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ORGANIC CHEMISTRY General, Organic, and Biochemistry 9th Edition Katherine J. Denniston Joseph J. Topping Danaè R. Quirk Dorr Robert L. Caret Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display 13.1 Structures and Physical Properties 13.1 Structure and Physical Properties • Aldehydes and ketones are polar compounds • The carbonyl group is polar – The oxygen end is electronegative 1 13.1 Structure and Physical Properties Hydrogen Bonding in Carbonyls • Aldehydes and ketones cannot form intermolecular hydrogen bonds • However, water can form hydrogen bonds to them Physical Properties Carbonyls boil at • Higher temperatures than: – Hydrocarbons – Ethers • Lower temperatures than: – Alcohols 13.2 Nomenclature and Common Names 2 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 – 2-methyl 5 4 3 2-methylpentanal 2 1 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 13.2 Nomenclature and Common Names IUPAC and Common Names With Formulas for Several Aldehydes Examples of Ketones Simplest ketone MUST have 3 carbon atoms so that the carbonyl group is interior 2 • 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 2 Rules directly analogous to those for aldehydes • Base name: longest chain with the C=O hept • 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 5 4 3 2 1 4-methyl-2-heptanone 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 13.3 Important Aldehydes and Ketones 3 • Methanal (formaldehyde) - gas used in aqueous solutions as formalin to preserve tissue • Ethanal (acetaldehyde) - produced from ethanol in the liver causing hangover symptoms • Propanone (Acetone) - simplest possible ketone – Miscible with water – Flammable – Both acetone and methyl ethyl ketone (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 13.3 Important Aldehydes and Ketone Vanillin and 2-Octanone 13.3 Important Aldehydes and Ketone Benzaldehyde and Cinnamaldehyde 13.3 Important Aldehydes and Ketone Citral and α-Demascone 13.4 Reactions Involving Aldehydes and Ketones 4 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 13.4 Reactions Involving Aldehydes and Ketones 1° Alcohol Oxidation • An aldehyde is formed 4 13.4 Reactions Involving Aldehydes and Ketones 2° Alcohol Oxidation • A ketone is formed 4 13.4 Reactions Involving Aldehydes and Ketones 3° Alcohol • 3o alcohols cannot undergo oxidation: 4 13.4 Reactions Involving Aldehydes and Ketones Reactions of Aldehydes and Ketones 1. Oxidation and Reduction 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. Alcohols to give hemiacetals and acetals, 13.4 Reactions Involving Aldehydes and Ketones Oxidation of Aldehydes • Aldehydes are easily oxidized to carboxylic acids by almost any oxidizing agent 5 – So easily oxidized that it is often difficult to prepare them as they continue on to carboxylic acids 13.4 Reactions Involving Aldehydes and Ketones Visually 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 Tollen’s Test 13.4 Reactions Involving Aldehydes and Ketones Benedict’s Test • Benedict’s Test – Reagent is a buffered aqueous solution of Cu(OH)2 and sodium citrate – Reacts with aldehydes, but not 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 Benedict’s Test on Glucose • Benedict’s Test 13.4 Reactions Involving Aldehydes and Ketones Reduction of Carbonyls 7 • 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 13.4 Reactions Involving Aldehydes and Ketones Addition Reactions 7 • 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 Hemiacetal and Acetal Formation Generalized structure of a hemiacetal 7 Generalized structure of an acetal 13.4 Reactions Involving Aldehydes and Ketones Aldehyde and Alcohol Reaction • Product of the addition reaction is a hemiacetal followed by formation of an acetal 13.4 Reactions Involving Aldehydes and Ketones Ketone and Alcohol Reaction 13.4 Reactions Involving Aldehydes and Ketones Intramolecular Hemiacetal Formation • The aldehyde and alcohol functional groups can both be parts of the same molecule in an intramolecular reaction 13.4 Reactions Involving Aldehydes and Ketones Hemiacetals and Carbohydrates • This reaction is important in the chemistry of carbohydrates, where hemiacetals are readily formed: 13.4 Reactions Involving Aldehydes and Ketones Acetals and Carbohydrates • The acetal is formed between the hemiacetal hydroxyl group in blue with the alcohol hydroxyl group in red on a second molecule 13.4 Reactions Involving Aldehydes and Ketones Keto-Enol Tautomers 8 • 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 Reaction Schematic Summary of Aldehyde and Ketone Reactions 1. Aldehydes and ketones a. Oxidation of an aldehyde b. Reduction of aldehydes and ketones c. Addition reactions i. ii. Hemiacetal and acetal Cyclic Hemiactetal and acetal 2. Keto-enol tautomerization Summary of Reaction Equations