1. Aldehydes and Ketones

2. Introduction Aldehydes and Ketones are among the most widely occurring organic compounds, both in nature and in the chemical industry Both are used as solvents or as starting materials for other compounds Aldehydes –Used in resins, solvents, plasticizers, pharmaceuticals and dyes due to reactivity –Found in many sugars (e.g. glucose) and in several natural and synthetic hormones (e.g. vitamins A and B6) –Products of photochemical smog. Large amounts of aldehydes in the atmosphere result in the production of PAN (a notorious pollutant which causes eye irritation and which is toxic to plants) Ketones –Not as reactive as aldehydes –Naturally present in pheromones due to their characteristic odours –Used in the perfume industry –Used in the production of dyes –Found in many sugars (e.g. fructose) –Products of low carb diet Products of oxidation of amino acids, carbohydrates and fatty acids Accumulation in the blood may result in ketosis or even ketoacidosis (dangerous conditions) Careful monitoring in diabetic patients

3. The Carbonyl Group Aldehydes and ketones are characterized by the presence of the carbonyl group Aldehydes carbonyl group attached to the end carbon Ketones carbonyl group attached to the carbon that is not at the end of the chain

4. Examples AldehydesKetones The simplest: formaldehyde The simplest: acetone

5. Glucose vs. Fructose An aldehyde A ketone

6. Chemistry of the Carbonyl Group Carbonyl group is polar –Stronger intermolecular forces and higher boiling points than alkanes of similar molecular weight –BUT they cannot form H-bonds Aldehydes and ketones are usually soluble in water due to the polar carbonyl group (dipole-dipole forces) –BUT as the length of the carbon chain increases, the solubility decreases –Not as soluble as alcohols (due to the absence of a hydroxyl group in most ketones and aldehydes) Lower boiling points than analogous alcohols Formaldehyde is a gas at room temperature and acetone is a very volatile liquid –Other simple aldehydes and ketones are liquid

7. Nomenclature of Aldehydes Replace the terminal –e of the corresponding alkane with –al The parent chain must contain the –CHO group (a symbol for the carbonyl group at the end carbon) The –CHO carbon is numbered as carbon 1

8. Examples Ethanal (aka acetaldehyde) Propanal (aka propionaldehyde) 2-ethyl-4-methylpentanal Note: the longest chain in 2-ethyl-4-methylpentanal is a hexane, but it doesn’t include the –CHO group, therefore it is not considered the parent chain

9. Naming Complex Aldehydes -CHO group attached to a ring, the suffix –carbaldehyde is used cyclohexanecarbaldehydeBenzenecarbaldehyde (aka benzaldehyde)

10. Reactions of Aldehydes 1.Preparation 1.From oxidation of primary alcohols Reactant: pyridinium chlorochromate (PCC) Solvent: dichloromethane From reduction of esters Reactant: diisobutylaluminum hydride (DIBAH) Solvent: toluene (at -78 o C) PCC CH 2 Cl 2 + 1.DIBAH, toluene, -78 o C 2. H 3 O +

11. Reactions of Aldehydes 1.Combustion Aldehyde + O 2 → CO 2 + H 2 O 2.Hydrogenation of aldehydes to form primary alcohols 3.Oxidation to carboxylic acids

12. Ketone Nomenclature The terminal –e of the corresponding alkane is replaced with –one (pronounced oan) The parent chain is the longest one containing the ketone group The numbering begins at the end nearer the carbonyl carbon

13. Examples Propanone (aka acetone) 3- Hexanone 4-Hexen-2-one2,4-Hexanedione

14. Reactions of Ketones 1.Preparation 1.From oxidation of secondary alcohol PCC CH 2 Cl 2

15. Reactions of Ketones 1.Combustion Ketone + O 2 → CO 2 + H 2 O 2.Hydrogenation to produce secondary alcohols Note: Ketones do NOT undergo oxidation reactions

16. Qualitative Tests for Aldehydes and Ketones 1.Fehling’s Solution (copper (II) solution) –Oxidation reaction –Blue copper(II) solution forms an orange precipitate if there is a reaction. aldehydes + Cu 2+ → carboxylic acid + Cu (s) –(blue solution → orange precipitate) ketones + Cu 2+ → NO REACTION –(solution stays blue)

17. 2.Tollen’s Reagent (Silver Mirror Test) Silver ions in ammonia (NH 3 ) Commercially used to “silver mirror” glass. Qualitative Tests for Aldehydes and Ketones