I
L.
HOWARD STEINBERG and D.
HUNTER
Research Division, United States Borax & Chemical Corp., Los Angeles 5, Calif.
reparation and Rate of Hydrolysis of Boric Acid Esters
,The possibility of preparing stable esters b y consideration of the stereochemical requirements in the hydrolysis prompted the work reported which is concerned with the synthesis of the boric acid esters from 38 primary, secondary, and tertiary alcohols, glycols, and phenols and the determination of their rates of hydrolysis in water and aqueous dioxane. Six new esters of high hydrolytic stability are included.
HSBOB
B20j
literature contains numerous references to the preparation of organic esters of boric acid utilizing a wide variety of alcohols, glycols, and phenols ( 7 7 , 73, 75, 77, 27, 25, 30, 34). Many of the references include isolated and qualitative statements as to the hydrolyric stability of the esters (75, 77, 25, 26, 34). Scattergood and coworkers (30) have formulated some generalizations as to the rate of hydrolysis of some aliphatic esters based on qualitative observations, and more recently isolated examples of quantitative data have appeared (6, 70). T H E
Preparation of Esters
+ 3ROH
+
+ 3ROH
-+
(RO)zB
r
+ 3H20
+ H8BO.j
Excess alcohol and added toluene or benzene were then removed by distillation and the residual ester was purified by distillation or recrystallization. Wherever necessary, manipulations werc performed in a dry box. Table I records the preparative data. Boron analyses were performed by hydrolysis and titration of boric acid in the presence of mannitol (35). Because of prolonged periods of reflux necessary for complete hydrolysis, the stable esters. 21, 23, 26-32, and 42, were analyzed by fusion with sodium carbonate at 850" C., solution in water, and titration of the boric acid. Triethanolamine borate, triisopropanolamine borate, and the phenolic borates, 36-40, were also
The general procedure for the preparation of the esters involved the reaction of boric acid with the appropriate alcohol or phenol.
analyzed by the fusion method to avoid interference of the amine or phenols in the final titration. In the case of the two alkanolamine borates, the samples were dissolved in concentrated hydrochloric acid for 72 hours prior to the carbonate fusion. The melting and boiling points of 15, 16, 19, 23, 26-31, and 42 are determined by and vary with the stereoisomers present. The structure of the trialkyl borates is well documented. However, only partial evidence exists on the structure of the glycol biborates. Both tri-(hexylene glycol) biborate (2-methyl-2,4pentanediol) and tri-(octylene glycol) biborate (2,5 - dimethyl - 2,5 hexanediol) are capable of existence in two forms (I and 11). One indication that structure I is the favored form is the failure to produce a biborate from 2,5-dimethyl-3-hexyne2,5-diol. only polymeric material resulting. The acetylenic diol is incapable of structure I because the linearity of the triple bond prevents 7-membered ring formation (38), but it could form the 14-membered rings of structure 11. Further evidence indicates that 1,3diols lead to structure I, Trhereas 1,4diols give structure 11. Partial hydrolysis of the biborate from 2-methyl2,4-pentanediol led to the isolation of 111, whereas the corresponding 7-membered ring could not be demonstrated to be present from the partial hydrolysis of the biborate from 2,5-dimethyl-2,5hexanediol.
-
CHI CHa C H s C/ H \/
I
O\
R
(R0)jB
The Lvater of the reaction bras removed as (A) a binary azeotrope with excess alcohol (2); (B) a binary azeotrope with added toluene; or (C) a ternary azeotrope with excess alcohol and benzene (21). A packed column (12 X 3 j 4 inches) was employed to remove the ternary azeotrope effectively. Excess boron oxide (D) was also employed in place of the boric acid, in which case the water of the reaction is consumed by the excess reagent (37).
