14. ALCOHOLS Flashcards
(18 cards)
Explain the difference in physical properties between alcohols and alkanes
Alkanes have non-polar bonds due to the similar electronegativity of the carbon and hydrogen atoms.
Therefore alkanes are non-polar.
The intermolecular forces between non-polar molecules are very weak london forces.
Alcohols contain polar O-H bonds due to the difference in electronegativity between the oxygen and hydrogen atoms.
Therefore alcohols are polar.
The intermolecular forces are weak London forces, however there will also be much stronger hydrogen bonds between the polar O-H groups.
Compare volatility between alcohols and alkanes
incl brief explanation
Alcohols are less volatile than alkanes.
Both alcohols and alkanes contain weak London forces, however alcohols also contain strong hydrogen bonds between the polar O-H groups.
These strong hydrogen bonds require more energy to overcome.
Compare melting point in alcohols and alkanes.
incl brief explanation
Alcohols have higher melting points than alkanes.
Both alcohols and alkanes contain weak London forces, however alcohols also contain strong hydrogen bonds between the polar O-H groups.
These strong hydrogen bonds require more energy to overcome.
Compare water solubility between alcohols and alkanes.
incl brief explanation
Alcohols have greater water solubility than alkanes.
Alcohols can form strong hydrogen bonds with water, alkanes cannot.
How does solubility differ with chain length?
Solubility decreases as chain length increases.
This is due to the influence of the -OH bond becoming relatively smaller as chain length increases.
What reactions do alcohols undergo?
- Combustion reactions
- Oxidation reactions
- Eliminations reactions
- Substitution reactions
What is the oxidising mixture for oxidising primary and secondary alcohols?
a solution of potassium dichromate (VI), K2Cr2O7, acidified with dilute sulfuric acid, H2SO4
What happens to the oxidising mixture if the alcohol is oxidised?
Orange -> green
How can you ensure an aldehyde is formed instead of a carboxylic acid?
Distill the aldehyde out of the reaction mixture as it is formed.
This prevents any further reaction with the oxidising agent
How are carboxylic acids formed (from oxidation of alcohols)?
Primary alcohol heated strongly under reflux with an excess of acidified potassium dichromate (VI).
Why is excess potassium dichromate (VI) used when trying to form a carboxylic acid?
To ensure that all of the alcohol is oxidised
Why do you heat under reflux when forming a carboxylic acid?
Ensures that any aldehyde formed initially in the reaction also undergoes oxidation to the carboxylic acid
What is a dehydration reaction?
Any reaction in which a water molecule is removed form the starting material.
Tell me about an alcohol elimination reaction
An alcohol is heated under reflux in the presence of an acid catalyst (conc. sulfuric acid or conc. phosphoric acid.
The products are an alkene and water.
Why is “alkene” less soluble in water than “alcohol”? [2]
“alkene” does not contain OH groups
and therefore does not form hydrogen bonds with water
Suggest why “alcohol with more OH groups” is less volatile than “other alcohol” [2]
“alcohol with more OH groups” has more OH groups
so stronger hydrogen bonding between molecules
Suggest why “alcohol with more branching” has a lower boiling point than “straight chain alcohol” (structural isomers) [2]
“alcohol with more branching” has weaker london forces
“alcohol with more branching” has less surface contact
State 6 uses of methanol
- As a fuel
- As a solvent
- As a petrol additive (to improve combustion)
- Feedstock
- Manufacture of biodiesel
- Manufacture of esters
