Chemistry 3231 – Organic Chemistry III

Laboratory Report for Experiment #1 – "Stereoselective Reduction of Benzoin"

Objective: The objective of this reaction is to study the procedure of a stereoselective reduction of compounds. In this case we are reducing Benzoin to 1,2-diphenyl-1,2-ethanediol. Due to the different ways this compound can be reduced we will use a specific reducing agent, Sodium Borohydride (NaBH4) and specific conditions (acidic and cold) we will try to obtain the meso isomer (ie, that both chiral centres are the mirror images of one another, if flipped).

Theory: In this reaction the use of Sodium Borohydride (NaBH4) will cause the reduction of the Benzoin so that the Sodium cation will chelate with the oxygen atoms of both of the central carbons (the carbon linkage between the 2 phenyl groups), that way it will in a sense ‘freeze’ the stereo isomer into a meso fashion. Now the carbonyl will be reduced and the resulting molecule will be a meso product. Even though there will be some of the other isomers made the favoured product will be the meso compound. Another way to look at the product is through what is known as Cram’s Rule. Cram’s Rule states that:

"If the molecule is observed along it’s axis… …the oxygen of the carbonyl orients itself between the small and the medium sized groups. The rule is that the incoming group preferentially attacks on the side of the plane containing the small group"

So, when we look at the projection of the benzoin compound we see that the carbonyl oxygen is oriented (for steric purposes) between the hydrogen and hydroxyl of the next carbon:

and that the NaBH4 will attack from the plane of the hydrogen causing the phenyl group to orient between the hydrogen and phenyl of the other carbon thus leaving the hydroxyl groups in the meso isomer.

The meso isomer. A meso isomer is a molecule with 2 chiral centres in which the attached functional groups on each are the same.

A meso compound is one whose molecules are superimposable on their mirror images even though they contain chiral centres.

For the example of 2,3 dichlorobutane we see that the last 2 enantiomers are not superimposable with each other. Now for the first stereoisomer we find that if we were to take the mirror image it would be a mirror image plus the ‘flipped’ version of it again. So it can either be the (2R,3S) or the (2S,3R) isomer, either is correct, and relate to the same molecule. This we call a meso isomer. For benzoin we have pretty much the same thing. Phenyl groups replacing the methyl and hydroxyl the chloro.

From Cram’s rule we find that if we line up the molecules we would see that the meso compound is the more favourable product. Now that is not to say that none of the other products will not be formed.

Table of Chemicals:

Reagents/Products Amount Used MW Concentration Comments
Benzoin 1.97 g 212.098 N/A The chemical being reduced (0.00928815924714047280030929098812813 mol used).
Ethanol 20 mL 46.0684 Absolute Used to dissolve the Benzoin.
Sodium Borohydride 0.44 g 37.8314 N/A Used to reduce the Benzoin. Added to the solution slowly.
Hydrochloric Acid 1 mL 36.4609 6M Used to set reaction conditions.
Water 30 mL, then a further 10 mL 18.0148 N/A  
Acetone/Hexane A few mL N/A 1:1 mixture Used to dissolve product and have it recrystallize.
1,2-diphenyl-1,2-ethanediol Measured 0.8536 g 214.2626 N/A Product. Observed Melting Point – 137.6oC
Nujoll A few mL N/A N/A Used make a mull of the product so it can be used in the IR spectrogram.

Experimental Procedure: followed the lab manual (Chemistry 3231 Lab Manual, Department of Chemistry, Lakehead University) in all steps. Refer to Lab Notebook for exact steps/procedures.

Results:

Observations:

In the IR spectrum (attached), we see a larger OH absorption (a much broader absorption). In comparison to the IR spectrum of the original reagent, (if we had it) we would have noticed that the region for the alcohol would have been much shorter. From the melting point we can tell that the end product was indeed the meso compound (it was the closest melting point). From the low yieldage, we can conclude that although the meso compound was the favoured product, there must have been large portions that were the other 2 isomers formed.

Yield Data:

Since the benzoin reacts to make only one product (being a reduction reaction) the

ratio is 1:1.

%Yield = (0.00398 / 0.0092) x 100%

= 0.42922 x 100%

= 42.9220%

Spectral Data:

From the run of the IR spectrum on the product we notice that benzoin’s carbonyl peak is now absent and a slightly more pronounced OH stretch. From this we must deduce that the carbonyl must have been reduced.

Reference(s):