Organic Chemistry

When the large substituents are on the same side of the molecule, their electron

clouds can interfere with each other, causing strain in the molecule and making it less

stable. You saw in Section 2.11 that this kind of strain is called steric strain.When the

large substituents are on opposite sides of the molecule, their electron clouds cannot

interact, and the molecule has less steric strain.

The heat of hydrogenation of cis-2-butene, in which the two alkyl substituents are

on the same sideof the double bond, is similar to that of 2-methylpropene, in which

the two alkyl substituents are on the same carbon. The three dialkyl-substituted

alkenes are all lessstable than a trialkyl-substituted alkene and are all morestable than

a monoalkyl-substituted alkene.

PROBLEM 29

Rank the following compounds in order of decreasing stability:

trans-3-hexene,cis-3-hexene, 1-hexene,cis-2,2,5,5-tetramethyl-3-hexene

4.12 Reactions and Synthesis

This chapter has been concerned with the reactions of alkenes. You have seen why

alkenes react, the kinds of reagents with which they react, the mechanisms by which

the reactions occur, and the products that are formed. It is important to remember that

when you are studying reactions, you are simultaneously studying synthesis. When

you learn that compound A reacts with a certain reagent to form compound B, you are

learning not only about the reactivity of A, but also about one way that compound B

can be synthesized.

For example, you have seen that alkenes can add many different reagents and that, as a

result of adding these reagents, compounds such as alkyl halides, vicinal dihalides,

halohydrins, alcohols, ethers, and alkanes are synthesized.

Although you have seen how alkenes react and have learned about the kinds of

compounds that are synthesized when alkenes undergo reactions, you have not yet

seen how alkenes are synthesized. Reactions of alkenes involve the additionof atoms

(or groups of atoms) to the two carbons of the double bond. Reactions that lead to

the synthesis of alkenes are exactly the opposite—they involve the eliminationof

atoms (or groups of atoms) from two adjacent sp^3 carbons.

sp^2

AB

CC >

H 3 C H

H

alkyl substituents

are trans

CH 3

CC

H 3 C H

H 3 C

alkyl substituents

are on the same sp^2 carbon

H

CC ∼

H 3 CCH 3

H

alkyl substituents

are cis

H

relative stabilities of dialkyl-substituted alkenes

174 CHAPTER 4 Reactions of Alkenes

3-D Molecule:

cis-2-Butene

3-D Molecule:

trans-2-Butene

CC

H

H

HHH

H

C

H

C

H

cis-2-butene

CC

HH

H

C

H

H

HH

C

H

trans-2-butene

the cis isomer has steric strain the trans isomer does not have steric strain