Alkyl group orienting effect

Substituents with -I,+M effects such as halogen show a similar orientation effect to that of alkyl groups. If the substituent is in the 2-position, substitution occurs at C-5 if it is in position 3, substitution occurs preferentially at C-2 or if steric requirements of the group or electrophile intervene, then substitution occurs at C-5. 

In comparison with the decomposition of taws-azoalkanes 20 a much larger group size effect has been found for the thermolysis rates of a few c/s-azoalkanes 24. Due to the repulsion of the free electron pairs on the two nitrogen atoms and due to steric interaction between the cis oriented alkyl groups cis azoalkanes 24 decom-... 

The effect of crown ethers on the geometrical orientation in base-promoted E2 eliminations has been studied by several groups. Bartsch et al. (1973) have investigated the effect of several parameters on the potassium alkoxide-promoted eliminations of HBr from 2-bromobutane (Table 45). In the absence of crown ethers the relative amount of 1-butene formed increases, while the trans/cis ratio of the 2-butenes decreases, with decreasing solvent polarity. Furthermore, the proportion of 1-butene increases and the trans/cis ratio decreases on increasing the base concentration. These effects were explained in terms of steric interactions between the base and a- and /7-alkyl groups in the... 

In a reactivity study conducted by competitive methods, the influence of 3-alkyl groups (Me and Et) has been found to be rather peculiar.135 3-Alkylpyridines are attacked at the adjacent 2-position more easily than pyridine itself in spite of the weak adverse electronic effect of the substituent. However, at the 6-position the attack occurs more slowly, as expected. Of the several possible reasons for this behavior, probably the most likely is the establishment of a weak attractive interaction between the a carbon of the 3-alkyl group and phenyllithium, which would favor orientation for attack at the 2- rather than the 6-position and overcome the small electronic effect of the group. 

When no important 7r-electron effect is possible, as with alkyl groups, the orientation effect of a substituent is controlled by its polar effect and the degree to which it polarizes the bonding electrons of the ring. Alkyl groups... 

Recently, Crabb, Turner, and Newton (68) have observed that perhydropyrido-[1.3]oxazine exists as a =9 1 mixture of the trans and the cis forms 97 and 98. The cis form 98 has two anomeric effects (-2.8 kcal/mol), two gauche forms of butane (1.8 kcal/mol) and one gauche form of -propyl ether (0.4 kcal/mol) whereas the trans form 97 has only one anomeric effect (-1.4 kcal/ mol). On that basis, the trans form 97 should be more stable than the cis form 98 by about 0.8 kcal/mol, in agreement with the experimental result. Katritzky and co-workers (69) have also shown that 1-oxa-3,5-diaza and 1,3-dioxa-5-aza cyclohexane derivatives exist respectively in the conformations 99 and 100. With an alkyl group in the axial orientation, both conformations gain two anomeric effects. 

The presence of bulky alkyl groups results in a shift of half-wave potentials towards more negative values this effect is interpreted rather as a hindrance of the attainment of the optimum conformation in the transition state than as a direct hindrance of the approach of the electron. An example is provided by the w-alkyl-substituted acetophenones (72). When one of the substituents is a phenyl group (68), the steric effects are much less pronounced, probably because the phenyl groups are orientated plane-parallel to the electrode surface, which forces the remaining alkyl groups into an orientation away from the electrode, where they cannot interfere with the orientation of the carbonyl group. 

If the alkyl substituent is in position 3, the 2-position is the most favored from the electronic point of view, being ortho to the alkyl group and alpha to the heteroatom. The other a position (5) is, however, free from steric effects. Consequently, reactions of 3-alkyl thiophenes, -furans, and -pyrroles yield a mixture of 2- and 5-substituted isomers, the composition of which depends on the nature of the heteroatom, the electrophile, and the size of the alkyl group. Table XXI reports the isomer distribution in the acylation of 3-methyl-, 3-isopropyl-, and 3-i-butylthiophenes and shows the importance of the steric factor in determining the orientation. 

Another example of the application of gas-phase NMR techniques to conformational equilibria is the investigation of conformational effects on the relative maxima in the N-0 bond internal rotation potential function in primary nitrites of one to five carbon atoms [27,22]. Alkyl nitrites (ONOR) exist as mixtures of syn and anti conformers which differ in the orientation of the alkyl group with respect to the nitrosyl group. 

C4 has resulted in a drastic reduction in the extent of multilayer adsoiption. The reason for this unusual physisorpdon behaviour is that the molecules in the localized monolayer are oriented to allow directional hydrogen bonding between their hydroxyls and the hydrated alumina surface. The alkyl groups are therefore directed away from the surface and thereby provide an effective low-energy screen against further adsorption. 

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