Introductory Organic and Biochemistry

Quiz: Oxidation and reduction.

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This quiz helps you see the relationship between two ways of identifying oxidation and reduction: counting electrons (using oxidation numbers) vs using the count of O and H atoms. It is relevant to several chapters, and can probably be done at any time from "Alkenes" on through "Acids". It does require knowing the basic structure of the various functional groups.

For Ouellette, this quiz is relevant to Ch 3, 4, 8, 10, 12. If you need help with the functional groups, see the listing in Table 1.4, p 19. If you are using another book, these are the chapters on alkanes, alkenes, alcohols, and simple and complex carbonyl compounds.

Starting in Ouellette Ch 4 we will discuss oxidation and reduction of organic compounds. The general topic of redox is introduced in the background chapters, Sect 2.4.

Chemists define oxidation as the loss of electrons, and reduction as the gain of electrons. It is common in general chemistry to deal with the electron count by using oxidation numbers. A brief reminder how to do this is in the next paragraph; it should be adequate even if you have not dealt with oxidation numbers before. (Ouellette does not introduce oxidation numbers.) However, organic chemists often just count H and O atoms instead. Oxidation is the gain of oxygen (quite reasonably) -- or, equivalently, the loss of hydrogen. Reduction, of course, is the opposite: gain of H or loss of O.

These two systems for identifying oxidation and reduction are equivalent. This follows from the common convention that the oxidation number of O in compounds is -2 and that of H is +1. (There are exceptions to those assignments, but they are not relevant here. All you need to assign oxidation numbers on this quiz is those two conventions; then calculate the oxidation number for the C atoms, using the principle that the total of all oxidation numbers in a molecule must be zero.)

Example: What is the average oxidation number of the C atoms in a compound with the formula C2H4O? 4 H have total +4, 1 O has total -2. Together the H and O have total +2. Therefore the total for the carbon atoms must be -2. Since there are 2 C, each C must be -1 in this molecule.

The purpose of this quiz is to help you see this equivalence, by using both systems to rank the oxidation state of some simple organic molecules. (For complex organic molecules, oxidation numbers become unwieldy.) (You are not formally responsible for oxidation numbers in this course. However, you are responsible for the rest of this.)

For more on redox... See the page on Balancing organic redox reactions. There are various ways to balance redox equations, and the ones you learn in general chem are sometimes not convenient with redox equations involving organic chemicals, or other complex chemicals. This page focuses on an approach that is especially suitable with organic chemicals: emphasizing H and O atoms. Thus it uses the material from this quiz in the context of balancing equations.

This is intended as a closed book quiz. If you need to look up some structures, that is ok. For example, you might use the table of functional groups. You shouldn't look up matters dealing with oxidation and reduction while taking this quiz.

1. Consider the following compounds, each of which contains one C atom: methane, methanol, methanal, methanoic acid, carbon dioxide.

a. Assign an oxidation number to the C atom in each of these compounds, using the common convention for O and H (as stated above).

b. Put these compounds in order from least oxidized to most oxidized, based on the oxidation number of the C, from part a.

c. Put these compounds in order from least oxidized to most oxidized, based on the organic chemistry convention of counting H and O atoms (as described above).

d. Compare the lists from parts b and c. They should be the same. If not, check your work, and ask for help if needed.

2. Consider the following compounds, each of which contains two C atoms: ethane, ethene, ethyne, ethanol.

Repeat parts a-d (as described for #1) for this set of compounds.

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Last update: September 15, 2011