The Chemistry of Sugars in Boric Acid Solutions - ACS Publications

12

The

C h e m i s t r y o f S u g a r s in B o r i c

Acid

Solutions TERRY E. ACREE

Downloaded by EMORY UNIV on February 29, 2016 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1971-0117.ch012

New York State Agricultural Experiment Station, Cornell University, Geneva, Ν. Y. 14456 The reactions of boric acid solutions withdiolshave been used for almost a century to examine structural differences among carbohydrates. The complexity of these reactions seems to arise not only from simple structural differences but also from differences in carbohydrate configuration and conformation. The precise nature of these reactions is not clear. Recent studies of the chemistry of polyol-boric acid solutions have clarified some aspects of these reactions that have important bearing on the structure of carbohydrates in solution. Nevertheless, some of the most fundamental ques tions about the nature of the reaction are still unanswered. T n 1842 Biot ( I ) observed that the addition of certain sugars increased the acidity of boric acid solutions. Research during the next one hundred years revealed that this phenomenon results from the formation of sugar-boric acid esters which were stronger acids than boric acid and that there are certain configurational requirements which sugars must satisfy to react with boric acid. Reviews of the research done during this period have been presented by Roy et al. (2), Boeseken (3), Weser (4), and Nies and Campbell (5). Some of the findings of this early work are: A

1) Boric acid w i l l react with diols which satisfy certain spatial re quirements 2) For vicinal diols to react, the dihedral angle between the hy droxyl groups must be between 0° and 60°. (Using paramagnetic reso nance ( P M R ) spectroscopy, Lenz and Heeschen ( β ) reported that the complexes of D-glucose and D-xylose with boric acid had a dihedral angle of 48°. This angle was derived from the coupling constant observed for the anomeric proton, and such a derivation has yet to be substantiated. Probably a more generally useful description of the configurational re208 In Carbohydrates in Solution; Isbell, Horace S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.


12.

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Sugars in Boric Acid Solutions

209

striction of the diol is that the oxygen-oxygen distance must be between 2.2-2.5 A . Such a description also includes 1,3 diols, etc. ) 3) A t equilibrium the diol-boric acid ester or complex can have a diol to boron ratio of 1 to 1, 2 to 1, or both. 4) The rate of esterification and hydrolysis of these complexes is rapid, and equilibrium occurs within one second. Most of the work on the boric a c i d - d i o l reaction during the last twenty years has been done to determine the coordination number of the diol (number of diol molecules) i n the complex and to evaluate the equi librium constant (often called a stability constant) for a number of d i o l boric acid reactions. Several techniques have been used to study these questions, including polarimetry (7), optical rotatory dispersion (8), polarography (9), conductivity (3), vapor pressure osmometry (JO), and electrochemistry ( I I , 12, 13). The most frequently studied system has been the electrochemical ( p H ) titration of boric acid or borax solutions with various diols. During this recent research several conflicting reports and opposing hypotheses have appeared. Three questions about the diol-boric acid reaction which have been particularly baffling are discussed here. These are: 1) Does the diol react with trigonal boric acid or with the tetrahedral borate anion? 2) H o w are data from titrations with diols to be interpreted to indi cate the diol coordination number or numbers and the equilibrium constants for the reaction? 3 ) What does the diol-boric acid reaction imply about the chemistry of sugars in solution? Before examining these three questions, some of the properties of boric acid in solution should be discussed. Some Aspects of the Chemistry of Boric Acid Solutions

In concentrated solutions boric acid forms a complex mixture of polyborate ions. In such solutions data obtained from titrations are complicated by the presence of these complex ions. Aqueous boric acid solutions below 0.2M seem to have negligible amounts of polyborate ions (14). M u c h of the early data reported i n the literature was collected using more concentrated solutions (3). Thus, the ionization of boric acid can be described by the following equation: B ( O H ) + H 0 = B ( O H ) - + H+ 3

2

4

I

and the equilibrium constant is ( B » (B(OH),) (B(OH) -) 4

In Carbohydrates in Solution; Isbell, Horace S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

t t

210

CARBOHYDRATES

IN

SOLUTION

The p K value for this dissociation is about 9.14. Reports of lower values (15) may result from the formation of polyborate ions or from the pres ence of chloride ions. If a knowledge of K is important i n a particular study, the possible effect of chloride ion should be investigated before it is introduced into the system. Dilute boric acid solutions probably contain only two boroxy species, trigonal boric acid and tetrahedral borate anion.


a

The Reactive Boroxy Species. Most of the previous work on the boric a c i d - d i o l reaction has included the assumption that a diol reacts with the borate anion and not with undissociated boric acid. Whether or not this assumption is correct i n most cases would not have affected the general conclusions of the work. However, the assumption has been stated in most of the previous literature for one reason or another. A n example of some recent data which was interpreted to support this assumption is the work of Knoeck and Taylor (12). Using P M R spectra of mannitol-boric acid solutions, they observed that a decrease in p H resulted i n a decrease i n the mannitol-boric acid complex concen tration. These results are supported by B nuclear magnetic resonance ( N M R ) spectroscopy (16) which showed that the complex between mannitol and boric acid increased with increasing p H . However, Knoeck and Taylor (12) reasoned that since an increase i n p H resulted i n an increase in the borate anion concentration, as w e l l as an increase i n the complex concentration, the diol reacts only with the borate ion to produce the complex. n

To understand how these conclusions are not justified by the data used to support them, consider the overall equilibrium expressions for the two possible reactions, HB + D = BD + H

III

Β + D = BD,

IV

where H B , B, H , D , and B D are boric acid, borate ion, hydrogen ion, diol, and complex, respectively. The equilibrium constants for the two reaction schemes are ( B P ) (H) (B) (D) and


V

12.

