Core Practical 6: Investigating chlorination of 2-methylpropan-2-ol

Question 1

2-methylpropan-2-ol is

Answer 1

flammable and harmful

Question 2

concentrated hydrochloric acid is

Answer 2

corrosive and the fumes of HCl it produces are toxic and corrosive
therefore, carry this practical out in a fume cupboard or well ventilated room

Question 3

2-chloro-2-methylpropane is

Answer 3

flammable

Question 4

calcium chloride is

Answer 4

an irritant

Question 5

the method for the initial reaction to produce crude 2-chloro-2-methylpropane is:

Answer 5

• note that 2-methylpropan-2-ol is a tertiary alcohol, so can be converted to a halogenoalkane without using PCl5 but concentrated HCl acid instead
• mix 10cm3 of 2-methylpropan-2-ol with 35cm3 of concentrated hydrochloric acid in a conical flask, swirl and release the bung at intervals to release pressure, for 20 minutes
• two layers will form in the flask, the upper layer being the crude product

Question 6

after making the impure product in the conical flask, the steps for obtaining a pure sample of 2-chloro-2-methylpropane is:

Answer 6

• add 6g of anhydrous calcium chloride to the conical flask (this ensures any unreacted alcohol, 2-methylpropan-2-ol, is removed to the aqueous layer on the bottom
• pour the contents of the flask into a separating funnel and allow the layers to settle, then run off and discard the lower layer
• add about 20cm3 of sodium hydrogencarbonate (add in portions, not in one go) to the funnel, place the bung, invert the funnel several times, then tilt the funnel upside down and open the tap to release the pressure form the CO2 gas produced from the neutralisation reaction with the acid
• discard the lower aqueous layer and repeat the previous steps with washings of NaHCO3
• runoff and discard the aqueous (bottom) layer, ensuring none remains
• runoff the organic layer into a small conical flask, and add a spatula full of anhydrous sodium sulfate and swirl the flask until the liquid is completely clear
• decant the liquid into a pear-shaped flask
• set up the apparatus for simple distillation, and distil the liquid at the fractional range of 50-52° C

Question 7

the set up for simple distillation is:

Answer 7

• attach the pear-shaped flask to a linking tube and clamp the condenser so that it is laterally downwards
• place the thermometer through the bung at the top of the linking tube, but leave the end of the condenser open to drip into a conical flask or beaker
• sit the pear-shaped flask in a water bath and commence heating
• monitor the temperature and begin collection of the product boiling over between 50-52° C, then stop heating once the thermometer reads a higher temperature

Question 8

Write an equation for the reaction of 2-methylpropan-2-ol with concentrated hydrochloric acid

Answer 8

(CH3)3COH + HCl → (CH3)3CCl + H2O

Question 9

What is removed from the crude product when it is shaken with sodium hydrogencarbonate solution? Write an equation for any reaction that occurs.

Answer 9

-excess hydrochloric acid is removed through a neutralisation reaction with a base

NaHCO3 + HCl → NaCl + H2O + CO2

Question 10

“2-methylpropan-2-ol has a boiling temperature of 82 °C and is soluble in water. 2-chloro-2-methylpropane has a boiling temperature of 51 °C and is insoluble in water.”

Explain these differences [3]

Answer 10

2-methylpropan-2-ol can form hydrogen bonds, 2-chloro-2-methylpropane cannot. 2-methylpropan-2-ol is able to form hydrogen bonds with water and so it is soluble in water. 2-chloro-2-methylpropane cannot form hydrogen bonds with water and so is insoluble in water. Intermolecular hydrogen bonds in 2-methylpropan-2-ol are stronger than the dipole-dipole attractions and London forces between molecules in 2-chloro-2-methylpropane so more energy is required to separate the molecules.