0-b
B--0-R-
'
0
R
B/ I
'0 I
174
INDUSTRIAL AND ENGINEERING CHEMISTRY
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u; VOL. 49, NO. 2
FEBRUARY 1957
175
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176
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Relative Rates of Hydrolysis Table II. Rate of Hydrolysis of Boric Acid Esters in 60% Aqueous Dioxane at 2 1 Initial Concn., Reactants, Mole/Liter Run No. 1 2 3 4 5 6 7 8
9 10 I1 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
a
Ester
Ester
Trimethyl borate Triethyl borate Tri-n-propyl borate Triisopropyl borate Tri-( 1,3-dichloro-2-propyl) borate Tri-n-butyl borate Triisobutyl borate Tri-(P,i3,@-trichloro-terL-butyl)borate Tri-(hexylene glycol) biborate Triphenyl borate Tri-o-chlorophenyl borate Tri-o-cresyl borate Tri-(o-phenylphenyl) borate Tri-(0-cyclohexylphenyl) borate Tri-n-amyl borate Tri-(octylene glycol) biborate Tri-n-hexyl borate Tri-sec-butyl borate Tri-( 1-ethynylcyclohexyl) borate Tri-n-octyl borate Trioleyl borate Tri-n-dodecyl borate Tristearyl borate Tri-2-octyl borate Tri-(2-ethylhexyl) borate Tri-(methylisobutylcarbinyl) borate Tri-tert-butyl borate Tri-3-pentyl borate Tri-3-heptyl borate Tri-(2,6,8-trimethyl-4-nonyl)borate
0.0238 0.0216 0.0246 0.0231 0.0284 0.0238 0.0252 0.0088 0.0123 0.0235 0.0168 0.0247 0.0075 0.0142 0,0238 0.0107 0.0238 0.0230 0.0130 0.0238 0.0078 0.0128 0.0086 0.0110 0.0128 0.0156 0.0235 0.0228 0.0124 0.0046
Half life a t 5 5 O C. was 44.4 seconds
Table 111.
Run No.
31
@I
0.0117' 0.0106 0.0123 0.0116 0.0142 0.0117 0.0128 0.0044 0.0121 0.0118 0.0084 0.0121 0.0038 0.0071,
Half-Life (tlld, Sec.
Too fast to measure
1.0 1.5 2.9 3.5 13.7 16.0 16.7 21.3 21.7 136 213 242 428" 1220 1416 1.27 x 104 (3.53 hr., 550 C.)
= 156 X
lo4
C.
kl , Bee-.' x 104
>6930
6930 4620 2390 1980 506 433 415 326 319 51.0 32.6 28.6 16.2 5.68 4.90 0.546
sec.-1).
60% (Vol.) Aqueous Dioxane at 21 C. The following procedure was developed for the measurement of the hydrolysis rates. A weighed sample of ester (0.4 to 1.2 mmoles) was dissolved in 15 ml. of purified dioxane (9). A solution of 10 ml. of dioxane, 3 drops of phenolphthalein, 15 ml. of a 9.09 weight % solution of mannitol in water, and one half of the amount of 0.2457 or 0.1130N sodium hydroxide (approximately 2 ml.) necessary for neutralization of the boric acid resulting from complete hydrolysis were added a t zero time. The resulting solution was swirled to effect homogeneity and the time for fade of the indicator was recorded as the half life (Table 11). A blank determination with 1 mmole of boric acid and 0.5 meq. of sodium hydroxide under the above conditions was too fast to measure. Compensation for the phenols liberated in runs 10 to 12 with additional base still led to half lives which were too fast to measure. 91% (Vol.) Aqueous Dioxane at 21' and 55' C. The limited solubility of several of the esters in 6OY0 dioxane necessitated their hydrolyses in more concentrated dioxane. I n addition three other solid esters required an elevated temperature for homogeneity. Weighed samples of the esters (0.4 to 0.8 mmole) were dissolved in 70 ml, of dioxane containing 5 ml. of water and enough 0.1130N sodium hydroxide (approximately 2 ml.) to neutralize ong
Rate of Hydrolysis of Boric Acid Esters in 91% Aqueous Dioxane at 21 O and
Ester
Tri-tert-amyl borate
Initial Concn., Reactants, Mmole/Liter Sodium Time, Ester hydroxide Sec. X 10-8 Temperature, 55" C. 10.7
5.35
0.148
55' C.
Half Life Hydrolyzed,
(tl/d?
