Hyperconjugation secondary

The intensity of the bending mode also bears out the contention of Streitwieser et al. (54) that this motion, rather than the CH stretching motion, is of primary importance in hyperconjugative secondary isotope effects in electron deficient systems but the proviso must be added that its effect is felt through coupling with other displacements, and not directly through a decrease of the CH bending frequency itself. 

Secondary isotope effects at the position have been especially thoroughly studied in nucleophilic substitution reactions. When carbocations are involved as intermediates, substantial /9-isotope effects are observed. This is because the hyperconjugative stabliliza-... 

This reanangement is shown in orbital terms in Figure 5.8. The relevant orbitals of the secondary car bocation are shown in structure (a), those of the transition state for reanangement in (b), and those of the tertiary carbocation in (c). Delocalization of the electrons of the C—CH3 a bond into the vacant p orbital of the positively charged car bon by hyperconjugation is present in both (a) and (c), requires no activation energy, and... 

Draw a skeletal structure of the following carbocation. Identify it as primary, secondary, or tertiary, and identify the hydrogen atoms that have the proper orienta- tion for hyperconjugation in the conformation shown. 

Solvolytic experiments specifically designed to test Bartell s theory were carried out by Karabatsos et al. (1967), who were primarily interested in an assessment of the relative contributions of hyperconjugation and non-bonded interactions to secondary kinetic isotope effects. Model calculations of the (steric) isotope effect in the reaction 2- 3 were performed, as well as that in the solvolyses of acetyl chloride... 

As with carbocations, the stability order of free radicals is tertiary > secondary > primary, explainable by hyperconjugation, analogous to that in carbocations... 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

The rather confusing conformational dependences in Fig. 3.52 can be rationalized in a simple way. From the total of 7t-a and a-7t stabilizations at their respective coplanar alignments with the pi system, one can confirm that C—H bonds are better overall hyperconjugating groups than C—I bonds. The most favorable hyperconjugative alignment is therefore to place both C—H bonds maximally out-of-plane (i.e., C—F in-plane) at = 0°, whereas a secondary favorable alignment... 

From comparison of Figs. 3.51 and 3.52 one can judge that Em,) includes contributions other than the six primary hyperconjugative interactions of Fig. 3.52. These omitted contributions are principally of secondary cr-a type, to be discussed in Section 3.4.2. However, these weaker hyperconjugative interactions do not significantly alter the preferred conformational angles established by the dominant six interactions of Fig. 3.52. 

In the extreme carbocation limit of (3.163) and (3.164), the stereoelectronic secondary-hyperconjugation effects therefore blend seamlessly into ordinary pi-type conjugation phenomena (Section 3.3), the two extremes always being linked by electronic continuity. 

The 2-butyl cation is the smallest secondary cation that can be stabilized either by C-C or C-H hyperconjugation. Experimental results give evidence for two equilibrating isomers.33 MP2/6-311G(d,p) calculations show that the symmetrically hydrido-bridged structure 11 is marginally more stable than the partially methyl-bridged structure 10.34 35... 

Thus a tertiary carbocation like the above will give nine resonating structures while a primary will give only two hyperconjugative forms. This explains why tertiary carbocations are more stable than secondary which in turn is more stable than primary. This also explains why ethyl carbocation (CH3CIlf) is more stable than methyl carbocation (CH )-... 

Secondary 0-deuterium KIEs and the case for negative ion hyperconjugation 202... 

Secondary 0-deuterium KIEs due to hyperconjugation in carbene and radical reactions 210... 

A value of ku/kD = 1.07 per /3-D was observed when the deuteriums were on the bridge at C-12 [5] and the dihedral angle between the p-orbital of the carbocation and the Cp—H bonds was approximately 30° and hyperconjuga-tion could occur. When [6] was used, the dihedral angle was 90° and there was no overlap between the empty p-orbital of the carbocation and the Cp—H bonds i.e. no hyperconjugation could occur and only a small inverse, inductive KIE, kH/fcD = 0.99, was observed. This study and other studies by Shiner and co-workers (Shiner, 1970b) have established that the maximum secondary /3-deuterium KIE in any system is observed when the dihedral angle is either 0° or 180°, i.e. where the overlap between the Cp—H and the p-orbital on the... 

Although the geometric relationship suggested by Shiner and by Sunko and their co-workers clearly demonstrates that hyperconjugation is the major contributor to the secondary /3-deuterium KIE in carbocation reactions, Williams (1985) has suggested that there is a significant inductive component to these KIEs. Williams used ab initio MO methods to calculate the geometries of the substrates and the isopropyl carbocation formed in a gas-phase heterolysis (30) of series of isopropyl derivatives at the RHF/4-31G level. 

An examination of the EIEs in Table 30 shows that the trend in the EIEs cannot be explained by hyperconjugation alone. If these KIEs were determined by hyperconjugation alone, the /3-deuterium EIE would be expected to decrease as the substrate acquired more cationic character at the a-carbon, i.e. as the Ca—Cp bond shortened. However, this trend was not observed. As the leaving group improves along the series X = H, OH, F, OH2+, the a-carbon of the substrate becomes more planar, the Ca—Cp bond shortens and the Cp—H bond in the substrate becomes shorter, i.e. the amount of cationic character at the a-carbon increases and the secondary /3-deuterium EIE increases. This trend in the EIE is obviously not consistent with a model where hyperconjugation is the sole contributor to the EIE. 

SECONDARY /3-DEUTERIUM KIEs AND THE CASE FOR NEGATIVE ION HYPERCONJUGATION... 

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