Ch 23 organic chem

Ch 23 organic chem - 1 Structures of Aldehydes &...

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Unformatted text preview: 1 Structures of Aldehydes & Ketones 2 Both aldehydes and ketones contain a carbonyl ( C=O) group. O C R H Ar aldehydes O C R R Ar ketones O C R Ar O C Ar 3 O C H In a linear expression, the aldehyde group is often written as: CHO O C H3C H is equivalent to CH3CHO 4 In the linear expression of a ketone, the carbonyl group is written as: CO O C H3C CH3 is equivalent to CH3COCH3 5 Naming Aldehydes & Ketones 6 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. 7 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. 8 Naming Aldehydes O C H3C ethanal O C 1 2 3 4 5 6 CH2CH2CHCH2CH3 CH3 9 H ethanal H 4-methylhexanal 4-methyhexanal 10 Common Names for Aldehydes O C H H formaldehyde O C H CH3 acetaldehyde O C H benzaldehyde 11 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. 12 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. 13 Naming Ketones O C H3C CH3 propanone O 1 2 H3CH2C C 3 4 5 6 7 8 CH2CH2CHCH2CH3 CH3 14 O C 1 3 4 5 2 H3C CH2CH2CH3 2-petanone 2-pentanone 6-methyl-3-octanone Common Names for Ketones O C H3C CH3 propanone O C H3C CH2CH3 butanone acetone methyl ethyl ketone, MEK 15 Bonding and Physical Properties 16 Bonding The carbon atom of the carbonyl group is sp2-hybridized and is joined to three other atoms by sigma bonds. The fourth bond is made by overlapping p electrons of carbon and oxygen to form a pi bond between the carbon and oxygen atoms. 17 Bonding Because the oxygen atom is considerably more electronegative than carbon, the C=O group is polar. Many of the chemical reactions of aldehydes and ketones are due to this polarity. C + O 18 Properties Unlike alcohols, aldehydes and ketones cannot 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. 19 Effect of Hydrogen Bonding on Physical Properties Alcohols can undergo hydrogen bonding resulting in higher boiling points and higher solubility. Aldehydes and ketones cannot undergo hydrogen bonding resulting in lower boiling points and lower solubility. 20 Mole Weight Boiling point oC 21 Chemical Properties of Aldehydes & Ketones 22 Reactions of Aldehydes & Ketones Oxidation aldehydes only Reduction aldehydes and ketones Addition aldehydes and ketones 23 Oxidation of Aldehydes Aldehydes are easily oxidized to carboxylic acids by a variety of oxidizing agents, including (under some conditions) oxygen of the air. 3 R O C H + Cr2O72- + 8 H+ R 3 O C OH + 3 Cr3+ + 4H2O 24 Tollens' Silver Mirror Test Tollens' reagent, which contains Ag+, oxidizes aldehydes, but not ketones. Ag+ is reduced to metallic Ag, which appears as a "mirror" in the test tube. O C R + 2 Ag+ H NH3 H2O O C R + 2 Ag (s) O-NH4+ 25 Ag+ + e Ag(s) 26 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 + 2 Cu H blue +2 O NaOH H2O C R + 2 Cu2O (s) O-Na+ brick red 27 Fehling and Benedict Tests Benedict's reagent, which contains Cu2+ ions in an alkaline medium, reacts with aldehydes that have an adjacent OH group. an aldehyde is oxidized to a carboxylic acid, while Cu2+ is reduced to give brick red Cu2O(s). 28 Increasing amounts of reducing sugar green orange red brown 29 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 O C R + 2 Ag+ R NH3 H2O no reaction + 2 Cu+2 NaOH no reaction H2O R 30 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. 31 Reduction of Aldehydes & Ketones Aldehydes and ketones are easily reduced to alcohols using LiAlH 4, NaBH4 , or H2/Ni . Aldehydes yield primary alcohols (1) while ketones yield secondary alcohols ( 2) . 32 Addition Reactions of Aldehydes & Ketones Common addition reactions: Addition of alcohols hemiacetal, hemiketal, acetal, ketal Grignard preparations of alcohols 2,4-dinitrophenylhydrazine (2,4-DNP) 33 Addition of Alcohols Aldehydes react with alcohols and a trace of acid to yield hemiacetals as shown here. 34 Addition of Alcohols In the presence of excess alcohol and a strong acid such as dry HCl, aldehydes or hemiacetals react with a second molecule of the alcohol to yield an acetal. 35 Intramolecular Addition of Alcohols Cyclic hemiacetals or hemiketals can form when the alcohol and the carbonyl group exist within the same molecule . 36 Addition of Alcohols to Aldehydes and Ketones OH R OR' H hemiacetal C OH R OR' R hemiketal C R H OR' C OR' acetal R R OR' C OR' ketal 37 Grignard preparations of alcohols A Grignard reagent is an organic magnesium halide. It can be either an alkyl or an aryl compound (RMgX or ArMgX). Grignard (pronounced green yard) reagents were first prepared in France around 1900 by Victor Grignard (1871-1935). 