Aldehydes and Ketones PDF

Organic Chemistry 7: Aldehydes and Ketones 1 Chapter Outline 1 Structures of Aldehydes and Ketones 4 Chemical Properties 2 Naming Aldehydes and Ketones 5 Common Aldehydes and Ketones 3 Physical Properties 6 Condensation Polymers 2 Structures of Aldehydes & Ketones 3 • Both aldehydes an...

Organic Chemistry 7: Aldehydes and Ketones 1 Chapter Outline 1 Structures of Aldehydes and Ketones 4 Chemical Properties 2 Naming Aldehydes and Ketones 5 Common Aldehydes and Ketones 3 Physical Properties 6 Condensation Polymers 2 Structures of Aldehydes & Ketones 3 • Both aldehydes and ketones contain a carbonyl ( C=O) group. O O C C R Ar H H aldehydes R O O O C C C R Ar R ketones Ar Ar 4 • The general formula for the saturated homologous series of aldehydes and ketones is: CnH2nO 5 •In a linear expression, the aldehyde group is often written as: CHO O C H3C H is equivalent to CH3CHO 6 •In the linear expression of a ketone, the carbonyl group is written as: CO O C H3C CH3 is equivalent to CH3COCH3 7 1 2 6 Glucose Fructose 8 Naming Aldehydes & Ketones 9 IUPAC Rules for Naming Aldehydes 1. To establish the parent name, select the longest continuous chain of carbon atoms that contains the aldehyde group. 2. The carbons of the parent chain are numbered starting with the aldehyde group. Since the aldehyde group is at the beginning (or end) of a chain, it is understood to be number 1. 10 IUPAC Rules for Naming Aldehydes 3. Form the parent aldehyde name by dropping the –e from the corresponding alkane name and adding the suffix –al. 4. Other groups attached to the parent chain are named and numbered as we have done before. 11 Naming Aldehydes O C H3C H ethanal O H C 1 2 3 4 5 6 CH2CH2CHCH2CH3 4-methyhexanal CH3 12 13 Common Names for Aldehydes O O C C H H formaldehyde H CH3 acetaldehyde O C H 14 benzaldehyde Dialdehydes • In dialdehydes, the suffix –dial is added to the corresponding hydrocarbon name. O O HCCH2CH2CH butanedial 15 IUPAC Rules for Naming Ketones 1. To establish the parent name, select the longest continuous chain of carbon atoms that contain the ketone group. 2. Form the parent name by dropping the –e from the corresponding alkane name and add the suffix –one. 16 IUPAC Rules for Naming Ketones 3. If the chain is longer than four carbons, it is numbered so that the carbonyl group has the smallest number possible; this number is prefixed to the parent name of the ketone. 4. Other groups attached to the parent chain are named and numbered as we have done before. 17 Naming Ketones O O C H3C CH3 propanone C 1 3 4 5 2 H3C CH2CH2CH3 2-petanone O 1 2 H3CH2C C 3 4 5 6 7 8 CH2CH2CHCH2CH3 6-methyl-3-octanone CH3 18 Common Names for Ketones O O C C H3C CH3 propanone acetone H3C CH2CH3 butanone methyl ethyl ketone, MEK 19 Physical Properties 20 Properties • Unlike alcohols, aldehydes and ketones cannot form hydrogen-bond to themselves, because no hydrogen atom is attached to the oxygen atom of the carbonyl group. • Aldehydes and ketones, therefore, have lower boiling points than alcohols of comparable molar mass. 21 • The carbonyl group of aldehydes and ketones is polar due to more electronegative oxygen atom therefore aldehydes and ketones have higher boiling points compared to nonpolar alkanes of similar molecular weight. • The carbonyl oxygen can form hydrogen bonds with water therefore low molecular weight aldehydes and ketones are soluble in water. 22 23 Chemical Properties of Aldehydes & Ketones 24 Reactions of Aldehydes & Ketones • Oxidation – aldehydes only • Reduction – aldehydes and ketones • Addition – aldehydes and ketones 25 Oxidation of Aldehydes • Aldehydes are easily oxidized to carboxylic acids by a variety of oxidizing agents, including (under some conditions) oxygen of the air. O 3 3 + Cr2O72- + 8 H+ C R H O + 3 Cr3+ + 4H2O C R OH 26 Tollens test • The Tollens test (silver-mirror test) for aldehydes is based on the ability of silver ions to oxidize aldehydes. O C R Aldehyde O + 2 Ag+ H NH3 H2O Sliver nitrate + 2 Ag (s) C R O-NH4+ Metallic silver 27 Fehling and Benedict Tests • Fehling and Benedict solutions contain Cu2+ ions in an alkaline medium. • In these tests, the aldehyde group is oxidized to an acid by Cu2+ ions. O C R O +2 + 2 Cu H blue NaOH H2O + 2 Cu2O (s) C R O-Na+ brick red 28 Tollens, Fehling & Benedict Tests • Because most ketones do not give a positive with Tollens, Fehling, or Benedict solutions, these tests are used to distinguish between aldehydes and ketones. O C R + 2 Cu+2 NaOH no reaction H2O R O C R + 2 Ag+ R NH3 H2O no reaction 29 Biochemical Oxidation of Aldehydes • When our cells ‘burn’ carbohydrates, they take advantage of the aldehyde reactivity. • The aldehyde is oxidized to a carboxylic acid and is eventually converted to carbon dioxide, which is then exhaled. • This stepwise oxidation provides some of the energy necessary to sustain life. 30 Reduction of Aldehydes & Ketones • Aldehydes and ketones are easily reduced to alcohols. O H2/Ni heat C R RCH2OH primary alcohol H O H2/Ni heat C R R OH RCHR secondary alcohol 31 Addition Reactions of Aldehydes & Ketones • Common addition reactions: – Addition of alcohols • hemiacetal, hemiketal, acetal, ketal – Addition of hydrogen cyanide (HCN) • cyanohydrin – Aldol Condensation (self-addition) 32 Addition of Alcohols • Aldehydes react with alcohols in the presence of a trace of acid to form hemiacetals: OH O + CH3OH C H CH2CH3 propanal H+ HC CH2CH3 OCH3 1-methoxy-1-propanol (propionaldehyde methyl hemiacetal) 33 Addition of Alcohols • In the presence of excess alcohol and strong acid such as dry HCl, aldehydes or hemiacetals react with a second molecule of the alcohol to give an acetal: OH HC CH2CH3 + CH3OH OCH3 OCH3 dry HCl HC CH2CH3 +H2O OCH3 1,1-dimethoxypropane (propionaldehyde dimethyl acetal) 34 Hemiacetal D-Glucose An intramolecular (within molecule) reaction occurs between and – OH group in glucose molecule giving glucose a cyclic hemiacetal structure. A similar reaction can occur between hemiketal and ketal. of ketones and –OH group giving 35 Quiz Just like glucose, what will be the predominant structure of fructose inside cell ? 36 Quiz Just like glucose, what will be the predominant structure of fructose inside cell ? 37 Quiz Just like glucose, what will be the predominant structure of fructose inside cell ? hemiketal 38 Quiz 39 Quiz 40 41