Functional groups hyperconjugation

Hydroxy group (Section 9.1) The OH functional group. Hyperconjugation (Section 7.14B) The overlap of an empty p orbital with an adjacent o bond. 

An alkyne is less reactive than an alkene. A vinyl cation is less able to accommodate a positive charge, as the hyperconjugation is less effective in stabilizing the positive charge on a vinyl cation than on an alkyl cation. The vinyl cation is more stable with positive charge on the more substituted carbon. Electrophilic addition reactions allow the conversion of alkenes and alkynes into a variety of other functional groups. 

The second type of overlap, the n-type, is characteristic for molecules that already possess a a-bond. In this approach, the two orbitals are parallel rather than collinear. Not only does this overlap pattern describe such important functional groups as aUcenes, alkynes, aromatics, and carbonyl derivatives, but the n-type overlap often plays a key stereoelectronic role even in molecules without a double bond. For example, the tt-overlap is important in vicinal hyperconjugative interactions (Figure 2.5), providing a stereoelechonic basis to such phenomena as the anomeric effect, ganche effect, and c -effect (vide infra). 

As mentioned above, the reactivity of alkoxyallenes is governed by the influence of the ether function, which leads to the expected attack of electrophiles at the central carbon C-2 of the cumulene. However, the alkoxy group also activates the terminal double bond by its hyperconjugative electron-withdrawing effect and makes C-3 accessible for reactions with nucleophiles (Scheme 8.3). This feature is of particular importance for cyclizations leading to a variety of heterocyclic products. The relatively high CH-acidity at C-l of alkoxyallenes allows smooth lithiation and subsequent reaction with a variety of electrophiles. In certain cases, deprotonation at C-3 can also be achieved. 

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