Organic Chemistry

Question 1

Describe a test for the presence of the -OH group.

Answer 1

React with PCl5.

Steamy (white) fumes of HCl gas form.

Question 2

Describe how to oxidise alcohols using:

a) distillation
b) reflux

Answer 2

a) distillation: collecting products according to boiling point (aldehydes & ketones)
Product (e.g. Ketone) distils off at lower boiling temp than reactant
b) reflux: converting alcohol to carboxylic acid (goes to completion)
Vertical condenser prevents vapours from leaving the flask.

Question 3

Describe how you can prepare halogenoalkanes from alcohols and hydrogen halides.

Answer 3

Place NaBr, alcohol and water in a flask. Run conc H2SO4 into flask and reflux. Set up distillation & collect layers. Organic layer with product is purified and redistilled.
Organic layer: unreacted alcohol, bromine & alkene
Aqueous layer: unreacted alcohol, sulfur oxides, HBr & water

Question 4

Why should you use phosphoric acid instead of sulphuric acid for iodoalkanes?

Answer 4

Sulphuric acid is a strong oxidising agent so it oxidises iodide ions to iodine and produces little HI.

Question 5

How and why does the rate of hydrolysis of halogenoalkanes change down the group?

Answer 5

Chloroalkane

Question 6

How can you form the following from a halogenoalkane:

a) alkene
b) alcohol
c) diol
d) nitrile
e) amine

Answer 6

a) alkene - KOH/NaOH in ethanol
b) alcohol - warm KOH/NaOH aqueous
c) diol - alcoholic KOH to alkene, then dilute KMnO4 / H+
d) nitrile - hot NaCN/KCN in ethanol
e) amine - heat NH3 in a sealed tube or warm conc NH3 in ethanol

Question 7

How can you make a halogenoalkane from:

a) an alkane
b) an alkene
c) an alcohol

Answer 7

a) alkane - Br2/Cl2 and UV light
b) alkene - HBr (aq)
c) alcohol - anhydrous PCl5

Question 8

How does a refrigerant work?

Answer 8

A refrigerant is a liquid that changes state from liquid to gas (vaporises) and back by absorbing heat and transferring heat to the outside.

Question 9

Give properties of a good refrigerant.

Answer 9

1. High enthalpy of vaporisation
2. Not toxic or flammable or corrosive
3. High density in vapour form
4. Works at temp below refrigerator

Question 10

How does a refrigerant work as a fire retardant in fire extinguishers?

Answer 10

They absorb heat from the fire
They prevent oxygen from reaching fire
They absorb free radicals in combustion propagation

Question 11

What is flammability?

Answer 11

A measure of ability of a fabric to burn and propagate a flame or fire

Question 12

What is a CFC?

Answer 12

A compound containing C-Cl and C-F bonds

Question 13

What is the problem with CFCs with the ozone layer?

Answer 13

CFCs cause the depletion of the ozone layer.
Chlorine free radicals are formed which breakdown ozone, and are regenerated.
A chain reaction takes place where one free radical destroys several ozone molecules.
Ozone absorbs UV light so depletion means more UV reaches the Earth’s surface and increases risk of skin cancer and eye cataracts.

Question 14

Give an advantage and disadvantage of using HCFCs and butane.

Answer 14

HCFCs: most are broken down in lower atmosphere so pose smaller risk to ozone layer. HCFCs are potent greenhouse gases despite low conc.
Butane: is a good alternative to CFCs but it is flammable.

Question 15

State what you would observe when sodium reacts with water or ethanol.
What is the difference between the two solvents?

Answer 15

Observation:
Effervescence, sodium getting smaller as it dissolves and mixture getting warmer.
Sodium is less dense than water, so sodium floats.
Sodium is more dense than ethanol, so it sinks.
Reaction between water is more vigorous than with ethanol.

Question 16

Why do tertiary halogenoalkanes undergo SN1 while primary halogenoalkanes undergo SN2?

Answer 16

Tertiary: have stable carbocations due to electron donating methyl groups. They experience steric hinderance (blocking of OH- by large groups)
Primary: do not have steric hinderance so they form unstable carbocations.