1 SULFONATE AND INORGANIC ESTER DERIVATIVES OF ALCOHOLS 443. |. R2C L CR2. ". H. carbocation X_. Lewis acid base association Br nsted acid base (halide ion) H2O. reaction R2C L CR2 $R. " ". $. R. C A C + H3O|. $ $. X H ( ). alkyl halide R R. alkene Notice that the principles you've studied in Chapter 9 for the substitutions and eliminations of alkyl halides are valid for other functional groups in this case, alcohols. SULFONATE AND INORGANIC ESTER . DERIVATIVES OF ALCOHOLS. the LOH group into a good leaving group.
2 However, if the alcohol molecule contains a group When an alkyl halide is prepared from an alcohol and a hydrogen halide, protonation converts must be used for other reasons, different ways of converting the LOH group into a good leav- that might be sensitive to strongly acidic conditions, or if milder or even nonacidic conditions ing group are required. Methods for accomplishing this objective are the subject of this section. A. SULFONATE ESTER DERIVATIVES of Alcohols Structures of SULFONATE Esters An important method of activating alcohols toward nu- cleophilic substitution and b-elimination reactions is to convert them into SULFONATE esters.
3 SULFONATE esters are DERIVATIVES of sulfonic acids, which are compounds of the form RL SO3H. Some typical sulfonic acids are the following: cL SO3H H3C LcL SO3H. H3C L SO3H. methanesulfonic acid benzenesulfonic acid p-toluenesulfonic acid (The p in the name of the last compound stands for para, which indicates the relative positions of the two groups on the benzene ring. This type of nomenclature is discussed in Chapter 16.). A SULFONATE ESTER is a compound in which the acidic hydrogen of a sulfonic acid is replaced by an alkyl or aryl group.
4 Thus, in ethyl benzenesulfonate, the acidic hydrogen of benzenesul- fonic acid is replaced by an ethyl group. O O. cL S L O L H cL S L O L C2H5. S S. acidic hydrogen S S. O O. benzenesulfonic acid ethyl benzenesulfonate (a SULFONATE ESTER ). 444 CHAPTER 10 THE CHEMISTRY OF ALCOHOLS AND THIOLS. Sulfur has more than the octet of electrons in these Lewis structures. Such octet expansion is com- mon for atoms in the third and higher periods of the periodic table. Bonding in sulfonic acids and their DERIVATIVES is discussed further in Sec.
5 Organic chemists often use abbreviated structures and names for certain SULFONATE esters. Esters of methanesulfonic acid are called mesylates (abbreviated RL OMs), and esters of p- toluenesulfonic acid are called tosylates (abbreviated RL OTs). O. C2H5 L O L S L CH3 C2H5 L OMs S. is the same as S. O ethyl mesylate ethyl methanesulfonate O. CH3CH2 L CH L O L S L cL CH3 CH3CH2 L CH L OTs S. " ". is the same as S. CH3 O CH3. sec-butyl p-toluenesulfonate sec-butyl tosylate PROBLEM. Draw both the complete structure and the abbreviated structure, and give another name for each of the following compounds.
6 (a) isopropyl methanesulfonate (b) methyl p-toluenesulfonate (c) phenyl tosylate (d) cyclohexyl mesylate Preparation of SULFONATE Esters SULFONATE esters are prepared from alcohols and other sulfonic acid DERIVATIVES called sulfonyl chlorides. For example, p-toluenesulfonyl chloride, often known as tosyl chloride and abbreviated TsCl, is the sulfonyl chloride used to prepare tosylate esters. O. CH3(CH2)9OH + Cl L S L cL CH3 +. S. S. 2. 1-decanol O N. pyridine p-toluenesulfonyl (used as solvent). chloride (tosyl chloride.)
7 A sulfonyl chloride). O. CH3(CH2)9O L S L cL CH3 +. S. Cl_ ( ). S |. ". O N. Further Exploration Mechanism of SULFONATE ESTER decyl tosylate H. Formation (90% yield). This is a nucleophilic substitution reaction in which the oxygen of the alcohol displaces chloride ion from the tosyl chloride. The pyridine used as the solvent is a base. Besides catalyzing the re- action, it also neutralizes the HCl that would otherwise form in the reaction (color in Eq. ). SULFONATE AND INORGANIC ESTER DERIVATIVES OF ALCOHOLS 445.
8 PROBLEM. Suggest a preparation for each of the following compounds from the appropriate alcohol. (a) cyclohexyl mesylate (b) isobutyl tosylate Reactivity of SULFONATE Esters SULFONATE esters, such as tosylates and mesylates, are useful because they have approximately the same reactivities as the corresponding alkyl bro- mides in substitution and elimination reactions. (In other words, you can think of a tosylate or mesylate ESTER group as a fat bromo group.) The reason for this similarity is that SULFONATE anions, like bromide ions, are good leaving groups.
9 Recall that, among the halides, the weak- est bases, bromide and iodide, are the best leaving groups (Sec. ). In general, good leav- ing groups are weak bases. SULFONATE anions are weak bases; they are the conjugate bases of sulfonic acids, which are strong acids. O O. 2 L S L cL CH3 _3 O L S L cL CH. 2 _3 OTs 2. S S. 2 S 2 S 2. HO 3 or O O. p-toluenesulfonic acid: p-toluenesulfonate anion a strong acid (tosylate anion): (pKa < 1) a weak base Thus, SULFONATE esters prepared from primary and secondary alcohols, like primary and sec- ondary alkyl halides, undergo SN2 reactions in which a SULFONATE ion serves as the leaving group.
10 _. Nuc 3 CH2 L OTs 21 Nuc L CH2 + 3 OTs 21. _. " ". ( ). R R. nucleophile tosylate leaving group Similarly, secondary and tertiary SULFONATE esters, like the corresponding alkyl halides, also undergo E2 reactions with strong bases, and they undergo SN1 E1 solvolysis reactions in polar protic solvents. Occasionally we'll need a SULFONATE ESTER that is much more reactive than a tosylate or me- group is nicknamed the triflate group and it is abbreviated L OTf. sylate. In such a case a trifluoromethanesulfonate ESTER is used.