Ch19_Lecture

Ch19_Lecture - Chapter 19 Oxidation and Reduction...

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Chapter 19: Oxidation and Reduction Oxidation, [O], and reduction, [R], are opposites and both must occur simultaneously, hence redox reactions. Organic chemists will normally describe a reaction as either oxidation or reduction depending on the fate of the major organic component. Oxidation: more C-O bonds (or other atoms more electronegative than C) less C-H bonds loss of electrons increased oxidation state, e.g. +1 to +3 (see below) Reduction: more C-H bonds less C-O bonds (or other atoms more electronegative than C) gain of electrons decreased oxidation state, e.g. +1 to -1 (see below) Calculating Oxidation Number or State (there are several methods for doing this, pick the one that works for you !) Attached Atoms Method The oxidation state for the C atom is based on what atoms are attached. for attached C atoms, i.e. C-C bonds electrons shared, count 0 for attached X atoms, i.e. C-X bonds (X more electronegative), count -1 (per bond) for attached H atoms, i.e. C-H bonds (H is less electronegative than C), count +1 Add the total for atoms attached to the C in question, then switch the sign. Note: You may have learnt that O is always -2, however the -2 statement is wrong. ... for example in oxygen gas, the oxidation state is zero. It is better to count each bond to an O as -1 since there is a difference between C=O and C-O as seen by comparing the aldehyde and the alcohol in the diagram above. Lewis Structures Method The oxidation state is determined by assigning electrons to respective atoms. 1. Draw a proper Lewis structure with all lone pairs of electrons 2. Assign electrons in a bond to the most electronegative atom 3. In a bond between the same atom, the electrons are divided between the atoms 4. Substract the number of assigned electrons from the number valence electron for that atom to determine oxidation state Note: The algebraic sum of the oxidation states must equal the charge of the molecule.
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Reduction Reactions Reduction usually involves the addition of a molecule of H 2 across a double bond. Surface Hydrogenation with H
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Ch19_Lecture - Chapter 19 Oxidation and Reduction...

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