%
Hours
51.3
2.38
4860
(min.)
32 33 34 35 36
Tri-(2-phenylcyclohexyl) borate" Tri-(diisobutylcarbinyl) borate Tri-(2,6,8-trimethyl-4-nonyl)borate Tri-(2-cyclohexylcyclohexyl) boratea Tri-(dicyclohexylcarbinyl) borate
5.63 5.97 5.32 5.48 4.70
37 38 39 40 41
Tri-(2-phenylcyclohexyl) borate" Tri-(diisobutylcarbinyl) borate Tri-(2,6,8-trimethyl-4-nonyl)borate Tri-(2-cyclohexylcyclohexyl) borateD Tri-(dicyclohexylcarbinyl) borate
5.33 5.22 5.14 6.56 5.38
42 43 44 45 46
Tri-tert-amyl borate Tri-trans-(2-phenylcyclohexyl) borate Tri-(2-phenylcyclohexyl) borate" Tri-cis-(2-phenylcyclohexyl) borate Tri-(2,6,8-trimethyl-4-nonyl) borate
6.36 5.04 4.90 5.60 4.71
47 48 49
Tri-tert-amyl borate Tri-trans-(2-phenylcyclohexyl) borate Tri-(2-phenylcyclohexyl)boratea Tri-cis-(2-phenylcyclohexyl) borate Tri-(2,6,8-trimethyl-4-nonyl)borate
10.0 5.24 5.00 5.67 6.05
2.82 2.97 2.66 2.75 2.36
3.48 28.2 29.4 27.6 29.7
72.2 83.2 78.4 66.5 27.4
0.522 3.03 3.70 4.86 17.8
368 63.3 52.2 39.7 10.8
5.88 76.2 75.6 76.5 75.9
72.8 39.4 25.7 17.9 8.2
0.867 29.4 49.2 74.5 170
222 6.57 3.92 2.59 1.13
3.18 2.53 2.47 2.79 2.35
1.80 71.3 73.0 72.7 243
56.9 80.9 76.3 69.6 74.8
0.411 8.32 9.72 11.7 33.9
468 23.2. 19.8 16.4 5.67
.. .. .. ..
8.22 71.1 72.7 71.5 243
74.6 60.8 55.6 48.8 24.3
..
.. .. .. .. Temperature, 21° C.
50
51 0
. I
1.15 14.6 17.2 20.4 167
167 13.2 11.2 9.37 1.15
Cis-trans mixture.
VOL. 49, NO. 2
FEBRUARY 1957
177-
half of the boric acid resulting from complete hydrolysis of the ester, I n half the runs the base was eliminated, and 7 ml. of water was used. The solutions were maintained a t 21 " or 5 5 " C. for various periods, cooled to room temperature, and diluted with 300 ml. of water. Mannitol and phenolphthalein were added, and the boric acid was titrated with 0.1 130N sodium hydroxide. I n one case (run 36) the unconsumed base was back-titrated with 0.0240N hydrochloric acid. The per cent hydrolysis allows calculation of the half life and first-order rate constant (Table 111). Water at 21' C. (Heterogeneous). I n order more closely to approximate actual conditions of possible applications of the boric acid esters, the esters were subjected to hydrolysis by agitation in water. A solution of 50 ml. of water, 5 grams of mannitol, 4 drops of phenolphthalein, and one half the amount of 0.2457 or 0.1 130Ar sodium hydroxide
Table IV.
