Be careful – do not confuse the terms hyd rogenation and dehy drogenation with hydration and dehydration – the latter refer to the gain and loss of a water molecule (and are not redox reactions), while the former refer to the gain and loss of a hydrogen molecule. Because they occur in conjunction with the transfer of a proton, these are commonly referred to as hydrogenation and dehydrogenation reactions: a hydride plus a proton adds up to a hydrogen (H 2) molecule. Electrons in an organic redox reaction often are transferred in the form of a hydride ion – a proton and two electrons. In organic chemistry, redox reactions look a little different. You also know that oxidation and reduction reactions occur in pairs: if one species is oxidized, another must be reduced at the same time – thus the term ‘redox reaction’.Most of the redox reactions you have seen previously in general chemistry probably involved the flow of electrons from one metal to another, such as the reaction between copper ion in solution and metallic zinc: You are undoubtedly already familiar with the general idea of oxidation and reduction: you learned in general chemistry that when a compound or atom is oxidized it loses electrons, and when it is reduced it gains electrons. ![]() Oxidation and reduction of organic compounds – an overview So unlike metals, which are almost always in a positive oxidation state, the oxidation state of carbon can vary widely, from -4 (in CH4) to +4 (such as in CO2). So a carbon attached to 4 carbons has an oxidation state of zero. For carbon bonded to another carbon, the oxidation state is unaffected.(Certain non-metals are less electronegative than carbon, such as phosphorus, silicon or boron, but bonds from carbon to these elements are much less common.) For carbon bonded to a more electronegative non-metal X, such as nitrogen, oxygen, sulfur or the halogens, each C-X bond will increase the oxidation state of the carbon by 1.This means that every C-H bond will decrease the oxidation state of carbon by 1. In a C-H bond, the H is treated as if it has an oxidation state of +1. ![]() To calculate the oxidation state for carbon, use the following guidelines: Most of the redox reactions in this chapter involve a change in the oxidation state of the carbon bearing the functional group. As we begin to look at organic redox reactions, it is useful to consider how we define the oxidation state for carbon.
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