Understanding Oxidation - Reduction in Organic Chemistry

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In the Classroom

Understanding Oxidation–Reduction in Organic Chemistry Jean-Pierre Anselme Department of Chemistry, University of Massachusetts at Boston, Harbor Campus, Boston, MA 02125 Several years of teaching organic chemistry at the introductory level have made it obvious that understanding oxidation–reduction can be a difficult and sometimes traumatic experience for students. Mastery of the concepts and definitions of oxidation as the loss, and reduction as the gain of electrons (eqs 1–3) is relatively simple (and comforting). However, the realization that the application of this knowledge is not intuitively transferable in organic chemistry can be most disconcerting and often frustrating to beginning students. Reducing Agent Al0 → Al3+ + 3e–

(1)

electron (a total of two). The last stage is merely the acquisition of two protons to neutralize the double negative charge leading to the final product. 2–

C

C

C

C

(a)

2 e–

C

(b)

2 H+

C

(c)

H

H

C

C

(5)

A

Let us now examine in detail various examples of reduction of organic compounds.

“Active Metal” Reduction (Clemmensen Reduction, eq 6)

(aluminum is oxidized) C O + 2Zn0 + 4HCl

Oxidizing Agent 3e– + Cr6+ → Cr3+

(6)

(2)

C O + 4e–(from 2Zn0) + 4H+

(chromate is reduced)

Overall Equation Al0 + Cr6+ → Al3+ + Cr3+

(3)

The various explications of these two fundamental processes in organic chemistry seem to assume the aura of doublespeak in contrast to the universal simplicity of the definitions learned in freshman chemistry. In organic chemistry, the student is told that oxidation may variously be called “dehydrogenation”, “addition of oxygen”, and “loss of hydrogen”, while terms such as “hydrogenation”, “loss of oxygen”, and “addition of hydrogen” are used for reduction. The present approach may be useful in helping students bridge the gap between what appear to be unrelated, and even sometimes dichotomous, concepts. One of the difficulties involves the often-confused aspects of mechanisms and what shall be called “accounting” in this paper. The present contribution makes no pretense of dealing with mechanistic aspects of oxidation–reduction of organic compounds. Indeed, although reasonable mechanisms may sometimes be written for the reduction and the oxidation of organic compounds, equally often the intermediate steps and species can only be subjects of speculation. The discussions that follow are merely a detailed examination of how to “balance the books” on atoms and electrons. Reduction How can we look at the reduction of a double bond, often mysteriously written as shown in eq 4, in terms of gain of electrons? [H] C C

H H C C

CH2 + 2ZnCl2 + H2O CH2 + H2O (2Zn0 become 2Zn2+)

Zinc metal (oxidation state 0) provides the electrons that actually effect the reduction and is evidently itself oxidized to zinc (oxidation state +2) chloride. The first stage of the process, namely the conversion of the carbonyl group to the alcohol (eq 7), may be accounted for as illustrated earlier in eq 5 (except in this case, it is a >C=O instead of a >C=C