Application of lithium Aluminum Hydride to the Determination of Hydroxyl Groups GEORGE A. STENMARK and F. T. W E E Shell Development Co., Emeryrille, Calif.
Prior to the introduction of Lithium aluminum hydride as a reagent for active hydrogen, no satisfactorymethod existed for the determination of hydroxyl in the presence of a-epoxide groups. Because accurate functional analysis of epoxy compounds (as exemplified by epoxy resins) is frequently necessary, the application oC lithium aluminum hydride Cor this purpose was investigated, A n improved method was developed and has been found valuable f o r the rapid and accurate determination of hydroxyl in epoxy compounds. The method is also useful in analysis of other materials, where the more conventional hydroxyl methods are limited.
THE
reactions of lithium aluminum hydride with organic compounds and the methods used for their measurement have hem discussed in a recent review article (6). The authors' experience with the reagent has demonstrated its value for the measurement of active hydrogen, in particular the determination of alcoholic and phenolic hydroxyl groups. The reaction is uniquely applieahle to the 'determination of hydroxyl in the presence of epoxy groups as in Epon resins (Shell Development ie-mark), where other hydroxyl methods fail. The e is also useful for the analysis of hindered phenols lo not react under the conditions of the classical on methods. m aluminom hydride is usually employed for active I determinations in procedures involving measurement le ( 1 7 ) or pressure (3, IO) of hydrogen evolved in the Higuchi (6, 7) describes a titrimetric procedure in cess reagent is titrated with B standard solution of alcohol :. Because the latter procedure measures consumption of the results are affected by all reactions of lithium alumiiride, many of which do not involve active hydrogen.
. A. B. C.
100-ml.buret Leveling tube
J.
^ ^
Reactor (see Figure 3)
K. Three-way valve control for
Thermometer
D. Nitrogen dryer conthining
Delivdrite. to nitroKen source
rnercuw lift steel needle valves Nitrogen tlushing Control to tetrahydrofuran dispenser (see Figure 5 ) >\I. .4ir supK9y 0. Vheuum n.rm+ supply 1'.
L.
41.
M~~~~~~ reservoir, 2.m Tetrahydrofurhn layer 6. v e n t port H. Water-filled buret jacket 1. Stainlesssteel Hypotubing. 14-gaee Q . E.
I'.
Tlie Equipment employed in this laboratory is a modification of tho volnmomeasuring apparatus originally described by Zerexitinoff (17),a design which still appears good because of its simplicity. The major improvement made in this application lies in the srhtitntion of stainless steel hypodermic t,ubing for glass and rubber tubing, resulting in increased flexibility and a reduetion in dead space. APPARATUS
A photograph and a schematic diagram of the apparatus are shown in Figures 1 and 2. The d u d unit is designed from the standpoint of maximum convenience and efficiency.
-
Each unit consists of B 100-mi. water-jacketed gal ated in 0.2-ml. divisions and equipped with a fou. stopcock, a leveling tube located in B position adjacent to the w s buret. and a mercurv lift connected a t the too bv means of a &ree-wa$ cock to vacuim and air services and i t ti;e bottom to
~~~
~
~
as showd;; Figure 1, &d5/12 steel sphericii~iointse; sold&ed to its ends. The steel spherical joints are sealed to the glass joints of the four-way stopcock and of the reactor adapter (Figure 3) by sealing wax. The reaction flask has 8. capacity of approximately 50 ml., with a, side arm of approximately 10-ml. capacity, as illustrated in Figure 3. A standard-titper joint is provided on top of the side arm and the flask is attached to the system by means of an adapter. The 250-ml. flask for preparation of reagent has a stoppered joint on toD and a standard-taper joint and a stapcook near the
Figure 1. Dual lithium aluminum hydride apparatus
described by Zmgg and Lauer (is),&e shown in Figure 4. The openings in the 19/38 standard-taper joints are closed by insert-
1784
1785
