http://www.ck12.org Chapter 25. Organic Chemistry
Substitution Reactions
Asubstitution reactionis a reaction in which one or more atoms in a molecule are replaced with another atom or
group of atoms. Alkyl halides are formed by the substitution of a halogen atom for a hydrogen atom. When methane
reacts with chlorine gas, ultraviolet light can act as a catalyst for the reaction.
CH 4 (g)+Cl 2 (g)UV light
−→ CH 3 Cl(g)+HCl(g)The reaction produces chloromethane and hydrogen chloride. When the mixture is allowed to react for longer periods
of time, further substitution reactions may occur.
CH 3 Cl(g)+Cl 2 (g)UV light
−→ CH 2 Cl 2 (g)+HCl(g)The product above is dichloromethane. Further substitution produces trichloromethane and tetrachloromethane,
commonly called carbon tetrachloride. A mixture of products occurs in the reaction, with the relative amounts
dependent upon the time that the reaction is allowed to proceed. Chlorofluorocarbons are produced by reacting
chloroalkanes with HF, because the fluorine atom makes a stronger bond to the carbon atom than chlorine does.
CCl 4 (g)+HF(g)SbF−→^5 CCl 3 F(g)+HCl(g)The fluorine atom substitutes for a chlorine atom in the reaction.
As discussed previously, benzene is a fairly stable and unreactive molecule due to the electron delocalization around
the six-membered ring. The treatment of benzene with a halogen in the presence of an iron catalyst causes the
substitution of a halogen atom for a hydrogen atom. The resulting molecule is called an aryl halide.
C 6 H 6 (l)+Br 2 (l)
FeBr 3
−→ C 6 H 5 Br(l)+HBr(g)Alkyl groups can be introduced onto a benzene ring by the reaction of benzene with an alkyl halide, using aluminum
chloride as the catalyst. In the reaction below, benzene reacts with chloroethane to produce ethylbenzene.
C 6 H 6 (l)+CH 3 CH 2 Cl(g)
AlCl 3
−→C 6 H 5 CH 2 CH 3 (l)+HCl(g)The reaction of an alkyl halide with an inorganic hydroxide base at elevated temperature produces an alcohol. The
molecular reaction to produce methanol from iodomethane and sodium hydroxide is shown below.
CH 3 I(l)+NaOH(aq)^100◦C
−→CH 3 OH(l)+NaI(aq)