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Organic, Biochemistry, and Polymers 317
Quick Tip
alkane is normally present. The following is an example of a substitution reaction.
The symbol, hn, indicates light energy.
CH 3CH 3 Cl 2
hn
4
CH 3CH 2Cl HCl
In many organic reactions, compounds such as HCl are not included in the final
answer. The key to the organic reaction equation is what happens to the organic
compound and not perfecting a balanced chemical equation.
Aromatic hydrocarbons, like alkanes, undergo substitution reactions. The conditions
required for an aromatic compound to react are different than that for
the reaction of an alkane. Aromatic substitution reactions are beyond the scope
of this text.
Don’t Forget!
The separate resonance structures for the aromatic hydrocarbons show C=C
bonds. These may appear the same as similar bonds in alkenes, but resonance
negates any similarity in behavior.
Alkenes and alkynes typically react by addition reactions. As the name implies,
two or more molecules simply add together. Normally, one product results with
a formula that is simply the sum of the reactant pieces. Only the functional
group, the carbon-carbon double or triple bond, will change during the reaction.
Two typical addition reactions are hydrogenation and halogenation. In
hydrogenation, we add a hydrogen, H 2, molecule. Thus, C 2H 4 will become
C 2H 42 C 2H 6. Hydrogenation always requires a catalyst. The most common
catalysts are platinum, Pt, palladium, Pd, or nickel, Ni. In a halogenation reaction,
we add a halogen, either chlorine or bromine. No catalyst is necessary,
and, unlike the reaction with an alkane, light is not necessary. The formation of
a polymer, such as polypropylene, is an addition reaction. For this reason,
polypropylene is an example of an addition polymer.