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211


In these reactions the diol coordination number was assumed to be one. The following argument would be true no matter what the coordination number actually is. If Equations V and V I are rearranged and the dissociation equation for boric acid is substituted into V , Equations V I I and V I I I are obtained. (B)Ki K

=

(B)K

= ^

a

(BD) (D)

VII


and

2

VIII

Expressions V I I and V I I I are identical i n form; they differ only i n the meaning of the constants they contain. In both cases an increase in the borate ion concentration would result i n an increase i n the d i o l boric acid complex. Therefore, an examination of the effect of p H on the equilibrium concentrations of various components of the system can not be used to determine which of the two boroxy species actually reacts with the diol. This question is essentially a mechanistic one i n which the two candidates reaction with the diol mechanism are i n rapid equilibrium with each other. Such mechanistic questions can be more properly an swered by studying the kinetics of the reaction. A kinetic study of the reaction boric acid and tartaric acid has been reported (17). It was found that tartaric acid reacts with boric acid and not the borate ion. Also, the relaxation time for this reaction is near 20 msec. Such rapid reactions imply a low transition state energy, and mecha nisms proposed for this reaction should take this into account. A reason able mechanism can be postulated i n which the reactive species is boric acid. H

Η

I

ο

Η

I I O-D

Η-0—Β

ι

Ο—H

Η—Ο—Β—O-D

I

I

ο

I

Η

Ο

ι

Figure Η1. A mechanism for the re action between trigonal boric acid and a diol to produce a trigonal product


212

CARBOHYDRATES

IN

SOLUTION


Isotope labeling experiments indicate that the Β—Ο bond is broken and not the C—Ο bond i n the formation of the diol-boric acid complexes (18). This indicates that the initial step i n the mechanism may be an attack on the boron atom by an oxygen of the diol, followed by the release of water. This could occur without developing any charge sepa ration. If such a mechanism were correct, it would seem that an attack on the boron atom would be easier for trigonal boric acid than for the tetrahedral borate anion (Figure 1). However, it remains to be proved which of the boroxy species reacts with diols other than tartaric acid. Determination of the Coordination Number from Diol-Titration Experiments. Early in the research on the diol-boric acid reaction it was recognized that two possible complexes could form (3), one with a diol coordination number of one, the other with a coordination number of two. A generalized expression for the overall complex reaction can be written as follows, HB +

nD

n

= B D ~ + H+ n

IX

where η is the coordination number for the diol. The expression for the equilibrium condition is ( B P , - ) (H+) (HB) ( D )

χ

n

The evaluation of this equilibrium using p H titration requires the use of certain approximations. Most of the experiments in the literature have dealt with systems which consist of p H measurements of solutions con sisting of dissociable monoboric acid, a diol, and sometimes an additional cation such as sodium. W h i c h approximation is made depends upon the examination of these three conservation equations for the titration i n question. The conservation of charge: H+ + Na+ = B D „ - + Β - + O H ~

XI

The conservation of boron: Β, = B - + B D ~ + H B n

XII

The conservation of diol: D, = BD„- +

D


XIII

12.

213


ACREE

Through the years there has been some conflict i n the literature over the determination of the coordination numbers of various diol-boric acid complexes. Most of these conflicts can be resolved by making the proper approximations i n these three conservation equations as deter mined by the experimental conditions. Consider, for example, the titration of sodium tetraborate solution with a diol and suppose the following experimental conditions are met: ( N a + ) > > ( H ) and that ( D ) > > ( B , ) , where D , and B* are the total diol and boron concentrations respectively (e.g., 0.01M N a B 0 7 , 0.1-1M diol, and between p H 4-8). Applying these conditions to the conservation equations, X I becomes +

t


2

Na+ = B D ~ + B n

4

XIV

n

Substituting X I V into X I I yields B , = H B + Na+

XV

Since ( N a ) and (B ) are constants, H B is constant throughout the titration. For sodium tetraborate solutions the p H equals the p K . Sub stituting Ka for H i n the dissociation for boric acid yields +

t

a

+

B - = HB 0

where B " is the initial concentration of borate ion in the absence of diol. In the presence of diol (or more properly when the p H ( p K — 2) ), B" is about zero. Applying these last approximations to equation X I V yields 0

Na+ = B - = H B = B D " 0

n

and Equation X I I I becomes D, = D

XVI

By applying these approximations to Equation X , taking the negative logarithm, and finally rearranging, Equation X becomes p H ^ pK -

nlog(Di).

XVII

Thus, a plot of the p H vs. D w i l l give at high D* a slope (n) equal to the negative coordination number and an intercept ( p K ) equal to minus the log of the overall equilibrium constant. In many cases it has been found that complexes with coordination numbers of one and two are present at equilibrium. However, i n sodium tetraborate solution when the same general approximations are used, t

n


214

CARBOHYDRATES

IN

SOLUTION

only the resulting equations for the relationship between (D*) and ( H ) are somewhat more complicated (see Reference I I ) . M a n y of the titration experiments, however, have dealt with boric acid solutions i n which the only positive ion is H . In this case the con servation of charge Equation X I becomes +

+

H+ = B D - + B - , n

and at low p H and high D* H+ = B D " Downloaded by EMORY UNIV on February 29, 2016 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1971-0117.ch012

n

and D, = D, B, = H B . Therefore, Equation I X becomes p

H

=

_

(log*,, + log(BO) _

η

l o g D

The Chemistry of Sugars in Boric Acid Solutions - ACS Publications

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