The influence of alkyl substituents

The direction of elimination in the reaction of a series of alkyl bromides (118f changes from Saytzeff to Hofmann as the size of a beta alkyl substituent is increased, indicating a gradual increase in importance of steric interaction. 

Steric interactions have been invoked to explain orientation in some unimolecular eliminations . In a series of tertiary bromides (Table 19) a gradual increase in the size of one of the branches causes an increase in the amount of 

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The influence of alkyl substituents on the asynchronous transition-state structure of the BF3-catalyzed carbo-Diels-Alder reaction of a,/ -unsaturated aldehydes with 1,1-dimethyl-l,3-butadiene derivatives has been investigated by Dai et al. [13]. 

The influence of alkyl substituents on the ease of ring hydrogenolysis depends on ring size. Thus, over a platinum/pumice catalyst at 100°C,... 

Nieuwstad, Klapwijk, and van Bekkum (105) have added to the knowledge of aromatic hydrogenation by their study of the influence of alkyl substituents in the 1 and 2 positions of naphthalene on the rate. Tetrahydro-naphthalenes were the products of hydrogenation over palladium at 80°C. The selectivity of the reaction was also followed and expressed as the ratio of the rate constants for the saturation of the unsubstituted and substituted rings, respectively. Steric effects play an important role, and, beside steric hindrance by the bulky substituents, steric acceleration also has been observed, the latter being caused by a release of the strain between the 1-alkyl group and hydrogen in position 8. 

Furthermore, the influence of alkyl substituents on the UV transitions of 1,3-dithiolylium ions has been studied by the PPP method and compared with experimental values the agreement was good (71T4705). These compounds possess two UV absorption maxima of approximately equal intensities. The long wavelength band corresponds to a transition with a transition moment in the direction of the two-fold symmetry axis of (1), whereas the v r transition of the second band is polarized perpendicular to the first one (71T4705). 

Since the major reviews covering the parent 3-bicyclo[3.1.0]hexyl system appeared in the mid-1970s, several other papers dealing with the system have appeared. In one of these, Mjoberg and coworkers have concluded from analysis of far infrared and microwave data that the bicyclo[3.1.0]hexyl system exists in a boat form in its most stable conformation, and that the chair form in which cyclopropyl participation takes place must be vibrationally highly excited. Also, Walkowicz " has reported some further investigations of the influence of alkyl substituents on the chemistry of 3-bicyclo[3.1.0]hexyl tosylates. 

The scope of this process has been extended in a more detailed investigation to the synthesis of quinolizines [175] and the influence of alkyl substituents in various positions of the aminodialkene substrate on product diastereoselectivity was probed. Neodymium-based catalysts are particularly efficient for six-membered ring formation (40). The methodology has found further application in the synthesis of tri- and tetracyclic alkaloidal skeletons (41) [176]. 

The influence of alkyl substituents on the direction of cleavage in a series of 1-methyl-tricyclo[4.4.0.0 ]decan-3-ones 39 has been studied. In the absence of an additional substituent, the formation of the brominated spiro[4.5]decanone compound 41 was slightly favored. 

Of course, the physical properties of alkylphenols are comparable to phenol. The properties are strongly influenced by the type of alkyl substituent and its position on the ring. Alkylphenols, like phenol, are typically soflds at 25°C. Their form is affected by the size and configuration of the alkyl group, its position on the ring, and purity. They appear colorless, or white, to a pale yellow when pure (Table 1). 

The reaction is insensitive to steric effects, but kj increases with the number of alkyl substituents although it is not influenced by position of the alkyl groups (unlike oxidation by TI(III)) . In these respects Cr(VI) oxidation resembles bromination, chlorination and epoxidation and a symmetrical transition state of the type depicted is favoured. 

The perturbation <5ap(Rp) is positive for all alkyl groups, increasing with increasing size of the group Rp. The change S jt nd = j — j of the orbital energy j due to the inductive influence of alkyl substituents Rp in positions p of the parent molecule can be... 

Note that the shift < /J ind is always negative, meaning that the inductive influence of alkyl substituents leads to a reduction of the ionization energies IJ calc relative to the values calculated for the parent molecule. 

The influence of steric effects on the thermodynamic parameters of extraction has been discussed in numerous publications (see the review in Ref. 12 and references therein). For a small series of selected extractants, a decrease of log Kex as a function of the length of alkyl substituents has been discovered.6970 However, in other cases, this dependence passes through a maximum or is not regular at all.71 The point is that the increase in the volume and branching of the substituents leads to some changes of molecular properties decrease of solubility in the aqueous phase, increase of steric hindrance upon the complexation, decrease of the extractant s aggregation, etc.12 Some of these factors strengthen extraction, but others weaken it. 

Previous investigations (Brady, 1949 Grayson, 1952 Bonner, 1952) in the Purdue laboratories were concerned with the influence of alkyl groups on the rate of ionization of phenyldimethylcarbinyl chloride. These studies indicated that first-order rate constants could be determined with high accuracy for the solvolysis reaction. Moreover, the entropies of activation were invariant in this series of halides. These considerations led to further study of other substituted phenyldimethylcarbinyl chlorides in an attempt to gain a further understanding of the influence of substituent groups on relative reactivity and as a possible model reaction for the assessment of parameters for electron-deficient reactions. 

All aspects of the structure, reactivity and chemistry of fluorine-containing, carbon-based free radicals in solution are presented. The influence of fluorine substituents on the structure, the stability and the electronegativity of free radicals is discussed. The methods of generation of fluorinated radicals are summarized. A critical analysis of the reactivities of perfluoro-n-alkyl, branched chain perfluoroalkyl and partially-fluorinated free radicals towards alkene addition, H-atom abstraction, and towards intramolecular rearrangement reactions is presented. Lastly, a summary of the synthetically-useful chemistry of fluorinated radicals is presented. 

The influence of fluorine substituents on the stability of alkyl radicals derives from the same complex interplay of inductive and resonance effects that affects their structure. Simple orbital interaction theory predicts that substituents of the -X type (that is, electronegative substituents bearing lone pairs) should destabilize inductively by virtue of their group electronegativities, and stabilize by resonance to the extent of their ability to delocalize the odd electron. 

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