Preparation of a synthetic detergent

Objective

To learn how to prepare a synthetic detergent, through the sulfonation of an aromatic ring.

Background

Detergents are surface-active substances, i.e. they greatly reduce the surface tension of water when used at very low concentrations. Detergents have a hydrophilic polar head, such as salts of carboxylic or sulfonic acids, which allows their solubility in water and a carbon chain that gives this part of the molecule a hydrophobic character, thus helping to remove fats and oils mainly from clothes.

To facilitate the biodegradation of these detergents, linear rather than branched hydrocarbon chains are used in preference to branched ones. Soaps have traditionally been prepared by a saponification reaction of vegetable oils (glycerol esters), resulting in sodium salts of fatty acids.

 

In this experiment we will proceed to the preparation of a synthetic liquid detergent in the form of ammonium salt of triethanolamine, through a sulfonation reaction of dodecylbenzene (electrophilic aromatic substitution SEAr). Subsequently, with the dodecylbenzenesulfonic acid obtained, a solid detergent will be prepared by treatment with 40 % NaOH.

Experimental procedure

Place 10 g (11.4 ml) of dodecylbenzene in a 100 ml Erlenmeyer flask. Add dropwise for 30 min, stirring constantly 20 g (10 ml) of fuming H2SO4 (great caution in its handling) to the dodecylbenzene, maintaining the temperature between 40-45 ºC with a water/ice bath. Once the addition is finished, the reaction mixture is heated in a water bath at 50 ºC for 5 min, stirring constantly.

To cool the mixture to room temperature and to introduce it in a funnel of decanting until it separates in two phases. Once separated, carefully neutralize 2/3 parts of the obtained dodecylbenzenesulfonic acid, adding drop by drop a solution of NaOH at 40 %. The remaining part is neutralized with triethanolamine following the same procedure in order to obtain a liquid detergent.

Return to the Organic Synthesis Experiments.