V O L U M E 28, NO. 11, N O V E M B E R 1 9 5 6 ing 12-mm. sleeve-type rubber serum stopples. A 5-ml. hypodermic syringe fitted with a 22-gage stainless steel needle is used for transferring reagent from storage vessel to reaction flask. The storage and dispensing apparatus for tetrahydrofuran consists of a 1-liter borosilicate glass bottle having a 29/42 standardtaper joint into which two pieces of glass tubing are sealed, as shown in Figure 5. One piece of tubing, 12 mm. in inside diameter, has a standard-taper joint sealed a t the top, into which a pipet is inserted; the pipet is provided with a stopcock and a small drying tube. The other piece of tubing, 6 mm. in inside diameter, is connected t.o a drving tube, small glass T-tube, and the source of nitrogen. Wit,h nitrogen flowing, t’he solvent is forced into the pipet by placing a finger over the open end of the T-t,ube. REAGENTS
Tetrahydrofuran was chosen as a solvent because of its excellent solvent properties for many resinous materials as w l l as for lithium aluminum hydride. K i t h iinknown samples having more than the expected qiiantity of reducible functional groups, it is possible to consume all of the reagent without obtaining quantitative reaction of active hydrogen. For this reason the reagent solution is made up to contain :t trace of +phenylazodiphenylamine as an indicator to verify the presence of excess reagent. Higuchi (6) recommended the use of this compound as an indicato for titration of lithium aluminum hydride. The Lithium Aluminum Hydride Reagent is an approximately 2.5W solution in tetrahydrofuran. Connect the flasks as shown in Figure 3 with a glass filter plug inserted in the connecting piece and flush completely with nitrogen. Add approximat.ely 11 grams of lithium aluminum hydride, 100 ml. of purified tetrahydrofuran, and 0.15 ml. of 170 4-phenylazodiphenj-lamine solution to the reagent. preparation flask. Stopper the flask and, with t,he stopcocks open, swirl vigorously. After most of the lithium aluminum hydride has dissolved, close the stopcock and allow the solution t o st,and overnight’. Filter the reagent into the storage flask, maintaining slight nitrogen pressure t o facilit,ate the transfer. Disassemble the apparatus and close t,he storage flask by inserting a serum stopple into the standard-taper joint. CAUTIOS.Wear a face mask during this operation and keep the reagent off the skin. Detailed safet,y precautions should be obt,ained from the manufacturer, &letal Hydrides, Inc., Beverly, 11ass. Tetrahydrofuran. Prior t o purification, tetrahydrofuran should be tested for peroxide content by an iodometric method ( 1 5 ) and reject’ed for use in this technique if the peroxide oxygen is greater than 10 meq. per liter. Commercially available material iu previously unopened containers is generally eatisfactory i n this respect. 5 / 1 2 Spherical Connection
I19/38
Figure 3.
Reaction flask assembly
u B
A
Figure E . Diagram of apparatus for preparation and storage of lithium aluminum hydride reagent I
A. E.
C.
250-ml. flask for preparatlon of reagent 250-nil. flask for storage of reagent Connecting piece for filtration of reagent from A to B
Table I.
>laximum Sample Sizes
Estimated Hydroxyl Content, Eq./100 c.
Sample Size. Grams
n i
zn
0.5 1.0
0.4 0 2
To purify, place 1 liter of tet,rahydrofuran in a dry 2-liter roundbottomed flask fit.t,ed with a standard-taper joint. Displace t’he air from the flask with dry nitrogen and maintain a s l o ~ st,ream of nitrogen over the solvent. Add lithium aluminum hydride in small increments, with swirling. Continue the addition until a n excess is present, as indicated by the absence of furt,her gas evolut,ion (10 to 20 grams will usually suffice). Connect the flask t’o a distilling head and condenser, and distill the tet,rahydrofiiran, using a mantle-type heater. Discard the first, 50 ml. of distillate, and collect approsimately 800 ml. in a 1-liter borosilicate glass dispensing bottle (Figure 4) from which air is escluded by a slow stream of dry nitrogen. Discontinue the distillation when approximately 100 ml. of liquid remains in the flask. Protect the distillate from moisture by a drying tube immediately upon completion of the distillation. 4-Phenylazodiphenylamine.
1% solution in benzenra.
GENERAL OPERATING TECHKIQUES
Operation of Mercury Lift. The folloving operatiorip trated by Figure 2.
