Experiment 3 Preparation of tert-Butyl Chloride | Exercises Organic Chemistry

Experiment 3

Preparation of tert-Butyl Chloride

The purpose of this experiment is to prepare tert-butyl chloride (2-chloro-2-methylpropane) from tert-

butyl alcohol (tert-butanol) using an acid catalyzed dehydration reaction. (Note: the correct IUPAC name for

this compound is 2-methyl-2-propanol).

We will also learn how to use a separatory funnel, the use and purpose of a drying agent and the

technique of distillation. The reaction is shown in Figure 3.1

CH3C

CH3

OH +H Cl CH3C

H3C

CH3

O H

H2O

CH3C

CH3

O H

+Cl-

CH3C

CH3

t-butanol

(2-methyl-2-propanol)

hydrochloric

acid

t-butyl chloride

(2-methyl-2-chloropropane)

Figure 3.1 Preparation of t-Butyl Chloride

carbocation

The first step of the overall reaction is an acid-base reaction between the t-butanol and the hydrochloric

acid. The t-butanol is a weak base and the hydrochloric acid is a strong acid. The alcoholic oxygen becomes

fully protonated and so the equilibrium lies far to the right. In the second step we have the slow loss of water to

form a carbocation intermediate. This species is very reactive and is immediately attacked by the chloride ion

liberated in the first step to form the final product. This is an example of an SN1 reaction (Substitution

Nucleophilic Unimolecular).

Physical Constants

Compound

Mol. Wt (g/mol)

Density (g/mL)

b.p. (°C)

m.p. (°C)

2-Methyl-2-propanol

74.12

0.775

25 – 26

2-Chloro-2-

methylpropane

92.57

0.85

51 – 52

-25

Hydrochloric acid

37% soln in H2O

1.18

Proper Use of the Separatory Funnel

Set up the separatory funnel as shown in Figure 3.2. Use a small iron ring stand to hold the funnel. You

can also use a metal clamp attached to the top neck of the separatory funnel but the iron ring is more

convenient. ALWAYS KEEP A BEAKER UNDERNEATH THE SEPARATORY FUNNEL TO CATCH

ANY SPILLS. When the handle of the stopcock is parallel to the separatory funnel (see shown in Fig. 3.2), the

stopcock is in the open position. When the handle is perpendicular to the length of the funnel it is closed.

With the stopcock in the closed position, pour in your solution into the top of the funnel. Insert the glass

stopper. Hold the funnel in one hand by grasping the top around the glass stopper. Lift it from the iron ring and

SLOWLY invert the funnel, holding it with your other hand. When it is now upside down, the glass stopper

should be in the palm of one hand, holding it securely. IMMEDIATELY VENT the funnel by opening the

stopcock and pointing the end of the funnel AWAY from your lab neighbor. CAUTION: always vent the

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Experiment 3

Preparation of tert - Butyl Chloride

The purpose of this experiment is to prepare tert - butyl chloride (2-chloro- 2 - methylpropane) from tert -

butyl alcohol ( tert - butanol) using an acid catalyzed dehydration reaction. (Note: the correct IUPAC name for

this compound is 2-methyl- 2 - propanol).

We will also learn how to use a separatory funnel, the use and purpose of a drying agent and the

technique of distillation. The reaction is shown in Figure 3.

CH 3 C CH 3 CH 3 OH +^ H Cl CH 3 C H 3 C CH 3 O H H^ H 2 O CH 3 C CH 3 CH 3 H

(^) Cl- O H Cl CH 3 C CH 3 CH 3 Cl t - butanol ( 2 - methyl- 2 - propanol) hydrochloric acid t - butyl chloride ( 2 - methyl- 2 - chloropropane) Figure 3. 1 Preparation of t - Butyl Chloride carbocation

The first step of the overall reaction is an acid-base reaction between the t - butanol and the hydrochloric

acid. The t - butanol is a weak base and the hydrochloric acid is a strong acid. The alcoholic oxygen becomes

fully protonated and so the equilibrium lies far to the right. In the second step we have the slow loss of water to

form a carbocation intermediate. This species is very reactive and is immediately attacked by the chloride ion

liberated in the first step to form the final product. This is an example of an SN1 reaction (Substitution

Nucleophilic Unimolecular).