38 Grignard reagents are usually made by reacting an organic halide and magnesium metal in an ether solvent: RX ArX + + Mg Mg ether ether RMgX ArMgX X = Cl, Br, or I X = Br 39 In the Grignard reagent, the bonding electrons between carbon and magnesium are shifted away from the electropositive Mg to form a strongly polar covalent bond. As a result the charge distribution in the Grignard reagent is such that the organic group (R) is partially negative and the MgX group is partially positive. This charge distribution directs the manner in which Grignard reacts with other compounds. 40 The Grignard reagent is one of the most versatile and widely used reagents in organic chemistry. We will consider only its reactions with aldehydes and ketones at this time. Grignards react with aldehydes and ketones to give intermediate products that form alcohols when hydrolyzed. With formaldehyde, primary alcohols are formed; with other aldehydes, secondary alcohols are formed; with ketones, tertiary alcohols are formed. 41 Examples Grignard reagent + formaldehyde 1 ROH Grignard reagent + other aldehydes 2 ROH Grignard reagent + ketones 3 ROH CH 3 H2C O + CH 3 MgBr ether H2 O H2C OMgBr CH 3 CH 2 OH Formaldehyde 42 Examples H C O + CH 3 MgBr ether H C CH 3 OMgBr Benzaldehyde H2 O CHOH CH 3 + Mg(OH)Br 43 Examples CH 3 CCH 3 O + CH 3 CH 2 MgBr ether CH 2 CH 3 CH 3 CCH 3 OMgBr H2 O CH 2 CH 3 CH 3 CCH 3 OH + Mg(OH)Br Acetone 44 Explanation The Grignard reaction with acetone may be explained in this way. In the first step of the addition of ethyl magnesium bromide, the partially positive MgBr of the Grignard bonds to the oxygen atom, and the partially negative CH3CH2 bonds to the carbon atom of the carbonyl group of acetone. 45 CH 2 CH 3 CH 3 CCH 3 O + CH 3 CH 2 MgBr CH 3 CCH 3 _ + MgBr O 46 Explanation In the hydrolysis step, a proton [H+] from water bonds to the oxygen atom, leaving the hydroxyl group [OH] to combine with the +MgBr. So, the alcohol is formed. 47 CH 2 CH 3 CH 3 CCH 3 _ + MgBr O + H OH CH 2 CH 3 CH 3 CCH 3 OH + Mg(OH)Br 48 2,4-dinitrophenylhydrazine (2,4-DNP) H N H H N NO 2 NO 2 49 2,4-dinitrophenylhydrazine (2,4-DNP) The carbonyl carbon in both aldehydes and ketones reacts with 2,4-DNP to form heavy yellow to orange crystalline solids. These solids were used extensively for identification purposes before the use of spectrometers. The solid is purified by crystallization and its melting point compared to those of known structure. 50 51 Common Aldehydes & Ketones 52 Formaldehyde (Methanal) Formaldehyde is made from methanol by reaction with oxygen (air) in the presence of a silver or copper catalyst. 2 CH3OH + O2 Ag heat 2H2C=O + 2H2O Formaldehyde is widely used in the synthesis of polymers. 53 Acetaldehyde (Ethanal) Its principal use is as an intermediate in the manufacture of other chemicals, such as acetic acid and 1-butanol. 54 Acetone and Methyl Ethyl Ketone Acetone is used as a solvent in the manufacture of drugs, chemicals, and explosives. It is also used as a solvent. Methyl ethyl ketone (MEK) is also widely used as a solvent, especially for lacquers. 55 Aldehydes & Ketones in Nature 56 C C O C C C C H C O OH H C O Violet (Irone) salicylaldehyde (meadowsweet) O O C O CH 3 (CH 2 ) 4 C (Eucalyptus) Piperonal (Heliotrope) H O CH 3 (CH 2 ) 10 C (Citrus Fruits) O H CH 3 C Raspberries O C 57 CH 3 O C Benzaldehyde (Oil of Almonds) H O HO CH 3 O Vanillin C H O CH CH C H H3 C CH 3 Oil of Cinnamon CH 3 O 58 Camphor (Mothballs) CH 3 CH 3 O C CHCH 2 CH 2 C CH 3 Citral (Lemon Grass Oil) CH C H O C C C C CH 3 C C C C O C (C) n C n = 4 or 6 Alarm Pheromones in ants 59 C C H H C H Citral (Honey Bee Recruiting Pheromone) 60 Boll Weevil Sex Attractant C C O O C (C) C C 12 C O Musk Ox Sex Attractant (C) C C (C) n C m O m = 4, n = 10 m = 7, n = 7 m = 7, n = 9 61 Civet Cat Sex Attractant Condensation Polymers 62 Leo Baekeland (1863-1944) 63 Phenol-Formaldehyde Polymers (Bakelite) A phenolic is a condensation polymer made from phenol as shown here. This is a section of a phenolic ( i.e. Bakelite) which is an example of a thermosetting polymer. These polymers are used in electrical equipment because of their insulating and fire-resistant properties. 64 Bakelite products 65 Bakelite products GE Locomotive 66 67 ...
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