Run No. 52
necessary to neutralize the boric acid resulting from complete hydrolysis was added to a weighed sample of ester. The mixtures were agitated, and the time for fade of the indicator was recorded as the half time. Compensation for the liberated phenols (runs 53, 54, and 59) with additional base did not materially change the half times. The esters with half times greater than 2 days were back-titrated with 0.0240N hydrochloric acid within the first day. The half times were then calculated, assuming first-order kinetics. Table IV summarizes the heterogeneous hydrolysis data. Tri-(hexylene glycol) biborate may have undergone only partial hydrolysis to structure 111. Such a material was isolated in 897, yield from a neutral hydrolysis in the absence of mannitol; melting point 74.4-75.2' C. Analysis. Calcd. for CeH1303B: B, 7.52%. Found: B, 7.447,. The hydrolysis of the dodecyl borate
Ester Trimethyl borate
53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74
Tri-o-chlorophenyl borate Triphenyl borate Tri-n-amyl borate Triethyl borate Tri-n-hexyl borate Tri-( 1,3-dichloro-2-propyl) borate Tri-o-cresyl borate Tri-n-butyl borate Tri-n-octyl borate Tri-n-propyl borate Tri-(2-ethylhexyl) borate Triisobutyl borate Tri-(hexylene glycol) biborate Trioleyl borate Tri-(octylene glycol) biborate Tri-n-dodecyl borate Triisopropyl borate Tri-( 1-ethynylcyclohexyl) borate Tri-2-octyl borate Tri-see-butyl borate Tristearyl borate Tri-(methylisobutylcarbinyl) borate 75 TIi-tert-amyl borate 76 Tri-tert-butyl borate 77 Tri-3-heptyl borate 78 Tri-(P,P,P-trichloro-terfbutyl) borate 79 Tri-(2,6,8-trimethyl-4-nonyl) borate
Reactants Sodium Ester, hydroxide, mmoles meq. 2.27
1.13
1.02 1.08 0.735 1.41 0.558 0.987 0.660 0.800 0.456 1.29 0.538 0.932 0.588 0.463 0.529 0.323 1.06 0.787 0.719 0.868
0.510 0.540 0.368 0.699 0.278 0.493 0.330 0.399 0.227 0.646 0.270 0.468 0.581 0.232 0.530 0.162 0.530 0.392 0.360 0.555 0.435
0.703 0.992 0.911 1.22
sec. sec. sec. sec. sec. sec. sec. sec. sec. sec. sec. sec. min. min. min. min. min. min. min. min.
0.353 0.497 0.455 0.610
71.7 2.58 2.81 4.64
min. hr. hr. hr.
4430 2050 1890 1140
0.633
0.317
47.0 hr.
0.813
0.407 0.890
3.33 days (16.6y0, 21.03 hr.) 6.36 days (1.52%, 3.38 hr.) 15.3 days (3.88y0, 21.02 hr.) 18.6 days (3.2570, 21.35 hr.) 42.0 days (1.44y0, 21.1 hr.) 68.5 days (0.895%, 21.37 hr.) 221 days (0.27570, 21.2 hr.)
1.11
Tri-3-pentyl borate
81
Tri-( 2-cyclohexylcyclohexyl) boratea Tri-(dicyclohexylcarbinyl) borate
0.877
0.437
0.770
0.384
83
Tri-(2-phenylcyclohexyl) boratea
0.900
0.448
84
Tri-(diisopropylcarbinyl) borate
1.34
0.675
85
Tri-(diisobutylcarbinyl) borate
1.09
0.546
Cis-trans mixture.
178
,/
Too fast to measure
Relative Rate
6.0 7.7 7.8 11.8 12.0 13.5 18.3 21.7 38.8 70.0 82.5 106 3.80 8.59 9.60 9.68 10.4 16.1 53.7 58.0
80
Q
1
Half Time
>1.91 X >1.91 X 3.18 X 2.48 X 2.45 X 1.62 X 1.59 X 1.42 X 1.04 X 8.80 x 4.92 X 2.73 X 2.32 x 1.80 x 8.37 x 3.70 x 3.31 X 3.28 x 3.06 x 1.98 x 5920 5490
1.78
82
C.
