Determination of Alkoxy Groups in Alkoxysilanes by Acid-Catalyzed Acetylation J. A. MAGNUSON Silicone Producfs Depcrrfrnenf, General Electric Co
b The formation of alkylacetates by the perchloric acid-caiialyzed cleavage of alkoxysilanes by way of acetic anhydride in ethyl acetate solvent is quantitative. The reaction is complete in less than 2 minutes at room temperature. This method of analysis for alkoxysilanes is less complicated and more versatile than existing methods. It is expected that the usual acetylation reaction interferences operate.
are of considerable importance as intermediates in the manufacture of siloxane materials. Accurate alkoxy functional group analysis is necessary to ensure uniformity in the formulations to which these intermediates are added. € o r better product control, therefore, an attempt was made to develop a fast m d quantitative alkoxysilane method imitable for routine use in a quality ccintrol laboratory. The method developed is also an aid for confirming the composition of newly created silanes by analysis of their alkoxy contents. The alkoxy content of alkoxysilanes is often determined, fc~llowinghydroly.sis, by employing the conventional analytical methods 'or alcohols in aqueous solution (6). There are several methods available for rapidly determining alcohols in t'liis manner. The slow steps, however. are campletion of the hydrolysis and separation of the alcohol from those silanes or siloxanes n-hose other functional group:: (or the silanols formed) somewhat soliibilize the alcohol. r\ modified Zeist:l technique (1) has been iised successfully for the analysis of methoxy-, ethoxy- and propoxysilanes. The lim-ting factors of this technique are a time-consuming procedure and the molecular weight of the alkyl group that can be distilled as the iodide. .kccurate assays of aryloxysilanes capable of stoj chiometric bromination have been reported ( 7 ) . Also, infrared spectrophotonietry has been used to determine the riethoxy content of siloxane polymers ( 2 ) . A11 these methods, k.owever, do not possess the versatility herein proposed. This method, incorporating the accuracy and speed of a simple titrimetric procedure is essentially that developed LKOXYSILANES
., Waterford,
N.
Y.
by Fritz and Schenk (6) for the determination of organic hydroxyl groups, Both monomers and soluble high molecular weight siloxanes containing a wide variety of alkoxy and aryloxy groups can be quantitatively determined with equal ease. ilcetylation of alkoxy groups bonded to silicon by acetic anhydride is perchloric acid-catalyzed in an ethyl acetate solvent. Table I shows that the acetylation is quantitative in less than 2 minutes a t room temperature with 0.06M perchloric acid catalyst. Following hydrolysis of the excess acetic anhydride, the alkoxy content is calculated from the difference between the blank and sample titrations with alcoholic potassium hydroxide.
sides of the flask Kith 10 ml. of 3 to 1 pyridine-water solution and allow the hydrolysis t o proceed for about 10 minutes. Titrate nith alcoholic 0.LV potassium hydroxide. The mixed indicator end point is the color change from yellow to brownish-purple. The reagent blank is run in the same manner, but without the sample. The alkoxy content is calculated from the difference in volume between the sample and blank titrations. RESULTS A N D DISCUSSION
The following scheme of reactions is suggested for the acid-catalyzed acetylation of alkoxy groups bonded to the silicon atom: AcsO
EXPERIMENTAL
Reagents ( 2and Solutions. Acetic Anhydride M ) in Ethyl Acetate (0.06M in Perchloric Acid). Add 2.2 grams of 72% perchloric acid t o 150 ml. of ACS grade ethyl acetate in a 250-ml. glass-stormered flask. Add 5 ml. of XCS g r d e acetic anhydride to the flask and allow to stand a t room temDerature for 30 minutes. Cool the cdntents of the flask t o 5' C. and add 45 ml. of cold acetic anhydride. Keep the flask a t 5' C. for an hour, then allow the reagent t o come to room temperature. A yellow color will develop which does not impair the usefulness of the anhydride reagent. The reagent has been used successfully for about 3 weeks. Pota-sium Hydroxide (0.5N). Dilute 170 ml. of aqueous 45y0 potassium hydroxide solution to 4000 ml. with absolute methanol. Standardize againqt potasqium acid phthalate using the mixed indicator. Mixed Indicator Solution. Miy one part of 0.1% neutralizpd aqueou- crew1 red with three parts of O.lycneutralized aqueoui: thymol blue. PotasGum Acid Phthalate. Primary qtandard grade. Procedure. Accurately weigh a sample containing 4 to 5 meq. of the alkoxydane into a 126-ml. glassstoppered flask. Pipet 5 ml. of 2h1 acetic anhydride in ethyl acetate into the flask. Swirl the solution. .illow the reaction to proceed for about 5 minutes. (Organic-perchloric acid solutions must not be heated!) Then add 1 or 2 ml. of water and swirl the mixture again for a few seconds. Rinse the
+ H+
kq AcsOH+
AczOH+ Ac+
Ac+
+ =%OR
-*
(1)
+ AcOH
(2)
-SiO+R
(3)
40
=SiOfR
! AC
+ ACiOESiOAc
+ AeOR + A c +
(4)
or
3SiO'R I Ac
+ AcOH
-+
=SiOAc
+ AcOR + H C
The formation of acetylium (Ac+) from acetic anhydride and perchloric acid by steps 1 and 2 is reported by Burton and Praill (3, 4 ) . The first result in Table I1 illustrates that without the acid catalyst, acetylation of diphenyldimethoxysilane was not detected in a reaction time of 10 minutes. Acetoxysilanes from reaction mixtures, before hydrolysis, and after the perchloric
Table I. Effect of Time on Extent of Acetylation of Methyl-tris(2-methoxyethoxy)silane
0.0632 Perchloric acid. Time, see. 5; Reaction 15 96.1 30 100.0 60 100.3 100.3 120 240 100.4
VOL. 35,
NO. 10, SEPTEMBER 1963
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Table II. Effect of Perchloric Acid Catalyst Concentration upon Extent of Acetylation of Diphenyldimethoxysilane
(10-Minute reaction time; triplicate determinations) M HCIOi Reaction, None
