Displacement Reactions

FIELDS OF STUDY: Organic Chemistry; Inorganic Chemistry; Analytical Chemistry

ABSTRACT

Displacement reactions are defined, and their importance in preparative and analytical chemistry is described. Displacement reactions include many types of substitution reactions, such as ionic substitutions, covalent substitutions, and oxidation-reduction (redox) reactions.

A Molecular Exchange

The simplest way to recognize a displacement reaction is to look for elements in the molecular formulas of the reactants that have essentially traded places in the products. This is the basic feature of a displacement reaction and is typically readily apparent upon examining a chemicalreaction equation. The terms "displacement" and "replacement" are entirely interchangeable in describing such reactions. Another term for replacement is "substitution," though this is often used to refer specifically to the replacement of one functional group with another in an organic compound.

One particularly important type of displacement reaction occurs when one element undergoes oxidation by giving up electrons and another element undergoes reduction by accepting those electrons. This type of replacement reaction is known as a "redox reaction" (from reduction and oxidation).

The Electrochemical Series

Each element has a characteristic ability to accept or give up electrons, known as its "standard electrode potential." Elements are ranked by this potential in a list called the "electrochemical series," which places elements in descending order of their ability to be oxidized, that is, give up electrons to form a positively charged ion; conversely, they are in ascending order of their ability to be reduced, that is, accept electrons to form a negatively charged ion. In displacement reactions, atoms of an element that is higher in the electrochemical series will replace atoms of an element that is lower in the series and thus has less ability to give up electrons. When two different elements in the electrochemical series are connected to each other through a conductive medium such as an electrolytic solution (a solution containing dissolved ions), electrons will spontaneously flow from the higher-ranked material to the material that is lower in the series and thus has a greater ability to accept electrons. The material from which the electrons flow is called the cathode, and the material that the electrons flow toward is termed the anode.

This process extends to ions in solution, causing negatively charged ions, or anions, to flow toward the anode, while the positively charged ions, or cations, are drawn toward the cathode. The movement of electrons through a conductor is an electrical current and occurs only when there is an electromotive force or voltage differential between the anode and the cathode. Thus, the electrochemical series is the principle behind all batteries and electrochemical cells, as well as the transmission of electrical signals between cells and the extraction of energy from adenosine triphosphate (ATP) in respiration and glycolysis in biochemical systems. All of these processes depend on redox reactions.

Single- and Double-Displacement Reactions

Single-displacement reactions often involve the transformation of a neutral element into cations while cations of the other element are reduced to a neutral form. A simple example is the reaction that occurs when neutral zinc metal is placed into a solution of copper sulfate. The zinc atoms each release two electrons to form Zn²⁺ ions, while the Cu²⁺ ions in the solution each accept two electrons to form neutral copper atoms. Since the neutral copper atoms are not soluble in water, they precipitate out of the solution as copper metal, while the zinc ions are dissolved. The intense blue color of the copper ions in solution gradually disappears as they are replaced by the colorless zinc ions. If the materials are present in the correct proportions, a dark blue solution of copper sulfate with metallic zinc is transformed into a clear, colorless solution of zinc sulfate with metallic copper. The overall reaction equation for the process is

CuSO₄ + Zn → ZnSO₄ + Cu

which can be written in the equivalent ionic form as

Cu²⁺ + Zn → Cu + Zn²⁺

The replacement is not necessarily a one-to-one replacement in all cases, but it is always in accordance with the number of electrons being transferred.

In a double-displacement reaction, the elements replace each other in their respective compound formulas. For example, in the reaction between chromium sulfate and potassium hydroxide, the products are chromium hydroxide and potassium sulfate, according to the following reaction equation:

Cr₂(SO₄)₃ + 6KOH → 2Cr(OH)₃ + 3K₂SO₄

In this equation, the chromium ions and potassium ions replace each other in their respective compounds. Another way of describing the overall result would be to say that the sulfate and hydroxide ions have switched partners.

In organometallic compounds, ligands are ions or molecules that are coordinated in a geometric arrangement around central metal atoms. They may be ionic species, or they may be neutral molecules with appropriate electron pairs that can interact with vacant atomic orbitals on the metal atom. Ligands can be replaced by other species that have a better ability to coordinate to the central metal atoms. An example of this occurs when four of the six molecules of H₂O in the complex ion [Cu(H₂O)₆]²⁺ are replaced by neutral ammonia molecules to form the ion [Cu(NH₃)₄(H₂O)₂]²⁺, according to the following reaction equation:

[Cu(H₂O)₆]²⁺ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O

Double-displacement reactions are not normally part of organic reaction mechanisms in any but a stoichiometric sense, which requires all atoms that were present at the start of a reaction to be present after the reaction has occurred. Typically, only the desired product of an organic reaction mechanism is of value.

Substitution in Organic Compounds

In organic compounds, one common form of substitution is nucleophilic substitution, which can proceed by one of two mechanisms. In the S_N1 mechanism, an intermediate carbonium ion is formed by the loss of an appropriate anion from the parent compound. Addition of a nucleophilic species to the carbonium ion generates a new compound in which the departing anion has been replaced. In the S_N2 mechanism, the replacement operation is more direct. A nucleophilic species forms a bond to a carbon atom from one side, while the bond to a less potent nucleophile on the opposite side is weakened and lost. The net result is that the original atom or molecule attached to the carbon atom is replaced by a new atom or molecule. A simple example is the formation of pentanol from the reaction of 1-bromopentane and a hydroxide ion, according to the following reaction equation:

Br−CH₂CH₂CH₂CH₂CH₃ + OH⁻ → HO−CH₂CH₂CH₂CH₂CH₃ + Br⁻

Displacement reactions and substitution reactions are extremely useful in both the preparation of specific compounds and the analysis of materials. Both require understanding the specific reactions that take place and the product that is produced from different reactants under specific reaction conditions.

PRINCIPAL TERMS

• anion: any chemical species bearing a net negative electrical charge, which causes it to be drawn toward the positive pole, or anode, of an electrochemical cell.
• cation: any chemical species bearing a net positive electrical charge, which causes it to be drawn toward the negative pole, or cathode, of an electrochemical cell.
• double displacement: a substitution reaction in which the atoms of two elements exchange places in their respective compounds.
• product: a chemical species that is formed as a result of a chemical reaction.
• reactant: a chemical species that takes part in a chemical reaction.
• single displacement: a substitution reaction in which atoms of one element replace atoms of another element in a compound.

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