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Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 13 Aldehydes and Ketones Denniston Topping Caret 6th Edition Carbonyl Compounds Contain the carbonyl group Short forms C=O Aldehydes: R may be hydrogen, usually a carbon containing group O R C H RCHO Ketones: R contains carbon O RC R RCOR Structures of Aldehydes and Ketones 13.1 Structure and Physical Properties • Aldehydes and ketones are polar compounds • The carbonyl group is polar – The oxygen end is electronegative – Can hydrogen bond to water – Cannot form intermolecular hydrogen bond O C + O C Hydrogen bond H O H 13.1 Structure and Physical Properties Hydrogen Bonding in Carbonyls Bonding with H2O Intermolecular 13.1 Structure and Physical Properties Physical Properties Carbonyls boil at • Higher temperatures than: – Hydrocarbons – Ethers • Lower temperatures than: – Alcohols 13.2 Nomenclature and Common Names Naming Aldehydes • Locate the parent compound – Longest continuous carbon chain – Must contain the carbonyl group • Replace the final –e of the parent with –al • Number the chain with the carbonyl carbon as 1 • Number and name all substituents 13.2 Nomenclature and Common Names Naming Aldehydes What is the name of this molecule? 1. 2. 3. 4. Parent chain – 5 carbons = pentane Change suffix – pentanal Number from carbonyl end – L to R Number / name substituents – 4-methyl O 5 4 3 2 1 CH CH CH CH CH 3 2 2 CH3 4-methylpentanal 13.2 Nomenclature and Common Names Common Names of Aldehydes • These names are taken from Latin roots as are the first 5 carboxylic acids • Greek letters are used to indicate the position of substituents with the carbon atom adjacent or bonded to the carbonyl carbon being the a carbon O Cl CH3CH CH2CH2CH a -chlorovaleraldehyde 13.2 Nomenclature and Common Names IUPAC and Common Names With Formulas for Several Aldehydes 13.2 Nomenclature and Common Names Examples of Ketones Simplest ketone MUST have 3 carbon atoms so that the carbonyl group is interior • Base name: longest chain with the C=O • Replace the –e of alkane name with –one • Indicate position of C=O by number on chain so that C=O has lowest possible number 13.2 Nomenclature and Common Names IUPAC Naming of Ketones Rules directly analogous to those for aldehydes • Base name: longest chain with the C=O pent • Replace the –e of alkane name with –one • Indicate position of C=O by number on chain so that C=O has lowest possible number 2 Cl O 1 CH3CH CH2C2 CH 3 5 4 3 4-chloro-2-pentanone 13.2 Nomenclature and Common Names Common Names of Ketones • Based on the alkyl groups that are bonded to the carbonyl carbon – Alkyl groups are prefixes (2 words) followed by the word ketone – Order of alkyl groups in the name • Alphabetical • Size – smaller to larger O O CH3 C CH2 CH3 CH3 C CH3 Methyl ethyl ketone or Dimethyl ketone Ethyl methyl ketone 13.3 Important Aldehydes and Ketones • Methanal (b.p. –21oC) is a gas used in aqueous solutions as formalin to preserve tissue • Ethanal is produced from ethanol in the liver causing hangover symptoms • Propanone (Acetone) is the simplest possible ketone – Miscible with water – Flammable – Both acetone methyl ethyl ketone (MEK or butanone) are very versatile solvents 13.3 Important Aldehydes and Ketone Important Uses of Carbonyl Compounds • Used in many industries – Food chemicals • Natural food additives • Artificial additives – Fragrance chemicals – Medicines – Agricultural chemicals CH3O HO O CH Vanillin Vanilla beans O CH3 CH2 5 C CH3 2-octanone Mushroom flavor 13.3 Important Aldehydes and Ketone Other Important Carbonyls 13.4 Reactions Involving Aldehydes and Ketones Preparation of aldehydes and ketones • Principal means of preparation is oxidation of the corresponding alcohol – Primary alcohol produces an aldehyde – Secondary alcohol produces a ketone – Tertiary alcohol does not oxidize • This oxidation process removes two hydrogens • It is considered an oxidative elimination reaction 13.4 Reactions Involving Aldehydes and Ketones Distinguishing Types of Alcohol Oxidation • Upper box shows oxidation of a 1º alcohol – Must use a mild oxidizing agent or a carboxylic acid will be formed • Lower box oxidation of a 2º alcohol to a ketone • Tertiary alcohols cannot be oxidized 13.4 Reactions Involving Aldehydes and Ketones Reactions of Aldehydes and Ketones 1. Redox a. Aldehydes: oxidized to carboxylic acids b. Aldehydes