Which compounds are lewis acids

Patra , Nilangshu Mandal. Nitrenium Salts in Lewis Acid Catalysis. Angewandte Chemie , 7 , Angewandte Chemie International Edition , 59 7 , Basicity of N-heterocyclic carbene and its main-group analogues.

Computational and Theoretical Chemistry , , McGrady , Jose M. Chemistry — A European Journal , 25 21 , Synthesis of N-heterocyclic nitrenium NHN ions and related donor systems: Coordination with dmetal ions.

Inorganica Chimica Acta , , Akhmedov , Sadegh Faramarzi. Journal of Molecular Structure , , Lewis Superacids: Classifications, Candidates, and Applications. Chemistry — A European Journal , 24 68 , Angewandte Chemie , 37 , Habraken , Nicolaas P. Ehlers , Martin Lutz , J. Chris Slootweg. Angewandte Chemie International Edition , 57 37 , Stable Diporphyrinylaminyl Radical and Nitrenium Ion.

Helium has two electrons. If a Lewis base or nucleophile donates a pair of electrons to a proton, the proton will obtain a Noble gas configuration. There is something about hydrogen cations that is not so simple, however.

They are actually not so common. Instead, protons are generally always bound to a Lewis base. Many of the other elements commonly found in compounds with hydrogen are more electronegative than hydrogen. As a result, hydrogen often has a partial positive charge. Remember, that is one of the reasons that atoms can act as Lewis acids: with a partial positive charge, an atom becomes electrophilic.

If hydrogens are almost always bonded to other atoms, then the Lewis acid-base interactions we have looked at so far are slightly different here.

Instead of two compounds coming together and forming a bond, we have one Lewis base replacing another at a proton. What makes a molecule or an atom or ion a Lewis base? It must have a pair of electrons available to share with another atom to form a bond.

The most readily available electrons are those that are not already in bonds. Bonding electrons are low in energy. Non-bonding electrons are higher in energy and may be stabilized when they are delocalized in a new bond. Ammonia, NH 3 , has a lone pair and is a Lewis base. It can donate to compounds that will accept electrons. Lewis bases may be anionic or neutral. The basic requirement is that they have a pair of electrons to donate.

Examples of Lewis bases include halide ions such as bromide or chloride. To the right of the halides in the periodic table are Noble gases such as neon.

Noble gases do have lone pairs, but are stable enough that they do not usually react. They are not very good Lewis bases. To the left of the halides, however, are other examples in oxygen and nitrogen compounds. Water also has lone pairs and is a common Lewis base, and so is hydroxide ion, HO-. Figure 2. Note that neon, although it has nonbonding electron pairs or lone pairs, does not usually act as a Lewis base. One column further to the left in the periodic table from nitrogen is carbon.

Carbon does not normally have a lone pair. For example, methane, CH 4 , has all of its valence electrons in bonding pairs. These bonding pairs are too stable to donate under normal conditions. Methane is not a Lewis base. Figure 3. Neither of these compounds has a lone pair, and neither is a good Lewis base. Therefore, by defining a species that donates an electron pair and a species that accepts an electron pair, the definition of a acid and base is expanded.

As of now you should know that acids and bases are distinguished as two separate things however some substances can be both an acid and a base.

You may have noticed this with water, which can act as both an acid or a base. This ability of water to do this makes it an amphoteric molecule. Water can act as an acid by donating its proton to the base and thus becoming its conjugate acid, OH-.

You may have noticed that the degree to which a molecule acts depends on the medium in which the molecule has been placed in.

Water does not act as an acid in an acid medium and does not act as a base in a basic medium. Thus, the medium which a molecule is placed in has an effect on the properties of that molecule. Other molecules can also act as either an acid or a base. For example,. Note how the amphoteric properties of the Al OH 3 depends on what type of environment that molecule has been placed in. Lewis Acids Lewis acids accept an electron pair.

The types of bonds formed in complex ions are called coordinate covalent bonds, as electrons from the ligands are being shared with the central atom.

Because of this, complex ions are sometimes referred to as coordination complexes. This will be studied further in upcoming chapters. The equilibrium constant for the reaction of the components of a complex ion to form the complex ion in solution is called a formation constant K f sometimes called a stability constant.

The inverse of the formation constant is the dissociation constant K d , the equilibrium constant for the decomposition of a complex ion into its components in solution. We will work with dissociation constants further in the exercises for this section. Appendix K and Table 2 are tables of formation constants. In general, the larger the formation constant, the more stable the complex; however, as in the case of K sp values, the stoichiometry of the compound must be considered.

As an example of dissolution by complex ion formation, let us consider what happens when we add aqueous ammonia to a mixture of silver chloride and water. More silver chloride then dissolves. If the concentration of ammonia is great enough, all of the silver chloride dissolves. Dissociation of a Complex Ion Calculate the concentration of the silver ion in a solution that initially is 0.

Solution We use the familiar path to solve this problem:. We write the equilibrium as a formation reaction because Appendix K lists formation constants for complex ions.

Both Q and K f are much larger than 1, so let us assume that the changes in concentrations needed to reach equilibrium are small. Thus 0. Because only 1. A Lewis acid is a species that can accept an electron pair, whereas a Lewis base has an electron pair available for donation to a Lewis acid.