MCAT Organic Chemistry Review
Analyzing Organic Reactions
4.3 Oxidation–Reduction Reactions
Another important class of reactions are oxidation–reduction (redox) reactions, in which the oxidation states of the reactants change. Oxidation state is an indicator of the hypothetical charge that an atom would have if all bonds were completely ionic. Oxidation state can be calculated from the molecular formula for a molecule. For example, the carbon in methane (CH4) has an oxidation state of –4 because the hydrogens each have an oxidation state of +1. This is the most reduced form of carbon. In carbon dioxide (CO2), each of the oxygen atoms has an oxidation state of –2, and the carbon has an oxidation state of +4. This is the most oxidized form of carbon. For an ion, the oxidation state is simply the charge—so Na+ and S2– would have oxidation states of +1 and –2, respectively. Carboxylic acids are more oxidized than aldehydes, ketones, and imines, which in turn are more oxidized than alcohols, alkyl halides, and amines.
We won’t need to know too much about how to assign oxidation states in organic chemistry, but should know the definitions of oxidation and reduction. Oxidation refers to an increase in oxidation state, which means a loss of electrons. In organic chemistry, it is often easier to view oxidation as increasing the number of bonds to oxygen or other heteroatoms (atoms besides carbon and hydrogen). Reduction refers to a decrease in oxidation state, or a gain in electrons. In organic chemistry, it is often easier to view reduction as increasing the number of bonds to hydrogen.
OXIDIZING AGENTS AND REACTIONS
As we mentioned above, oxidation refers to an increase in oxidation state. Oxidation of a carbon atom occurs when a bond between a carbon atom and an atom that is less electronegative than carbon is replaced by a bond to an atom that is more electronegative than carbon. In practice, this usually means decreasing the number of bonds to hydrogen and increasing the number of bonds to other carbons, nitrogen, oxygen, or halides.
One can organize the different functional groups by “levels” of oxidation:
· Level 0 (no bonds to heteroatoms): alkanes
· Level 1: alcohols, alkyl halides, amines
· Level 2: aldehydes, ketones, imines
· Level 3: carboxylic acids, anhydrides, esters, amides
· Level 4 (four bonds to heteroatoms): carbon dioxide
The oxidizing agent is the element or compound in an oxidation–reduction reaction that accepts an electron from another species. Because the oxidizing agent is gaining electrons, it is said to be reduced. Good oxidizing agents have a high affinity for electrons (such as O2, O3, and Cl2) or unusually high oxidation states (like Mn7+ in permanganate, MnO4−; and Cr6+ in chromate, CrO42−).
Primary alcohols can be oxidized by one level to become aldehydes, or can be further oxidized to form carboxylic acids. This reaction commonly proceeds all the way to the carboxylic acid when using strong oxidizing agents such as chromium trioxide (CrO3) or sodium or potassium dichromate (Na2Cr2O7 or K2Cr2O7), but can be made to stop at the aldehyde level using specific reagents such as pyridinium chlorochromate (PCC). Secondary alcohols can be oxidized to ketones.
A number of oxidation reactions and the relevant oxidizing agents are shown in Figure 4.7. Note that the goal at this point should not be memorization of these reactions, but recognition of two themes: oxidation reactions tend to feature an increase in the number of bonds to oxygen, and oxidizing agents often contain metals bonded to a large number of oxygen atoms.
Figure 4.7. Oxidation Reactions and Common Oxidizing Agents
REDUCING AGENTS AND REACTIONS
Conversely, reduction refers to a decrease in oxidation state. Reduction of a carbon occurs when a bond between a carbon atom and an atom that is more electronegative than carbon is replaced by a bond to an atom that is less electronegative than carbon. In practice, this usually means increasing the number of bonds to hydrogen and decreasing the number of bonds to other carbons, nitrogen, oxygen, or halides.
Good reducing agents include sodium, magnesium, aluminum, and zinc, which have low electronegativities and ionization energies. Metal hydrides, such as NaH, CaH2, LiAlH4, and NaBH4, are also good reducing agents because they contain the H– ion.
Aldehydes and ketones will be reduced to primary and secondary alcohols, respectively. This reaction is exergonic, but exceedingly slow without a catalyst. Amides can be reduced to amines using LiAlH4. This same reducing agent will reduce carboxylic acids to primary alcohols and esters to a pair of alcohols. Examples of reduction reactions are shown in Figure 4.8. Again, the focus is not on memorization, but on recognizing that reduction reactions tend to feature an increase in the number of bonds to hydrogen, and reducing agents often contain metals bonded to a large number of hydrides.
Figure 4.8. Reduction Reactions and Common Reducing Agents
Note that many of the common oxidizing and reducing agents include transition metals. This is because transition metals can often take on many different oxidation states. Their low ionization energies and presence of d-orbitals allow them to give up and accept electrons easily. Transition metals and periodic trends are discussed in Chapter 2 of MCAT General Chemistry Review.
MCAT Concept Check 4.3:
Before you move on, assess your understanding of the material with these questions.
1. What are some characteristics of good oxidizing agents? List some examples of common oxidizing agents.
2. What are some characteristics of good reducing agents? List some examples of common reducing agents.
3. List the following carbon-containing compounds from most oxidized carbon to most reduced: methane, carbon dioxide, ketone, alcohol, carboxylic acid
· Most oxidized:
· Most reduced: