Derivatization of Alcohols Using (bmim)HSO4: A Green Approach for the Undergraduate Chemistry Laboratory

Alcohol is one of the functional groups detected as part of qualitative organic analysis. The final identification of alcohol involves the preparation of suitable crystalline derivative, usually the 3,5-dinitrobenzoate. According to the standard procedure, 3,5-dinitrobenzoic acid is first converted to 3,5-dinitrobenzoyl chloride by reaction with either phosphorous pentachloride (PCl5) or thionyl chloride (SOCl2). However, in this reaction, hazardous side products like phosphorous oxychloride (POCl3), hydrochloric acid (HCl) and sulphur dioxide (SO2) are produced. In the present method, we report the direct and benign conversion of alcohols to the corresponding 3,5-dinitrobenzoates using ionic liquid, (bmim)HSO4 under microwave irradiation.


Introduction
Qualitative analysis of an unknown organic compound for both undergraduate and postgraduate students is an essential part of the chemistry syllabus. This systematic analysis includes preliminary examination, tests for functional groups and preparation of a suitable crystalline derivative. Alcohols, which are mostly liquids, are converted to the corresponding 3,5-dinitrobenzoates and the identification is done through the melting point determination of this derivative. [1,2] The common method of conversion of an alcohol to the 3,5-dinitrobenzoate has the disadvantage of not adhering to the principles of green chemistry. Green chemistry is applying strategies and procedures to reduce or eliminate the usage or production of harmful substances to minimize the damage to the environment. [3] Chemists have begun to create chemical processes that are safer and less polluting by employing the twelve principles of green chemistry. [4,5] Keeping in mind the various principles of green chemistry, there is a need to modify the current method and develop a safe and benign procedure for producing the alcohol derivatives. In the current work, 3,5-dinitrobenzoate derivatives of common alcohols are prepared via a microwave-assisted synthesis using the ionic liquid, (bmim)HSO 4 In recent years, the use of microwave assisted organic synthesis in the undergraduate laboratory has gained much popularity [6,7]. These reactions provide a green approach to chemical synthesis. Generally, microwave assisted reactions offer numerous key advantages, such as, improved energy efficiency, a shorter reaction time and in many cases, reactions may be performed under solventless conditions. [8,9] Ionic liquids are commonly known as designer solvents. They are primarily organic salts with melting points below 100°C and therefore are also known as liquid organic salts. Ionic liquids, in general, display physical properties close to fluids or liquids at ambient temperatures and today, they serve as an excellent alternate solvent for organic synthesis. The attractiveness of use of ionic liquids as solvents in organic synthesis has grown substantially during the past years. [10] (bmim)HSO 4 is an acidic ionic liquid which is used for the preparation of 3,5-dinitrobenzoate from alcohol.

Conventional Method for Synthesis of 3,5-dinitrobenzoate
Prepared the 3,5-dinitrobenzoyl chloride by combining 1.0 g 3,5-dinitrobenzoic acid with 1.5 g phosphorus pentachloride (PCl 5 ) with constant stirring (in a fume cupboard). Next, in a dry boiling tube kept in a heated water bath, the 3,5-dinitrobenzoyl chloride so prepared is mixed with desired alcohol (1 mL). The product is washed with saturated sodium bicarbonate (NaHCO 3 ) solution and water, followed by recrystallization with alcohol. The full procedure generally takes the undergraduate student 45 to 60 minutes to complete.

Green Synthesis of 3,5-dinitrobenzoates of Alcohols Using (bmim)HSO 4
In a clean and dry round bottom flask, equimolar amounts of 3,5-dinitrobenzoic acid and the desired alcohol were combined with 3 mL of the ionic liquid, (bmim)HSO 4 . The mixture was heated under microwave (Milestone Start Synth) irradiation for 3 minutes at 70°C. Added ice cold water to the reaction mixture for precipitating the 3,5-dinitrobenzoate. To eliminate any unreacted 3,5-dinitrobenzoic acid, the precipitated ester was filtered and washed with an aqueous sodium bicarbonate solution. The pure 3.5-dinitrobenzoate derivative was obtained through recrystallization in alcohol. This microwave assisted preparation takes the undergraduate student only 15-20 minutes from start to finish. The various 3,5-dinitrobenzoates were characterized via melting point.

Result and Discussion
The microwave assisted preparation of the 3,5dinitrobenzoate derivatives of alcohols using the ionic liquid, (bmim)HSO 4 [ Figure 1] is associated with several advantages, which are highlighted below: • Toxic chemicals like PCl 5 and SOCl 2 are not used.
• Generation of hazardous by-products such as hydrogen chloride, phosphorous oxychloride and sulphur dioxide are eliminated. • The above protocol allows the reduction in time and energy. The reaction times are significantly lower in the microwave assisted method compared to conventional two-step process. • This modified method works for some secondary alcohols as well, though the yields were generally low. Tertiary alcohols however, failed to undergo the microwave assisted reaction.  Various alcohols which are routinely available in the common undergraduate laboratory were used to prepare the 3,5-dinitrobenzoates and identification was through melting point confirmation and listed in Table 1.
Results of this microwave assisted esterification with ionic liquid is overall a greener, cleaner and safer process. The protocol is in accordance with the principles of green chemistry. The procedure proceeds with less chemicals used, generates less waste, is time saving, energy efficient and in general gave better yields.

Conclusion
The synthesis of 3,5-dinitro benzoate derivatives of simple alcohols is reported using a simple and environmentally friendly approach. Harmful chemicals are no longer used and undesired by-products are no longer formed. This method works well for most primary and secondary alcohols. The method adheres to the basic green chemistry concepts, making it simple to adopt in an undergraduate chemistry lab.