coordination of the aromatic ring can be attributed to steric effects that make carbonyl coordination difficult. Therefore, metallic iridium nanoparticles prepared in ILs may serve as active catalysts for the hydrogenation of carbonyl compounds in both solventless and biphasic conditions. 

The solubilities of aromatic compounds in the ionic liquid are dramatically higher than those of saturated compounds. Benzene has a solubility of 4.9mol/mol of ionic liquid, and thiophene has a solubility of 6.7mol/mol of ionic liquid. A dramatic steric effect was observed on the solubility of aromatics the alkyl-substituted aromatics showed reduced solubility. Although the solubility of hexene in the ionic liquid is considerably lower than that of the aromatics, it is still measurably higher than that of hexane. Similar structure-solubility relationships characteristic of organic molecules were observed with the ionic liquids [BMIM]BF4, [BMIM]PFg, and [EMIM]BF4 (Fig. 10) (27). 

A rough prediction of desorption and recovery can be made by extrapolation from similar compounds however, until we know more about the factors involved, this practice can lead to difficulties as illustrated in Table II. The recovery of naphthalene and biphenyl from charcoal are much lower than one would predict from the recoveries of other aromatic compounds, and this stronger attraction may be related to bond angles or steric effects. The order of recovery, ortho (84%) > meta (81%) > para (76%) methyl biphenyl, supports this theory. One may predict from this information that the recovery of benzene would be very poor, which is not the case. 

Reduction of aldehydes and ketones. Earlier work on amine borane reagents was conducted mainly with tertiary amines and led to the conclusion that these borane complexes reduced carbonyl compounds very slowly, at least under neutral conditions, and that the yield of alcohols is low. Actually complexes of borane with primary amines, NHj or (CH3)3CNH2, reduce carbonyl compounds rapidly and with utilization of the three hydride equivalents. BH3 NH3 is less subject to steric effects than traditional complex hydrides. A particular advantage is that NH3 BH3 and (CH3)3CNH2 BH3 reduce aldehyde groups much more rapidly than keto groups, but cyclohexanone can be reduced selectively in the presence of aliphatic and aromatic acyclic ketones. 

In summary, the most important factors influencing the pKa of a given acid or base function are inductive, resonance, and steric effects. The impact of a substituent on the pK3 depends critically on where the substituent is located in the molecule relative to the acid or base group. In one place, a given substituent may have only one of the mentioned effects, while in another location, all effects may play a role. It is quite difficult, therefore, to establish simple general rules for quantifying the effect(s) of structural entities on the pK3 of an acid or base function. Nevertheless, in certain restricted cases, a quantification of the effects of substituents on the pKa value is possible by using LFERs. In the next section, we discuss one example of such an approach, the Hammett correlation for aromatic compounds. First, however, a few comments on the availability of experimental pKiz values are necessary. 

Steric effects on the reactivity of benzoic acid derivatives are a little more complicated. The first significant point is that the aromatic carboxylic acids and esters are often some two orders of magnitude less reactive than the corresponding aliphatic compounds. Chapman et al. 7, have explained this difference in terms of three co-operative factors (i) the stabilization of the initial state by delocalization in the case of the aromatic compounds, (ii) inductive electron-withdrawal by the ring, which is significant in esterification... 

In aromatic compounds carbon-13 shifts are largely determined by mesomeric (resonance) and inductive effects. Field effects arising from through-space polarization of the n system by the electric field of a substituent, and the influences of steric (y) effects on the ortho carbon nuclei should also be considered. Substituted carbon (C-l) shifts are further influenced by the anisotropy effect of triple bonds (alkynyl and cyano groups) and by heavy atom shielding. 

The Hammett equation also fails for open-chain aliphatic derivatives. For example, there is no simple linear relationship between log K for a scries of substituted ethanoic acids (RCH2C02H) and log k for the hydrolysis rates of similarly substituted ethyl ethanoates (RCH2C02C2H5). The freedom of motion available to a flexible open-chain compound permits a much wider range of variations in steric effects than for meta- and para-substituted aromatic compounds. 

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