11. Acids and Bases: The Lewis Definition Lewis acids are electron pair acceptors and Lewis bases are electron pair donors Brønsted acids are not Lewis acids because they cannot accept an electron pair directly (only a proton would be a Lewis acid) The Lewis definition leads to a general description of many reaction patterns but there is no scale of strengths as in the Brønsted definition of pK a

12. Lewis Acids: How to recognize them The Lewis Acids accept a pair of electrons when they form a bond to a base (electron pair donor). They can be positively charged or neutral but are never negatively charged Group 3A elements, such as BF 3 and AlCl 3, are Lewis acids because they have unfilled valence orbitals and can accept electron pairs from Lewis bases All organic cations (such as CH 3 + ) and metal cations, such as Mg 2+, are Lewis acids Transition-metal compounds, such as TiCl 4, FeCl 3, ZnCl 2, and SnCl 4, are Lewis acids There are some special cases such as halogens (Br 2,Cl 2, and I 2 ) which are good Lewis acids even though they have plenty of lone pairs.

13. Organic compounds that have highly polarized covalent bonds are called electrophiles, and are Lewis Acids at the atom that is the positive end of the dipole (usually a C). However, they can also serve as Lewis bases at the lone pairs of the heteroatom.

14. Lewis Bases Lewis bases can accept protons as well as Lewis acids, therefore the definition encompasses that for Brønsted bases. They may be neutral, negatively charged, but are never negatively charged. Most oxygen- and nitrogen-containing organic compounds are Lewis bases because they have lone pairs of electrons

15. Lewis bases only Lewis acids or bases