Chapter 14.2 - Reactions of alcohols Flashcards
What are the four reactions of alcohols?
- Combustion
- Oxidation
- Dehydration
- Substitution
What does the combustion of alcohols form?
Carbon dioxide and water
Write the symbol equation of the complete combustion of ethanol and balance
C₂H₅OH(l) + 3O₂(g) -> 2CO₂(g) + 3H₂O(g)
Is combustion an exothermic or endothermic reaction?
Exothermic reaction, releasing a large quantity of energy in the form of heat
What effect does increasing carbon chain length have on heat release?
As the number of carbon atoms in the alcohol chain increases the quantity of heat released per molecule also increases
What does the oxidation of alcohols form?
- Aldehydes
- Carboxylic acids
- Ketones
What alcohols can be oxidised?
Primary and secondary
What is the oxidising agent?
Acidified potassium dichromate - K₂Cr₂O₇|H₂SO₄
Written as [O] in equations
What colour change occurs if oxidation has taken place?
Orange to green
- cr 6+ becomes cr 3+
What can primary alcohols be oxidised into?
Aldehydes and carboxylic acids
Describe the conditions of the oxidation of a primary alcohol into an aldehyde
- Heat
- With K₂Cr₂O₇|H₂SO₄
- Distil
Describe the conditions of the oxidation of a primary alcohol into a carboxylic acid
- Heat strongly
- With excess K₂Cr₂O₇|H₂SO₄
- Reflux
Why is an excess of K₂Cr₂O₇|H₂SO₄ used when oxidising a primary alcohol into a carboxylic acid?
It ensures that all of the alcohol is oxidised
Why are primary alcohols heated strongly under reflux to form carboxylic acids?
To ensure that any aldehydes formed initially in the reaction also undergoes oxidation to the carboxylic acid
What are secondary alcohols oxidised into?
Ketones only
Describe the conditions of the oxidation of a secondary alcohol into a ketone
- Heat
- K₂Cr₂O₇|H₂SO₄
- Reflux
Why do tertiary alcohols not undergo oxidation? Is there a colour change?
They do not have a hydrogen atom attached to the carbon atom, so there is no colour change
What does the dehydration/elimination of alcohols form?
Alkenes
Describe the conditions of the dehydration of an alcohol into an alkene
- Heat
- Concentrated H₂SO₄ or H₃PO₄
- Reflux
What does the substitution of alcohols form?
Haloalkanes
Describe the conditions of the substitution of an alcohol into a haloalkane
- Heat
- NaX
- H₂SO₄
- Reflux
What is the one common by-product of reactions of alcohols?
H₂O
- water
Write out the reaction between a sodium halide and sulfuric acid when substituting alcohols, bromine for example
NaBr (s) + H₂SO₄ (aq) -> NaHSO₄ (aq) + HBr (aq)
-> because hydrogen halides are toxic
Write out the overall equation of the reaction of propan-2-ol with NaBr and H₂SO₄
CH₃CHOHCH₃ + NaBR + H₂SO₄ -> CH₃CHBrCH + NaHSO₄ + H₂
