Alcohols Flashcards
Compare the physical properties of alcohols with alkanes.
Alcohols are less volatile, have higher melting points and greater water solubility than the corresponding alkanes.
Explain the polarity of the bonds in both the alkanes and alcohols and the effect that the bonds have on the strength of the intermolecular forces.
The alkanes have non-polar bonds because the electronegativity of hydrogen and carbon are very similar. Alkane molecules are therefore non-polar. Alcohols have a polar O-H bond because of the difference in electronegativity of the oxygen and hydrogen atoms. Alcohol molecules are therefore polar.
Why do alcohols have a lower volatility than the alkanes with the same number of carbon atoms?
In alcohols, intermolecular hydrogen bonds hold the alcohol molecules together. In alkanes, London forces hold the molecules together. It takes far more energy to overcome the hydrogen bonds so alcohols have a lower volatility than alkanes
Why are alkanes not soluble in water but alcohols are?
Alkanes are non-polar molecules and cannot form hydrogen bonds but alcohols can form hydrogen bonds with the OH groups in water and so are soluble.
True or false? Solubility decreases the longer the chain becomes.
True
What are the products of complete combustion of alcohols?
Carbon dioxide and water
What is the most common oxidising agent?
Potassium dichromate (K2Cr2O7)
What must the potassium dichromate be reacted with before it can act as an oxidising mixture?
Dilute sulphuric acid.
What colour change would you expect to see if the alcohol is oxidised?
From orange to green.
Describe what happens when a primary alcohol is oxidised by gently heating with acidified potassium dichromate.
An aldehyde (C=O, C-H at the end) is formed and the colour of the solution changes from orange to green.
If an aldehyde is heated under reflux, with an excess of acidified potassium dichromate, what product is formed?
A carboxylic acid (C=O, C=OH at the end)
Describe what happens when a secondary alcohol is oxidised by gently heating with acidified potassium dichromate.
A ketone (C=O on middle carbon) is produced and the solution will change from orange to green.
What colour change will occur when a tertiary alcohol is reacted with an acidified oxidising agent? Why?
No colour change will occur because no reaction takes place.
What is a dehydration reaction of an alcohol?
A reaction which causes a molecule of water to be removed from the starting material.
What conditions are required for a dehydration reaction to take place?
An alcohol must be heated under reflux in the presence of an acid catalyst such as concentrated sulphuric acid or concentrated phosphoric acid.
