The Foundations of Chemistry

The principal reactions of alkenes and alkynes are addition reactions rather than substi-
tution reactions. For example, contrast the reactions of ethane and ethylene with Cl 2.

ethane: CH 3 XCH 3 Cl 2 88nCH 3 XCH 2 ClHCl (substitution, slow)

ethylene: (addition, rapid)

Carbon–carbon double bonds are reaction sitesand so represent functional groups.Most
addition reactions involving alkenes and alkynes proceed rapidly at room temperature. By
contrast, many substitution reactions of the alkanes require catalysts and high tempera-
tures.
Bromine adds readily to the alkenes to give dibromides. The reaction with ethylene is

The addition of Br 2 to alkenes is used as a simple qualitative test for unsaturation.
Bromine, a dark red liquid, is dissolved in a nonpolar solvent. When an alkene is added,
the solution becomes colorless as the Br 2 reacts with the alkene to form a colorless
compound. This reaction may be used to distinguish between alkanes and alkenes.
Hydrogenationis an extremely important addition reaction of the alkenes. Hydrogen
adds across double bonds at elevated temperatures, under high pressures, and in the pres-
ence of an appropriate catalyst (finely divided Pt, Pd, or Ni).

catalyst

CH 2 UCH 2 H 2 888888nCH 3 XCH 3

heat

Some unsaturated hydrocarbons are converted to saturated hydrocarbons in the manu-
facture of high-octane gasoline and aviation fuels. Unsaturated vegetable oils can also be
converted to solid cooking fats (shortening) by hydrogenation of most of the carbon–
carbon double bonds present (Figure 27-21). Most of the double bonds that remain are
the trans- isomer.

The hydration reaction(addition of water) is another very important addition reaction
of alkenes. It is used commercially for the preparation of a wide variety of alcohols from
petroleum by-products. Ethanol, the most important industrial alcohol, is produced indus-
trially by the hydration of ethylene from petroleum, using H 2 SO 4 as a catalyst.

H 2 SO 4

H 2 CUCH 2 H 2 O888888nH 3 CXCH 2 OH

3H 2

Ni catalyst

heat

olein (an oil, liquid) stearin (a fat, solid)

H 2 COC(CH 2 ) 7 CH CH(CH 2 ) 7 CH 3

O

H 2 COC(CH 2 ) 16 CH 3

O

O

O

HCOC(CH 2 ) 16 CH 3

H 2 COC(CH 2 ) 16 CH 3

HCOC(CH 2 ) 7 CH CH(CH 2 ) 7 CH 3

O

H 2 COC(CH 2 ) 7 CH CH(CH 2 ) 7 CH 3

O

H 2 CCH 2 Br 2 CH 2

Br Br

CH 2 1,2-dibromoethane

(ethylene dibromide)

H 2 CCH 2  Cl 2 CH 2

Cl Cl

CH 2

Figure 27-21 Hydrogenation of

the olefinic double bonds in

vegetable oil converts it to solid fat.

27-17 Addition Reactions 1089

Think of H 2 O as HOH.

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 11.4, Hydrocarbons and

Addition Reactions.