12-12.7 - Chapter. 12 -A Lewis acid-base reaction converts...

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Chapter. 12 -A Lewis acid-base reaction converts a lone pair on a base and an empty orbital on an acid into a covalent bond. -A Lewis acid is a substance that has an empty orbital that it can use to share a lone pair to form a bond. -A Lewis base is a substance that has a lone pair that it can share in a covalent bond. -A Lewis acid-base reaction is the conversion of the lone pair on the base and the empty orbital of the acid into a covalent bond between the acid and the base. -The product of a Lewis acid-base reaction is a covalent bond between the acid and the base. Both bonding electrons come from the base and the bonding orbital is produced by the overlap of the empty orbital on the acid and the filled orbital on the base. -If the broad classification of Lewis is used for acid-base reactions, then almost all chemical reactions can be classified as either acid-base or redox reactions. In one, a base shares its electrons with an acid; in the other, a reducing agent transfers its electrons to an oxidizing agent. -A Lewis base contains a lone pair. The strength of the base increases as the electron density on the atom containing the lone pair increases. - Lewis bases A Lewis base must contain at least one lone pair of electrons. All anions are Lewis bases, but not all Lewis bases are anions. The lone pair is frequently, but not always, located on oxygen or nitrogen atoms. - The strength of the base depends upon the electron density in the region of the lone pair, the greater the electron density the stronger the base. Consequently, the strength of a base depends upon the groups around the lone pair. For example, consider the relative base strengths of the following, which are basic due to the lone pairs on the oxygen atom: CH 3 O 1- > HO 1- > ClO 1- CH 3 O 1- is the strongest base because the CH 3 group pushes electron density onto the oxygen atom. ClO 1- is the weakest because the electronegative chlorine atom removes electron density from the oxygen. -
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Lewis acids Lewis acids must contain an empty orbital that can be used to share an electron pair in a covalent bond to a Lewis base. The Lewis acidic atom gains an electron region when it forms a bond to a Lewis base, so Lewis acids must contain an atom that is capable of adding an electron region. Since atoms that obey the octet rule cannot have more than four electron regions, Lewis acids that obey the octet rule must contain an atom with less than four electron regions. All cations are Lewis acids, but not all Lewis acids are cations -AlCl 3 is electron deficient because aluminum has only six valence electrons. Molecules with electron deficient atoms are strong Lewis acids. SO 3 and CO 2 are not electron deficient, but the central atom in each has less than four electron regions (three around S and two around C), so they are Lewis acids. -Cations are good Lewis acids only if their empty orbital is fairly low in energy. The
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12-12.7 - Chapter. 12 -A Lewis acid-base reaction converts...

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