Chapter 14: Alcohols

• OH functional group; hydroxyl group • CnH2n+1OH is the general formula Methanol - used for combustion (fuel); chemical feedstock - starting material in many industrial syntheses Ethanol - alcoholic drinks, fuel, solvents, feedstock

Alcohols are: - less volatile - have higher melting points - have greater water solubility

Alkanes: - Alkanes have non-polar bonds (as C and H have very similar electronegativity) - Alkane molecules are non-polar - Alkanes only have weak London forces Alcohols: - alcohols have a polar O-H bond due to difference in electronegativity of O and H, and Oxygen has 2 lone pairs of electrons - Alcohol molecules are polar - Alcohols will have weak London forces AND much stronger hydrogen bonds

• Hydrogen bonds between alcohol molecules in liquid state • require more energy to break in order to turn the alcohol liquid into a gas than just London forces in alkanes • Alcohols with the same number carbon atoms have lower volatility than alkanes.

Volatility is a measure of how easily a substance is converted from a liquid to a gas. low boiling point = low volatility high boiling point = high volatility

• Alcohols are soluble in water because • hydrogen bonds can form between the polar -OH groups of alcohol and water molecules • alkanes are non-polar, so cannot form hydrogen bonds with the water

As carbon chain increases, solubility of alcohol decreases because as hydrocarbon chain increases, the influence of the -OH group gets relatively smaller

• Primary alcohol: -OH group attached to 2 H atoms and 1 alkyl group • Secondary alcohol: -OH group attached to 1 H atom and 2 alkyl groups • Tertiary alcohol: - OH group attached to 3 alkyl groups

• alcohols burn in plenty supply of oxygen (i.e. complete combustion) • exothermic reaction - releases large quantities of energy • as number of carbon atoms increases (chain increases), heat released per mole increases, because longer chain alcohols will have a higher enthalpy change (more negative)

Primary and secondary alcohols can be oxidised by an oxidising agent. Conditions: - acidified potassium dichromate (VI), K2Cr2O7 - dilute sulphuric acid, H2SO4 (to acidify the potassium dichromate)

If alcohol is oxidised, the solution goes from orange to green. BECOMES GREEN. This is because dichromate (VI) ions (orange solution) are reduced to chromium (III) ions (green solution). Cr2O7 2- –> Cr3+

• aldehyde • carboxylic acid

• oxidation of primary alcohol • gentle heating • acidified potassium dichromate (VI) • aldehyde is distilled out of reaction mixture as it forms (water is also produced).

• oxidation of primary alcohol • heated strongly under reflux • excess of acidified K2Cr2O7, so all of the alcohol is oxidised. (water is also produced).

You heat the primary alcohol under reflux, so that any aldehyde formed initially undergoes oxidation to the carboxylic acid

The continual boiling and condensing of a reaction mixture back to the original container to ensure that the reaction takes place without the contents of the flask boiling dry.

Boiling dry is when liquid boils without sufficient replenishment (of the liquid), leading to localised overheating, which is when the surface beneath overheats. This leads to damage of the equpiment. You don’t want that. (obvs)

You form ketones

• Oxidisation of secondary alcohols • oxidised to ketones • use acidified dichromate (VI) ions • heated under reflux to ensure reaction goes to completion • dichromate (VI) ions go from orange to green

TRICK QUESTION - YOU CAN’T OXIDISE TERTIARY ALCOHOLS. This is because tertiary alcohols don’t contain the necessary H atom to form a carbonyl group.

Any reaction in which a water molecule is removed from the starting material.

• alcohol is heated under reflux • presence of acid catalyst e.g. sulphuric or phosphoric acid (h2so4 or h3po4) • produces alkene + water

Heat an alcohol under reflux in the presence of a phosphoric acid catalyst