Sodium borohydride is a selective reducing agent [1]. In this experiment you will condense 3-nitroan — Physical Chemistry — Thermodynamics Chemistry Question
Selective Reduction of a Highly Unsaturated Imine
Sodium borohydride is a selective reducing agent [1]. In this experiment you will condense 3-nitroaniline with cinnamaldehyde to produce the highly unsaturated intermediate A (an imine) [1]. This is then selectively reduced with sodium borohydride to produce B [1]. The structure of B can be deduced from the 1H NMR spectrum [1].
[VISUAL]
The experiment illustrates the classic method of imine formation (azeotropic removal of water) [1].
Experimental
Chemicals
* 3-nitroaniline, solid [1]
* Cinnamaldehyde, liquid [1]
* sodium borohydride, solid [1]
* hexane liquid [1]
* ethyl acetate, liquid [1]
Procedure
Place 3-Nitroaniline (2.76 g) and absolute ethanol (20 cm3) in a 100 cm3 round bottomed flask, together with a few anti-bumping granules [2]. Set up the flask for distillation as shown above using an isomantle or steam bath as the heat source [2]. [VISUAL] Use a graduated measuring cylinder to collect the distillate [2].
Add dropwise a solution of cinnamaldehyde (2.9 g) in absolute ethanol (5 cm3) through the thermometer inlet [2, 3]. Turn on the heat source and distil off approx. 22 cm3 of solvent over a period of about 30 minutes [3]. During the distillation dissolve with stirring sodium borohydride (0.76 g) in 95% ethanol (20 cm3) [3].
After the 22 cm3 of solvent has distilled off, disconnect the apparatus [3]. Set aside a small sample of the residue A which remains in the flask for thin layer chromatography [3]. Then add 95 % ethanol (20 cm3) to the flask to dissolve the remaining residue [3]. To this solution of A add VERY CAREFULLY the sodium borohydride solution [3]. This must be added slowly and with constant swirling of the reaction flask (vigorous effervescence occurs) [3, 4]. After the addition, heat the mixture under reflux for 15 minutes, then cool the flask and pour the contents into water (50 cm3) [4]. The product B, should crystallise out slowly on standing in an ice bath [4]. Recrystallise your product from 95% ethanol [4].
Record the yield of your product [4]. Run a thin layer chromatogram of your product B and the sample of A on a silica plate using hexane/ethyl acetate 1 : 1 as the eluent [4].
Record the Rf value of each. Measure and record the m.p. of B. Predict the structure of B using the 1H NMR spectrum given below [5].
[VISUAL]
In the preparation of A why is absolute ethanol and not 95% used? [5]
Model Answer
Water is product of the reaction of imine A formation [5]. The use of aqueous ethanol as a solvent will negatively influence equilibrium [5, 6].
Why is the solvent removed during the reaction? [5]
Model Answer
Ethanol forms azeotropic mixture with water [6]. Produced water is thus removed from the reaction mixture what shifts the equilibrium towards intermediate A [6].