4,4'-Dimethyl-1,1'-biphenyl (CAS NO.: ) could be produced through the following synthetic routes.

A. (4-methylphenyl)magnesium bromide (CAS NO.: ). A 500-ml., three-necked, round-bottomed flask equipped with a reflux condenser protected by a drying tube, a mercury-sealed mechanical stirrer, and a 250-ml., pressure-equalizing dropping funnel fitted with a gas-inlet tube is thoroughly purged with dry nitrogen and charged with 6.25 g. (0.256 g.-atom) of magnesium turnings and 50 ml. of anhydrous tetrahydrofuran. The dropping funnel is charged with a solution of 42.7 g. (30.5 ml., 0.250 mole) of in 100 ml. of anhydrous tetrahydrofuran. Approximately 10 ml. of the 4-bromotoluene solution is added to the flask, and the contents are stirred until the Grignard reaction commences. When the initial vigorous reaction has subsided the remainder of the 4-bromotoluene solution is added at a rate such that the mixture refluxes gently. Generally the addition is complete at the end of 1 hour, and almost all of the magnesium has dissolved. The mixture is refluxed for a further hour and cooled. The yield of (4-methylphenyl)magnesium bromide is about 95%.

B. 4,4'-Dimethyl-1,1'-biphenyl (CAS NO.: ). A 1-l., three-necked, round-bottomed flask equipped with a reflux condenser protected by a drying tube, a mercury-sealed mechanical stirrer, and a gas-inlet tube is charged with 101 g. (0.356 mole) of (I) bromide and 400 ml. of anhydrous . The slurry is stirred vigorously while a stream of dry nitrogen is passed through the apparatus. The reflux condenser is temporarily removed, and the solution of (4-methylphenyl)magnesium bromide is added to the flask as rapidly as possible through a large filter funnel fitted with a loose plug of glass wool. A black solid precipitates almost immediately from solution. The reflux condenser is replaced, and the contents of the flask are refluxed with stirring for 4 hours under a nitrogen atmosphere. The reaction mixture is then cooled, filtered, and the metallic thallium washed with 200 ml. of diethyl ether. The organic layer is washed once with 100 ml. of 0.1 N hydrochloric acid and once with 100 ml. of water, then dried over anhydrous sodium sulfate.

The organic solvent is removed by distillation under reduced pressure, giving 4,4'-dimethyl-1,1'-biphenyl contaminated with a small amount of bis(4-methylphenyl)thallium bromide. The crude product is dissolved in 30 ml. of benzene, and the solution is filtered through a short column of alumina using 250 ml. of benzene as eluent. Distillation of the benzene under reduced pressure leaves 19–21 g. (80–83%) of 4,4'-dimethyl-1,1'-biphenyl as a colorless solid, m.p. 118–120°.

Notice: Thallium salts are very toxic. This preparation should be carried out in a well-ventilated hood. The operator should wear rubber gloves.
Benzene has been identified as a carcinogen. OSHA has issued emergency standards on its use. All procedures involving benzene should be carried out in a well-ventilated hood, and glove protection is required.