addition reaction

How to calculate the oxidation numbers (or oxidation states) of C atoms in organic compounds ?

Method : Imagine that the molecule is cut into segments by breaking all bonds between two C atoms. The sum of oxidation numbers for all elements in each segment is equal to 0.

Example 1: CH2=CH-CH2Cl
In each segment of CH2, CH and CH2Cl, the sum of oxidation number is 0.
Oxidation numbers of H and Cl are +1 and -1 respectively.
In CH2 segment, oxidation number of C = -2.
In CH segment, oxidation number of C = -1.
In -CH­2­Cl segment, oxidation number of C = -1.

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In terms of oxidation number (or oxidation state), addition reaction is redox.

Example 2 : CH2=CH2 + Cl2 → CH2Cl-CH2Cl
The oxidation number of C increases from -2 to -1.
The oxidation number of C decreases from 0 to -1.
The reaction is redox.

Example 3 : CH2=CH2 + HCl → CH3-CH2Cl
The oxidation number of C on the left decreases from -2 to -3.
The oxidation number of C on the right increases from -2 to -1.
The reaction is redox.

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In organic chemistry, oxidation numbers are seldom used. Firstly, only the major organic reactant is considered as oxidized or reduced, and neglecting all other reactant(s) (the term reagent is usually used). Secondly, oxidation is the gain in oxygen or loss in hydrogen, while reduction is the loss in oxygen or gain in hydrogen.

In such view, addition reactions in examples 2 and 3 above are not oxidation or reduction.

Similarly, many addition reactions are not oxidation or reduction, but a few addition reactions are oxidation or reduction.

Example 4 : CH2=CH2 + H2 → CH3-CH3
In the reacton, CH2=CH2 undergoes reduction (gain of H).

Example 5 : CH2=CH2 + [O] + H2O → CH2OH-CH2OH
In the reaction, CH2=CH2 undergoes oxidation (gain of O)

addition reaction 不等於 redox 架!

明顯的分別是:
addition reaction 係加 ions 落去，然後c=c bond 轉做c-c bond( organic chem)
e.g.

H2C=CH2+ Br2 ~>BrH2C-CH2Br

係carbon compound 中 無失去任何野，

however,
redox reaction 分為two part:
1. reducing agent (moecule) 進行 oxidation, then 會
+oxygen or -hydrogen or -electron
also oxidation number 會increase 左

2. oxidizing agent (moecule) 進行 reduction, then 會
-oxygen or +hydrogen or +electron
also oxidation number 會decrease 左

e.g.

zn ~> zn 2+ & cu 2+ ~> cu
by displacement reaction undergo redox reaction