Hyperconjugative stabilisation

Data for aliphatic aldehyde enolisation are very scarce, probably because the enolisation process is often complicated by oxidation and hydration. Nevertheless, the rate constants for base- and acid-catalysed iodination of R R2CHCHO were determined in aqueous chloroacetic acid-chloroacetate ion buffers (Talvik and Hiidmaa, 1968). The results in Table 4 show that alkyl groups R1 and R2 increase the acid-catalysed reactivity in agreement with hyperconjugative and/or inductive effects. This contrasts with aliphatic ketones for which steric interactions are important and even sometimes dominant. Data for base-catalysis are more difficult to interpret since a second a methyl group, from propionaldehyde to isobutyraldehyde, increases the chloroacetate-catalysed rate constant. This might result from a decrease of the a(C—H) bond-promoted hyperconjugative stabilisation of the carbonyl compound... 

The results were interpreted in terms of anchimeric assistance, by Me3Si, to give the bridged intermediate, 4. A possible alternative explanation is that the open-chain, hyperconjugatively stabilised carbonium ion, 7, is formed, but rapidly undergoes facile 1,2 silyl migration63 before being trapped by bromide ion. 

Fig. 3.21 The high lying Walsh orbital of cyclopropane (a) and the accepting orbital of the cationic centre (b) involved in the strong hyperconjugative stabilisation of the cyclopropylcarbinyl cation (c)...

C AH° = — 26 cal mol per D] on the other shows that the dimethyl-bicyclo[2.1.1]hexyl cation falls in the intermediate range. The decreasing size of the CDj-isotope effect on (85)-(87) indicates a decreasing demand for C—H hyperconjugative stabilisation due to increasing a-stabilisation in... 

This reflects the relative ease with which the C—H bond in the alkane precursor will undergo homolytic fission, and more particularly, decreasing stabilisation, by hyperconjugation or other means, as the series is traversed. There will also be decreasing relief of strain (when R is large) on going from sp3 hybridised precursor to essentially sp2 hybridised radical, as the series is traversed. The relative difference in stability is, however, very much less than with the corresponding carbocations. 

The carbocation intermediate can be stabilised by neighbouring alkyl groups through inductive and hyperconjugation effects. However, it can also be stabilised by sharing the positive charge with the bromine atom and a second carbon atom. 

Any positively charge species like carbocations are inherently reactive and unstable. The more unstable they are, the less easily they are formed and the less likely the overall reaction. Any factor that helps to stabilise the positive charge (and by inference the carbocation) will make the reaction more likely. The three ways in which a positive charge can be stabilised are (i) inductive effects, (ii) hyperconjugation, and (iii) delocalisation. 

Electronic factors also help in the formation of the carbocation because the positive charge can be stabilised by the inductive and hyperconjugative effects of the three alkyl groups ... 

One case where C—C bonds are exceptionally effective in hyperconjugation is in the stabilisation provided by a cyclopropyl substituent to an empty p orbital. The cyclopropylmethyl cation is actually better stabilised than an allyl cation, as judged by the more rapid ionisation of cyclopropylmethyl chloride 2.19 than of crotyl chloride 2.20. In this case, hyperconjugation appears, unusually, to be better than n conjugation. 

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