Lewis acids with polar multiple bonds

Lewis Acids with Polar Multiple Bonds Molecules that contain a polar double... 

Lewis Acids with Polar Multiple Bonds Molecules that contain a polar double bond also function as Lewis acids. As the electron pair on the Lewis base approaches the partially positive end of the double bond, one of the bonds breaks to form the new bond in the adduct For example, consider the reaction that occurs when SO2 dissolves in water. The electronegative O atoms in SO2 withdraw electron density from the central S, so it is partially positive. The O atom of water donates a lone pair to the S, breaking one of the ir bonds and forming an S—O bond, and a proton is transferred from water to that O. The resulting adduct is sulfurous acid, and the overall process is... 

Simple pi bonds are usually not good enough nucleophiles to react with polarized multiple bonds. If a Br0nsted acid or a Lewis acid is added to improve the electron sink, then addition can occur via the lone-pair-stabilized carbocation as the sink. Figure 8.7 shows a mechanistic example from a short synthesis of the human hormone estrone. 

Aromatic rings are usually not good enough nucleophiles to react with polarized multiple bonds. If a Br0nsted acid or a Lewis acid is added to improve the electron... 

In the Lewis acid-base definition, an acid is any species that accepts a lone pair to form a new bond in an adduct. Thus, there are many more Lewis acids than other types. Lewis adds include molecules with electron-deficient atoms, molecules with polar multiple bonds, and metal cations. 

The most important point to remember in classifying into sources and sinks is to stay flexible. Almost all electron sinks have lone pairs that can serve as sources. Lone pairs on sinks can complex with Lewis acids or be protonated. A polarized multiple bond can serve as an electron-withdrawing group, making protons on adjacent atoms acidic. Looking at both reactive partners can help you decide. If one partner is clearly a sink, like sulfuric acid, then the other partner must serve as a source, even though it may be a polarized multiple bond sink like a carbonyl protonate the carbonyl lone pair as a source. 

Write down the Lewis structure of the reactants, complete with formal charges, and draw any major resonance forms. Look for leaving groups, polarized single and multiple bonds, acids and bases. Classify into generic sources and sinks and then rank them. The reaction usually occurs between the best source and sink. Above all, note if the medium is acidic or basic. In basic media, find the best base, and then locate any acidic hydrogen within range (not more than 10 p Ta units above the pATabH of Ihe base). In acidic media, identify the best sites for protonation. Likewise, do not create a species that is more than 10 units more acidic than your acid. Understand what bonds have been made or broken, but do not lock into an arbitrary order as to which occurred first. 

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