Preparation Of 1-bromobutane

Preparation of 1-Bromobutane

In this experiment you will prepare 1-bromobutane (n-butyl bromide) from nbutanol

(1-butanol) using a substitution reaction under acidic conditions. This is an SN2

reaction. The mechanism is shown in Figure 7.1. The purpose of the sulfuric acid is to

first protonate the weakly basic hydroxyl group and thereby convert it into a good leaving

group. It can then leave as neutral water.

CH3CH2CH2CH2 OH

CH3CH2CH2CH2

H

H

O

H O S

O

OH

O

CH3CH2CH2CH2 Br

CH3CH2CH2CH2 O

H

H

H2O

O S

O

OH

O

Na+

+ +

Na+ Br - + +

Figure 7.1 Preparation of 1-Bromobutane

+

1-butanol

1-bromobutane

sulfuric acid

sodium bromide

There is a side reaction that occurs that we cannot prevent: this is the formation of

HBr from the reaction of the sodium bromide, which is the source of the bromide

nucleophile, and the sulfuric acid, which we need in order to protonate the OH group of

the alcohol to make it into a good leaving group. As shown in Figure 7.2, a significant

equilibrium concentration of HBr will form. HBr, like HCl, is a gas that is quite soluble

in cold water, but not in hot water. Since you are going to bring the mixture to a boil,

HBr will escape from the apparatus. Being a strong acid, this gas will cause damage to

lungs when inhaled and will certainly sting your eyes. This is not an uncommon situation

in chemistry, but it can be handled safely. Your apparatus must be vented appropriately,

directly into the air vent above your bench (see Figure 7.3 for the proper set-up).

H O S

O

OH

O

H Br Na+ O S

O

OH

O

Na+ Br - + +

Figure 7.2 Formation of Hydrobromic Acid

(gas)

Physical Constants

Compound Mol. Wt (g/mol) Density

(g/mL)

b.p. (oC) m.p. (oC)

n-Butanol 74.14 0.810 117.7 -90

1-Bromobutane 137.03 1.276 100-104 -112

Sulfuric acid 98.08 1.840

Sodium bromide 102.90 solid 747

Procedure:

The set-up is shown in Figure 7.3. Place 25 mL of 65% sulfuric acid solution in

your 250 mL round bottom flask. Be careful to use the 65% H2SO4 and NOT the 85%

material that is available for a subsequent part of the experiment. Using the wrong

concentration leads to unpredictable and potentially dangerous results. Cool the flask to

5 -10 oC in an ice bath. Be sure to clamp your flask in place since an unsupported flask

in an ice bath can tip over when the ice melts.

When the solution is cold (5 -10 oC) slowly add 10 mL of 1-butanol a few mL at a

time using a pipette or medicine dropper. After each addition thoroughly mix and cool

the solution in the ice bath. When all of the 1-butanol has been added, weigh out 12.5 g

of solid sodium bromide (NaBr) in a small beaker. Slowly add the solid sodium bromide

in small portions. A good way to do this is to crease a piece of weighing paper and use

this as a funnel, scraping the solid material into your flask using your spatula. As you

add the solid NaBr, swirl the flask in order to thoroughly mix it. The sodium bromide

will only partially dissolve. When all of the NaBr has been added use your spatula to

remove any solid material that is adhering to the inside of the ground glass joint of your

250 mL round bottom flask. "Wet" the spatula in the solvent and use this to dissolve the

solid material or wipe the joint with a piece of wet paper towel several times. It is very

important to remove this solid so that you get a tight fit between the 250 mL flask and the

distilling column. Otherwise HBr gas may escape into the laboratory.

Attach the distilling column and glass absorption trap as shown in Figure 7.3.

Add 2-3 boiling chips and heat the flask to a gentle reflux using a setting of about 60 on

the rheostat. Again, the exact setting depends on the size of the heating mantle, etc. Do

not heat too quickly as the reaction can get out of control and HBr gas can escape. If you

do see HBr gas escaping immediately lower the heating mantle from the flask and call for

assistance