For example, you face a problem: perform the following chain of transformations: ethylene - ethane - chloroethane - ethyl alcohol.
Start with the first link of the chain ethylene - ethane.Please write to their chemical formula: C2H4 and C2H6.Ethylene - is unsaturated hydrocarbon (alkene) having in its composition a double bond, ethane - a saturated hydrocarbon (alkane).It differs from ethylene having two additional hydrogen atoms.Obviously, what way can be
converted one into another product of the reaction is as follows:

C2H4 + H2 = C2H6.

reaction proceeds at elevated temperatures and pressures and in the presence of a catalyst.
second link in the chain: ethane - chloroethane.Again, write their chemical formula: C2H6 and S2N5Cl.Your job - to replace one hydrogen atom in the molecule of ethane to chlorine atom.How can I do that?Saturated hydrocarbons are reacted with halogens, to UV irradiation is extremely conducive to the formation of free radicals.They play the role of the initiator of the reaction.It looks like this:

C2H6 + Cl2 = C2H5Cl + HCl.
third link in the chain - the production of ethyl alcohol from chloroethane.Their formulas, respectively: C2H5OH and S2N5Cl.That is, you need to replace the chlorine atom in the molecule chloroethane by hydroxyl group.How can you achieve this?Effects on chloroethane aqueous solution of strong alkali.The reaction proceeds as follows:

S2N5Cl + NaOH = C2H5OH + NaCl.

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