Reactions of aldehydes and ketones Sunday, 11 November 2018

Reactions of aldehydes and ketones Lesson Objective: To know some reactions of aldehydes and ketones Success criteria: To understand the reactions of carbonyl compounds with lithium tetrahydridoaluminate (lithium aluminium hydride) in dry ether (in equations the reducing agent can be represented by [H]) To understand the reactions of carbonyl compounds with HCN, in the presence of KCN, as a nucleophilic addition reaction, using curly arrows, relevant lone pairs, dipoles and the evidence of optical activity to show the mechanism To understand the reactions of carbonyl compounds with iodine in the presence of alkali

Reduction of aldehydes and ketones Remember from year 12, primary alcohols can be oxidised to aldehydes and secondary alcohols can be oxidised to ketones. Using a reducing agent you can reduce aldehydes back to primary alcohols. Equation [H] = the reducing agent

Reduction of aldehydes and ketones Remember from year 12, primary alcohols can be oxidised to aldehydes and secondary alcohols can be oxidised to ketones. Using a reducing agent you can reduce ketones back to secondary alcohols. Equation [H] = the reducing agent

Reduction of aldehydes and ketones Remember from year 12, primary alcohols can be oxidised to aldehydes and secondary alcohols can be oxidised to ketones. Suitable reducing agents are LiAlH4 (lithium tetrahydrialuminate (III) also known as lithium aluminium hydride) in dry ether. It is a very powerful reducing agent which reacts violently with water bursting into flames. [H] = the reducing agent Ethers are a class of organic compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula R–O–R′, where R and R′ represent the alkyl or aryl groups.

Reactions of aldehydes and ketones Lesson Objective: To know some reactions of aldehydes and ketones Success criteria: To understand the reactions of carbonyl compounds with lithium tetrahydridoaluminate (lithium aluminium hydride) in dry ether (in equations the reducing agent can be represented by [H]) To understand the reactions of carbonyl compounds with HCN, in the presence of KCN, as a nucleophilic addition reaction, using curly arrows, relevant lone pairs, dipoles and the evidence of optical activity to show the mechanism To understand the reactions of carbonyl compounds with iodine in the presence of alkali

Reaction of carbonyls with hydrogen cyanide Hydrogen cyanide reacts with carbonyl compounds to produce hydroxynitriles (compounds with a CN and an OH group). It is carried out in an aqueous alkaline solution containing potassium cyanide (KCN). The KCN increases the concentration of the CN- ions (cyanide ions), allowing the reaction to happen quicker. It is a nucleophilic addition reaction A nucleophile attacks the molecule and adds itself. Safety hydrogen cyanide is a highly toxic gas. The reaction should be carried out in a fume cupboard, wearing gloves, goggles and a lab coat

Reaction of carbonyls with hydrogen cyanide Mechanism The CN- ion attacks the slightly positive carbon atom and donates a pair of electrons to it. Both electrons from the double bond transfer to the oxygen. H+ bonds to the oxygen to form the hydroxyl group (OH) The carbon chain is increased by one C atom

Reaction of carbonyls with hydrogen cyanide Information about the optical activity of the hydroxynitrile can provide evidence for the reaction mechanism The groups surrounding the carbonyl carbon in a ketone or aldehyde are planar This means that the nucleophile (CN- ion) can attack it from either side. So there is an equal chance of it attacking from above or below the plane. When you react an aldehyde or asymmetric ketone with CN- you get a racemic mixture of two optical isomers. This is because the carbonyl group gets attacked equally from each side producing equal amounts of the two optical isomer products The product is a racemic mixture and optically inactive.

Reactions of aldehydes and ketones Lesson Objective: To know some reactions of aldehydes and ketones Success criteria: To understand the reactions of carbonyl compounds with lithium tetrahydridoaluminate (lithium aluminium hydride) in dry ether (in equations the reducing agent can be represented by [H]) To understand the reactions of carbonyl compounds with HCN, in the presence of KCN, as a nucleophilic addition reaction, using curly arrows, relevant lone pairs, dipoles and the evidence of optical activity to show the mechanism To understand the reactions of carbonyl compounds with iodine in the presence of alkali

Reaction of carbonyls with iodine Triiodomethane, CHI3, is an insoluble yellow solid It forms in reactions between certain carbonyl compounds and an alkaline solution of iodine It will form if there’s a methyl carbonyl group present in the carbonyl compound reactant. This means that ethanol is the only aldehyde that will react with iodine. Examples

Reactions of aldehydes and ketones Lesson Objective: To know some reactions of aldehydes and ketones Success criteria: To understand the reactions of carbonyl compounds with lithium tetrahydridoaluminate (lithium aluminium hydride) in dry ether (in equations the reducing agent can be represented by [H]) To understand the reactions of carbonyl compounds with HCN, in the presence of KCN, as a nucleophilic addition reaction, using curly arrows, relevant lone pairs, dipoles and the evidence of optical activity to show the mechanism To understand the reactions of carbonyl compounds with iodine in the presence of alkali

Reactions of aldehydes and ketones questions Substance Q reacts to give an orange precipitate with 2,4- dinitrophenylhydrazine. It produces a secondary alcohol when reduced. It reacts with iodine to give a yellow precipitate. The molecular formula of Q is C7H14O. Use the information to draw a possible structure for Q. Explain how each piece of information is useful. [4 marks] 2,4-DNPH contains carbonyl Reaction with iodine contains a methyl compound Reduction to secondary alcohol must be ketone Page 79 CGP book

Reactions of aldehydes and ketones questions Substance Q reacts to give an orange precipitate with 2,4- dinitrophenylhydrazine. It produces a secondary alcohol when reduced. It reacts with iodine to give a yellow precipitate. The molecular formula of Q is C7H14O. b) Suggest and explain how the precipitate formed when Q reacts with 2,4-DNPH reagent could be used to confirm your suggested structure. [2 marks] You can measure the melting point of the precipitate formed with 2,4- DNPH Each carbonyl compound gives a precipitate with a specific melting point which can be looked up in tables Page 79 CGP book

Reactions of aldehydes and ketones questions Substance Q reacts to give an orange precipitate with 2,4- dinitrophenylhydrazine. It produces a secondary alcohol when reduced. It reacts with iodine to give a yellow precipitate. The molecular formula of Q is C7H14O. c) Draw the structure of the substance produced when Q reacts with LiAlH4 in dry ether. [1 mark] Page 79 CGP book

Reactions of aldehydes and ketones Lesson Objective: To know some reactions of aldehydes and ketones Success criteria: To understand the reactions of carbonyl compounds with lithium tetrahydridoaluminate (lithium aluminium hydride) in dry ether (in equations the reducing agent can be represented by [H]) To understand the reactions of carbonyl compounds with HCN, in the presence of KCN, as a nucleophilic addition reaction, using curly arrows, relevant lone pairs, dipoles and the evidence of optical activity to show the mechanism To understand the reactions of carbonyl compounds with iodine in the presence of alkali