Organic Chemistry

172 CHAPTER 4 Reactions of Alkenes

The details of the mechanism of catalytic hydrogenation are not completely under-

stood. We know that hydrogen is adsorbed on the surface of the metal and that the

alkene complexes with the metal by overlapping its own porbitals with vacant orbitals

of the metal. Breaking the bond of the alkene and the bond of and forming the

C H bonds all occur on the surface of the metal. The alkane product diffuses away

from the metal surface as it is formed (Figure 4.6).

The heat released in a hydrogenation reaction is called the heat of hydrogenation.

It is customary to give it a positive value. Hydrogenation reactions, however, are

exothermic (they have negative values). So the heat of hydrogenation is the pos-

itive value of the of the reaction.

Because we do not know the precise mechanism of a hydrogenation reaction, we can-

not draw a reaction coordinate diagram for it. We can, however, draw a diagram showing

the relative energies of the reactants and products (Figure 4.7). The preceding three cat-

alytic hydrogenation reactions all form the same alkane product, so the energy of the

productis the same for each reaction. The three reactions, however, have different heats

of hydrogenation, so the three reactantsmust have different energies. For example,

3-methyl-1-butene releases the most heat, so it must be the leaststable (have the greatest

energy) of the three alkenes. In contrast, 2-methyl-2-butene releases the least heat, so it

must be the moststable of the three alkenes. Notice that the greater the stability of a com-

pound, the lower is its energy and the smaller is its heat of hydrogenation.

If you look at the structures of the three alkene reactants in Figure 4.7, you will see

that the most stable alkene has two alkyl substituents bonded to one of the carbons

and one alkyl substituent bonded to the other carbon, for a total of three alkyl sub-

stituents (three methyl groups) bonded to its two carbons. The alkene of interme-

diate stability has a total of two alkyl substituents (a methyl group and an ethyl group)

sp^2

sp^2

sp^2

¢H°

¢H°

¬ s

p s H 2

CH 3 C CHCH 3 + H 2

CH 3

2-methyl-2-butene

Pt/C

CH 3 CHCH 2 CH 3

CH 3

CH 2 CCH 2 CH 3 + H 2

CH 3

2-methyl-1-butene

Pt/C

CH 3 CHCH 2 CH 3

CH 3

CH 3 CHCH CH 2 + H 2

CH 3

3-methyl-1-butene

Pt/C

CH 3 CHCH 2 CH 3

CH 3

heat of hydrogenation

26.9 kcal/mol

28.5 kcal/mol

30.3 kcal/mol

−26.9

−28.5

−30.3

− 113

− 119

− 127

kcal/mol kJ/mol

∆H°

the alkene approaches the

surface of the catalyst

the π bond between the two

carbons is replaced by two

C H σ bonds

hydrogen molecules settle

on the surface of the catalyst

and react with the metal atoms

H

C

H

H

H

HH

H H

H H HHCC

H

H

H

C H

C

H H

C

H

H H

H

H

H

H

H

H

H

H H

Figure 4.6
Catalytic hydrogenation of an
alkene.

The most stable alkene has the smallest
heat of hydrogenation.