Determination of Acetals in SIR: A method was desired for determining the acetal (carbonyl content of Cs and Cla alcohols below the 100p.p.m. level. Because very sensitive photometric methods itre available for determining aldehyde and ketone carbonyl using 2,4dinitrophenylhydrazine (3, 4),these methods were investigated for their application to the determination of acetal carbonyl. Acetals hydrolyze t o aldehydes under acid ,conditions and,- therefore, are determined with the aldehyd2s when the acidcatalyzed phenylhydrazine method is used. However, acetals are stable in basic conditions. To determine the aldehyde carbonyl and the acetal carbonyl separately, i t was necessary t o remove the aldehyde under basic conditions, then acid hydrolyze the acetal t o an aldehyde and proceed with the phenylhydrazine method. Chaiken and Brown ( 2 ) reported t h a t aldehydes and ketones are readily reduced t o alcohol in an alkaline solution of sodium borohydride. Brown (1) also presented data showing that organic oxides are not redured by sodium borohydride. Therefore, i t appeared feasible t h a t aldehydes could be reduced t o alrohols in basic solution and the acetal carbonyl content could be determined by acid hydrolysis and photometry. This paper describes a method for determining acetals in the presence of aldehydes where the total carbonyl content is below 100 p.p.m. EXPERIMENTAL
Procedure.
An alkaline sodium borohydride solution was prepared by dissolving 10 grams of sodium borohydride in 100 ml. of nqurous potassium hydroxide solution ( I 00 grams per liter). 9 fresh solution was prepared daily. Ten m,lliliters of the alcoliol sample was pipcttcd into a 50ml. Erlenmryer flask and 10 rnl. of the sodium borohydride solution was added. The flask was placed in a const'ant temperature bath maintained a t 75" C. for 4 hours. The mixture was then transferred to a 25-nil. separatory funnel, the aqueous phase was removed, and the alcohol phase was scrubbed twice with water to re-nove any borohydride dissolved in the dcohol. Sodium borohydridr reart3 violently rvith concentrated acids and must be removed. A tube (10 cm. x 0.6 cm.) containing a small pluq of gl in one m i \vas filled t o a hei cm. with grauular anhydrous sodium sulfate and the alcohol was pei~olated through the colunin to rpnio\ e dissolved water. The eluate 1 a.crt'al carbonyl in accordance with the photometric technique developed by Lappin and Clark ( 3 ) . Because the
C, and C,, Alcohols
extinction coefficient varies with different carbonyls, the specific carbonyl compound to be determined should be available for calibration.
about 15y0complete. The solution was neutralized and a n aldehyde-free product was obtained by vacuum distillation. The product was examined by infrared spectroscopy which showed that
DISCUSSION
Acetaldehyde diethyl acetal is one of the most reactive acetal compounds, and i t is readily hydrolyzed to acetaldehyde and ethanol. T o determine whether sodium borohydride solution reduces this acetal, synthetic blends were prepared by adding acetaldehyde diethyl acetal to octvl alcohol and reacting for 4 hours at 80' C. The data shown in Table I illustrate t h a t alkaline sodium borohydride does not reduce acetaldehyde diethyl acetal under the conditions of the reartion. Carbonyl-free octyl and decyl alcohols were obtained by reacting 500 ml. of alcohol with an equal volume of alkaline sodium borohydride at 75" C. for 4 hourq, scrubbing twice with water, drying, and storing under prepurified nitrogen. Octyl aldehyde was blended with the octyl alcohol and decyl aldehyde with decyl alcohol a t various concentrations and then reacted with sodium borohydride reagent. These data shorn that the carbonyl content was redured to about 1 p.p.m. n hich was the lower limit of the photometric method. Thus, octyl and decyl aldehydes were completely reduced by borohydride treatment. The octyl acetal [C7H&H(OC8Hli) was prepared by heating 1.0 mole of octyl alcohol and 0.33 mole of octyl aldehi.de in the presence of 2 ml. of concentrated H2SO4catalyst a t 100' to 120' C. for 17 days. The progress of the reaction was monitored by gas liquid chromatography. Using the aldehyde peak as a guide, the reaction was estimated to be about f1.57~ complete. 1-nreacted aldehyde was removed from the reaction product by repeated aqueous sodium bisulfite eytraction. Examination of the aldehydefree product bv infrarcd spectroscopy showed that either acetal or aliphatic ether wa4 preqent. Both acetals and ethers displar C - 0 stretching vibrations between 1000 to 1200 r m . - l ( 5 ) . The decyl acetal [C9H,9CH(OCl~HI,)2] was prepared by heatinq 0.73 mole of decyl alcohol and 0.22 molc of dccyl aldehyde with 2 ml. of conwntrated H2S0, as a catalyst a t 100" to 120" C. for 25 days. Again. the progrrsq of the reaction n-as monit ored by follolving the increase of the acetal peak and the decrease of the aldehyde peak by gas chromatography. Based on the diminution of the aldehyde peak. the reaction was estimated to be
Table I. Analysis of Octyl and Decyl Alcohols for Aldehyde and Acetal Carbonyl Carbonyl content,, ' p.p. . m Before After boroborohydride hydride treattreatSample ment ment
Acetaldehyde diethyl acetal in octyl alcohol
27 27 11 11 7 7
31 25 11 13 7 6
Octyl aldehyde in octyl alcohol
95
1 1 3
95 78 78
2
10 10
