15.14
Lewis Acids and Bases
In 1923, G. N. Lewis proposed a generalized definition of acid-base behavior in which acids and bases are identified by their ability to accept or to…
The Brønsted-Lowry model defines acids and bases in terms of protons, where acids are proton donors, and bases are proton acceptors. In contrast, the Lewis model defines acids and bases in terms of electron pairs, where Lewis acids are electron-pair acceptors, and Lewis bases are electron-pair donors.
In a Brønsted acid, like acetic acid, the hydrogen can also act as a Lewis acid because it has an empty orbital to accept electrons donated from a base, like water, acting as a Lewis base.
The advantage of the Lewis model is that it allows scientists to classify a greater number of compounds as acids—including the ones that do not have ionizable protons. For example, boron trifluoride cannot be classified as an acid by the Brønsted-Lowry model because it does not contain hydrogen.
However, boron trifluoride possesses an incomplete octet with an empty orbital that can accept an electron pair from a Lewis base, such as ammonia, and, therefore, can act as a Lewis acid.
The resultant product formed by such Lewis acid-base reactions is called a Lewis acid-base adduct.
Some molecules, like carbon dioxide, can rearrange their electrons to act as a Lewis acid.
For example, in the reaction between water and carbon dioxide, an electron-pair moves from the carbon-oxygen pi bond to the terminal oxygen of the carbon dioxide.
The resultant empty orbital on the carbon atom allows it to accept the electron pair from a water molecule and act as a Lewis acid. As the water molecule donates the electron pair, it acts as a Lewis base.
In further rearrangement, a proton is transferred from the water oxygen to the terminal oxygen of the carbon dioxide, resulting in the formation of the carbonic acid adduct.
Small metal cations, like Al(III), can reacquire electron pairs and act as Lewis acids. For example, Al(III) accepts lone pairs of electrons from water and forms hexaaquaaluminum ions. Here, water molecules donate electron pairs and act as a Lewis base.
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Q1: How do Lewis acids and bases differ from Brønsted-Lowry acids and bases?
The Brønsted-Lowry model defines acids as proton donors and bases as proton acceptors. The Lewis model defines acids as electron-pair acceptors and bases as electron-pair donors. Lewis theory is broader, classifying more compounds as acids, including those without ionizable protons. Brønsted-Lowry acid-base reactions represent a subcategory of Lewis reactions where the acid is specifically H+.
Q2: Why is boron trifluoride considered a Lewis acid?
Boron trifluoride cannot be classified as a Brønsted-Lowry acid because it lacks hydrogen. However, the boron atom has only six electrons in its valence shell, creating an incomplete octet with an empty orbital. This empty orbital accepts electron pairs from Lewis bases like ammonia, allowing boron trifluoride to act as a Lewis acid.
Q3: What is a Lewis acid-base adduct?
A Lewis acid-base adduct is the product formed when a Lewis acid accepts an electron pair from a Lewis base, creating a coordinate covalent bond. In this bond, one atom provides both bonding electrons. For example, when ammonia donates an electron pair to a hydrogen ion, the resulting ammonium ion is a Lewis acid-base adduct.
Q4: How can carbon dioxide act as a Lewis acid?
Carbon dioxide can rearrange its electrons to act as a Lewis acid. An electron pair moves from the carbon-oxygen pi bond to the terminal oxygen, creating an empty orbital on the carbon atom. This empty orbital then accepts an electron pair from a water molecule, which acts as a Lewis base, forming carbonic acid.
Q5: What role do metal cations like Al(III) play in Lewis acid-base chemistry?
Small metal cations like Al(III) act as Lewis acids by accepting electron pairs from Lewis bases. For example, Al(III) accepts lone pairs of electrons from water molecules, which donate electron pairs and act as Lewis bases. This interaction forms hexaaquaaluminum ions, demonstrating how metal cations participate in coordinate covalent bonding.
Q6: What is a coordinate covalent bond?
A coordinate covalent bond, also called a dative bond, forms when one atom provides both bonding electrons to the bond. This occurs in Lewis acid-base reactions when a Lewis base donates an electron pair to a Lewis acid. Examples include water combining with a hydrogen ion to form a hydronium ion.
Q7: How does the Lewis model expand the definition of acids beyond the Brønsted-Lowry model?
The Lewis model allows scientists to classify a greater number of compounds as acids, including those without ionizable protons. Any compound with an empty orbital that can accept an electron pair qualifies as a Lewis acid. This broader definition encompasses nonmetal oxides, metal cations, and other electron-deficient species that the Brønsted-Lowry model cannot classify as acids.