4.7 aldehydes and ketones, Exams of Chemistry

Download 4.7 aldehydes and ketones and more Exams Chemistry in PDF only on Docsity! 1 4.7 ALDEHYDES AND KETONES FACTFILE: GCE CHEMISTRY Learning Outcomes Students should be able to: 4.7.1 recall the molecular and structural formulae of aldehydes and ketones, including branched structures, with up to six carbon atoms in the main chain; 4.7.2 explain the boiling points and solubility of aldehydes and ketones by making reference to intermolecular forces; 4.7.3 recall that aldehydes and ketones can be prepared from the corresponding primary or secondary alcohol using a suitable oxidising agent 4.7.4 recall the reaction of aldehydes and ketones with hydrogen cyanide; 4.7.5 describe the mechanism for the nucleophilic addition reaction of hydrogen cyanide and propanone; 4.7.6 explain why racemic mixtures can be produced when hydrogen cyanide reacts with aldehydes and ketones; 4.7.7 recall the reaction of aldehydes and ketones with 2,4-dinitrophenylhydrazine; 4.7.8 recall the preparation of 2,4-dinitrophenylhydrazones for identification purposes with reference to melting point determination; 4.7.9 recall that aldehydes and ketones can be distinguished using acidified potassium dichromate(VI), Fehling's solution and Tollens’ reagent (with Fehling's solution and Tollens’ reagent viewed as Cu2+ and Ag+ respectively); 4.7.10 recall that aldehydes and ketones can be reduced using lithium tetrahydridoaluminate(III) (lithal); Nomenclature The functional group present in both aldehydes and ketones is the carbonyl group C=O which is polar. Aldehydes The carbonyl group is always at the end of the chain in aldehydes and so a positional number is not needed. The names of aldehydes are based on the carbon skeleton, with the ending changed to – anal. C O δ δ– H C O H C C O H H H H C O H C H H H H H C methanal ethanal propanal 2 FACTFILE: GCE CHEMISTRY / 4.7 ALDEHYDES AND KETONES Oxidation Aldehydes and ketones are the products of the oxidation of alcohols. Aldehydes can be oxidised into carboxylic acids, and ketones cannot be oxidised. 1. Oxidation using acidified potassium dichromate(VI) The orange dichromate(VI) ion is reduced to the green chromium(III) ion, Cr3+ by the aldehyde: The oxidising agent is represented by [O] Example: Oxidation of an aldehyde Condition: warm with acidified potassium dichromate(VI) solution. Observation: orange solution changes to green solution. 2. Oxidation by Fehling’s solution and Tollens’ reagent. Fehling’s solution and Tollens’ reagent are mild oxidising agents which are used to distinguish between aldehydes and ketones. Aldehydes are oxidised and ketones are not. This is 3-chlorobutanal, the C=O is at the end of the chain in position1,hence it is an aldehyde the chloro group is on position 3 Ketones The names of ketones are based on the carbon skeleton, with the ending changed to –anone. The carbonyl group can be at any position on the chain, except for the end hence propanone is the simplest ketone. A number for the position of the CO group is needed from 5 carbon atoms upwards. Physical properties 1. The smaller aldehydes and ketones are soluble in water. This is due to hydrogen bonding between polar H of water and the lone pair on the oxygen of the carbonyl group. Solubility decreases as chain lengths increase as the longer hydrocarbon parts of the molecules start to get in the way. By forcing themselves between water molecules, they break the relatively strong hydrogen bonds between water molecules and so solubility decreases. 