Relative Rates of Hydrolysis of Boric Acid Esters in Water ot 21
INDUSTRIAL AND ENGINEERING CHEMISTRY
113
lo7 lo7
lo6 106 106 106 106 106 106 106 106 106 105 106 104
104 104 104 104 104
Table V. Hydrolysis of Triisopropanolamine Borate in Water at 25" C. Time, Hr. 0
0.37 3.55 5.83 23.3 94.7 120 172 264 458 571
Concn. of, Ester (a - x ) , Mole/Liter 0.0954 0.0948 0.0925 0.0916 0.0904 0.0897 0.0896 0.0894 0.0891 0.0582 0.0877
Hydrolyzed,
7% 0
0.62 2.96 3.94 5.17 5.89 6.02 6.28 6.53 7.48 7.99
was performed at 35' C. to prevent solidification of dodecyl alcohol and erratic results. The half time at 21 O C. was calculated from the value at 35" C. ( 3 64 minutes) assuming a rate doubling for every 10 temperature increase. Alkanolamine Borates in Water at 25 " C. The nitrogen-containing watersoluble ester. triisopropanolamine borate, was subjected to hydrolysis by dilution of a 1.8986-gram sample to 100 ml. in a volumetric flask. Ten-milliliter aliquots mere periodically removed and titrated with O.0235OA1-hydrochloric acid. The data are given in Table 1'. The sensitivity of triethanolamine borate to acid (6) precludes the possibility of following the hydrolysis in neutral solution by titration of the amine ; however, reproducible values may be obtained by back-titration of unconsumed hydrochloric acid. Consequently a 0.7207-gram (4.59-mmole) sample of triethanolamine and 19.4 ml. of 0.2367,V hydrochloric acid (4.59 meq.) were rapidly mixed and diluted to 50 mi. Five-milliliter aliquots werr removed and rapidly titrated with 0.024955 sodium hydroxide. Table VI summarizes the data.
Discussion Scope a n d Limitations. 4 point which the present work does not explore is the nature of the actual species neutralized by the sodium hydroxide. I t has been assumed to be the boric acid-mannitol complex which would result from
66.3 34.8 14.4
Table VI. Acid-Catalyzed Hydrolysis of Triethanolamine Borate in Water at
25" C.
11.9 5.27 3.23
Time, See. 0
1
152 322 467 617
Concn. of Ester (a - z), Hydrolyzed, Mole/Liter % 0.0918 0 0.0360 60.8 0.0184 80.0 0.00698 92.4 0.00395 95.7
complete hydrolysis of the ester. However, the possibility of more than momentary existence of partially hydrolyzed species, (R0)zBOH and ROB(OH)2, cannot be ruled out, since 1menthoxyboric acid, CloHISOB(0H) 2, has been isolated by partial hydrolysis of 1-menthyl metaborate trimer (24). Such species would be expected to be capable of at least partial titration in the presence of mannitol, as has been performed with the similar boronic (79, 33) and borinic acids (20). The partially hydrolyzed species (111) can be quantitatively titrated in the presence of mannitol. The effects of the changing ionic and basic strength of the medium due to the reaction of the sodium hydroxide with the boric acid produced have also been neglected. The hydrolysis of esters of inorganic acids in general is accelerated by alkali, but the effect is not very marked (23). The basic catalysis of the boric acid ester hydrolyses is made evident by comparison of the rates of groups 1 and 2 and groups 3 and 4 of Table 111. In the presence of base, the average rate a t 55" C. increased by a factor of 9.9 and a t 21" C. by a factor of 2.6. The hydrolyses in 6070 dioxane are further complicated by the presence of the mannitol. Any basic catalysis present initially is diminished as the hydrolysis proceeds because of neutralization of the base by the acidic boric acid-mannitol complex. This effect is made evident when the mannitol is added to the system a t some point after zero time. The half life of 2-ethylhexyl borate was diminished from 213 to 115 seconds when the mannitol was added after 50 seconds and further diminished to 107 seconds when the mannitol was added after 100 seconds. The exact function of the base in the hydrolysis is not elucidated. If the sole function of the hydroxyl ion were to compete with water in an attack upon the boron atom of the ester, an over-all acceleration of rate would result as shown above. However, an inverse relationship of half life and concentration would also be a necessary consequence. This follows from the integrated expression for the second-order reaction rate constant, kl. 2.303 b(a - x ) k) = ___ b ) log t(a a(b X )
