Strong Bases



 

Learning Objective

  • Recognize strong bases and their chemical properties.

Key Points

    • In chemistry, a base is a substance that can either accept hydrogen ions (protons) or, more generally, donate a pair of valence electrons; it can be thought of as the chemical opposite of an acid.
    • Strong bases are commonly, though not exclusively, formed from the hydroxides of alkali metals and alkaline earth metals.
    • Superbases are stronger than hydroxide ions and cannot be kept in water; they provide examples of bases that do not contain a hydroxide ion (and are therefore strong Lewis and/or Bronsted-Lowry bases, but not Arrhenius bases).

Terms

  • basea proton acceptor, or an electron pair donor
  • solvatea complex formed from solvent molecules attaching to a solute
  • dissociationthe process by which compounds split into smaller constituent molecules, usually reversibly

As discussed in the previous concepts on bases, a base is a substance that can: donate hydroxide ions in solution (Arrhenius definition); accept H+ ions (protons) (Bronsted-Lowry definition); or donate a pair of valence electrons (Lewis definition). In water, basic solutions have a pH higher than 7.0, indicating a greater concentration of OH than H+.

Strong Arrhenius Bases

A strong Arrhenius base, like a strong acid, is a compound that ionizes completely or near-completely in solution. Therefore, the concentration of hydroxide ions in a strongly basic solution is equal to that of the undissociated base. Common examples of strong Arrhenius bases are the hydroxides of alkali metals and alkaline earth metals such as NaOH and Ca(OH)2. Strong bases are capable of deprotonating weak acids; very strong bases can deprotonate very weakly acidic C–H groups in the absence of water.

Sodium hydroxide pelletsSodium hydroxide pellets, before being suspended in water to dissociate.

Some common strong Arrhenius bases include:

  • Potassium hydroxide (KOH)
  • Sodium hydroxide (NaOH)
  • Barium hydroxide (Ba(OH)2)
  • Caesium hydroxide (CsOH)
  • Sodium hydroxide (NaOH)
  • Strontium hydroxide (Sr(OH)2)
  • Calcium hydroxide (Ca(OH)2)
  • Lithium hydroxide (LiOH)
  • Rubidium hydroxide (RbOH)

The cations of these strong bases appear in the first and second groups of the periodic table (alkali and earth alkali metals). Generally, the alkali metal bases are stronger than the alkaline earth metal bases, which are less soluble. When writing out the dissociation equation of a strong base, assume that the reverse reaction does not occur, because the conjugate acid of a strong base is very weak.

Superbases (Lewis bases)

Group 1 salts of carbanions (such as butyllithium, LiC4H9, which dissociates into Li+ and the carbanion C4H9), amides (NH2), and hydrides (H) tend to be even stronger bases due to the extreme weakness of their conjugate acids—stable hydrocarbons, amines, and hydrogen gas. Usually, these bases are created by adding pure alkali metals in their neutral state, such as sodium, to the conjugate acid. They are called superbases, because it is not possible to keep them in aqueous solution; this is due to the fact they will react completely with water, deprotonating it to the fullest extent possible. For example, the ethoxide ion (conjugate base of ethanol) will undergo this reaction in the presence of water:

CH3CH2O + H2O → CH3CH2OH + OH

Unlike weak bases, which exist in equilibrium with their conjugate acids, the strong base reacts completely with water, and none of the original anion remains after the base is added to solution. Some other superbases include:

  • Butyl lithium (n-BuLi)
  • Lithium diisopropylamide (LDA) (C6H14LiN)
  • Lithium diethylamide (LDEA)
  • Sodium amide (NaNH2)
  • Sodium hydride (NaH)
  • Lithium bis(trimethylsilyl)amide, ((CH3)3Si)2NLi

Superbases such as the ones listed above are commonly used as reagents in organic laboratories.