For example, you face the task: perform the following chain of transformations: ethylene – ethane – chlorate – ethyl alcohol.
Start with the first link of the chain: ethylene – ethane. First, write their chemical formulas: С2Н4 and С2Н6. Ethylene is unsaturated hydrocarbon (alkene) having in its composition a double bond, and ethane – is the ultimate hydrocarbon (an alkane). It differs from ethylene by the presence of two additional hydrogen atoms. Obviously, the way you can turn one product into another, the reaction proceeds according to the following scheme:

С2Н4 + N2 = С2Н6.

The reaction takes place at elevated temperature, pressure and in the presence of a catalyst.
The second link of the chain: ethane – chlorate. Again, write their chemical formulas: С2Н6 and С2Н5Cl. Your task is to replace one hydrogen atom in the molecule of ethane to chlorine atom. How can I do that? Saturated hydrocarbons react with Halogens under the action of UV radiation, promoting the formation of highly active free radicals. They play the role of initiator of the reaction. It looks like this:

С2Н6 + Cl2 = C2H5Cl + HCl.
The third link of the chain – obtaining ethanol from chloretone. Their formulas, respectively: C2H5OH and С2Н5Cl. That is, you need to replace the chlorine atom in the molecule chloretone to the hydroxyl group. How can you achieve? The impact on chlorate aqueous solution of a strong alkali. The reaction proceeds according to the following scheme:

С2Н5Cl + NaOH = C2H5OH + NaCl.

As you can see, you have reached the goal, that is, started with ethylene, came to ethanol.