-
-
where a, 6 , and x are the initial concentration of ester, initial concentration of base, and concentration of ester consumed, respectively. At 50% reaction or at the half life, x = I / Z U
The half life is inversely proportional to the difference in initial concentrations of ester and base (a - 6). I n actuality
there is a tendency toward direct proportionality of the half life and ( a - b). As initial ester and base concentrations in the ratio of 2 to 1 are increased, t1/2 increases. Although these problems have not been resolved, a comparison of hydrolysis rates was made on the basis of the pseudo-unimolecular reaction rate constants, kl, calculated by use of the integrated form of the expression for a first-order reaction ki
2.303
= - log
t
Table VII. Relative Rates of Hydrolysis of Boric Acid Esters in Aqueous Dioxane
2(a - x)
where a is the initial concentration of the ester and ( a - 2) is the concentration of the ester remaining at time t. Such first-order rate constants appear in Tables I1 and 111. I n support of this arbitrary procedure are the identical relative orders of the esters for the hydrolyses in both basic and neutral solution (Table 111). Comparison of Rates. The rates at 21 " C . in 60% dioxane (Table 11) and at 55 " C. in 91% dioxane (Table 111) were adjusted to a common basis as follows. Comparison of the half lives of tri-(2,6,8trimethyl-4-nonyl) borate at 55 " C. in 60% dioxane (run 30) and 91% dioxane (run 34) shows the hydrolysis to proceed 1.05 times faster in the 60y0 dioxane. Comparison of the half lives for tritert-butyl borate in 60y0dioxane at 21 " and 55" C. (run 27) shows the reaction to proceed 9.63 times faster at the elevated temperature. Therefore, to compare the rates a t 21 " C. in 60% dioxane to the base rate of tri-(dicyclohexylcarbinyl) borate at 55" C. in 91% dioxane (run 36), the rates at 21" C. in 6OY0dioxane are multiplied by 1/ 1.05 X 9.63 = 9.17. Table VI1 records the relative rates. Hydrolyses in Aqueous Dioxane. The relative rates of the aliphatic and alicyclic esters are in the order predicted by the steric requirements in the nucleophilic attack of water (or hydroxyl ion) on the central boron atom. \
'0:
H
H
RO
H
/
+
$-OR /
RO
.0-
\
\ +
A+
'-
/
H
5 Rate constant increased by factor of 18.6 to compensate for temperature differential (runs 32 and 44, Table 111).
*
Cis-trans mixture.
OR I
OR /
P-
- -B I
The greater the bulk of the alcohol the slower the rate of hydrolysis. Thus in the normal alkyl borates: methylethyl propyl butyl > amyl > hexyl > octyl > dodecyl > stearyl. Branching of the alkyl chain produces a further decrease in rate: n-octyl > 2-ethylhexyl. In the esters derived from straightchain secondary alcohols, (RlRzCHO)sB, the bigger the R1 and Rz groups the slower the rate of hydrolysis: isopropyl (RI = R2 = methyl) > sec-butyl (R1 = methyl, R 2 = ethyl) > 2-octyl (R1 = methyl, R 2 = hexyl) > 3-pentyl (R, = R Q = ethyl) > 3-heptyl (R1 = ethyl,
- -
Relative Ester Rate Trimethyl borate Triethyl borate Tri-n-propyl borate Triisopropyl borate Tri-( 1,3-dichloro-2-propyl) borate Tri-n-butyl borate Triisobutyl borate Tri-(P,P,&trichloro-tert>5.87 X IO6 butyl) borate Tri-(hexylene glycol) biborate Triphenyl borate Tri-o-chlorophenyl borate Tri-o-cresyl borate Tri-(o-phenylphenyl) borate Tri-(o-cyclohexylphenyl) borate I 5.87 x 105 Tri-n-amyl borate Tri-(octyiene glycol) biborate 3.92 X lo5 Tri-n-hexyl borate 2.02 x 105 Tri-sec-butyl borate 1.68 x 105 Tri-( 1-ethynylcyclohexyl) borate 4.29 x 104 Tri-n-o ctyl borate 3.67 x 104 Trioleyl borate 3 . 5 3 x 104 Tri-n-dodecyl borate 2.77 x 104 Tristearyl borate 2.71 x 104 Tri-2-octyl borate 4.33 x 103 Tri-(a-ethylhexyl) borate 2.77 x 103 Tri-(methylisobutylcarbinyl) borate 2430 Tri-tert-butyl borate 1370 Tri-3-pentyl borate 483 Tri-tert-amyl borate 449 Tri-3-heptyl borate 415 Tri-trans-(2-phenylcyclohexyl) borate 40.0a Tri-(2-phenylcyclohexyl) borateb 34.1 Tri-cis-(2-phenylcyclohexyl) borate 28.2' Tri-(diisobutylcarbinyl) borate 5.86 Tri-(2,6,8-trimethyl-4-nonyl) borate 4.83 Tri-(2-cyclohexylcyclohexyl) borate 3.67 Tri-(dicyclohexylcarbinyl) 1 borate
-:OR
+ HO-B
OR
4
f
ROH
Hz.OO 'R
etc.
