Phase Transfer Catalysis Chem 237

The Williamson ether synthesis between 2-naphthol and allyl bromide in the presence of strong base and benzyltriphenylammonium chloride as a phase transfer catalyst in dichloromethane solution is known to proceed in reasonable yield. The need for a phase transfer catalyst would be removed if a polar aprotic solvent such as DMSO were used as a reaction solvent. Why is DMSO not used as a solvent for this reaction?

Because of risks associated with toxicity.

List the procedural steps, from start to finish, that are required to convert 2-naphthol and allyl bromide into allyl 2-naphthyl ether.

The reaction is performed on a small scale by mixing the alcohol, the haloalkane, and the phase transfer catalyst in a conical vial. Sodium hydoxide (base) is added to initiate the reaction. The reaction flask is covered and stirred to conduct reaction to prevent evaporation of the solvent. The progress of the reaction is monitored by TLC so that no time is wasted. The reaction solution is then dried over calcium chloride to remove any remaining water. The product is purified by column chromatography and collected by evaporation.

Sodium hydroxide can cause severe damage to skin and eyes.

True

The Williamson ether synthesis between 2-naphthol and allyl bromide in the presence of strong base and benzyltriphenylammonium chloride as a phase transfer catalyst in dichloromethane solution is known to proceed in reasonable yield. Why will the reaction fail to work if the solvent is switched to ethanol while retaining the same reactants, phase transfer catalyst, and base?

Because the base would deprotonate ethanol and create a second nucleophile.

In the reaction to produce allyl 2-naphthyl ether, 2-naphthol is first reacted with aqueous sodium hydroxide to make sodium 2-naphtholate, which then goes on to react with allyl bromide and benzyltributylammonium chloride phase transfer catalyst, with dichloromethane as a solvent. Note: the boxes in the diagram represent the layers inside the reaction flask part-way to completion. Place the labels in the appropriate box. (If a molecule has significant solubility in both layers, it may be used more than once).

Dichloromethane has a density of 1.33 g/mL, while water has a density of 1.00 g/mL. Dichloromethane will be the lower layer in the separatory funnel. Allyl bromide is soluble in dichloromethane, while the ionic compounds (NaOH and sodium naphtholate) will all be soluble in the aqueous layer. 2-naphthol has a solubility of 0.74 g/mL in water. Once it is deprotonated to naphtholate by base, its solubility in the aqueous phase will increase significantly. Benzyltributylammonium chloride acts as a phase transfer catalyst because its ammonium salt is soluble in water and its alkyl chains are soluble in organic solvents.

Gloves and goggles are not required when handling sodium hydroxide.

False

Lachrymators like allyl bromide are specifically dangerous because of their explosion hazard.

False

The primary risk associated with dichloromethane is its flammability.

False

Unused allyl bromide can be disposed of in the aqueous waste.

False

A reaction was performed in which 0.47 g of 2-naphthol was reacted with a slight excess of allyl bromide to make 0.48 g of allyl 2-naphthyl ether. Calculate the theoretical yield and percent yield for this reaction.

TY: 0.60g PY: 80%

A Williamson ether synthesis between 2-naphthol and allyl bromide is performed in the presence of a phase transfer catalyst and strong base in dichloromethane solution. The reaction mix contains: unreacted allyl bromide, unreacted 2-naphthol, benzyltributylammonium chloride, allyl naphthyl ether, and dichloromethane. The reaction mix is passed through a silica gel mini column. Drag and drop the components of the mixture that elute from the column into the blue box next to the sample vial.

The silica gel stationary phase is highly polar and will trap the polar components in the mixture. Only the *allyl naphthyl ether* will elute with *dichloromethane* from the column.

Match the procedural step to its purpose by dragging each step in the written procedure for the reaction of 2-naphthol with allyl bromide and a phase transfer catalyst in strong base into the appropriate box.

The two immiscible layers can be converted into an emulsion if the agitation is too rapid. Alcohols are weak nucleophiles. Alcohols can be deprotonated to form stronger alkoxide nucleophiles in the presence of a strong base such as sodium hydroxide. Because 2-naphthol has a pKa of 9.5, it is easily deprotonated by hydroxide once they are in the same phase. Using an excess of allyl bromide reactant will allow the reaction to proceed to completion in under an hour. Silica gel is a highly polar stationary phase. The non-polar ether product passes through the column because it is not attracted to the silica, but the polar by-products are trapped by the silica. Solid anhydrous calcium chloride is a drying agent.