M15Q8: Lewis Acids and Bases

Learning Objectives

  • Identify Lewis acids and bases.

| Key Concepts and Summary | Glossary | End of Section Exercises |

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 donate a pair of electrons and form a coordinate covalent bond.

A coordinate covalent bond (or dative bond) occurs when one of the atoms in the bond provides both bonding electrons. For example, a coordinate covalent bond occurs when a water molecule combines with a hydrogen ion to form a hydronium ion. A coordinate covalent bond also results when an ammonia molecule combines with a hydrogen ion to form an ammonium ion. Both of these equations are shown here.

This figure shows two reactions represented with Lewis structures. The first shows an O atom bonded to two H atoms. The O atom has two lone pairs of electrons. There is a plus sign and then an H atom with a superscript positive sign followed by a right-facing arrow. The next Lewis structure is in brackets and shows an O atom bonded to three H atoms. There is one lone pair of electrons on the O atom. Outside of the brackets is a superscript positive sign. The second reaction shows an N atom bonded to three H atoms. The N atom has one lone pair of electrons. There is a plus sign and then an H superscript positive sign. After the H superscript positive sign is a right-facing arrow. The next Lewis structure is in brackets. It shows an N atom bonded to four H atoms. There is a superscript positive sign outside the brackets.

A Lewis acid is any species (molecule or ion) that can accept a pair of electrons, and a Lewis base is any species (molecule or ion) that can donate a pair of electrons.

A Lewis acid-base reaction occurs when a base donates a pair of electrons to an acid. The product contains a newly-formed coordinate covalent bond between the Lewis acid and the Lewis base. The following equations illustrate the general application of the Lewis concept.

The boron atom in boron trifluoride, BF3, has only six electrons in its valence shell. Being short of the preferred octet, BF3 is a very good Lewis acid and reacts with many Lewis bases; a fluoride ion is the Lewis base in this reaction, donating one of its lone pairs:

This figure shows a reaction with two reactants and one product. The first reactant is labeled "Lewis base" and is a flourine atom with 8 dots around it and a negative charge. The second reactant has a central boron atom single bonded to three flourine atoms 120 degrees apart. Each flourine has 6 electron dots. This reactant is labeled "Lewis acid." The product has a central boron atom single bonded to four fluorine atoms, each with 6 electron dots. The structure is bracketed with a minus charge indicated.

In the following reaction, each of two ammonia molecules, Lewis bases, donates a pair of electrons to a silver ion, the Lewis acid:

This figure contains a reaction with two reactants and a product. The first reactant has a central nitrogen atom single bonded to three hydrogen atoms, one traditional line, one a wedge, and the last a dash. There are two of these reactants in the balanced reaction. This first reactant is labeled "Lewis base". The second reactant is Ag with a positive charge. It is labeled "Lewis acid". The product has a central nitrogen single bonded to three hydrogens and a single bond to the right to Ag, which is single bonded to the right to a nitrogen atom, which is single bonded to three hydrogen atoms. The structure is bracketed with a positive charge indicated.

Key Concepts and Summary

The final definition of acids and bases we will learn in this course is the most comprehensive as they only require molecules to transfer a pair of electrons, rather than a proton. Lewis acids are defined as accepting a pair of electrons and Lewis bases as donating a pair of electrons. A Lewis acid tends to be more positively charged and a Lewis base more negatively charged (although this is not robust enough for all scenarios!)

Glossary

coordinate covalent bond
a covalent bond where one of the atoms in the bond provides both bonding electrons, rather than each atom providing a single electron to form the bond
Lewis acid
a species (molecule or ion) that can accept a pair of electrons
Lewis base
a species (molecule or ion) that can donate a pair of electrons

Chemistry End of Section Exercises

  1. Write the Lewis structures of the reactants and product of each of the following equations, and identify the Lewis acid and the Lewis base in each:
    1. CO2(g) + OH–(aq)  ⇌  HCO3–(aq)
    2. B(OH)3(aq) + OH–(aq)  ⇌  B(OH)4–(aq)
    3. I–(aq) + I2(aq)  ⇌  I3–(aq)
    4. AlCl3(s) + Cl–(aq)  ⇌  AlCl4–(aq)
    5. O2-(aq) + SO3(aq)  ⇌  SO42-(aq)
  2. For the reaction, NH3 + H+  →  NH4+, identify the correct statement(s) below. Select all that apply.
    1. H+ is acting as a Lewis acid.
    2. NH3 is acting as a Lewis base.
    3. NH3 is acting as a Bronsted base.
  3. Consider the reaction mechanism step shown below. Circle the substance acting as a Lewis acid and put a rectangle around the substance acting as a Lewis base.
    A reaction is shown. On the reactant side, a Br- anion reacts with CH2(Br+)C(H)CH3 bromonium cation. The product formed is 1,2-dibromopropane (CH2BrCHBrCH3).

