Dao 4

QuynhTrang Dao Hanh NguyenApril 3, 2015

Reduction of a Ketone to an AlcoholIntroductionThe reduction of carbonyl group yields primary or secondary alcohol.1 In the experiment, Benzhydrol will be synthesized from Benzophenone using sodium borohydrite (NaBH4). Instead of using LiAlH4 like in theory, in reality NaBH4 is often used because it is less reactive with water than LiAlH4 while LiAlH4 reacts violently with water, and this might ruin the experiment.1 BenzophenoneBenzhydrolThe mechanism of the reaction is based on hydride transferring. In the first step, H detaches from BH4- ion and adds to the ketone group creating an alkoxide ion.2 In the second step, H in water is added to alkoxide to form alcohol.2

Experimental 1.016g benzophenone, 15mL of 95% EtOH and a stir bar were added to 100mL round bottom flask. The flask was gently heated in a sand bath, which was on top of the stir plate. When benzophenone was completely dissolved, the flask was pull up from the sand bath but still on top of the stir plate, and was let to cool down. Preliminary TLC was taken. 0.201g NaBH4 was slowly added to the flask. After all NaBH4 was added, 4 comparison TLCs was taken every 5 minutes (before and after NaBH4 was added). The flask was taken off and placed into ice bath. 0.1% HCl was slowly added to the flask for the solution to precipitate. The solid was dried by vacuum filtration. IR spectrum was taken with TA help. The weight of white solid was recorded to be 0.872g. DataFigure 1: Benzophenone and NaBH4 reaction.

BenzophenoneBenzhydrol

Figure 2: IR Spectrum of Benzophenone

Figure 3: IR Spectrum of the solution after reaction

Figure 4: TLC plates every 5 minutes

Before Before Rf = = 0.70 Rf = = 0.62

After AfterRf = = 0.72 Rf = = 0.62

Rf = = 0.50 Rf = = 0.47

Before BeforeRf = = 0.68 Rf = = 0.69CompoundsBenzophenoneNaBH495% EtOHBenzhydrol

Formula Weight 182.22 g/mol37.8 g/mol46 g/mol184.23 g/mol

Mole ratio41NA4

Mole Needed5.49 x 10-3 mol5.49 x 10-3 molNA5.49 x 10-3 mol

Quantity Needed (g or mL)1.0 g0.208 g15 mL1.01g

Quantity Measured1.016 g0.201 g15 mL0.872g

Mp/Bp (C)47-51 CNANA65-67 C

Limiting ReagentYesNoNANo

After AfterRf = = 0.56 Rf = = 0.53

Table 1: Reagents and properties

ConclusionAccording to the IR spectra of Benzophenone before the reaction, there was a strong and sharp peak in the 1680 1715 cm-1 position, which represented the Ketone group (-CO). After the reaction, the IR spectra of the solution did not have any strong sharp peak in that position any more, but there was a strong and broad peak in the 3370.64 cm-1 position, which represented the hydroxyl group (-OH). The IR spectrum proved that the solution collected after the reaction was Benzhydrol. After the first 5 minutes, there were 3 spots on the TLC plate: one of the Benzophenone (Rf value = 0.70), and two of the reacting solution A1 (Rf value = 0.72 and 0.50). The first spot of A1 represented Benzophenone with approximately equal Rf values. The second spot of A1 represented Benzhydrol. The reaction was progressing. In the second TLC plate (next 5 minutes), there were also 3 spots, and the results appeared to be almost the same. The reaction had not completed yet. In the third TLC plate, there were only 2 spots: one of the Benzophenone before reaction (Rf value = 0.68), and one of the reacting solution A3 (Rf value = 0.56). The one spot on A3 represented Benzhydrol, and it proved that Benzophenone was not in the solution any more. On the forth TLC plate, there were also 2 spots. The reaction had completed. The amount of Benzhydrol collected was expected to be 1.01g while in reality, 0.872g of Benzhydrol was collected. Therefore, the percentage yield was 86.3%. The experiment was successful.

References1. Thaxton, A.; Lindsay, M.; Hickey, S.Organic Chemistry Lab 1; 2nd ed.; The University of New Orleans: New Orleans, 2015; p.107.2. Masterorganicchemistry.com,. 'Sodium Borohydride: Reducing Agent For Aldehydes And Ketones Master Organic Chemistry'. N.p., 2015. Web. 13 Apr. 2015. http://www.masterorganicchemistry.com/2011/08/12/reagent-friday-sodium-borohydride-nabh4/