Question

Experiment . the reduction of bencil (March 2020) Your name Sodium borohydride is a convert recent for the reduction of dehydes and ketones to 1" and 2 alcohols, respectively. The mechanism of the reaction - (below) is straightforward and is driven by boron's affinity for oxygen. In addition, given borohydride's remarkably low reactivity with OH groups under neutral and especially basic conditions, this reaction can be done in alcohol solvents. This means that (unlike LAH or Grignard reactions) no separate hydrolysis step is required; boron very much wants a bond to oxygen, but is not particular about which oxygen. So the excess of alcohol solvent (usually methanol) rapidly replaces the reduction product In this experiment, we will be using borohydride to reduce the diketone known as benzil, followed by crystallization 2102 AM For this experiment you will complete this worksheet in Word, and submit it to your TA by email Procedure: Weigh 500 mg record exact weight below) of benzill into a 50 mL Erlenmeyer flask, and add a medium size stir bar. Add 5 mL of ethanol. To the stirred suspension, add approx. 100 mg (record exact weight below) of sodium borohydride (weigh) and cap loosely with the small yellow cap provided (not airtight!). Exact amount of benzil used: 0.5849 Amount of ethanol used: 5.2mL Exact amount of sodium borohydride used: 0.1449 Observations: The Sun saution became wam and began as a murky yellow color, after the first few minutes OF Stirring the mixturé looked like an off white color. After Stirning For 10 minutes it looked more likea After stirring 10 minutes, 5 mL of water was added, then 10 mL of saturated aqueods sodium chloride solution. The flask was heated moderately on a stirring hotplate until the solid dissolved, then set aside to cool to room temperature. Crystallization occurred water the Amount of water used: 5.2 ml Arteradding water Solution looked like a Foggy 136 Amount of NaCl soln. used: 10 ml yellowish white. Addition of lo saturated Naci made the solution et a cloudy white clor. As the solution ? To boil the white color rose to the top and disappeared. Recrystal C. Recrystallization occured, crystals grew as it is cooled, and the solution turned back into a Foggy white color. Ac e bath it looked like a give-like White again or to mingeant White, the solution

Answer 1

Experiment # 6 Ans:            

The above reduction of benzyl gives two major products, viz., (+)-benzoin & hydrobenzoin, The hydrobenzoin exists in (+) & meso forms. After recrystallization we will get the pure Hydrobenzoin and the supernatant liquid will have the impurities including the benzoin. The reaction products are given below:

OH Benzil (210-2 g/mol ) (+)Bergen (I) Hycloberzoin Meso- rychobenzoin

The recrystallized material will not be 100 % pure, and can have impurities of benzil (unreacted), the benzoin and all the isomers of hydrobenzoin. This due to the fact that they also have some solubility in the water.

Yield Calculation: The reduction yields the product w.r.t. starting material in 1:1 molar ration, if no other products are considered to be formed. Assuming this, we will calculate the yields, theoretical from the product molar masses and % experimental yield from the weight of the crystals obtained. The steps are as follows:

It is given that:

Weight of benzil taken             = 0.584 g

Wight of the crystals = watch glass net wt. – watch glass tare wt.

Product weight obtained          = 97.989 – 97.656 = 0.333 g (practical yield)

Now, since, 210.2 g/mol of benzil yields 214.3 g/mol of the hydrobenzoin

So, the 0.584 g benzil will give            = [(214.3 / 210.2) * 0.584] g of hydrobenzoin   

Theoretical yield = 0.595 g of hydrobenzoin

Percent yield    = (product wt. obtained in g/ theoretical yield in g) * 100

                        = (0.333 / 0.595) * 100

                        = 55.97 %

Percent yield = = 55.97 % Hydrobenzoin

Comment on GC-MS spectra:

The second peak having very nearly identical mass is due to the benzoin impurity in the extract, whose molecular weight is just 1 amu less than the hydrobenzoin.