Organic Chemistry

372 CHAPTER 10 Substitution Reactions of Alkyl Halides

10.4 The Reversibility of an Reaction

Many different kinds of nucleophiles can react with alkyl halides. Therefore, a wide

variety of organic compounds can be synthesized by means of reactions.

It may seem that the reverse of each of these reactions can also occur via nucleophilic

substitution. In the first reaction, for example, ethyl chloride reacts with hydroxide ion

to form ethyl alcohol and a chloride ion. The reverse reaction would appear to satisfy the

requirements for a nucleophilic substitution reaction, given that chloride ion is a nucle-

ophile and ethyl alcohol has an leaving group. But ethyl alcohol and chloride ion

do notreact.

Why does a nucleophilic substitution reaction take place in one direction, but not in

the other? We can answer this question by comparing the leaving tendency of in

the forward direction and the leaving tendency of in the reverse direction. Com-

paring leaving tendencies involves comparing basicities. Most people find it easier to

compare the acid strengths of the conjugate acids (Table 10.3), so we will compare the

acid strengths of HCl and HCl is a much stronger acid than which means

that is a much weaker base than (Remember, the stronger the acid, the

weaker is its conjugate base.) Because it is a weaker base, is a better leaving

group. Consequently, can displace in the forward reaction, but cannot

displace in the reverse reaction. The reaction proceeds in the direction that al-

lows the stronger base to displace the weaker base (the best leaving group).

If the difference between the basicities of the nucleophile and the leaving group is

not very large, the reaction will be reversible. For example, in the reaction of ethyl bro-

mide with iodide ion, is the leaving group in one direction and is the leaving

group in the other direction. The reaction is reversible because the values of the

conjugate acids of the two leaving groups are similar ( of of

see Table 10.3).

You can drive a reversible reaction toward the desired products by removing one of

the products as it is formed. Recall that the concentrations of the reactants and

products at equilibrium are governed by the equilibrium constant of the reaction

CH 3 CH 2 Br ++I− CH 3 CH 2 I Br−

an SN2 reaction is

reversible when the

basicities of the leaving

groups are similar

HI=-10;

pKa HBr=-9; pKa

pKa

Br- I-

HO-

HO- Cl- Cl-

Cl-

Cl- HO-.

H 2 O. H 2 O,

HO-

Cl-

HO-

N N

CR

an alcohol

a thiol

CH 3 CH 2 Cl ++HO− CH 3 CH 2 OH Cl−

CH 3 CH 2 Br ++HS− CH 3 CH 2 SH Br−

an ether

a thioether

CH 3 CH 2 ICH++RO− 3 CH 2 OR I−

CH 3 CH 2 Br ++RS− CH 3 CH 2 SR Br−

a primary amine

CH 3 CH 2 Cl ++−NH 2 CH 3 CH 2 NH 2 Cl−

an alkyne

a nitrile

CH 3 CH 2 Br ++−C CH 3 CH 2 CBrCR −

CH 3 CH 2 ICH++−C 3 CH 2 CI−

SN 2

SN 2

An reaction proceeds in the
direction that allows the stronger
base to displace the weaker base.

SN 2