Answers to Chemistry End of Section Exercises

  1. (a) This figure shows a chemical reaction modeled with structural formulas. On the left side is a structure with a central C atom. O atoms, each with two unshared electron pairs, are double bonded to the left and right sides of the C atom. Following a plus sign is another structure in brackets which has an O atom with three unshared electron dot pairs single bonded to an H atom on the right. Outside the brackets is superscript negative sign. Following a right pointing arrow is a structure in brackets that has a central C atom to which 3 O atoms are bonded. Above and slightly to the right, one of the O atoms is connected with a double bond. This O atom has two unshared electron pairs. The second O atom is single bonded below and slightly to the right. This O atom has three unshared electron pairs. The third O atom is bonded to the left of the C atom. This O atom has two unshared electron pairs and an H atom single bonded to its left. Outside the brackets to the right is a superscript negative symbol.
    (b) This figure shows a chemical reaction modeled with structural formulas. On the left side is a structure that has a central B atom to which 3 O atoms are bonded. The O atoms above and below slightly right of the B atom each have an H atom single bonded to the right. The third O atom is single bonded to the left side of the B atom. This O atom has an H atom single bonded to its left side. All O atoms in this structure have two unshared electron pairs. Following a plus sign is another structure which has an O atom single bonded to an H atom on its right. The O atom has three unshared electron pairs. The structure appears in brackets with a superscript negative sign. Following a right pointing arrow is a structure in brackets has a central B atom to which 4 O atoms are bonded. The O atoms above, below, and right of the B atom each hav an H atom single bonded to the right. The third O atom is single bonded to the left side of the B atom. This O atom has an H atom single bonded to its left side. All O atoms in this structure have two unshared electron pairs. Outside the brackets to the right is a superscript negative symbol.
    (c) This figure illustrates a chemical reaction using structural formulas. On the left, two I atoms, each with 3 unshared electron pairs, are joined with a single bond. Following a plus sign is another structure which has an I atom with four pairs of electron dots and a superscript negative sign. Following a right pointing arrow is a structure in brackets that has three I atoms connected in a line with single bonds. The two end I atoms have three unshared electron dot pairs and the I atom at the center has two unshared electron pairs. Outside the brackets is a superscript negative sign.
    (d) This figure illustrates a chemical reaction using structural formulas. On the left, an A l atom is positioned at the center of a structure and three Cl atoms are single bonded above, leftt, and below. Each C l atom has three pairs of electron dots. Following a plus sign is another structure which has an F atom is surrounded by four electron dot pairs and a superscript negative symbol. Following a right pointing arrow is a structure in brackets that has a central A l atom to which 4 C l atoms are connected with single bonds above, below, to the left, and to the right. Each C l atom in this structure has three pairs of electron dots. Outside the brackets is a superscript negative symbol.
    (e) This figure illustrates a chemical reaction using structural formulas. On the left is a structure which has an S atom at the center. O atoms are single bonded above and below. These O atoms have three electron dot pairs each. To the right of the S atom is a double bonded O atom which has two pairs of electron dots. Following a plus sign is an O atom which is surrounded by four electron dot pairs and has a superscript 2 negative. Following a right pointing arrow is a structure in brackets that has a central S atom to which 4 O atoms are connected with single bonds above, below, to the left, and to the right. Each of the O atoms has three pairs of electron dots. Outside the brackets is a superscript 2 negative.
  2. All three choices are correct.
  3. In the reaction of Br- anion with CH2(Br+)C(H)CH3 bromonium cation, forming 1,2-dibromopropane (CH2BrCHBrCH3), a rectangle is drawn around Br- anion and an oval is drawn around the bromonium cation.
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