Hyperconjugation and Reactivity

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. 

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... 

Traditionally, the focus has been on polar and resonance effects, based on VB ideas about structure, and the emphasis is on partial charges arising from polar bonds and resonance/hyperconjugation. However, in MO theory, we use the idea of perturbations. The question asked is, How does a substituent affect the energy and shapes of the orbitals, with particular attention to the HOMO and LUMO, the frontier orbitals. Ultimately, substituents affect structure and reactivity by changing the electron density distribution. From the concept of electronegativity, we know that bonds have dipoles,... 

Overall, this section provides multiple examples of the power of negative n o hyperconjugation (which sometimes evolves into conjugation) in control of stability and reactivity. In the following section, we will show that n conjugation can have multiple consequences for reactivity as well by analyzing the variety of chemical phenomena that take advantage of delocalization (or lack of thereof) with the amide n-system. 

The hyperconjugative interactions and their role in the structure and reactivity of organic molecules have been outlined in a review by Alabugin et al In particular, the authors discuss an influence of the homoanomeric effects in aza-, oxa-, thio-, and selenaheterocycles on the /hc couplings measured for these compounds. 

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