Core Practical 4: Investigation of hydrolysis rates of halogenoalkanes

Question 1

halogenoalkanes are

Answer 1

flammable and harmful

Question 2

ethanol is ……………… therefore you should

Answer 2

flammable

should obtain hot water from a kettle rather than using a Bunsen burner to heat the water bath

Question 3

silver nitrate is

Answer 3

corrosive

Question 4

the method for testing the rates of hydrolysis of halogenoalkanes based on the halogen atom present, but with the same structure, is:

Answer 4

• set up a water bath at 50 degrees C by filling a 250cm beaker up by 3/4
• in 3 test tubes, add 5cm3 of ethanol
• in the first test tube, add four drops of 1-iodobutane, and do the same for the second and third test tubes with 1-bromobutane and 1-chlorobutane, then label the tubes
• loosely pace a bung on each test tube and place the tubes in the water bath
• add 5cm3 of silver nitrate to three new test tubes, and place them in the water bath
• once all six tubes have reached the temperature of the water bath, remove the bung from the tube containing the 1-iodobutane and ethanol mixture, and add the silver nitrate from one of the test tubes, starting a stopwatch as soon as you do this, then record the time it takes for the mixture to go cloudy as a precipitate appears
• repeat with the other two halogenoalkanes, and record their times

Question 5

the method for testing the rates of hydrolysis of halogenoalkanes based on the structure, but with the same halogenoalkane, is:

Answer 5

• set up a water bath at 50 degrees C by filling a 250cm beaker up by 3/4
• in 3 test tubes, add 5cm3 of ethanol
• in the first test tube, add four drops of 1-bromobutane, and do the same for the second and third test tubes with 2-bromobutane and 2-bromo-2-methylpropane, then label the tubes
• loosely pace a bung on each test tube and place the tubes in the water bath
• add 5cm3 of silver nitrate to three new test tubes, and place them in the water bath
• once all six tubes have reached the temperature of the water bath, remove the bung from the tube containing the 1-bromobutane and ethanol mixture, and add the silver nitrate from one of the test tubes, starting a stopwatch as soon as you do this, then record the time it takes for the mixture to go cloudy as a precipitate appears
• repeat with the other two halogenoalkanes, and record their times

Question 6

what is the trend of reactivity for:

i) the different halogeoalkanes
ii) different structures

explain!

Answer 6

i) For the different halogenoalkanes:
rate is greatest for 1-iodobutane, and slowest for 1-chlorobutane
Iodine is a better leaving group than bromine or chlorine as its larger atom means I had a lower charge density and so is more weakly attracted to the carbocation than Br of Cl
this means the activation energy for the reaction is lower, so the rate is greater for I
ii)For the different structures:
rate is greatest for 2-bromo-2-methylpropane (tertiary) and slowest for 1-bromobutane (primary)
The tertiary structure has a three electron-releasing methyl groups which ‘push’ electrons towards the carbocation, so reducing its charge and stabilising it.

Question 7

Write an equation for the reaction of 1-bromobutane with water:

Answer 7

CH3CH2CH2CH2Br + H2O → CH3CH2CH2CH2OH + HBr

Question 8

Explain why ethanol is used in these reactions:

Answer 8

halogenoalkanes are immiscible with water and so ethanol is used to dissolve the halogenoalkane to ensure it dissolves with the water molecules

Question 9

Explain why water is able to act as a nucleophile.

Answer 9

-Water has lone pair(s) of electrons on the oxygen atom.

Question 10

Explain why water is used as the nucleophile rather than hydroxide ions?

Answer 10

-if OH- ions were used, a precipitate of silver hydroxide, AgOH, a brown solid, would form instantly