Physical Constants

Compound Mol. Wt (g/mol) Density (g/mL) b.p. (°C) m.p. (°C)

2 - Methyl- 2 - propanol 74.12 0.775 83 25 – 26

2 - Chloro- 2 -

methylpropane

Hydrochloric acid 37% soln in H 2 O 1.

Proper Use of the Separatory Funnel

Set up the separatory funnel as shown in Figure 3.2. Use a small iron ring stand to hold the funnel. You

can also use a metal clamp attached to the top neck of the separatory funnel but the iron ring is more

convenient. ALWAYS KEEP A BEAKER UNDERNEATH THE SEPARATORY FUNNEL TO CATCH

ANY SPILLS. When the handle of the stopcock is parallel to the separatory funnel (see shown in Fig. 3.2), the

stopcock is in the open position. When the handle is perpendicular to the length of the funnel it is closed.

With the stopcock in the closed position, pour in your solution into the top of the funnel. Insert the glass

stopper. Hold the funnel in one hand by grasping the top around the glass stopper. Lift it from the iron ring and

SLOWLY invert the funnel, holding it with your other hand. When it is now upside down, the glass stopper

should be in the palm of one hand, holding it securely. IMMEDIATELY VENT the funnel by opening the

stopcock and pointing the end of the funnel AWAY from your lab neighbor. CAUTION : always vent the

separatory funnel immediately when you invert it. Pressure can build up and the contents of the funnel can

shoot out uncontrollably.

Once you have vented the separatory funnel, close the stopcock and shake it GENTLY AT FIRST 2 - 3

times. Then VENT the funnel again. Now you can shake it more vigorously several times. Then vent it again.

You may hear the sound of escaping gases when you open the stopcock to vent. Once this subsides then it is

safe to shake the separatory funnel vigorously for 2-3 minutes. It is very important to shake the funnel very

thoroughly to ensure intimate mixing of the two layers.

When you have finished shaking, replace the separatory funnel back in the iron ring and REMOVE

THE GLASS STOPPER before draining off the bottom layer. Otherwise you will create a vacuum inside the

separatory funnel and it will not drain.

19 / 22 size Ground glass stopper; be sure to remove the stopper when draining the funnel; otherwise a vacuum develops Empty beaker under separatory funnel to catch spills. iron ring connected to metal bar on bench Stopcock (Note: stopcock shown here is in open position because the handle is vertical.) 125 mL separatory funnel Figure 3. 2

Note that if there are two layers in a separatory funnel it is because the two liquids contained in the

funnel are not soluble in each other. In most of our experiments, including the one today, we have a relatively

non-polar organic layer – usually containing the product that we want to isolate – and a polar aqueous layer.

The more dense layer is on the bottom. In many cases, this is the water layer (recall that oil floats on the

surface of water), but not always!! Be careful. When using the separatory funnel, ALWAYS SAVE BOTH

LAYERS until you are sure that you have isolated the correct layer. Never throw anything away until you are

finished with your experiment. To test which layer is which – organic or aqueous – simply add a little water to

the layer that you think is the aqueous layer. If you see only one layer then you were correct in thinking that

this was the aqueous layer.

Experimental Procedure

In your separatory funnel, place 35 mL of concentrated hydrochloric acid that has first been cooled to

5°C in an ice bath. Add 12.0 mL of t - butanol and swirl gently with the stopper off the separatory funnel.

You want to mix the t-butanol and the hydrochloric acid thoroughly. You can also stir the mixture with your

glass stirring rod.

Once the layers have been thoroughly mixed, allow the separatory funnel to stand with the stopper off

for 20 minutes. You will gradually see two layers forming. (What are they?)

Appendix 3.1 Distillation

Theory and Background:

Distillation is the separation of liquids by vaporization and condensation of the vapor back to the liquid

phase. It is an excellent and convenient way to purify liquids. The liquid to be purified is put into the distilling

flask which is heated to the boiling point of the liquid. The vapors then rise up to the thermometer at the top of

the three-way adapter which has a sidearm attached to the cold condensing column. When the hot vapors hit the

cold condensing column they condense back to liquid and are collected in the receiving flask.

The boiling point of a liquid is defined as the temperature at which its vapor pressure equals the

atmospheric pressure. It is characterized by vigorous bubbling of the liquid as it vaporizes. The actual

temperature of a boiling liquid is sometimes higher than the boiling point of the compound due to superheating,

which is ameliorated by the use of boiling chips.

