PROPERTIES OF SUBSTANCES LLOYD N. FERGUSON Howard University, Washington, D. C. S T u n m T s often learn during their second or third year of college chemistry that hydrogen honds are of sufficient strength to cause noticeable effects upon certain properties of substances. For instance, the relatively high boiling points of hydroxylic compounds is attributed t o an association of the liquid molecules by means of hydrogen honds. An extra amount of energy is required to separate the liquid molecules from each other in vaporization. Although students encounter hydrogen bonding two or three times in the elementary o r g a ~ cchemistry course, few are aware of the variety of physical properties which are affected by hydrogen bonds. It is hoped that this paper may provide a brief interpretive survey of these various properties and thereby serve t o emphasize the significance and prevalence of hydrogen bonding. The treatment will be confined to the presentation in one discussion of information hitherto scattered in the literature. The reader should consult other references1 for discussions of the theory and general characteristics of hydrogen bonds themselves. As an introduction and for common understanding, it can be said that hydroxylic and amino compounds associate, primarily in the liquid and solid states. They form polymeric aggregates with a pattern, for example, like that of alcohols
where R represents an alkyl group and n is an indefinite number. By contrast, a few classes of compounds, notably the carboxylic acids, amides, and oximes, tend to form dimers predominantly. 0--H4
// R-C \
\
//C-R 0-H--0 Carboxylic acid dimer
/
R&=N~
0-H
o-Fluaraphenol
Dibenzoylmethane
chelatia while the intermolecular hydrogen bonding is referred to as association. Hydrogen bonds occur only when the hydrogen atom is covalently bonded to a highly electronegative atom. I n such cases, the covalent bonds have much ionic character with the hydrogen atom carrying a formal positive charge. It thus has a coulombic attraction for a negatively-charged atom, the hydrogen bond owing its strength to this attraction. Normally, the C-H bond is not sufficiently polarized t o produce a hydrogen bond. Two exceptions are in HCN and HCCl,, where the electron distributions in the molecules are such as to make carbon quite electronegative. This gives the C-H bonds considerable ionic character. The result is that a highly associated state for HCN and a feebly associated state for HCCla exists due to the hydrogen honds. TRANSITION TEMPERATURES
The effect of association upon boiling points is easily illustrated. For example, ethyl alcohol, which is associated, has a higher boiling point (7g0) than its functional isomer, dimethyl ether (b. p. = -25"). The latter compound cannot form the hydrogen bonds to bind ether molecules together. Similarly, methyl and ethyl esters of carboxylic acids boil a t lower temperatures than the respective carboxylic acids, in spite of the fact that the esters have the larger molecular weights. In this case, the acids are associated, while the esters are monomeric and vaporize more easily. For the same reasons, propylenimine boils at a higher temperature than its isomer, N-methylethylenimine, because the molecules of the former compound are associated through N-H---N bonds.
'..
CHa
CH-CH
/N=CR2 H-0 Oxime dimer
I
/
I
Hydrogen bonding may occur between two atoms of H CH, the same molecule as in o-fluorophenol or in a ,%diketone. Propylenimine, b. p. = 67" N-Methylethylenimine, b. p. = 270 his intramolecular hydrogen bonding is a form of Chelation has just the opposite effect upon transition For example, nitration of a substance 1 F E R G ~ S ~ N L. , N., c t ~ l e o t r o structures n of organio ~ ~ temperatures. 1 ~ . usually brings about an elevation in its boiling point, cules,"Prentice-Hall, Inc., New York, 1952, p. 52-68. 267
JOURNAL OF CHEMICAL EDUCATION
but the nitration of resorcinol in the 2 position, and of catechol in the 3 position actually lowers their boiling point^.^ These decreases are due to their chelated structures. H
where practically only monomeric and dimeric species are present at low pressures. The data permit a determination of the dimer-monomer equilibrium constants as well as an evaluation of the energy and eutropy of association. WATER SOLUBILITY
The sublimation temperatures of chelated compounds are usually lower than those of associated compounds and this often permits their separation. For example, 4(7)-nitrobenzimidazole sublimes a t atmospheric pressure whereas its isomer, 5(6)-nitrobenzimidazole, does not.' The 4(7) isomer is chelated and therefore is less associated than the 5(6) isomer which cannot chelate. Cryoscopic measurements also verify that the 4(7) isomer is less associated in solution.
