Nucleophilic Hydride Lab

Abstract: The purpose of this laboratory is to convert benzophenone to diphenylmethanol. This was done by using sodium borohydride to reduce benzophenone. The product was then crystalized from hexanes then using the melting point and IR testing it was possible to characterize it. The melting point range was…
Introduction:
This lab is possible because of oxidation and reduction reactions. In organic chemistry oxidation is the loss of electron density around a carbon, while reduction is an increase in electron density around a carbon. More electronegative atoms such chlorine have a strong affinity for electrons while conversely low electronegative atoms have a low affinity for electrons. This unequal affinity for electrons causes an unequal
distribution of electrons in a molecule. The unequal distributions causes dipoles or a negative charge around the electronegative atoms and a positive charge around the non-electronegative. An example of such a reaction is: Figure 1

An example of a reduction reaction is:
Figure 2 A catalytic hydrogenation reductions are a specific type of reduction reaction that require high temperature and pressure along with a metal catalyst and H2. A major short coming of this sort of reduction is that it is non-selective, therefore it will break every Pi bond in a molecule. This limits its use for synthesis reactions. An example of such a reaction is:
Figure 4 The type of reduction used in this lab is a nucleophilic hydride reduction.
Nucleophilic hydride is unstable by itself so it must be used through a hydride reagent such a NaBH4 or LiAlH4. The hydrogen is more electronegative than boron and aluminum giving it a negative charge. This causes the hydrogen to act like a hydride. This is important because it causes it causes it to be selective. A hydride has a negative charge so it will attack the positive carbon in a carbonyl group. This makes it more useful in synthesis reactions because if a molecule has a carbonyl group and carbon-carbon double bond, it will only attack the polar carbonyl group. LiAlH4 is much more reactive than NaBH4 because Al is more electronegative than boron which forms a stronger dipole making hydrogen more negative making it more nucleophilic. As explained both NaBH4 and LiAlH4 perform the same task, but because of LiAlH4’s higher reactivity they are used in different situations. NaBH4 can be used in alcoholic solvent while LiAlH4 can’t be because it will deprotonate and produce H2. Also, LiAlH4 can reduce ketones, aldehydes, carboxylic acids as well as esters. In contrast, NaBH4’s lower reactivity causes it only effective at reducing ketones, and aldehydes. In this lab NaBH4 will be used, an example of such a reaction using NaBH4 as a reducing agent
is:
Figure 5
(Note: the reduction is followed by the protonation of oxygen, this will be discussed during the discussion)
Reagent Table:
Chemical Name Structure Molecular
Formula Molecular Weight (g/mol) Amount Used Density (g/ml) Melting Point (°C) Boiling Point (°C)
Safety
Benzophenone C13H10O 182.217 5.51 g 1.11 47.9 305.4 Harmful if swallowed, inhaled. Skin, eye irritant1
Diphenylmethanol C13H12O 184.24 — — 65-67 297-298 Irritant to skin, eyes, flammable1
Sodium Borohydride NaBH4 37.83 1.00 g 1.0740 400 500 Corrosive to skin, irritant to skin eyes. Hazardous if ingested or inhaled.1
Methanol CH3OH 32.04 50 ml 0.7918 –97 64.7 Skin, eye irritant, Hazardous if ingested or inhaled. 1 Flammable
6M Hydrochloric Acid H—Cl HCl 36.46 20 ml 0.909 -59 108 Corrosive to skin and eyes, hazardous if ingested 1
Diethyl Ether C4H10O 74.1 100 ml 0.71 -116.3 34.5 Skin, eye irritant, Hazardous if ingested, inhaled1
Water H2O 18.02 50 ml 1.00 0.00 100 none Anhydrous Magnesium Sulfate MgSO4 120.415 ~ 1 g 2.66 1124 — Hazardous if ingested1

Hexanes C6H14 86.18 250 ml 0.6548 -95 69 Skin permeator, eye irritant, flammable 1

Experimental: First a drying tube was prepared. This was done by placing a small piece of cotton on the bottom of the tube, it was important that the cotton was not packed to tightly into the tube. Next the tube was filled with chunky anhydrous then the top was plugged with a small piece of cotton. A thermometer adapter was then placed into the bottom of the trying tube to fit on the reflux apparatus. After the drying tube was assembled, 5.52g of benzophenone along with a few boiling stones were added to 100mL round bottom flask. At this point 1.0g of NaBH4 was taken and iced, and slowly added to the benzophenone. Next the condenser was attached to the round bottom flask and the drying tube creating the reflux apparatus as seen in figure 6
Figure 6 The apparatus was heated over a steam bath for 20 minutes, then 6M HCL was added until the litmus paper turned blue showing that it was acidic. At this point the following reaction was complete:

The mixture was then added to a 250mL Erlenmeyer flask where 50mL of diethyl ether was added. Water was then added, but there was still a large amount of precipitate so vacuum filtration was used to separate it from the mixture. It was then added to a separatory funnel where the aqueous layer was extracted twice with 50mL of ether. The organic layer was placed in a separate flask, which was washed with 50mL of water. The ether phase was then dried with anhydrous sodium for 10 minutes then decanted into a beaker. It was then boiled over a steam bath to evaporate the ether. Following this the crude solid was recrystallized from hexanes, then vacuum filtration was used to collect the product.

References:
1. “MSDS.” Science Lab.Com Chemicals and Laboratory Equipment. 2005. Web. 24 Mar. 2014 http://www.sciencelab.com/msdsList.php