2 Aldehydes and ketones have van der Waals’ forces and dipole dipole attractions between the molecules. This means that the boiling points will be higher than those of similarly sized hydrocarbons - which only have van der Waals’ forces between the molecules. Boiling points increase as the molecules get bigger – this is because the bigger the molecule, the greater the number of electrons and the stronger the van der Waals’ forces between the molecules . C O H C H H Cl H C C H H H CC H HO C H H H pentan-2-one CC H HH H H CC OH C H H H pentan-3-one CC H HH HH H Solution Method Result Equation Fehling’s solution Add a few drops of the unknown solution to 1cm3 of freshly prepared Fehling’s solution reagent in a test tube. Warm in a water bath If the unknown is an aldehyde a red ppt occurs. Solution remains blue for a ketone. Cu2+  e- → Cu+ Tollens’ reagent Add a few drops of the unknown solution to 1 cm3 of freshly prepared Tollens’ reagent in a clean test tube. Warm in a water bath. If the unknown is an aldehyde a silver mirror occurs on the inside surface of the test tube. Solution remains colourless for a ketone. Ag+  e- → Ag CH3CHO ethanal (aldehyde)  [O] → CH3COOH ethanoic acid (carboxylic acid)[O] primary alcohol aldehyde carboxylic acid [O] [O] secondary alcohol ketone no further oxidation [O] Cr2O  14H+  6e- → 2Cr3+  7H2O2 7 5 FACTFILE: GCE CHEMISTRY / 4.7 ALDEHYDES AND KETONES Condition: room temperature, 2,4-dinitrophenylhydrazine solution in acid Observations: orange/ yellow precipitates are produced This reaction produces a 2,4-dinitrophenylhydrazone which is named by putting the name of the organic compound in front. Aldehydes and ketones are often liquids and difficult to identify – so they are reacted with 2,4-dinitrophenylhydrazine. Solid crystals of the 2,4-dinitrophenylhydrazone are formed and the melting point can be found (after purification by recrystallisation) and compared to a data book to identify the original aldehyde and ketone. Laboratory preparation of 2,4-dinitrophenylhydrazones 1. Place 5 cm3 of 2,4-dinitrophenylhydrazine solution ( Brady’s reagent) in a suitable container. 2. Add some drops of the test liquid eg. propanal or the test solid dissolved in ethanol. 3. If crystals do not form add some dilute sulfuric acid and warm the mixture. 4. Cool the mixture in iced water. 5. Filter off the crystals using suction filtration. 6. Recrystallise. 7. Determine the melting point. Recrystallisation method 1. Dissolve the impure crystals in the minimum volume of hot solvent. 2. Filter when hot by gravity filtration, using a hot funnel, to remove insoluble impurities. 3. Cool and allow to crystallise - the impurities are left in the solution. 4. Dry the crystals between filter paper/in a cool oven/in a dessicator. Suction filtration method 1. Place filter paper in a Buchner funnel. 2. Place Buchner funnel in a Buchner flask. 3. Attach the flask to a suction pump and suck air through the flask. This is faster than normal filtration and the solid is left quite dry. Melting point method 1. Place some solid in a capillary tube which is sealed at one end. 2. Heat slowly (using melting point apparatus). 3. Record the temperature at which the solid starts and finishes melting. 4. Compare the temperature to data book/tabulated values. A solid which melts within a range of 1-2 oC indicates that the solid is pure. Buchner funnel Filter paper Buchner flask to suction pump Apparatus for suction filtration 6 FACTFILE: GCE CHEMISTRY / 4.7 ALDEHYDES AND KETONES 2 An organic compound, X, gives an orange crystalline product with 2,4-dinitrophenylhydrazine, but does not give a precipitate on heating with Fehling's solution. Which one of the following formulae is that of X? A HCHO B C6H5CH2CHO C CH3COCH3 D CH3CH2CO2H Revision Questions 1 Which one of the following compounds produced a silver mirror when heated with Tollens’ reagent? A CH3COCH3 B CH3CO2H C CH3CHO D CH3CO2CH3 [1] 3 a) Name the compound CH3CHO and state its functional group. [1] b) Name and outline a mechanism for the reaction of CH3CH2CHO with HCN. [4] c) State and explain how a racemic mixture is formed in this reaction. [2] [1] 7© CCEA 2020 FACTFILE: GCE CHEMISTRY / 4.7 ALDEHYDES AND KETONES 4 Some Scotch whiskies have the fragrance of cut grass. This smell is attributed to cis hex-3-enal, an unsaturated aldehyde a) Suggest a structure for cis hex-3-enal. [2] [2] b) What would be observed if cis hex-3-enal is warmed with Fehling's solution? c) Write the equation for the reaction between cis hex-3-enal and 2,4-dinitrophenylhydrazine. Represent the structure of cis hex-3-enal as. [3] C O There is no requirement to draw the full structure.