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Acid-Base Behavior in Aprotic Organic Solvents UNITED STATES DEPARTMENT OF COMMERCE • C. R. Smith, Secretary NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director Acid-Base Behavior in Aprotic Organic Solvents Marion Maclean Davis Institute for Materials Research National Bureau of Standards Washington, D.C. 20234 , 105 . U^S^ National Bureau of Standards.Monograph r » » Issued August 1968 For sale by the Superinlendent <if Documents. U.S. Government Printing Office Washington, D.C. 20402 - Price $2.25 NOV 2 9 1368 1^4 2 5 '46 Library of Congress Catalog Card No. 67-62078 Foreword During the past 50 years the American chemical industry has made available for common uses a great variety of organic solvents, in some of which acids and bases behave very differently than in water. For example, the order of relative strengths of a series of acids or bases may be altered by a change of solvent. This is especially evident when acid-base behavior in water is compared with that in hydrocarbons and halogenated hydrocarbons. To the latter two groups, which are often called "inert" or "aprotic" solvents, belong important liquids like benzene, toluene, cyclo- hexane, and carbon tetrachloride. Industrially important materials such as drycleaning solvents, lubricants, motor fuels, refrigerants, and transformer oils are additional examples. From 1941 to the end of 1965 the National Bureau of Standards maintained, in response to requests from industry and other Government agencies, a research program designed to ascertain and explain acid-base behavior in aprotic organic solvents, as well as to develop methods and reference materials for determining total acid and base content and relative strengths of acids and bases in such media. Supplementing the experimental program, a record was maintained of rele- vant work being performed in other laboratories. The Office of Naval Research and the Air Force Office of Scientific Research contributed financial support. This Monograph consists of a comprehensive, critical survey of present knowledge in this area, based on numerous kinds of chemical and physical measurements. More than 90 tabulations of data are included. The conclusions reached have broad scientific applicability, and the Monograph will be a useful guide both for those concerned with practical chemical programs and those en- gaged in fundamental researches. Its detailed discussion of the varied types of hydrogen bonds which form in aprotic media will be valuable to scientists attempting to develop an improved theory of hydrogen bonding. A. V. ASTIN, Director. iii Contents Page Page 1. Introduction 1 b. The differential vapor pressure (DVP) method... 51 2. Acid-base concepts 2 4.3.4. Spectral absorbance in the visible and ultraviolet 2.1. Present-day concepts 2 regions 52 2.2. Formulation of acid-base reactions in ionizing a. Bromophthalein magenta 53 solvents 2 b. Halogen derivatives of phenolsulfonephthalein... 58 2.3. Formulation of acid-base reactions in nonionizing c. Nitrophenols 58 solvents 3 d. a-Naphthol, jS-naphthol, and phenol 61 2.4. Hantzsch's views 4 4.3.5. Spectral absorbance in the infrared 62 2.5. The role of the solvent in ionization 4 a. Measurements of crystals....- 62 3. Classification of organic solvents 5 b. Results using dry aprotic solvents 64 3.1. Dielectric constants of pure solvents 5 c. Results with wet aprotic solvents 73 3.2. Dielectric constants of mixed solvents 6 d. Further discussion 75 3.3. Br0nsted's eight classes 8 4.3.6. Nuclear magnetic resonance spectroscopy 76 3.4. Comparisons of aprotic solvents 9 a. Measurements of crystals 76 3.4.1. Aromatic solvents as bases 9 b. Studies of solutions 76 3.4.2. Unsaturated aliphatic hydrocarbons as bases 11 4.3.7. Concluding discussion of hydrogen-bonded ion 3.4.3. Halogen compounds as bases 12 pairs 77 a. Alkyl halides 12 4.4. Hydrogert bonding of cations to neutral proton b. Carbon tetrachloride 13 acceptors 79 3.4.4. Carbon tetrachloride as an electron acceptor 13 4.4.1. Introduction 79 3.4.5. C-H containing compounds as proton donors 13 4.4.2. Solvated cations 79 a. Chloroform and related solvents 13 a. Solvated hydronium ions 79 b. Acetylenic hydrocarbons 14 b. Miscellaneous solvated cations 81 c. Other studies of C-H containing solvents 16 4.4.3. Homoconjugate cations 82 3.4.6. Chemical reactivity of halogenated hydro- a. Studies using aprotic or dipolar aprotic solvents... 