Rz
= butyl). In every case, the esters derived from secondary alcohols hydrolyze more slowly than their primary counterparts. The tertiary alcohols in turn give rise to more stable esters than do the secondary alcohols: sec-butyl > tert-butyl, 3-pentyl > tert-amyl. In the tertiary series itself, I-ethynylcyclohexyl with two groups tied back in a ring hydrolyzes more rapidly than tert-butyl, which in turn hydrolyzes more rapidly than the more bulky tert-amyl. The glycol biborates follow the secondary-tertiary trend. Hexylene glycol
VOL. 49, NO. 2
0
FEBRUARY 1957
179
biborate with one secondary and one tertiary oxygen-bearing carbon hydrolyzes more rapidly than octylene glycol biborate which possesses two tertiary linkages. I n addition, hexylene glycol biborate with its six carbon atoms tied back in a ring and away from the boron atom is more susceptible to hydrolysis than the analogous open-chain methyl isobutyl carbinyl borate. Cis-2-phenylcyclohexyl borate hydrolyzes more slowly than the trans isomer. These results are in accord with Vavon and coworkers (36), who found that cis-2-cyclohexylcyclohexyl succinate saponified more slowly than the trans succinate. T h e primary requisite for stability in the present group of aliphatic and alicyclic esters appears to be the shielding of the boron atom with bulky groups below and above the plane of the BO3 grouping. Cyclohexylcyclohexyl groups serve this purpose, as do isobutyl or larger groups. T h e symmetry of the dicyclohexylcarbinyl group compared to the 2-cyclohexylcyclohexyl grouping allows for equal distribution of the bulk below and above the BO3 plane and, indeed, the ester derived from the former is more stable than the 2-cyclohexylcyclohexyl borate. I n turn, the 2-cyclohexylcyclohexyl group affords more protection than the less bulky 2-phenylcyclohexyl group and is therefore more stable. Electronic and polar effects can substantially effect the hydrolysis rates. (p,P,p trichloro tert butyl) Tri borate might be expected to hydrolyze more slowly than its unsubstituted counterpart, tri-tert-butyl borate, because of the added bulk of nine chlorine atoms. However, the inductive effect of the halogen atoms serves to increase the electrophilic reactivity of the boron atom sufficiently and produce an over-all acceleration of rate. Analogous reasoning could be applied to the triisopropyl borate and tri-(1,3-dichloro-2-propyl) couple. Both, unfortunately, hydrolyze too rapidly to be measured by the present method. The aromatic borates (phenyl, ochlorophenyl, and cresyl) also hydrolyze too rapidly to be measured. This is not unexpected in view of the lowered electron density on the boron atom due to the withdrawal of electrons by the aromatic nucleus (7).
-
-
-
-
-1.06
I
0
I
100
I
200
I
300
I
400
I
500
I
600
t, hr.
Figure 1.