The boiling point is a function of the atmospheric pressure. As we lower the atmospheric pressure we

also lower the boiling point. This relationship is shown in Fig 3.4 for acetone. At 760 mm Hg (atmospheric

pressure) the boiling point of acetone is 56.2 °C, but at 500 mm Hg it is only about 45 °C.

boiling point, °C vapor pressure, mm Hg 200 400 800 600 10 20 30 40 50 60 Figure 3. 4 Vapor-pressure-boiling point diagram for Acetone [(CH 3 )C=O] volume of distillate Temperature of the distillate boiling point Figure 3. 5 Volume versus temperature curve of the distillate in the distillation of a pure liquid

When a reasonably pure liquid is distilled, the observed temperature rises rapidly to the boiling point

(see Figure 3.5). Once the distillation apparatus has reached thermal equilibrium, the boiling point remains

relatively constant, not changing more than 1-2 ° during the distillation. When there is a significant drop in

temperature, it is a good signal that the desired compound has all distilled. But be careful here. Very often the

temperature can drop as the distillation proceeds because the volume of distillate decreases as you get toward

the end. Usually you need to increase the heat in order to drive all of the distillate up the column.

General Procedure for Set-up of Distillation Apparatus

The set-up of the apparatus for distillation is shown in Figure 3.6. You really need to use only two metal

clamps. Those with three fingers are best. Follow these steps:

1. First set up the heating mantle on the iron ring at a convenient height (12-16 inches) above the bench top.

Make sure it is the correct size for the flask. Attach it to one of the vertical bars on the lab bench.

2. Then clamp the distilling flask securely in the heating mantle so that the flask is touching the bottom of the

heating mantle. Be sure to add your boiling chips!

3. Add the three-way adapter. You do not need a clamp for this, since it is positioned vertically.

4. Attach the condensing column to the three-way adapter, using one of the blue plastic clamps to secure the

joint. Note that the smaller ring of the plastic clamp fits over the inner tube of the three-way adapter and the

Expanded view of the correct position for the thermometer: tip of the thermometer is just even with the side arm of the 3 - way adaptor Connect plug from heating mantle to Rheostat inlet heating mantle Water in Water out thermometer 25 mL flask 100 mL flask boiling chips Condensing column vacuum adaptor metal clamp Figure 3. 6 three-way adaptor

Experiment 3 Preparation of tert-Butyl Chloride, Exercises of Organic Chemistry

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Partial preview of the text

Download Experiment 3 Preparation of tert-Butyl Chloride and more Exercises Organic Chemistry in PDF only on Docsity!

Experiment 3

Preparation of tert - Butyl Chloride

The purpose of this experiment is to prepare tert - butyl chloride (2-chloro- 2 - methylpropane) from tert -

butyl alcohol ( tert - butanol) using an acid catalyzed dehydration reaction. (Note: the correct IUPAC name for

this compound is 2-methyl- 2 - propanol).

We will also learn how to use a separatory funnel, the use and purpose of a drying agent and the

technique of distillation. The reaction is shown in Figure 3.

The first step of the overall reaction is an acid-base reaction between the t - butanol and the hydrochloric

acid. The t - butanol is a weak base and the hydrochloric acid is a strong acid. The alcoholic oxygen becomes

fully protonated and so the equilibrium lies far to the right. In the second step we have the slow loss of water to

form a carbocation intermediate. This species is very reactive and is immediately attacked by the chloride ion

liberated in the first step to form the final product. This is an example of an SN1 reaction (Substitution

Nucleophilic Unimolecular).

Physical Constants

Compound Mol. Wt (g/mol) Density (g/mL) b.p. (°C) m.p. (°C)

2 - Methyl- 2 - propanol 74.12 0.775 83 25 – 26

2 - Chloro- 2 -

methylpropane

Hydrochloric acid 37% soln in H 2 O 1.

Proper Use of the Separatory Funnel

Set up the separatory funnel as shown in Figure 3.2. Use a small iron ring stand to hold the funnel. You

can also use a metal clamp attached to the top neck of the separatory funnel but the iron ring is more

convenient. ALWAYS KEEP A BEAKER UNDERNEATH THE SEPARATORY FUNNEL TO CATCH

ANY SPILLS. When the handle of the stopcock is parallel to the separatory funnel (see shown in Fig. 3.2), the

stopcock is in the open position. When the handle is perpendicular to the length of the funnel it is closed.

