These are detailed Revision Notes for Topic 3.7 and 3.8 of AQA A-Level Chemistry (Aldehydes, Ketones, Carboxylic acids and derivatives). They were written by me using a combination of the textbook and class notes. I will also be uploading the other topics and creating bundles.
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Compounds Containing the Carbonyl Group
26.1 Introduction to Aldehydes and
Ketones: 26.1 Introduction to aldehydes and ketones
- The carbonyl group is a carbon-oxygen
double bond. This group is in both 26.2 reactions of the carbonyl group in aldehydes and ketones
aldehydes and ketones. 26.3 carboxylic acids and esters
- In ketones this group is in the middle
of the carbon chain, and in aldehydes 26.4 reactions of carboxylic acids and esters
this group is on the end carbon. 26.5 acylation
- Aldehydes are named using the suffix
-al, ketones are named using the suffix -one.
- The carbon oxygen double bond is strongly polar, so they have permanent dipole-dipole forces.
- Their boiling pints are higher than hydrocarbons but lower than alcohols because in alcohols there is
hydrogen bonding between the molecules.
- Shorter chain molecules are soluble as they can form hydrogen bonds with water, as the carbon chain
increases in length, they become less soluble.
- Because of the polar double bond, addition reactions are possible.
26.2 Reactions of the Carbonyl Group in Aldehydes and Ketones:
Nucleophilic addition reactions:
- Carbonyl compounds can be used to make
hydroxynitriles by a nucleophilic addition reaction.
- This reaction is important as it increases the carbon
chain by one carbon.
- This reaction will produce a racemic mixture, as
the :CN- ion can attack from above or below
the C=O group.
Oxidation:
- Aldehydes can be oxidised to carboxylic acids.
- This is usually done by acidified potassium
dichromate (VI), K2Cr2O7/H+
- Ketones cannot be easily oxidised because a C-C bond must be broken.
- As only ketones can be oxidised, tests can be done to distinguish them.
- The Silver Mirror Test (Tollens’ Reagent)
o When an aldehyde is warmed with Tollens’ Reagent a silver mirror is formed, as it is oxidised to
a carboxylic acid.
o Ketones give no reaction to this test.
- Fehling’s Test
o When an aldehyde is warmed with Fehling’s solution, a brick red precipitated is produced as it
oxidises to a carboxylic acid.
o Ketones give no reaction to this test.
Reduction:
- Aldehydes can be reduced to primary alcohols and ketones can be reduced to secondary alcohols.
- The reducing agent is normally NaBH4 in aqueous solution, which forms the :H- ion
- [H] is used to represent reduction in equations
- This is a nucleophilic addition reaction.
, 26.3 Carboxylic Acids and Esters:
- The functional group of carboxylic acids is -COOH. They are named using the -oic acid suffix.
- Because carboxylic acids have two different functional groups on the same carbon atom (-OH and C=O)
this changes the properties of each group. The -OH in the carboxylic acid is much more acidic than in
alcohols.
- Carboxylic acids can form hydrogen bonds with water, so up to 4 carbons long they are soluble in
water.
- Carboxylic acids can also form hydrogen bonds with itself in the solid state, so they
have high melting points.
- Esters are derived from carboxylic acids. The general formula is RCOOR’.
- Esters are named with the alcohol it was derived from as the prefix and the carboxylic
as the second word. E.g., methyl ethanoate.
- Esters are volatile and have fruity smells
- They are used as solvents and plasticisers.
26.4 Reactions of Carboxylic Acids and Esters:
Reactions of Carboxylic acids:
- If the hydrogen of the -OH group is lost, you are left with a negative carboxylate ion This
negative charge is shared over the whole of the carboxylate group.
- This delocalisation of electrons makes the ion more stable, so they are weak acids.
- They react more slowly than strong acid because there are fewer H+. However, they are still strong
enough to release carbon dioxide from sodium hydrogencarbonate (NaHCO3). This distinguishes them
from alcohols. CH3COOH(aq) + NaHCO3(aq) àCH3COONa(aq) + H2O(l) + CO2(g)
- Carboxylic acid + base à salt + H2O
o Ethanoic acid + sodium hydroxide à sodium ethanoate + water
o CH3COOH(aq) + NaOH(aq) à CH3COONa(aq) + H2O(l)
- Carboxylic acid + metal à salt + H2
o Ethanoic acid + magnesium à magnesium ethanoate + hydrogen
o 2CH3COOH(aq) + Mg(s) à (CH3COO)2Mg + H2
- Carboxylic acid + carbonate à salt + CO2 + H2O
o Ethanoic acid + sodium carbonate à sodium ethanoate + water + carbon dioxide
o 2CH3COOH(aq) + Na2CO3(aq) à 2CH3COONa(aq) + H2O(l) + CO2(aq)
Formation of Esters:
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