:ire
iIIus-
-1. T o raise the mercury level in the buret, close valve Q, twit three-nay valve K to “air” position, and open valve P . B. To lower the mercury level in the buret, close valve Q, t m n three-way valve K to “vacuum” position, and open valve P . C. To raise or loiver the mercury levels in both buret and leveling tube, carry out operation -4or B r i t h valve Q open. D. To level the mercury columns, open valve Q, turn thrcev ay valve K to “vacuum,” and open valve P . When the mercuri level in the leveling tube is belolv that in the buret, turn three-\\-a;. valve K to “air,” and close valve P to a low flow. Close valve P completely when the columns have risen to the same level. Buret Maintenance. Place a layer of purified tetrahydrofuran in the buret to ensure saturation of the contained gas. Replace the tetrahydrofuran weekly with fresh material. Do not allow the liquid to come in contact with the buret stopcock, as it will dissolve the lubricant. Testing for Leaks. Prior to performing any analyses, test the apparatus for leaks. Attach a reaction flask to the apparatus. Raise the mercury in the buret to a level near the top, venting the gas to the atmosphere. Turn the stopcock to connect the buret with the reaction flask. Lower the mercury level in the buret t o approximately the 20-ml. mark and note the volume. K a i t 3 to 4 minutes and again note the volume. If a change is observed, carefully relubricate the buret stopcock and glass joints and test again. Read the top level of the tetrahydrofuran in the buret when measuring the volume. Glassware Preparation. Dry all glassn are in an oven a t 110” C. for 30 minutes and store in a desiccator. Sample Size. Use an amount of sample calculated to yield 60 to 80 ml. of hydrogen; maximum sample sizes for typical anhydrous samples are given in Table I. Limit the size of sample to 0.5 gram, unless the behavior of the material with respect t o side reactions is known.
ANALYTICAL CHEMISTRY
1786 Table 11. Reaction of Alcohols and Phenols
Compound Cetyl alcohol 4-Met hyl-4-pentene-2-01 tert-Amyl alcohol Ethylene glycol Phenol 2,6-Dibutyl-4-methylphenol 2 4 6-Tributylphenol 2:2:Bis(p-hydroxyphenyl) propane Tris(p-Hydroxyphenyl)methane a
b c
Hydroxyl Value, Eq./l00 G. BY independent Theoretical method, av. B y LiAIH4 0.413 0,410” 0.411,0.412,0.413 0,999 0,9846 0,982 1.135 1,124c 1.11 3.223 3,194= 3.188 1.064 .... 1.062, 1.064
0.454 0.381
0 . 4 5 2 , O .449d
0 . 4 5 6 , O . 454
0.389d
0.376,0.373
0.874
0.8638
0.861
1.026
1.Id
1.020
Acetic anhydride method (14). Acetyl chloride method (1.2). Boron trifluoride-Fischer reagent method (3).
d e
Ethylenediamine titration ( 5 ) . Calculated from calorimetric purity measurement (13).
PROCEDURES
General Procedure. Into a dry reaction flask previously flushed with dry nitrogen, weigh, to the nearest milligram, the appropriate amount of sample as given in Table I . Add 5.0 ml. of tetrahydrofuran, stopper the flask, and swirl gently until the material has dissolved. By means of a hypodermic syringe, introduce 5 ml. of lithium aluminum hydride reagent into the side arm of the flask through the standard-taper joint on the top of the side arm and cap the standard-taper joint. Flush the gas-measuring apparatus with nitrogen for approximately 5 minutes, uncap the joints on the reaction flask, and immediately attach the flask t o the system. Allow nitrogen to pass through for approximately 1 minute a t a slow rate, shut off the nitrogen, and cap the small joint on the side arm. Turn the buret stopcock to connect with the reaction flask, immerse the flask completely in an ice slurry kept in a Dewar flask, and balance the levels of mercury. Allow several minutes for the system to equilibrate; equilibrium is established when the levels of mercury in the buret and leveling tube remain constant for approximately 5 minutes. Turn the buret stopcock to connect with the atmosphere, and force the liquid level in the buret up to the zero mark, expelling the nitrogen from the buret into the atmosphere. Turn the buret stopcock to connect with the reaction flask, set the three-way valve of the mercury lift on “Vacuum,” and open the lower needle valve. Raise the reaction flask out of the ice bath, open the upper needle valve slightly, and tip the flask so that a portion of the reagent in the side arm flows into the sample solution. Swirl the flask and continue adding reagent in small increments, with sxvirling, until all has been used. Maintain the system a t as near atmospheric pressure as possible a t all times. When all gas evolution