Carboxylic acids and their derivatives
Carboxylic acids contain the carboxyl functional group (-COOH). Their systematic names all end with -oic acid.
Remember: When counting carbons in the longest chain to name them, include the carbon in the functional group. The carboxylic carbon is assigned #1. When two acid groups are present, the ending is -dioic acid.
Acid-Base: Carboxylic acids partially dissociate in aqueous solution to form hydrogen ions (H+) and carboxylate ions (RCOO-). Remember: Acid + Base = Salt + Water
Esterification: Carboxylic acids react with alcohols in the presence of a strong acid catalyst (e.g. conc. sulphuric acid) when heated under reflux. This esterification reaction is reversible and comes to equilibrium during refluxing.
Remember: Carbonate + Carboxylic Acid = Salt + Water + Carbon Dioxide
Afterwards, test with limewater, which will turn milky in the presence of the carbon dioxide.
The OH group in alcohols, phenols and carboxylic a
The hydroxyl group (-OH) occurs in alcohols, phenols and carboxylic acids.
In alcohols, the -OH is attached to an aliphatic (non-aromatic) carbon chain. Phenols are compounds with one or more -OH groups attached directly to a benzene ring.
Remember: When neutral iron(III) chloride solution is added to a phenol/phenol derivative, a purple complex is formed. This does not happen with alcohols.
Alcohols, phenols and carboxylic acids are all weak acids. They react with water, producing oxonium ions (H30^+).
Acidic strength from weakest to strongest: ethanol < water < phenol < ethanoic acid
Strength of acids is determined by comparing stability of the anion.Phenoxide and carboxylate ions are more stable than hydroxide and ethoxide ions because the negative charge on the ion can be delocalised across several atoms.
Reaction with NaOH(aq): Ethanol (no reaction), Phenol (forms a salt), Ethanoic acid (forms a salt). Reaction with carbonates: Only ethanoic acid reacts. Fizzes - CO2 is released.
Esters are made by heating an alcohol and a carboxylic acid under reflux.
Requires an acid catalyst - conc. sulphuric acid or conc. hydrochloric acid.
Also known as a condensation reaction and is reversible; eventually reaches equilibrium.
When reaction is reversed, the ester is hydrolysed. Dilute sulphuric acid is a more effective catalyst in this case.
To make esters from phenols, an acyl chloride (at room temperature) or acid anhydride (heated under reflux) is required. Anhydrous conditions are essential.
Names of esters end in -oate. First part is from the alcohol, second part is from the acid used to make the ester.
Remember: The group attached to the C=O is always from the acid.
Hydrolysis is breakdown of a molecule by water. Can be carried out using an acid catalyst (sulphuric acid) or alkali catalyst (sodium hydroxide). Alkaline is preferred - reaction goes to completion. With alkaline, a carboxylate salt is formed.
Which reactions have the highest atom economy?
% Atom economy = (Mr of useful product / Mr of reactants used) x 100
Reactions are either:
Rearrangement and addition reactions have 100% atom economy. Nothing else is formed/lost during the process.
Substitution involves replacing one group in a molecule with another. Always two products formed, so atom economy is less than 100%.
Elimination involves removing a group from a molecule, so the atom economy is always less than 100%. Condensation reactions involve addition followed by elimination, so they are also less than 100%.
Aldehydes and ketones
Both contain the carbonyl group. Aldehydes end in -al and ketones end in -one.
Aldehyde: Reactant is primary alcohol, oxidising agent is acidified potassium dichromate(VI). Excess alcohol is used, aldehyde is distilled off straight away to prevent further oxidation to a carboxylic acid.
If primary alcohols are heated under reflux with acidified potassium dichromate(VI), the corresponding carboxylic acid is formed.
Ketone: Reactant is a secondary alcohol, heated under reflux with acidified potassium dichromate(VI). In both cases, a colour change from orange-green is seen.
Only aldehydes are further oxidised with APD(VI) because of the hydrogen atom on the carbonyl group. Aldehydes distinguished from ketones by reacting with Fehling's Solution; the aldehyde is oxidised to a carboxylic acid, copper ions reduced to copper(I) oxide, orange/brown precipitate form. Ketones don't react with Fehling's.
Sodium Tetrahydridoborate (NaBH4) reduces carbonyl compounds back to the alcohol.
Characterising acids is done by their ability to transfer H+ ions (protons). A base is a substance that accepts H+ ions. This is known as the Bronsted-Lowry theory.
In the reaction below, a proton is transferred from HNO3 to H2O. HNO3 is acting as the acid; H2O is acting as the base.
In many cases, the donation of a proton is reversible. When compounds alternate between being the acid or base, they are called a conjugate acid-base pair.
If a substance can act as an acid and a base, it is described as amphoteric.
Acids which are powerful H+ donors are strong acids, and weak acids are poor H+ donors. A strong acid has a weak conjugate base, and vice versa.
Indicators are weak acids. In order to behave as an indicator, the conjugate acid and base must be different colours.
Used to determine the structure of molecules.
The ions produced when molecules have been bombarded are Molecular Ions (M+). The molecular ion is often unstable and breaks up, in a process called fragmentation. Only positive ions from fragmentation pass through the spectrometer to the detector.
- The x-axis shows the mass-to-charge ratio, m/z
- The y-axis shows intensity
- Every line in the spectrum represents a positively charged ion. One line is the molecular ion, the other lines are fragments of the molecular ion or isotope peaks.
- Identify the molecular ion peak. At low resolution, this is the peak with highest m/z. This peak gives the relative molecular mass of the compound.
- List the masses of the other major peaks, then find the difference in mass between these peaks and the molecular ion peak.
- Identify any isotope peaks.