What is the general formula of an alcohol?
CnH2n+1OH
When naming an alcohol, how are they named?
The OH has the smallest number before it. Say you have a 5 carbon length chain and the OH is on the 2 and 3 one depending on where you’re reading from. The OH will be 2-OH.
OH is always on the parent chain regardless if there’s a longer chain.
OH takes priority over alkenes and alkanes, but ketones, aldehydes and esters take priority over OH.
What are the physical properties of alcohols in comparison with alkanes?
Alcohols are less volatile, have a high boiling point and high solubility in water (polar solvents).
Alkanes are more volatile, have a low boiling point and a low solubility in water (polar solvents).
Why are there differences in the physical properties of alcohols and alkanes?
Alkanes are non-polar molecules because the difference in electronegativity for C and H is very low and so the bonds are non-polar. This means that there is no dipole acting across the atoms of the compound. The intermolecular forces between the non-polar molecules are very weak London forces.
Regarding alcohols, the hydroxyl is polar because there is a large difference in the electronegativities of hydrogen and oxygen. This creates a dipole across the atoms of the compound. Therefore, they have London forces and hydrogen bonds.
Why are alcohols less volatile with a higher boiling point than alkanes?
Hydrogen bonds and London forces act between alcohol molecules, whereas only London forces act between alkanes. Therefore, as more energy is required to overcome them, they have a lesser tendency to become a gas and have a higher boiling point.
Why are alcohols soluble in water but alkanes aren’t?
Methanol is polar, therefore it dissolves easily in polar solvents like water because they can form hydrogen bonds with the water as they are attracted to each other.
Alkanes are non-polar so they don’t dissolve easily in polar solvents because they cannot form hydrogen bonds with the water as they are not attracted to each other.
Why does solubility decrease when you lengthen the carbon chain of an alcohol?
The influence of the OH group becomes relatively smaller.
What are primary alcohols?
OH is attached to a carbon with two hydrogens and one alkyl group.
What are tertiary alcohols?
OH is attached to a carbon with three alkyl groups.
What are secondary alcohols?
OH is attached to a carbon with one hydrogen and two alkyl groups.
What are the reactions of alcohols?
Oxygen in a combustion reaction
Oxidising agents, e.g acidified potassium dichromate (H+K2Cr2O7)
Dehydration (elimination)
Substitution
What is the oxidation of alcohols?
Primary and secondary alcohols can be oxidised by an oxidising agent, often acidified potassium dichromate.
If the alcohol is oxidised, the orange dichromate ions go to green chromium ions.
Cr2O7 2- — Cr 3+
What can a primary alcohol oxidise into?
Aldehyde
Carboxylic acid
What is the oxidation of a primary alcohol into an aldehyde?
Alcohol + acidified potassium dichromate [O] = (DISTILLATION) = aldehyde + water
Why is the alcohol distilled when being made into aldehyde?
To prevent the aldehyde from oxidising further into a carboxylic acid.
What is the oxidation of a primary alcohol into a carboxylic acid?
Alcohol + 2[O] = (UNDER REFLUX AT HIGH TEMPERATURES) carboxylic acid + water
OR
Alcohol + [O] = (DISTILL) aldehyde + [O] = (REFLUX) carboxylic acid + water
What are secondary alcohols oxidised into?
Ketones using acidified potassium dichromate.
Alcohol + [O] = (REFLUX) ketone + water
What is the dehydration of alcohols?
Elimination reaction where a water molecule is removed from an alcohol.
Alcohol is heated under reflux in the presence of an acid catalyst e.g sulfuric or phosphoric acid.
What is a substitution alcohol reaction?
Hydrogen halides are toxic and therefore made in stitu (in place).
The alcohol is first reacted with a halogen carrier e.g NaBr, which which produces the hydrogen halide e.g HBr. Acid catalyst is required for this step.
Hydrogen halide then reacts with the alcohol to form haloalkane and water.
Alcohol + halogen carrier (+ acid) = hydrogen halide.
Alcohol + hydrogen halide = haloalkane + water