Reciprocal hydrogen honding between the OH group of hydroxylic compounds and water makes the hydroxylic substances tend to dissolve in water. As long as a compound has a large ratio of OH groups t o hydrocarbon groups, it will have a significant solubility in water. Thus, sugars and certain starches are very soluble in water, and even polyvinyl alcohol, a polymer of reasonably large molecular weight,
is quite soluble in water. Too, the success of nonionic detergents is due to the powerful solubilizing effect of hydroxyl groups. They are usually derivatives of diand triethanolamines or of the condensation products 0 - 0 of ethylene oxide with alcohols or phenols. Unlike the kN& H 0 H cationic or anionic "soaps," where the water-soluble portions of the molecules are ionic, the water-soluble portions of the nonionics contain neutral hydroxyl and amino groups which dissolve in water primarily as a result of hydrogen honding. Liquid ethers and esters, of course, are nonassociated which makes them have much lower boiling points than 4(7)-Nitrobeneirnidamle 5(6)-Nitrobenzimidazole their isomeric alcohols and acids. Water may form VAPOR PRESSURES hydrogen bonds with ethers and esters but ethers and At low pressures, a normal vapor will obey fairly esters do not have groups for also forming hydrogen closely the ideal gas law, P V = nRT, where n is the bonds with water. The lack of co-association of these number of moles of gas present. However, when a classes of compounds with water may or may not have substance is associated, there are fewer independent a significant effect upon their water solubilities (Tables molecules present and the vapor pressure or the vapor 1 and 2). I n the case of the ethers, the one-way hydrodensity is altered. Quantitative measurements of as- gen honding is sufficient to give dialkylethers and alsociation can be made by determining the deviation cohols comparable water solubilities, but not so for the from perfect-gas behavior. Many such studies have esters and acids. The failure to co-associate with water been madea4 particularlv for carboxylic acid vapors causes the esters to have much lower water solubilities HUNTER,L., Report of a Symposium on The Hydrogen than their isomeric acids. Bond, The Royal Institute of Chemistry, London, 1949, p. 1. It will he noted that if the OH group of an hydroxylic ~ A B I N G ~ I T ZJ ,. L., AND E. C. WAGNER, J . Am. Chem. Soe., compound is tied up by chelation, association with water 73,3030 (1951 1. T ~ a o n M. , D., ET AL., J . Am. Chem. Soc., 74, 4151 (1952). is hindered and the water solubility then approaches that of a similar compound which lacks the OH group. L. K. NASH,ET AL.. J. Am. Chem. Soe., 74,4654 (1952). TABLE 1 Boiling Points and Water Solubilities at Room Temperature of Some Isomeric Alcohols and Ethers*
Alcohol
B. p. ( T . )
Ether ~olubilit~~
R
* IT.)
~olutri~itg, %
n-Butyl 118 9 Diethyl 35 n-Pentyl 138 2.7 Ethvl n - ~ r o ~ v l 62-3 n-Hexyl 157 0.6 91 Isohexyl 158 v. 51. s. Diisobrobjil 69 Isohexyl 158 v. 81. 8. n-Propyl isopropyl 83 bOotyl 195 81. 8. Di-n-butyl 142 From LANGE, N. A,, "Hmdbwk of Chemistry," 6th ed., Handbook Publishers, Inc., Sandusky, Ohio, 1946.
7.5 sl. a. sl. s. 0.2 0.5 0.05
VOLUME 33, NO. 6, JUNE, 1956
One practical application of this contrasting effect of chelation and association upon physical properties is made in the separation by steam distillation of a mixture of ortho and para oxy- or amino-carbonyl or nitro compounds. In the case of the ortho isomers, association of the OH group with water molecules is diminished owing to its chelation with the adjacent carbonyl or H
\f
0
0-0
W
I
Salicylate ion
2,6'-Dihydrouybensoste ion
H\c/
C/I \
e
HO C ''
C
'?-
C ''
0-
/
\H
H
0 II
I
A
C
II
Maleate ion
Fumrtrate ion
for m a k acid. The COO- group of the fumarate ion has only a mild electrostatic effecton the dissociation of the second COOH group. BY contrast the maleate ion is*greatly stabilized by hydrogen bonding. This impedes the dissociation of the second proton and gives a large K 1 / K 2ratio. STEREOISOMERISM
When the anion of an acid isstabilized by intramolecular hydrogen bonding, there is a marked increase in the dissociation of the acid.* For illustration, the ionization constants ( X 105) for the isomeric hydroxy- and methoxybenzoic acids in water are:
Hydrogen bonding may be a strong factor in determining the spatial collfiguration of & molecule or in hindering isomerisation. For example, the trans configuration of indigo is stabilized by hydrogen bonds to
Para 2.9 3.4
It is apparent that the o-hydroxy isomer has an abnormally large dissociation. It is the only acid in this group whose anion has a structure which is stabilized by chelation. Its dissociation is 17 times that of benzoic acid, while that of 2,6'dihydroxyhenzoic acid is 800 times as large, owing to an even greater stabilization by hydrogeu bonding. For the same reason, the ratio of the first to the second ionisation constants of dicarboxylic 6
II
H
0
IONIZATION O f CARBOXYLIC ACIDS
Meta 8.3 8.2
b l b
0
nitro groups. Consequently, the ortho isomers are only sparingly water soluble and are much more volatile than the para isomers. As a combined result of decreased water solubilities and increased vapor pressures the nrtho isomers can be distilled so much more rapidly. than the para isomers that a practical separation of two isomers may be made. An easy method of differentiating between chelated and associated compounds consists of determining the differences in melting points of a substance when dry and when in with ~h~ depression in melting point when wet is much smaller for chelated than for associated compounds.
Ortho 105 8.1
/\p, C H
H
1 '
/
Hydmxybenzoic acid Methoxybenzoic acid
0.. 0
\ C/'\ H
acids may differ greatly for two geometric isomers." Thus, the ratio K , / K 2is 23.2 for fumaric acid and 20,200
\T
