Halogenation steric effects

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

The reaction is quite susceptible to steric effects since hindered secondary hydroxyl groups were found to be unreactive. The method can therefore be used to selectively replace a primary hydroxyl group by halogen in the presence of more hindered secondary hydroxyl groups in the same molecule. An example (70) is the reaction of 52 with triphenylphosphite methiodide which affords the 6-deoxy-6-iodo derivative 53 (60%) in which the C-2 hydroxyl group remains intact. 

An investigation of the competing halogen transfer from BrCCl3 and CCl45, 79 has shown that steric effects are also of importance in atom transfer reactions to alkyl and aryl radicals. Giese80 investigated very carefully the temperature depen-... 

Since the solvent properties of dimethyl sulfoxide are widely different from those of hydrocarbons and halogenated hydrocarbons, it may be difficult to compare the kinetic and thermodynamic data for the C02H group (Table 16) directly with others. However, heating the carboxylic acid (68, X = OH) in toluene affords the sp isomer almost exclusively. Probably, the observed results with the carboxylic acid derive from difficulty in the formation of a hydrogen bond owing to a steric effect, in addition to the nonplanar conformation of the carboxyl group relative to the naphthalene. 

The size of the alkyl groups in the haloalkane is Important. This is known as a steric effect. You will recall that, in the mechanism, the nucleophile attacks the carbon atom of the C-X bond from the side opposite to the halogen atom. In the case of a tertiary haloalkane, attack from that side is likely to be sterlcally hindered because three bulky alkyl groups will limit access to the atom. Hence tertiary haloalkanes are unlikely to react with nucleophiles via an Sj. 2 mechanism. Primary haloalkanes, on the other hand, have no more than one alkyl group attached to the halogen-bearing carbon atom and so access to the atom will be much easier. This suggests that primary haloalkanes are most likely to react with nucleophiles via an S. j2 mechanism. 

Steric effects are undoubtedly a factor in the reactivity of halogen substituted fumaric acid complexes to Cr+2 (18). The results of rate measurements for these species are shown in Table III. Specific rates are entered both for the (H+) independent path (path 1, ki) and for the path first order in (H+) (path 2, fo). 

Haloform reaction, 237, 296 Halogenation alkanes, 300, 323 alkenes, 179,186, 313 benzene, 138,316 ketones, 295 Hammett equation, 362 additional parameters, 374, 388, 395 derivation of, 362 deviations from, 375 empirical nature of, 395 implications of, 394 reaction pathway, and, 375 solvent effects and, 388 spectroscopic correlations, 392 standard reaction for, 362, 395 steric effects and, 361, 383 thermodynamic implications of, 394 Hammett plots, 359 change in rate-limiting step and, 383 change in reaction pathway and, 378... 

This is indeed borne out by correlating Vw for monatomic X with rv for the set including H and halogen, lOOr2 = 90.0, further inclusion of OH, SH, NH2, and PH2 results in lOOr2 = 95.3. Note however, that when the cylindrical groups, CN and C=CH are also included the lOOr2 value falls to 85.8. To demonstrate even more clearly the sharp difference between steric effects and volume parameters we... 

Steric effects (y shieldings) are operative for C-8 in 1-substituted naphthalenes. Table 4.55 illustrates this and also shows that shielding decreases with increasing size of the halogen [392], probably arising from an expanded X— C-l— C-9 bond angle. 

In addition to steric effects, other structural effects of R influence the Sn2 reactivity of RX. A double bond /3 to the halogen,6 as in 2-propenyl, phenylmethyl (benzyl), and 2-oxopropyl chlorides enhances the reactivity of the compounds toward nucleophiles. Thus the relative reactivities toward Iein 2-propanone are... 

The composition of the product mixture will depend on the relative rates of formation of the isomeric enols, provided that the halogenation step is not a reversible reaction. Barring any serious steric effects that influence the rate of reaction, the more rapidly formed enol generally is the more thermodynamically stable enol. 

The halogen of an a-halo aldehyde or an a-halo ketone is exceptionally unreactive in SN1-displacement reactions, but is exceptionally reactive in Sn2 displacements, compared with the halogen of alkyl halides having comparable potential steric effects. Similar behavior is observed with a-halo carboxylic acids and is discussed further in Chapter 18. 

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