Hydrolysis of triisopropanolamine borate
Heterogeneous Hydrolyses in Water. The divergence of solubilities, viscosities, state of aggregation, and wetting properties of the various borates precludes the validity of generalizations; however, some trends appear to be operative. T h e rates of hydrolysis of the butyl borates are as would be predicted on purely steric grounds: n-butyl > isobutyl > sec-butyl > tert-butyl, and once again the most stable members contain the flanking cyclohexyl or isobutyl groups. Alkanolamine Borates. A plot (Figure 1) of the log of the total ester concentration us. time (Tabel V) reveals a straight line after 50 hours. Multiplication of the slope (-2.03 X 10-5) by
Even the highly hindered aromatic borates (0-phenylphenyl and o-cyclohexylphenyl) hydrolyze a t a rate too fast to measure.
1 80
t
INDUSTRIAL AND ENGINEERING CHEMISTRY
I
0
-2.303 gives the first-order reaction rate constant for triisopropanolamine borate, 4.67 X 10-5 hr,-I, which is equivalent to a half-life of 618 days. Extrapolation to zero time indicates 5.4% of a more rapidly hydrolyzed impurity. The first-order reaction rate constant for the acidic hydrolysis of triethanolamine borateobtained from a plot (Figure 2) of the data of Table V I is 5.32 X set.-'. The half-life in the presence of one equivalent of hydrochloric acid is thus 130 seconds as compared to the half life of 181 seconds (with 0.5 equivalent of acid present) as found by Brown and Fletcher (6). The cryoscopic molecular weight data
bO
I
I
I
I
I
IO0
2 00
300
400
500
t, sec.
Figure
2. Hydrolysis of triethanolamine borate
I 600
of Hein and Burkhardt (74) for triethanolamine borate in neutral solution can be interpreted into a n approximate rate of hydrolysis. They found the freezing point depression of a water s o h tion (0.107,M) at 20’ C. to increase with time and level after 271 minutes. Assuming 99 to 99.99% hydrolysis a t that time, the half life would range from 40.8 to 20.4 minutes. A comparison of the rates of hydrolysis of triethanolamine borate and triisopropanolamine borate with their closest open-chain analogs, triethyl borate and triisopropyl borate, shows the nitrogencontaining “cage” structures (IV) to be far the more stable.
CHR
CHR
CHR
a rate of hydrolysis a t least comparable in order of magnitude to those of the open-chain analogs. T h a t this situation does not hold provides independent support for the existence of a transannular bond in molecules of this kind. I n addition, intramolecular coordination of amino nitrogen to boron has been suggested in P-aminoethyl diphenylborinate (20). Comparison of the half lives of 130 seconds a t 25’ C. for triethanolamine borate and 618 days a t 25’ C. for triisopropanolamine borate indicates the latter to hydrolyze 4.11 X l o 5 times slower than the triethanolamine borate. This deceleration of rate in proceeding from a primary to secondary alcohol far surpasses any decreases observed in the open-chain esters: n-octyl > 2-octyl by a factor of 8.5, n-amyl > 3-pentyl by a reactor of 1220, and n-hexyl > 3 heptyl by a factor of 487. It is hoped a more detailed study of the kinetics of the hydrolysis of triisopropanolamine borate and its apparent equilibration with its hydrolysis products may be presented later.
Summary
IV. R = H, CHI Brown and Fletcher (6) noted that triethanolamine borate could exist either in the cage structure IV or the tetrahedral structure V resulting from a transannular interaction of the “lone pair’’ of electrons on the nitrogen atom with the open sextet of the boron atom.
Thirty-eight boric acid esters from a variety of alcohols, glycols, and phenols have been synthesized, characterized, and subjected to hydrolysis in water, GO and 91% aqueous dioxane a t 21 ’and 55’ C. Six new esters of high hydrolytic stability are included. T h e relative rates of the aliphatic and alicyclic members are in the order predicted by the steric requirements in the nucleophilic attack of water (or hydroxyl ion) on the central boron atom. T h e rapid rate of hydrolysis of the highly hindered aromatic and chlorine substituted aliphatic esters indicates a shift from steric to electronic controlling factors for these members.
Acknowledgment
V.
R = H,CHa
From the inertness of the nitrogen atom toward methyl iodide and toward methanesulfonic acid in nitrobenzene, these authors concluded that a transannular bond must be present. Neglecting the solvent effects of water and GOO/, aqueous dioxane in the present work, triethanolamine borate hydrolyzes a t
