Superbase

A superbase is a chemical compound with a very high affinity for protons. In chemistry, an affinity is a strong attraction or pull. This means superbases are extremely effective at grabbing and holding onto protons. They are of great theoretical interest to researchers. They are also potentially valuable tools in organic synthesis. Organic synthesis is the process of building new organic molecules. Scientists have been describing and using superbases since the 1850s.

There are several ways to define a superbase. The International Union of Pure and Applied Chemistry, or IUPAC, calls a superbase a compound with very high basicity. A common example is lithium diisopropylamide, often called LDA. Many scientists group superbases into two broad categories: organic and organometallic. Others define them by comparing them to a specific molecule called proton sponge. Proton sponge is 1,8-bis(dimethylamino)naphthalene. To be a superbase by this definition, a species must have a higher absolute proton affinity than proton sponge. It must also have a higher intrinsic gas phase basicity. The absolute proton affinity of proton sponge is 245.3 kcal/mol. Its intrinsic gas phase basicity is 239 kcal/mol.

Organic superbases are mostly charge-neutral species. These molecules often contain nitrogen. In these compounds, the nitrogen acts as a proton acceptor. This means the nitrogen atom is the part that grabs the proton. Common organic superbases include amidines and guanidines. They also include phosphazenes and phosphanes. Some organic compounds act as proton chelators. These are called aromatic proton sponges or bispidines. Multicyclic polyamines like DABCO might also be included in this group. Even though they have huge proton affinity, some organic superbases show low nucleophilicity. Nucleophilicity is a measure of how much a species seeks out positive charges.

Organometallic superbases are another important group. These involve metals and organic groups. Some are called Lochmann–Schlosser superbases. These result from combining alkali metal alkoxides with organolithium reagents. One famous example is the Schlosser base. This is a mixture of n-butyllithium and potassium tert-butoxide. When these two are mixed, they form a mixed aggregate. This aggregate is more reactive than either reagent used by itself. You can also create these by exchanging a reactive metal for a hydrogen on a heteroatom. This can happen on atoms like oxygen or nitrogen. Metal amides, such as lithium diisopropylamide, are one example. Organolithium and organomagnesium compounds, known as Grignard reagents, are also examples. Unlike many organic superbases, organometallic superbases are generally strong nucleophiles.

Some scientists define superbases by how they are made. Caubère defines a superbase as a base resulting from mixing two or more bases. This mixing leads to new basic species with inherent new properties. This definition does not mean the new base is thermodynamically or kinetically stronger than the original bases. Instead, it means a new reagent is created by combining different characteristics. This creates a tool that behaves in ways the single bases could not.

Inorganic superbases exist as well. These are typically salt-like compounds. They feature small and highly charged anions. Examples include lithium hydride, potassium hydride, and sodium hydride. These inorganic superbases are usually insoluble in liquids. However, the surfaces of these materials are highly reactive. Because of this, scientists often use them in a thick mixture called a slurry. This allows the reactive surfaces to interact with other chemicals during synthesis.

Superbases play a major role in many chemical fields. They are frequently used in organocatalysis. This is a way to speed up chemical reactions using organic molecules. They are also essential for creating complex structures in organic synthesis. Even the strength of these bases can be studied through advanced math. Quantum-chemical calculations suggest that caesium oxide might be the strongest base. This shows how much there is still to learn about these powerful tools.