With the stopcock in the closed position, pour in your solution into the top of the funnel. Insert the glass

stopper. Hold the funnel in one hand by grasping the top around the glass stopper. Lift it from the iron ring and

SLOWLY invert the funnel, holding it with your other hand. When it is now upside down, the glass stopper

should be in the palm of one hand, holding it securely. IMMEDIATELY VENT the funnel by opening the

stopcock and pointing the end of the funnel AWAY from your lab neighbor. CAUTION : always vent the

separatory funnel immediately when you invert it. Pressure can build up and the contents of the funnel can

shoot out uncontrollably.

Once you have vented the separatory funnel, close the stopcock and shake it GENTLY AT FIRST 2 - 3

times. Then VENT the funnel again. Now you can shake it more vigorously several times. Then vent it again.

You may hear the sound of escaping gases when you open the stopcock to vent. Once this subsides then it is

safe to shake the separatory funnel vigorously for 2-3 minutes. It is very important to shake the funnel very

thoroughly to ensure intimate mixing of the two layers.

When you have finished shaking, replace the separatory funnel back in the iron ring and REMOVE

THE GLASS STOPPER before draining off the bottom layer. Otherwise you will create a vacuum inside the

separatory funnel and it will not drain.

Note that if there are two layers in a separatory funnel it is because the two liquids contained in the

funnel are not soluble in each other. In most of our experiments, including the one today, we have a relatively

non-polar organic layer – usually containing the product that we want to isolate – and a polar aqueous layer.

The more dense layer is on the bottom. In many cases, this is the water layer (recall that oil floats on the

surface of water), but not always!! Be careful. When using the separatory funnel, ALWAYS SAVE BOTH

LAYERS until you are sure that you have isolated the correct layer. Never throw anything away until you are

finished with your experiment. To test which layer is which – organic or aqueous – simply add a little water to

the layer that you think is the aqueous layer. If you see only one layer then you were correct in thinking that

this was the aqueous layer.

Experimental Procedure

In your separatory funnel, place 35 mL of concentrated hydrochloric acid that has first been cooled to

5°C in an ice bath. Add 12.0 mL of t - butanol and swirl gently with the stopper off the separatory funnel.

You want to mix the t-butanol and the hydrochloric acid thoroughly. You can also stir the mixture with your

glass stirring rod.

Once the layers have been thoroughly mixed, allow the separatory funnel to stand with the stopper off

for 20 minutes. You will gradually see two layers forming. (What are they?)

Appendix 3.1 Distillation

Theory and Background:

Distillation is the separation of liquids by vaporization and condensation of the vapor back to the liquid

phase. It is an excellent and convenient way to purify liquids. The liquid to be purified is put into the distilling

flask which is heated to the boiling point of the liquid. The vapors then rise up to the thermometer at the top of

the three-way adapter which has a sidearm attached to the cold condensing column. When the hot vapors hit the

cold condensing column they condense back to liquid and are collected in the receiving flask.

The boiling point of a liquid is defined as the temperature at which its vapor pressure equals the

atmospheric pressure. It is characterized by vigorous bubbling of the liquid as it vaporizes. The actual

temperature of a boiling liquid is sometimes higher than the boiling point of the compound due to superheating,

which is ameliorated by the use of boiling chips.

The boiling point is a function of the atmospheric pressure. As we lower the atmospheric pressure we

also lower the boiling point. This relationship is shown in Fig 3.4 for acetone. At 760 mm Hg (atmospheric

pressure) the boiling point of acetone is 56.2 °C, but at 500 mm Hg it is only about 45 °C.

When a reasonably pure liquid is distilled, the observed temperature rises rapidly to the boiling point

(see Figure 3.5). Once the distillation apparatus has reached thermal equilibrium, the boiling point remains

relatively constant, not changing more than 1-2 ° during the distillation. When there is a significant drop in

temperature, it is a good signal that the desired compound has all distilled. But be careful here. Very often the

temperature can drop as the distillation proceeds because the volume of distillate decreases as you get toward

the end. Usually you need to increase the heat in order to drive all of the distillate up the column.