has ceased, replace the reaction flask in the ice bath, close the upper needle valve, and equilibrate for about 10 minutes; establish atmospheric pressure in the buret. When the mercury levels in the buret and leveling tube remain constant for 5 minutes, read and record the buret volume, jacket temperature, and barometric pressure. The color of the reaction mixture should be red. If the solution is yellow, indicating depletion of reagent, repeat determination Tvith a smaller sample. Resin Procedure. Into a dry reaction flask previously flushed with dry nitrogen, weigh, to the nearest milligram, the appropriate amount of sample as given in Table I. Introduce a ‘/*-inch steel ball in the flask, add 5.0 ml. of tetrahydrofuran, swirl gently to dissolve the sample, and add 5.0 ml. of tetrahydrofuran. By means of a hypodermic syringe, introduce 5 ml. of lithium aluminum hydride reagent into the side arm of the flask, cap the standard-taper joint, and proceed as directed in the general procedure. After addition of the reagent, mix the solution thoroughly, swirling the flask in such a manner that the steel ball will aid in dispersing any precipitate which is formed. Immerse the flask in the ice bath, allow several minutes for equilibration, establish atmospheric pressure in the buret, and record the gas volume and jacket temperature. Allow the mixture to react until a constant volume is obtained; in general, resins require reaction times of from 0.5 to 2.5 hours. Mix the contents of the flask thoroughly several times during the reaction and take gas volume readings every 15 to 30 minutes. When analyzing materials which require extended reaction times i t is advisable to leave the three-m-ay valve in the “vacuum” position between volume readings. Additional Tests. BLANK. Rfake a blank determination exactly as described above, omitting the sample, and using the same volume of tetrahydrofuran as for the sample determination. The blank volume for 5 ml. of solvent is normally 1 to 5 ml. of gas.
WATER. Determine the water content of the sample by titration with Fischer reagent. The water determination should be made preferably on the same day the hydroxyl value is determined. ACIDITY. Determine the acidity of the material by titration with standard base to the phenolphthalein end point. CALCULATION
Calculate the hydroxyl value of the sample by means of the following equation: ( S - B ) 1.604 ( P - &) Hydroxyl value, eq./100 g. = ( T + 273)W(1000) n~here S = volume of gas evolved by the sample, milliliters B = volume of gas evolved by the blank, milliliters P = atmospheric pressure, millimeters 62 = vapor pressure of tetrahydrofuran, millimeters, calculated from the expression, Q = 6.82’ - 1 5” = jacket temperature, degrees centigrade W = weight of sample, grams C = water content, per cent by weight of the sample D = acid content of the sample, equivalents per 100 grams 1.604 = reciprocal of the gas constant 9 = equivalent weight of water in this determination
-($+.)
R E S U L T S AND DISCUSSION
Alcoholic and phenolic hydroxyl groups react to produce 1 mole of hydrogen per equivalent of hydroxyl. These reactions are rapid, in most cases reaching completion within 10 minutes a t 0” C. as shown in Table 11. Excellent accuracy is observed in the analysis of primary, secondary, and tertiary alcohols. Values obtained for phenol and highly substituted phenols indicate stoichiometric reaction. The method has been found especially useful for analysis of ortho-substituted phenols, which react only partially or not at all with esterification reagents such as acetyl chloride (12) or acetic anhydride (14). I n the authors’ experience, water reacts to a variable degree under the conditions of the method, giving 1.5 to 2.0 moles of hydrogen per mole. The extent of reaction is influenced by the presence of other reactants; water by itself in an inert solvent yields 1.5 moles of hydrogen per mole, in agreement with Baker and MacNevin’s observations (1); small amounts of water in the presence of other hydroxyl compounds produce 1.9 to 2.0 moles of hydrogen, as observed by Hochstein (8). When the method is applied t o samples low in water content, correction for water is made assuming equivalent weight of 9 (2 equivalents per mole).
Table 111. Reaction of Other Common Organic Functional Groups Compound Acetone Acetophenone Hexaldehyde Benzaldehyde Glycidyl phenyl ether Glycidol Epichlorohydrin Benzoic acid Oleic acid Ethyl acetoacetate Diethyl malonate
Moles/100 G . , Calcd. 1 724 0 833 1 00 0 943
Active Hydrogen, Eq./100 G. (LiAlH,)
