Organic Chemistry

464 CHAPTER 12 Reactions of Alcohols, Ethers, Epoxides, and Sulfur-Containing Compounds

protic polar solvent (Section 10.3). Recall that if a reactant in the rate-determining step is

charged, the reaction will be faster in a nonpolar solvent (Section 10.10).

Crown ethers have been used to dissolve many other salts in nonpolar solvents.

In each case, the positively charged ion is nestled within the crown ether and is

accompanied by a “naked”anion in the nonpolar solvent.

12.10 Thiols, Sulfides, and Sulfonium Salts

Thiolsare sulfur analogs of alcohols. Thiols used to be called mercaptans because

they form strong complexes with heavy metal cations such as mercury and arsenic

(they capture mercury).

Thiols are named by adding the suffix “thiol”to the name of the parent hydrocar-

bon. If there is a second functional group in the molecule, the SH group can be

indicated by its substituent name,“mercapto.”Like other substituent names, it is cited

before the name of the parent hydrocarbon.

Low-molecular-weight thiols are noted for their strong and pungent odors, such as

those associated with onions, garlic, and skunks. Natural gas is completely odorless

and can cause deadly explosions if a leak goes undetected. Therefore, a small amount

of a thiol is added to natural gas to give it an odor so that gas leaks can be detected.

Sulfur is larger than oxygen, so the negative charge of the thiolate ion is spread over a

larger volume of space, causing it to be more stable than an alkoxide ion. Recall that the

more stable the base, the stronger is its conjugate acid (Section 1.18). Thiols, therefore,

are stronger acids than alcohols, and thiolate ions are weaker bases than

alkoxide ions. The larger thiolate ions are less well solvated than alkoxide ions, so in

protic solvents thiolate ions are better nucleophiles than alkoxide ions (Section 10.3).

Because sulfur is not as electronegative as oxygen, thiols are not good at hydrogen bond-

ing. Consequently, they have weaker intermolecular attractions and, therefore, consider-

ably lower boiling points than alcohols have (Section 2.9). For example, the boiling

point of is 78°C, whereas the boiling point of is 37°C.

The sulfur analogs of ethers are called sulfidesor thioethers. Sulfur is an excellent

nucleophile because its electron cloud is polarizable (Section 10.3). Sulfides, there-

fore, react readily with alkyl halides to form sulfonium salts—a reaction that an ether

cannot undergo because oxygen is not as nucleophilic and cannot accommodate a

charge as easily.

Because it has a weakly basic leaving group, a sulfonium ion readily undergoes

nucleophilic substitution reactions. As with other SN 2 reactions, the reaction works best

CH 3 SCH 3 CH 3

CH 3

+ I CH 3 SCH 3 I−

+

dimethyl

sulfide

trimethylsulfonium iodide

a sulfonium salt

CH 3 CH 2 OH CH 3 CH 2 SH

CH 3 SCH 3 CH (^2) CH
3 OH

• Br CH 3 SCH 2 CH 3
  −


• Br−
  1 pKa= 102
  3-methyl-1-butanethiol
  CH 3
  CH 3 CHCH 2 CH 2 SH
  1-propanethiol
  CH 3 CH 2 CH 2 SH
  2-mercaptoethanol
  HSCH 2 CH 2 OH
  ethanethiol
  CH 3 CH 2 SH
  a thiol mercuric ion
  2 CH 3 CH 2 SH + Hg^2 + CH 3 CH 2 SSCHHg 2 CH 3 + 2 H+
  3-D Molecule:
  Methanethiol
  3-D Molecules:
  Dimethyl sulfide;
  Trimethylsulfonium cation
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