Organic Chemistry

PROBLEM 7

Under what circumstances can you assume that the less stable reactant will be the more

reactive reactant?

Why is the transition state for the first step of an electrophilic addition reaction for

an alkyne less stable than that for an alkene? The Hammond postulate predicts that the

structure of the transition state will resemble the structure of the intermediate

(Section 4.3). The intermediate formed when a proton adds to an alkyne is a vinylic

242 CHAPTER 6 Reactions of Alkynes • Introduction to Multistep Synthesis

Free energy

Progress of the reaction

∆G‡alkyne

alkene

vinylic

cation

alkyne

alkene

∆G‡ alkyl cation

an alkyne is less

reactive than an alkene

an alkyne is less

stable than an alkene

Figure 6.2N

Comparison of the free energies of

activation for the addition of an

electrophile to an alkyne and to an

alkene. Because an alkyne is less

reactive than an alkene toward

electrophilic addition, we know

that for the reaction of an

alkyne is greater than the for

the reaction of an alkene.

¢G‡

¢G‡

proton. In the second step of the reaction, the positively charged carbocation interme-

diate reacts rapidly with the negatively charged chloride ion.

Thus alkynes, like alkenes, undergo electrophilic addition reactions. We will see

that the same electrophilic reagents that add to alkenes also add to alkynes and that—

again like alkenes—electrophilic addition to a terminalalkyne is regioselective: When

an electrophile adds to a terminal alkyne, it adds to the spcarbon that is bonded to the

hydrogen. The addition reactions of alkynes, however, have a feature that alkenes do

not have: Because the product of the addition of an electrophilic reagent to an alkyne

is an alkene, a second electrophilic addition reaction can occur.

An alkyne is lessreactive than an alkene. This might at first seem surprising because

an alkyne is less stable than an alkene (Figure 6.2). However, reactivity depends on

which in turn depends on the stability of the reactant andthe stability of the tran-

sition state (Section 3.7). For an alkyne to be both less stable and less reactive than an

alkene, two conditions must hold: The transition state for the first step (the rate-limiting

step) of an electrophilic addition reaction for an alkyne must be less stable than the tran-

sition state for the first step of an electrophilic addition reaction for an alkene,andthe

difference in the stabilities of the transition states must be greater than the difference in

the stabilities of the reactants so that ¢Galkyne‡ 7¢Galkene‡ (Figure 6.2).

¢G‡,

CH 3 CCHCCH 3 CH 3 C CHCH 3 3 CCH 2 CH 3

Cl Cl

Cl

HCl HCl

a second electrophilic

addition reaction occurs

CH 3 C CCH 3 + H Cl CHCH 3 + Cl CH 3 C CHCH 3

+ −

CH 3 C

Cl

Alkynes are less reactive than alkenes in

electrophilic addition reactions.

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