Organic compounds containing oxygen — Eduqas A-Level Chemistry

This topic covers the chemistry of organic compounds that contain oxygen, including alcohols, phenols, aldehydes, ketones, carboxylic acids, and esters. You will explore their structures, nomenclature, physical properties, and chemical reactions, with a focus on reaction mechanisms such as nucleophilic addition and nucleophilic substitution. Understanding these compounds is essential because they are fundamental to many biological processes and industrial applications, from fuels and solvents to pharmaceuticals and polymers.

In the WJEC A-Level Chemistry specification, this topic builds on your knowledge of alkanes, alkenes, and haloalkanes, and introduces new functional groups that exhibit distinct reactivity. You will learn how the presence of oxygen atoms influences polarity, boiling points, and solubility, and how to interconvert between different oxygen-containing compounds. Mastery of this topic is crucial for success in organic chemistry questions, which often require you to predict products, propose mechanisms, and explain properties.

Beyond exams, these compounds are everywhere: ethanol is a biofuel and disinfectant, ethanoic acid gives vinegar its sour taste, and esters are responsible for many fruit flavours. By understanding their chemistry, you gain insight into how everyday substances are made and how they behave, making this topic both academically important and practically relevant.

• →Functional groups and nomenclature: recognise and name alcohols (primary, secondary, tertiary), phenols, aldehydes, ketones, carboxylic acids, and esters using IUPAC rules.
• →Oxidation reactions: primary alcohols oxidise to aldehydes then carboxylic acids; secondary alcohols oxidise to ketones; tertiary alcohols do not oxidise. Know reagents (e.g., acidified K₂Cr₂O₇) and colour changes.
• →Nucleophilic addition in carbonyl compounds: aldehydes and ketones undergo nucleophilic addition with HCN (forming hydroxynitriles) and with 2,4-DNPH (test for carbonyls). Understand the mechanism and why aldehydes are more reactive than ketones.
• →Esterification and hydrolysis: carboxylic acids react with alcohols in the presence of an acid catalyst to form esters (condensation reaction). Esters can be hydrolysed by acid or base (saponification).
• →Acidity of carboxylic acids and phenols: carboxylic acids are weak acids (react with carbonates to release CO₂); phenols are weaker acids but stronger than alcohols. Understand the effect of substituents on acidity.

• Distinguishing between primary, secondary, and tertiary alcohols
• Oxidation products of alcohols using acidified dichromate(VI)
• Nucleophilic addition mechanisms for aldehydes and ketones
• Use of Tollens' reagent and Fehling's reagent to distinguish aldehydes from ketones
• Acidity trends and reactions of carboxylic acids and phenols
• Esterification and hydrolysis reactions
• Identification of carbonyl groups using 2,4-dinitrophenylhydrazine
• Identification of methyl carbonyl groups using the triiodomethane (iodoform) test

• 💡Ensure you can draw and name all functional groups accurately
• 💡Memorize the specific reagents and conditions for each oxidation and reduction reaction
• 💡Practice writing mechanisms clearly, showing curly arrows and lone pairs
• 💡Be prepared to describe practical procedures like distillation and reflux in detail
• 💡Use the provided functional group tests to construct logical identification schemes for unknown compounds
• 💡When drawing mechanisms, always show curly arrows starting from a lone pair or a bond, and ensure the arrow points exactly to where the electrons are going. For nucleophilic addition, the arrow from the nucleophile must go to the δ+ carbon of the carbonyl group.
• 💡In oxidation questions, state the reagent (acidified potassium dichromate(VI)), the colour change (orange to green), and the conditions (warm gently for aldehyde, distill or reflux for carboxylic acid). This shows precise knowledge.
• 💡For esterification, remember the acid catalyst (concentrated H₂SO₄) and that the reaction is reversible. Use the equilibrium arrow (⇌) and mention that water is removed to drive the equilibrium to the right.

• Confusing the oxidation products of primary and secondary alcohols
• Incorrectly identifying the mechanism for nucleophilic addition
• Failing to distinguish between the conditions for oxidation to aldehydes versus carboxylic acids
• Misinterpreting the results of functional group tests like Tollens' or Fehling's
• Confusing the acidity of phenols with that of carboxylic acids
• Misconception: All alcohols can be oxidised. Correction: Only primary and secondary alcohols can be oxidised; tertiary alcohols have no hydrogen on the carbon bearing the –OH group, so they do not undergo oxidation under normal conditions.
• Misconception: Aldehydes and ketones both give positive tests with Tollens' reagent. Correction: Only aldehydes reduce Tollens' reagent (silver mirror); ketones do not because they lack a hydrogen atom on the carbonyl carbon.
• Misconception: Carboxylic acids are strong acids. Correction: Carboxylic acids are weak acids; they only partially dissociate in water. For example, ethanoic acid has a pH around 3-4 in dilute solution.