Estimation of acetyl groups

It is obvious that this method serves for the estimation of acetyl groups present in esters of alcohols or phenols. It also serves for the estimation of acetyl groups present in substituted primary or secondary amines. 

By use of benzene sulphonic acid or phosphoric acid. When the ester yields on hydrolysis products which become coloured in presence of alkali and air, Method 1 is inapplicable. If the acid produced on hydrolysis is volatile in steam, benzene sulphonic or phosphoric acid may be used as hydrolytic agent, and the acid (from the ester) after separation by steam distillation is titrated with standard alkali. (See Estimation of Acetyl Group, p. 479.)... 

The analyses which follow are arranged in the order in which they would be applied to a newly discovered substance, the estimation of the elements present and molecular weight deter-minations(f.e., determination of empirical and molecular formulae respectively) coming first, then the estimation of particular groups in the molecule, and finally the estimation of special classes of organic compounds. It should be noted, however, that this systematic order differs considerably from the order of experimental difficulty of the individual analyses. Consequently many of the later macro-analyses, such as the estimation of hydroxyl groups, acetyl groups, urea, etc. may well be undertaken by elementary students, while the earlier analyses, such as estimation of elements present in the molecule, should be reserved for more senior students. 

Estimation of the Number of Acetyl Groups in an Acetyl Ester. 

Jute and kenaf, like most vegetable fibers, contain a proportion of acetyl groups that are readily hydrolyzed by dilute alkali to acetic acid. Estimation of the quantity of acetic acid produced per unit weight of fiber then provides an index of the acetyl content. 

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. 

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... 

Chemical Analysis. The copolymers were acetylated by refluxing with acetic anhydride in pyridine, and the hydroxyl content was estimated by titration with alkali of the acetic acid produced during acetylation and by hydrolysis of excess anhydride. The procedure adopted was based on that described by Sorensen and Campbell (33). The results of the analyses are given in Table A. At the three lower concentrations of hydroxyl groups all results were averaged from five or more measurements. 

Estimation of the different types of hydroxyl groups can be performed readily on acetylated lignin samples (Fig. 5.4.9) (Robert and Brunow 1984,... 

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