Alcohols estimation with acetic anhydride

Estimation of alcohols and hydroxyl values by reaction with acetic anhydride (AA)... 

Estimation of benzyl alcohol by reaction with acetic anhydride. 

The use of qualitative information alone is not suf cient to correctly characterize an essential oil, and quantitative data are of extreme importance. Classical methods are generally focused on chemical groups and the assessment of quantitative information through titration is widely applied, for example, for the acidimetric determination of saponi ed terpene esters. Saponi cation can be performed with heat, and in this case, readily saponi ed esters are to be investigated, in the cold, and afterward, the alkali excess is titrated with aqueous hydrochloric acid thereafter, the ester number can be calculated. A further test is the determination of terpene alcohols by acetylating with acetic anhydride part of the acetic anhydride is consumed in the reaction and can be quanti ed through titration of acetic acid with sodium hydroxide. The percentage of alcohol can then be calculated. The latter method is applied when the alcoholic constituents of an essential oil are not well known in case these are established, the oil is saponi ed, and the ester number of the acetylated oil is calculated and used to estimate the free alcohol content. 

ALCOHOLS. The determination of alcohols depends upon their esterification with acetic anhydride and estimation of the proportion of the resulting acetates by hydrolysis with ethanolic potassium hydroxide. 

The excess of unchanged acetic anhydride is then hydrolysed by the addition of water, and the total free acetic acid estimated by titration with standard NaOH solution. Simultaneously a control experiment is performed identical with the above except that the alcohol is omitted. The difference in the volumes of NaOH solution required in the two experiments is equivalent to the difference in the amount of acetic add formed, i.e., to the acetic acid used in the actual acetylation. If the molecular weight of the alcohol is known, the number of hydroxyl groups can then be calculated. 

This method is precisely similar to the previous method used for the estimation of the number of hydroxyl groups in a polyhydric alcohol. A known weight of aniline is heated with a mixture of acetic anhydride and pyridine until acetylation is complete the excess of acetic anhydride remaining is... 

I. 4-methoxyacetophenone (30 //moles) was added as an internal standard. The reaction was stopped after 2 hours by partitioning the mixture between methylene chloride and saturated sodium bicarbonate solution. The aqueous layer was twice extracted with methylene chloride and the extracts combined. The products were analyzed by GC after acetylation with excess 1 1 acetic anhydride/pyridine for 24 hours at room temperature. The oxidations of anisyl alcohol, in the presence of veratryl alcohol or 1,4-dimethoxybenzene, were performed as indicated in Table III and IV in 6 ml of phosphate buffer (pH 3.0). Other conditions were the same as for the oxidation of veratryl alcohol described above. TDCSPPFeCl remaining after the reaction was estimated from its Soret band absorption before and after the reaction. For the decolorization of Poly B-411 (IV) by TDCSPPFeCl and mCPBA, 25 //moles of mCPBA were added to 25 ml 0.05% Poly B-411 containing 0.01 //moles TDCSPPFeCl, 25 //moles of manganese sulfate and 1.5 mmoles of lactic acid buffered at pH 4.5. The decolorization of Poly B-411 was followed by the decrease in absorption at 596 nm. For the electrochemical decolorization of Poly B-411 in the presence of veratryl alcohol, a two-compartment cell was used. A glassy carbon plate was used as the anode, a platinum plate as the auxiliary electrode, and a silver wire as the reference electrode. The potential was controlled at 0.900 V. Poly B-411 (50 ml, 0.005%) in pH 3 buffer was added to the anode compartment and pH 3 buffer was added to the cathode compartment to the same level. The decolorization of Poly B-411 was followed by the change in absorbance at 596 nm and the simultaneous oxidation of veratryl alcohol was followed at 310 nm. The same electrochemical apparatus was used for the decolorization of Poly B-411 adsorbed onto filter paper. Tetrabutylammonium perchlorate (TBAP) was used as supporting electrolyte when methylene chloride was the solvent. 

Carboxyl and hydroxyl groups can be estimated by standard titration methods. For the estimation of carboxyl groups, the sample is directly titrated with a standard alcoholic potassium hydroxide solution. To determine hydroxyl number, the sample is refluxed for 1 h with acetylating mixture (pyridine/acetic anhydride) and the excess acetic anhydride titrated back. The overall average functionality (fj (carboxyl and hydroxyl) is the sum of average carboxyl functionality (c ), main-chain hydroxyl functionality (h ) and branching-chain hydroxyl functionality (bj. 

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