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Negative hyperconjugation in silicon
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<p>Negative hyperconjugation is a theorized phenomenon in <a href="/facts/Organosilicon_chemistry/JvUwzDQ4">organosilicon compounds</a>, in which <a href="/facts/Hyperconjugation/9aMXnxT2">hyperconjugation</a> stabilizes or destabilizes certain <a href="/facts/Partial_charge/hmfe03j2">accumulations of positive charge</a>. The phenomenon explains corresponding peculiarities in the <a href="/facts/Stereochemistry/XkcRVNwd">stereochemistry</a> and <a href="/facts/Hydrolysis_constant/jTRaI2XA">rate of hydrolysis</a>. </p><p><a href="/facts/Period_3_element/1BE0Nc0K">Second-row elements</a> generally stabilize adjacent <a href="/facts/Carbanion/AyYH2udQ">carbanions</a> more effectively than their first-row <a href="/facts/Congener_(chemistry)/8qy2rvGG">congeners</a>; conversely they destabilize adjacent <a href="/facts/Carbocation/vIggUxTh">carbocations</a>, and these effects reverse one atom over. For <a href="/facts/Phosphorus/27xCeIqs">phosphorus</a> and later elements, these phenomena are easily ascribed to the element's greater <a href="/facts/Electronegativity/H7Vq7Uah">electronegativity</a> than carbon. However, Si has <a href="/facts/Electronegativities_of_the_elements_(data_page)/BBURTWVu">lower</a> electronegativity than carbon, <a href="/facts/Electron-donating/GDzV1DXL">polarizing the electron density onto</a> carbon. </p><p>The continued presence of second-row type stability in certain organosilicon compounds is known as the silicon α and β effects, after the corresponding <a href="/facts/Locant/HwU4BBRY">locants</a>. These stabilities occur because of a partial overlap between the C–Si <a href="/facts/Sigma_orbitals/CSyEe5CY">σ orbital</a> and the σ* <a href="/facts/Anti-bonding_orbital/9lhLNyt3">antibonding orbital</a> at the β position, lowering the <a href="/facts/Nucleophilic_substitution/MLLeSsnr"><i>SN</i></a> reaction <a href="/facts/Transition_state/34PsdqRd">transition state</a>'s energy. This hyperconjugation requires an <a href="/facts/Antiperiplanar/m7fvXqAs">antiperiplanar</a> relationship between the Si group and the leaving group to maximize orbital overlap. </p><p>Moreover, there is also another kind of silicon α effect, which is mainly about the <a href="/facts/Hydrolysis/JTvwIHPs">hydrolysis</a> on the silicon atom. </p>