CHEM 273L: Lab 7- The Grignard Reaction

Why would you expect triphenyl methanol not to dissolve in cold petroleum ether?

"Like dissolves like" - The alkane solvent doesn't have any polar groups or hydrogen bonding acceptors/donators so triphenyl methanol will probably not interact with it.

Why must the ether used in this reaction be anhydrous?

Water quenches Grignard reagents and can result in the by-product benzene upon protonation.

Why are Grignard reactions tricky to handle?

Yields are less than 100% because of various side reactions.

Write a mechanism for the formation of triphenyl methanol from the reaction of phenylmagnesium bromide and diethyl carbonate.

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Predict the diagnostic changes in the IR spectrum that will signal a successful reaction in this lab.

1. Appearance of the OH stretching absorption at 3500-3200 cm -1 2. Disappearance of the carbonyl stretching absorption at 1700 cm -1 3. Strengthened Csp2-H stretching absorption at 3100-3000 cm -1 due to the presence of three phenyl groups

Biphenyl and benzene are common side products in this Grignard reaction. Explain at which points in the purification procedure these impurities are removed.

1. Biphenyl and benzene is removed via TRITURATION with petroleum ether after the diethyl ether is evaporated away in the water bath to give the crude product. The triphenyl methanol will not dissolve in this solvent and thus will be separated from the biphenyl and benzene.

What is used in this reaction for trituration?

1. Petroleum Ether (for biphenyl) and 2. hot 2-propanol (for phenol)

List three possible safety risks specific to this experiment. Briefly describe precautions to address these risks.

1. The oven-dried reaction tubes will be hot when it is taken out --> use a napkin or tongs to grab it at first 2. Ether and its vapors are very flammable -> remove any sources of heat when evaporating the solvent 3. Heating solvents may cause them to suddenly pop/burst out of the top of the reaction tube --> point tubes away from the face when heating

Who discovered Grignard Reagents? And what is another name for Grignard reagents?

1. Victor Grignard 2. Organomagnesium compounds

When was Victor Grignard awarded the Nobel Prize in Chemistry?

1912

What is the temperature of the reaction mixture during reflux?

3 degrees Celsius (boiling point of diethyl ether)

What is trituration?

Exploits the different solubilities of the solid products and byproducts by adding a solvent and dissolving away one entity.

In Part II, if the reaction was slow could you use a heating mantle or heated stir plate to accelerate the reaction?

No, because the diethyl ether has a very low flash point and its vapors are heavier than air, so as it evaporates the vapors may fall onto the hot surface and cause an explosion.

What are the reactants and products of the second part of the experiment?

Phenylmagnesium bromide will react with benzophenone to create the salt of of triphenyl methanol, which will become the alcohol product when added with HCl.

What produces the black sludge at the bottom of the reaction vessel?

Polymerization reactions

The carbonyl stretching frequency of benzophenone is lower than the average carbonyl peak. Why is that so?

The carbonyl in the benzophenone is conjugated with the phenyl groups, and has more of a single bond character (as seen in resonance structures) since the electrons of the C-O pi bond can be delocalized throughout each phenyl group.

What is the main characteristic of a Grignard reagent?

A polarized carbon metal bond, with a negative charge on carbon and a positive charge on magnesium. Allows carbons to be nucleophiles.

What is dimerization/Wurtz Coupling and what does it produce?

A side reaction Grignard's can undergo with other grignards which gives biphenyl as a product.

What is oxidization?

A side reaction Grignard's can undergo with oxygen, which gives phenol as a product.

What is quenching?

A side reaction Grignard's can undergo with water, which results in protonation and gives benzene as a product.

During the TLC of the crude product, would you expect biphenyl or the triphenyl methanol to possess the higher Rf value?

Biphenyl will possess the higher Rf since it lacks the polar functional groups in triphenyl methanol and will interact less with the silica plate.

What are the reactants and products of the first part of the experiment?

Bromobenzene will react with Magnesium Metal to form phenylmagnesium bromide.

Write a balanced equation for the preparation of triphenyl methanol from phenylmagnesium bromide and benzophenone.

C6H5MgBr + C13H10O --> C19H15OMgBr + HCl --> C19H16O + MgBrCl