Effects of Alkyl Substitution

Substituents on the 2- and 6-positions of phenol rings greatly influence QM reactivity. Reaction rates for QMs derived from several of the phenols, shown in Fig. 10.1, were determined in methanolic or aqueous solutions and are listed in Table 10.1. Replacing a tert-butyl substituent of BHT by a methyl group (i.e., BDMP-QM) increased the rate of hydration by 60-70-fold at pH 7.4 and this 

C(CH3)3 C(CH3)3CH2OH C(CH3)3 C(CH3)2OH FIGURE 10.1 Structures and abbreviations of alkylphenols mentioned in the text. 

TABLE 10.1 Half-Lives (s) for QM Solvolysis in Methanol and Water  

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A comparison of the relative basicities of pyrrole, furan and thiophene may be made by comparing the pK values of their 2,5-di-t-butyl derivatives, which were found to be -1.01, —10.01 and —10.16, respectively. In each case protonation was shown by NMR to occur at position 2. The base-strengthening effect of alkyl substitution is clearly apparent by comparison of pyrrole and its alkyl derivatives, e.g. A-methylpyrrole has a pKa. for a-protonation of -2.9 and 2,3,4,5-tetramethylpyrrole has a pK of 4-3.7. In general, protonation of a-alkylpyrroles occurs at the a -position whereas /3-alkylpyrroles are protonated at the adjacent a-position. As expected, electron-withdrawing groups are base-weakening thus A-phenylpyrrole is reported to have a p/sTa of -5.8. The IR spectrum of the hydrochloride of 2-formylpyrrole indicates that protonation occurs mainly at the carbonyl oxygen atom and only to a limited extent at C-5. 

What conclusion can you draw about the effect of alkyl substitution on UV absorption maxima Approximately what effect does each added alkyl group have ... 

The effect of alkyl substitution on alcohol acidity is due primarily to solvation of the alkoxide ion that results from dissociation. The more readily the alkoxide ion is solvated by water, the more stable it is, the more its formation is energetically favored, and the greater the acidity of the parent alcohol. For example, the oxygen atom of an unhindered alkoxide ion, such as that from methanol, is stericallv accessible and is easily solvated by water. The oxygen... 

The main tables are designated by a letter and a number (a full list is given in the Contents at the beginning of this chapter). Tables in the text, which mostly contain effective molarities taken from the main tables, are designated by a number only. Every individual reaction is identified by a reference letter and two numbers. Thus B.6.3 refers to the base-catalysed cyclization of cyclohexyl 2-hydroxyethyl phosphate which is the third entry in Table B.6. The EM for this reaction is given in Table 10 in the discussion of the effects of alkyl substitution on ring-closure, and the reference number quoted there. Full details of the calculation of the EM, the rate constant on which it is based, the conditions used and the authors concerned can then be found by consulting Table B.6. 

Effects of alkyl substitution on EM s for lactonization of hydroxy acids... 

Price, W. C. The Effect of Alkyl Substitution on the Spectra and Ionization... 

Vesala, A. and Lonnberg, H. Salting-in effects of alkyl-substituted pyridinium chlorides on aromatic compounds, Acta Chem. 5canrf., A34 187-192, 1980. 

As one proceeds to more condensed thiophene derivatives (such as diben-zothiophene), the effects of alkyl substitution become much more signifi-... 

Abstract—A review of the literature is presented for the hydrolysis of alkoxysilane esters and for the condensation of silanols in solution or with surfaces. Studies using mono-, di-, and trifunctional silane esters and silanols with different alkyl substituents are used to discuss the steric and electronic effects of alkyl substitution on the reaction rates and kinetics. The influences of acids, bases, pH, solvent, and temperature on the reaction kinetics are examined. Using these rate data. Taft equations and Brensied plots are constructed and then used to discuss the mechanisms for acid and base-catalyzed hydrolysis of silane esters and condensation of silanols. Practical implications for using organofunctional silane esters and silanols in industrial applications are presented. 

Let us have a look at some instructive pH-rate profiles. That for acetophenone was already discussed in the section pH-Rate Profiles (Fig. 3). Its general shape is characteristic for the behavior of the enols of simple ketones and aldehydes. The enolization constants of aldehydes tend to be higher than those of ketones compare, for example, pA h(acetonc) = 8.33 and pA"E(acetaldehyde) 6.23. This is in line with the well-known stabilizing effect of alkyl substitution on double bonds, in particular of the polar C=0 bond, a-Substitution of ketones and aldehydes by alkyl or, better still, by aryl groups further stabilizes the enol, so that the enol content of 2,2-diphenylacetaldehyde reaches 10%.34... 

Different alkyl halides undergo Sn2 reactions at vastly different rates. The structure of the substrate is the most important factor in its reactivity toward Sn2 displacement. The reaction goes rapidly with methyl halides and with most primary substrates. It is more sluggish with secondary halides. Tertiary halides fail to react at all by the Sn2 mechanism. Table 6-5 shows the effect of alkyl substitution on the rate of Sn2 displacements. 

Styrene will act similarly but the effect of alkyl substitutents is almost entirely inductive. 

Two-laser two-photon results revealed photoisomerization of the cation E,E-11 to its stereoisomer Z,E-11, which undergoes thermal reversion with a lifetime of 3.5 ps at room temperature. Absolute rate constants for reaction of styrene, 4-methylstyrene, 4-methoxystyrene and /i-methyl-4-methoxystyrene radical cation with a series of alkanes, dienes and enol ethers are measured by Laser flash photolysis [208]. The addition reactions are sensitive to steric and electronic effects on both the radical cation and the alkene or diene. Reactivity of radical cations follows the general trend of 4-H > 4-CH3 > 4-CH3O > 4-CH30-jff-CH3, while the effect of alkyl substitution on the relative reactivity of alkenes toward styrene radical cations may be summarized as 1,2-dialkyl < 2-alkyl < trialkyl < 2,2-dialkyl < tetraalkyl. 

EFFECT OF ALKYL SUBSTITUTION OF THE RATES OF THE CLAISEN REARRANGEMENT AT 170... 

By studying several epoxides Gee et al. [15] were able to show the large steric effect of alkyl substitution on the rate of reaction. The second order rate coefficient for attack at a —CHj— (in EO or in PO), k2, was... 

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