Common Types of Organic Reactions

There are tons of types of organic reactions that occur in nature or laboratory. In this chapter only the very basic types of reactions will be introduced.

The addition reaction, as its name implies, is simply the addition of an atom or a group to compound. For example, a hydrogen molecule can be added to acetylene to produce ethylene, and ethylene can again be added to a hydrogen molecule to form ethane, as shown in the figure below:

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However, ethane cannot participate in an addition reaction with hydrogen, because ethane is fully saturated.

A famous application of the addition reaction is the bromination of hydrocarbons. It is used to determine whether a hydrocarbon is saturated or not. Bromine $(\text{Br}_2)$ itself has a unique brownish color. If bromine is added to an unsaturated hydrocarbon (e.g. alkene or alkyne) solution, then bromine will add to the hydrocarbon. As bromine is consumed, the solution will lose the brownish color. However if bromine is added to a saturated hydrocarbon solution, bromine will not react with the hydrocarbon, and thus discoloration cannot be observed.

The substitution reaction, can be simply thought of replacing an atom with a different one. For example, the reaction below illustrates the substitution of a bromine atom for a hydrogen atom in benzene:

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Substitution usually takes place in saturated compounds or aromatic rings, as these compounds rarely participate in addition reactions. On the other hand, unsaturated compounds like alkenes or alkynes tend to participate in addition reactions rather than substitution reactions.

Oxidation is the process of losing electrons, whereas reduction is gaining electrons. When it comes to organic chemistry, most redox reactions involve the gaining/losing hydrogen or oxygen atoms. Gaining an oxygen atom usually means oxidation, and losing one means reduction. On the other hand gaining a hydrogen atom means reduction, while losing one means oxidation. A well known example is the oxidation and reduction of alcohols, aldehydes, ketones, and carboxylic acids.

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In the figure above, $[\text{O}]$ means gaining an oxidation, i.e. oxidation. Likewise $[\text{H}]$ means gaining two hydrogen atoms, which is equivalent to reduction. Primary alcohols oxidize to form aldehydes, which then can oxidize again to produce carboxylic acids. Reversely, carboxylic acids can be reduced to form aldehydes, which again can be reduced to primary alcohols. Secondary alcohols can oxidize once to form ketones.

Esterification is a condensation reaction between an alcohol and a carboxylic acid to produce an ester. One alcohol molecule will react with one carboxylic acid molecule to produce one ester molecule and one water molecule. During the process, a water molecule is formed, because the hydroxyl group from the carboxylic acid and a hydrogen atom from the alcohol are removed while the ester is formed, as shown in the figure below:

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120 grams of acetic acid $(\text{CH}_3\text{COOH})$ reacts with 68 grams of $^{18}\text{O}$-methanol $(\text{CH}_3^{18}\text{OH})$ to form methyl acetate. Calculate the amount of water (in grams) during this reaction. Use the atomic weights

$$\text{H}=1, \text{C}=12, \text{O}=16, ^{18}\text{O}=18.$$

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First note that 120 grams of acetic acid is 2 moles and 68 grams of $^{18}\text{O}$-methanol is also 2 moles. Thus the esterification reaction will produce 2 moles of methyl acetate and 2 moles of water. Observe from the figure above that the 18-oxygen all stays in methyl acetate, instead of water. In other words, all oxygen atoms in the water molecules are 16-oxygen atoms, and therefore the amount of water formed is $2\times18=36$ grams. $_\square$