and ketones are reduced to alcohols: aldehydes to primary alcohols and ketones to secondary alcohols 2. Addition a. Hydrogen to give alcohols b. Alcohols to give hemiacetals, acetals, hemiketals, and ketals c. Aldehydes/ketones to give aldol (-hydroxy carbonyl) products 13.4 Reactions Involving Aldehydes and Ketones Oxidation of Aldehydes • Aldehydes are easily oxidized to carboxylic acids by almost any oxidizing agent – So easily oxidized that it is often difficult to prepare them as they continue on to carboxylic acids – Susceptible to air oxidation even at room temperature – Cannot be stored for long periods 13.4 Reactions Involving Aldehydes and Ketones Distinguishing Aldehydes From Ketones Visual tests for the aldehyde functional group based on its easy oxidation are: • Tollen’s Test – Silver ion is reduced to silver metal – Use a basic solution of Ag(NH3)2+ – The silver metal precipitates and coats the container producing a smooth silver mirror 13.4 Reactions Involving Aldehydes and Ketones Distinguishing Aldehydes From Ketones • Benedict’s Test – Reagent is a buffered aqueous solution of copper(II) hydroxide and sodium citrate – Reacts with aldehydes, but not generally with ketones – Cu2+ is reduced to Cu+ • Solution of Cu2+ is a distinctive blue color • Color fades during the reaction as Cu+ precipitates as the red solid, copper(I) oxide, Cu2O 13.4 Reactions Involving Aldehydes and Ketones Reduction of Carbonyls • Both aldehydes and ketones are readily reduced to alcohols – Reduction occurs with hydrogen as the reducing agent • Classical reaction is hydrogenation – React with hydrogen gas – Requires a catalyst – Ni, Pt, Pd – Occurs with heat and pressure O OH CH3CH2C CH3 CH3CH2CH CH3 H2 Pt, O Pd, Ni OH CH3CH2CH2CH CH3CH2CH2CH2 13.4 Reactions Involving Aldehydes and Ketones Addition Reactions • Principal reaction is the addition reaction across the polar C=O double bond – Very similar to the addition hydrogenation of alkenes – Requires catalytic acid in the solution • Product of the reaction is a hemiacetal – Hemiacetals are quite reactive – Undergo a substitution reaction with the –OH group of the hemiacetal is exchanged for another –OR group from the alcohol – Reaction product is an acetal – This reaction is reversible 13.4 Reactions Involving Aldehydes and Ketones Formation of Hemiacetal or Hemiketal • Product of the addition reaction is a hemiacetal (above) or a hemiketal (below) O CH3CH2CH2CH + CH3OH OH CH3CH2CH2CH Hemiacetal (ketal) carbons O CH3 H+ are part of both alcohol and ether functions and are a new functional group OH O H+ CH3CH2C CH3 CH3CH2C CH3 + CH3OH O CH3 13.4 Reactions Involving Aldehydes and Ketones Recognizing Hemiacetals, Acetals, Hemiketals, and Ketals 13.4 Reactions Involving Aldehydes and Ketones Keto-Enol Tautomers • Tautomers are isomers which differ in the placement of: – A hydrogen atom – A double bond – The keto form has a C=O while the enol form has a C=C. • The keto form is usually the most stable R1 OH H O R1 C C R3 C C R2 R3 R2 H O H C CH H OH H C C H H 13.4 Reactions Involving Aldehydes and Ketones Aldol Condensation • Self-addition or condensation • Uses two molecules of the same aldehyde or ketone • The a carbon of the second molecule adds to the carbonyl carbon of the first molecule • Strong base such as hydroxide catalyzes the reaction • Very complex reaction occurring in multiple steps 13.4 Reactions Involving Aldehydes and Ketones Condensation of an Aldehyde • An aldol has an –OH to the carbonyl group OH O O O OHCH3CH CH2CH CH3CH + CH3CH a C, 2nd molecule original aC carbonyl carbon of first molecule becomes alcohol carbon in aldol 13.4 Reactions Involving Aldehydes and Ketones Aldol Condensation: Aldolase Dihydroxyacetone phosphate + D-glyeraldehyde-3-phosphate 1 2- 1 2CH2 OPO3 CH2 OPO3 2 2 C O C O 3 H C OH aldolase H 3C OH 4 H Bond formed H5C O H 4 H5C O HC OH 26 HC OH OPO CH 3 2 26 CH2 OPO3 D-fructose-1,6-bisphosphate a carbon (3) adds to carbonyl carbon (4) Reaction Schematic Carbonyl Oxidation Carboxylic Acid Addition Reduction If aldehyde If aldehyde If ketone Hemiacetal - Acetal 2º Alcohol 1º Alcohol If ketone Hemiketal - Ketal Summary of Reactions 1. Aldehydes and ketones a. Oxidation of an aldehyde b. Reduction of aldehydes and ketones c. Addition reactions i. Hemiacetal and acetal ii. Hemiketal and ketal 2. Keto-enol tautomerization 3. Aldol condensation Summary of Reactions