<Li> HCl + NH → NH Cl </Li> <P> As with the acetic acid reactions, both definitions work for the first example, where water is the solvent and hydronium ion is formed by the HCl solute . The next two reactions do not involve the formation of ions but are still proton - transfer reactions . In the second reaction hydrogen chloride and ammonia (dissolved in benzene) react to form solid ammonium chloride in a benzene solvent and in the third gaseous HCl and NH combine to form the solid . </P> <P> A third, only marginally related concept was proposed in 1923 by Gilbert N. Lewis, which includes reactions with acid - base characteristics that do not involve a proton transfer . A Lewis acid is a species that accepts a pair of electrons from another species; in other words, it is an electron pair acceptor . Brønsted acid - base reactions are proton transfer reactions while Lewis acid - base reactions are electron pair transfers . Many Lewis acids are not Brønsted - Lowry acids . Contrast how the following reactions are described in terms of acid - base chemistry: </P> <P> In the first reaction a fluoride ion, F, gives up an electron pair to boron trifluoride to form the product tetrafluoroborate . Fluoride "loses" a pair of valence electrons because the electrons shared in the B--F bond are located in the region of space between the two atomic nuclei and are therefore more distant from the fluoride nucleus than they are in the lone fluoride ion . BF is a Lewis acid because it accepts the electron pair from fluoride . This reaction cannot be described in terms of Brønsted theory because there is no proton transfer . The second reaction can be described using either theory . A proton is transferred from an unspecified Brønsted acid to ammonia, a Brønsted base; alternatively, ammonia acts as a Lewis base and transfers a lone pair of electrons to form a bond with a hydrogen ion . The species that gains the electron pair is the Lewis acid; for example, the oxygen atom in H O gains a pair of electrons when one of the H--O bonds is broken and the electrons shared in the bond become localized on oxygen . Depending on the context, a Lewis acid may also be described as an oxidizer or an electrophile . Organic Brønsted acids, such as acetic, citric, or oxalic acid, are not Lewis acids . They dissociate in water to produce a Lewis acid, H, but at the same time also yield an equal amount of a Lewis base (acetate, citrate, or oxalate, respectively, for the acids mentioned). Few, if any, of the acids discussed in the following are Lewis acids . </P>

What is a general characteristic of a lewis acid
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