82 carbons 16 b. Studies of solid salts of N-oxides 85 3.4.7. "Dipolar aprotic" solvents 17 4.4.4. Miscellaneous heteroconjugate cations 87 3.4.8. Concluding discussion of solvents 18 4.5. Hydrogen bonding of anions to neutral proton 4. The role of hydrogen bonding in aprotic media 19 donors 88 4.1. Self-association of nitrogen-containing bases 19 4.5.1. Introduction 88 ' 4.1.1. Aliphatic amines : 20 4.5.2. Homo- and heteroconjugation of OH ions 89 4.1.2. Aromatic amines 21 4.5.3. Homoconjugate aliphatic and aromatic car- 4.1.3. Amidines, guanidines, and miscellaneous heter- boxylate ions 89 cyclic bases 22 a. Solid acid carboxylates 89 4.2. Self-association of acids 24 b. Homoconjugation of carboxylates in solutions 91 4.2.1. Phenols 25 4.5.4. Homoconjugate phenolate ions 97 a. Self-association of phenol in aprotic solvents 25 a. Examples of homoconjugation 97 b. Hindered phenols 25 b. Examples of non-homoconjugation 100 c. Intramolecular hydrogen bonding in phenols 28 4.5.5. Homoconjugation of halide ions 102 d. The deuterium isotope effect in phenols 30 a. Homoconjugation in solutions 102 4.2.2. Carboxylic acids 31 b. Homoconjugation of X~ in solids 104 a. General discussion 31 4.5.6. Homoconjugation of other organic and inorganic b. Dimerization constants 31 acids c. Hetero-association 34 a. Perchloric acid 105 d. Solvent effects on dimerization 35 b. Other acids 105 e. Chelated acids 35 4.5.7. Heteroconjugate anions 105 f. Thermodynamic constants 36 a. Chemical behavior 105 g. Possible equilibrium between open-chain and b. Physical measurements 108 cyclic dimers 37 4.6. Concluding discussion of hydrogen bonding 112 h. Thiolobenzoic acid 37 4.6.1. Definition of a hydrogen bond. Systems which 4.2.3. Nitric acid 37 form hydrogen bonds 112 4.2.4. Phosphorus-containing organic acids 38 4.6.2! Some recent concepts of hydrogen bonding 113 a. Nature of aggregates formed 38 5. Acidity and basicity scales in aprotic organic solvents... 114 b. Solvent effects 38 5.1. Early attempts to determine relative acidities 114 c. Monomer-dimer equilibrium constants 39 5.2. Log Kbha scales of acidity and basicity 115 4.2.5. Sulfinic and sulfonic acids 40 5.3. Methods used in determining values of log A^bha 116 a. Sulfinic acids, R-S(= 0)0H 40 5.4. Tables of log ^bha, and AS obtained using aprotic b. Sulfonic acids, K-SO2OH 40 solvents 118 4.2.6. tert. -Butyl hydroperoxide and deuteroperoxide. 5.4.1. Aromatic and substituted aromatic hydrocar- Deuterium isotope effects on hydrogen bond- bons 118 ing by Br0nsted acids 40 a. Benzene 118 4.3. Hydrogen-bonded ion pairs 41 b. Anisole and chlorobenzene 123 4.3.1. Conductance 41 c. Miscellaneous aromatic solvents 125 a. Walden's work 42 5.4.2. Aliphatic hydrocarbons 126 b. Wynne-Jones' postulate 42 5.4.3. Carbon tetrachloride 126 c. Investigations of Kraus, Fuoss, and associates... 43 5.4.4. Chloroform 128 d. Summary 45 5.4.5. Miscellaneous aprotic solvents 129 4.3.2. Dielectric constants 46 6. Acid-base titrations in completely aprotic solvents 133 a. Dielectric polarization of substituted ammonium 6.1. Introduction 133 picrates and halides in benzene 46 6.2. Titrations with indicator dyes in completely aprotic b. Dielectric polarization of other acid-base com- solvents 133 plexes in benzene 47 6.3. Instrumental procedures for detecting end-points in 4.3.3. CoUigative properties 49 completely aprotic solvents 133 a. Cryoscopy 50 6.3.1. Conductance titrations 133 iv Contents — Continued Page Page 6.3.2. Dielectrometric titrations 133 b. p-Toluenesulfonic acid 135 6.3.3. Photometric titrations 134 c. Picric acid 135 6.3.4. Thermometric titrations 135 d. Trichloroacetic acid 135 6.3.5. Titrations by the DVP method 135 e. Diphenyl phosphate 135 6.3.6. Cryoscopic titrations 135 6.4.2. Bases 135 6.3.7. Other possible instrumental procedures 135 a. Aliphatic amines 135 a. Refractive index 135 b. Amidines 136 b. Density 135 c. Derivatives of guanidine 136 c. Optical rotation 135 6.4. Reference acids and bases for aprotic organic 7. List of references 137 solvents 135 6.4.1. Acids 135 a. Hydrogen chloride 135 8. Author index 145 V Acid-Base Behavior in Aprotic Organic Solvents Marion Maclean Davis A unified picture of acid-base behavior in aprotic organic solvents is presented, based on an extensive survey of the Hterature and experimental results of the author and associates. Evidence given to support this picture includes data pertaining to colligative properties of acids, bases, and salts and also conductance, dielectric constants, distribution between immiscible solvents, and spec- tral absorption in the infrared, visible, and ultraviolet. The acids upon which attention is centered are proton-donor compounds that are measurably ionized in water, such as aliphatic and aromatic carboxylic acids, substituted phenols, and mineral acids. The bases of principal interest are likewise compounds capable of forming ions in water, for example, aliphatic and aromatic amines and derivatives of guanidine or pyridine. The solvents emphasized are hydrocarbons and halohydrocarbons, but data for dipolar aprotic solvents (for example, acetone, acetonitrile, and nitrobenzene) are included. Con- trasts in acid-base behavior and in acidity and basicity scales in aprotic and water-like solvents are discussed.
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