Question

Add 7.0 mL of N,N-dimethylaniline and 50 mL of dichloromethane (CH2Cl2, DCM) to a 400- mL beaker.

Place the beaker in an ice-water bath. Add a magnetic stir bar and thermometer. Clamp the thermometer in

place. Turn on the stirring motor.

Add 1.1 equivalents of a solution (~1 M) of bromine (Br2) in DCM (already prepared in the hood) with a

plastic disposable pipet. If the temperature of the solution rises above 25 °C stop adding bromine solution.

Allow the temperature of the solution to cool to 20 °C. Continue addition of the bromine solution with the

pipet such that the temperature does not rise above 25 °C. The orange color of the bromine solution should

dissipate during addition. At the same time, vapor may appear above the surface of the reaction solution.After addition of bromine is completed the color of the solution may still be orange. This is expected

because you used an excess of bromine. Add an aqueous solution of saturated sodium bicarbonate

CAREFULLY. The reaction of bicarbonate with the hydrogen bromide side product will cause bubbling

and foaming! Try to keep the bubbling and foaming froth from escaping the beaker and the temperature of

the solution from exceeding 20 °C.

After you have added 150 mL of bicarbonate solution, check the pH of the solution with pH paper. If the

solution is still acidic, keep adding bicarbonate, 10 mL at a time.

When the solution has been neutralized you will notice that there are two layers in the beaker. Carefully

pour these two layers through a funnel into a 250-mL separatory funnel. Remove the bottom layer (the

organic layer) into a clean, dry 250-mL Erlenmeyer flask. If you have too much liquid your may have to

do this in two steps.

Add enough magnesium sulfate (white solid in the balance room) to cover the bottom of the flask

containing the organic solution and swirl occasionally to dry the organic solution. This should require

about five minutes. A dry solution is clear to look through and not cloudy. The white solid should be

loose, not clumped (think of a snow globe, that’s what you want). When you have determined the

water has been removed from the organic layer, filter through fluted filter paper or carefully decant the

organic layer away from the magnesium sulfate into a clean, dry and pre-weighed 250-mL Erlenmeyer

flask. Put a wooden stick (from the side shelf) into the flask and evaporate the dichloromethane on a

steam bath. When bubbling has ceased, remove the flask from the steam bath and allow it to cool.

Crystallization should occur. Weigh the flask with crystals in it to determine your crude yield.

-> What is the theoretical yield?

Answer 1

The reaction of N,N-dimethylaniline with 1.1 eq. of bromine gives para-bromo-N,N-dimethyl aniline as the major product as shown in the following figure:

One equivalent of N,N-dimethylaniline gives one equivalent of para-bromo-N,N-dimethyl aniline product. Weight of 7.0mL of N,N-dimethylaniline can be calculated as follows:

Weight of aniline = Density x volume

Weight of aniline = (0.956 g/mL) x (7.0 mL)

Weight of anline = 6.69 g

From the balanced equation it can be seen that, one mole of N,N-dimethyl aniline (i.e. 121 g) gives one mole of para-bromo-N,N-dimethyl aniline (i.e. 200g). Thus, weight of para-bromo-N,N-dimethyl aniline that can be obtained from 6.69 g of N,N-dimethyl aniline can be calculated as follows:

Weight of product = (200g / 121g ) x 6.69g

Weight of product = 11.1 g

Therefore, theoretical yield of given reaction is 11.1 g