Elucidating Organic Molecules (Cambridge (CIE) A Level Chemistry): Revision Note

Elucidating Organic Molecules

• Students should be able to identify organic functional groups and recall their properties and the reactions that they undergo

Properties of functional groups

• In addition to the functional groups mentioned in the AS course, students should also be familiar with additional functional groups and their properties including:

  • Arenes

  • Halogenoarenes

  • Carboxylic acids (and derivatives)

  • Phenols

  • Amides

  • Amino acids

  • Acyl chlorides

Overview of additional functional groups

These functional groups are extra to those already covered in 21.1 Organic Synthesis

Reactions of functional groups

• Students should also be able to recall:

  • The reactions by which these functional groups can be produced including:

    • Benzoic acid

    • Acyl chlorides

    • Amides

  • The reactions that these functional groups undergo

Producing functional groups

Benzoic acid

• Benzoic acid is produced, from methylbenzene, in two steps: 

  1. An oxidation reaction by refluxing with hot alkaline KMnO4 to form potassium benzoate

  2. A protonation reaction with dilute HCl

Production of benzoic acid

Benzoic acid is produced from methylbenzene in 2 steps

Acyl chlorides

• Acyl chlorides are formed by an electrophilic substitution reaction of the parent carboxylic acid

• There are various possible reagents:

  • Solid PCl₅ - producing the acyl chloride along with POCl₃ and HCl

  • Liquid PCl₃ and heat - producing the acyl chloride and H₃PO₃ 

  • Liquid SOCl₂ - producing the acyl chloride along with SO₂ and HCl

Production of acyl chlorides

Using ethanoic acid as an example, the reactions all produce acyl chlorides with different byproducts due to the different reagents

Amides

• Amides are formed by a condensation reaction, at room temperature, of an acyl chloride

• The possible amides that can be produced are substituted amides and non-substituted amides

  • Non-substituted amides are produced by the reaction with ammonia

  • Substituted amides are produced by the reaction with primary amines

Production of substituted and non-substituted amides

Using propanoyl chloride as an example, the reaction with ammonia produces a non-substituted amide while the reaction with primary amines produces substituted amides

Reacting functional groups

Electrophilic substitution reactions of arenes

• You should be able to provide the mechanisms for specific electrophilic substitution reactions of benzene:

  • The electrophile, E⁺, for a halogenation reaction, is a positive halogen ion, X⁺  

  • The electrophile, E⁺, for a nitration reaction, is the nitronium ion, NO₂⁺  

  • The electrophile, E⁺, for an alkylation reaction, is a carbocation, R⁺  

  • The electrophile, E⁺, for an acylation reaction, is an acyl group, RCO⁺  

    • An acyl group is an alkyl group containing a carbonyl, C=O group

Specific electrophilic substitution reactions of arenes

Using different electrophiles, E⁺, the mechanisms for halogenation, nitration, alkylation and acylation are shown

Hydrogenation of arenes The hydrogenation of benzene is an addition reaction Benzene is heated with hydrogen gas and a nickel or platinum catalyst to form cyclohexane Hydrogenation of benzene

The hydrogenation of benzene causes a complete loss of aromaticity

Esterification of phenol

• The esterification reaction of carboxylic acids with phenol is slow, so acyl chlorides are used

• The reaction between ethanoyl chloride and phenol is similar to the reaction of ethanoyl chloride and ethanol, although it is not as vigorous

  • The products of this reaction are phenyl ethanoate and hydrogen chloride gas

• The reaction between benzoyl chloride and phenol is also similar to the reaction of ethanoyl chloride and ethanol

  • The products of this reaction are phenyl benzoate and hydrogen chloride gas

Formation of esters from the reaction of alcohols with acyl chlorides

The first part of the ester name comes from the alcohol and the second part of the ester name comes from the acyl chloride

• To make the reactions with acyl chlorides and aryl chlorides occur in a more timely fashion, phenol is converted into sodium phenoxide by heating with sodium hydroxide

  • This forms the phenoxide ion which is more reactive

  • The hydrogen chloride product is replaced by water and sodium chloride 

Acid / base reactions of phenol

Phenols dissolve in alkaline solutions to undergo acid-base reactions with bases forming a soluble salt and water Molten phenols also react vigorously with reactive metals such as sodium (Na) in an acid-base reaction forming a soluble salt and hydrogen gas  Acid / base reactions of phenol

Phenols behave as weak acids in alkaline conditions and molten phenols react vigorously with reactive metals to form a soluble salt and hydrogen gas

Nitration of phenol

Phenols undergo electrophilic substitution reactions with dilute nitric acid (HNO₃) at room temperature to give a mixture of 2-nitrophenol and 4-nitrophenol When concentrated HNO₃ is used, the product will be 2,4,6-trinitrophenol instead

Nitration of phenol

Phenols undergo nitration when reacted with dilute HNO₃ at room temperature

Bromination of phenol

Phenols undergo electrophilic substitution with bromine water at room temperature forming a white precipitate of 2,4,6-tribromophenol

Phenol in bromination reactions

Phenols undergo bromination when reacted with bromine water at room temperature