Chemeketa Community College Aldehydes and Ketones Chapter 23 Larry Emme Chemeketa Community College

Structures of Aldehydes & Ketones

Both aldehydes and ketones contain a carbonyl ( C=O) group. aromatic aromatic aromatic aromatic

In a linear expression, the aldehyde group is often written as: CHO

In the linear expression of a ketone, the carbonyl group is written as: CO

Naming Aldehydes & Ketones

IUPAC Rules for Naming Aldehydes To establish the parent name, select the longest continuous chain of carbon atoms that contains the aldehyde group. 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.

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.

Naming Aldehydes ethanal 4-methylhexanal

IUPAC Names of Several Aldehydes Formula (condensed) IUPAC Name HCHO Methanal CH3CHO Ethanal CH3CH2CHO Propanal CH3CH2CH2CHO Butanal CH3CHCHO 2-Methyl propanal  CH3

Common Names for Aldehydes Acetic acid Formic acid Benzoic acid [O]

IUPAC Rules for Naming Ketones To establish the parent name, select the longest continuous chain of carbon atoms that contain the ketone group. Form the parent name by dropping the –e from the corresponding alkane name and add the suffix –one.

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.

Naming Ketones 2-pentanone

Common Names for Ketones dimethyl ketone

Bonding and Physical Properties

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.

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.

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.

Effect of Hydrogen Bonding on Physical Properties 20 20

Boiling Points of Aldehydes, Ketones, and Corresponding Alcohols. Name Mole Weight Boiling Point (°C) 1-Propanol 60 97 Propanal 58 49 Propanone 56 1-Butanol 74 118 Butanal 72 76 Butanone 80 1-Pentanol 88 138 Pentanal 86 103 2-Pentanone 102

Chemical Properties of Aldehydes & Ketones

Reactions of Aldehydes & Ketones Oxidation aldehydes only Reduction aldehydes and ketones Addition

Oxidation of Aldehydes Aldehydes are easily oxidized to carboxylic acids by a variety of oxidizing agents, including (under some conditions) oxygen of the air.

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. Ag+ + e– → Ag(s) 25

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). 27

green  orange  red  brown Increasing amounts of reducing sugar green  orange  red  brown

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.

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.

Reduction of Aldehydes & Ketones Aldehydes and ketones are easily reduced to alcohols using LiAlH4, NaBH4 , or H2/Ni . Aldehydes yield primary alcohols (1) while ketones yield secondary alcohols ( 2) . 31 31

Addition Reactions of Aldehydes & Ketones Common addition reactions: Addition of alcohols 2,4-dinitrophenylhydrazine (2,4-DNP)

Addition of Alcohols Aldehydes react with alcohols and a trace of acid to yield hemiacetals as shown here. 33 33

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. 34 34

Intramolecular Addition of Alcohols Cyclic hemiacetals or hemiketals can form when the alcohol and the carbonyl group exist within the same molecule . 35 35

Utility of the Hydroxyl Functional Group

2,4-dinitrophenylhydrazine (2,4-DNP)

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.

Common Aldehydes & Ketones

Formaldehyde (Methanal) Formaldehyde is made from methanol by reaction with oxygen (air) in the presence of a silver or copper catalyst. 2 CH3OH + O2  2H2C=O + 2H2O Formaldehyde is widely used in the synthesis of polymers. Ag heat

Acetaldehyde (Ethanal) Its principal use is as an intermediate in the manufacture of other chemicals, such as acetic acid and 1-butanol.

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.

Aldehydes & Ketones in Nature

Condensation Polymers

Leo Baekeland (1863-1944)

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. 55 55

Bakelite products

Bakelite products GE Locomotive

The End

End of Chapter Learning Checks Try these after you have reviewed the chapter

Learning Check Write the structure of butyl ethyl ketone. 61 61

Learning Check Answers Write the structure of butyl ethyl ketone. 62 62

Learning Check Give names for the following compounds. 63 63

Learning Check Answers Give names for the following compounds. 64 64

Learning Check Write the structure of cyclohexanone. 65 65

Learning Check Answers Write the structure of cyclohexanone. 66 66

Learning Check Rank these molecules based on boiling point from lowest to highest. 3-hexanol (molar mass = 102 g/mol) 3-hexanone (molar mass = 100 g/mol) heptane (molar mass = 100 g/mol) 67 67

Learning Check Answers Rank these molecules based on boiling point from lowest to highest. 3-hexanol (molar mass = 102 g/mol) 3-hexanone (molar mass = 100 g/mol) heptane (molar mass = 100 g/mol) 68 68

Learning Check Write the structures of the products, or indicate no reaction, for the following. 1. Propanone in the Tollens test 2. Pentanal in the Benedict’s test 69 69

Learning Check Answers Write the structures of the products, or indicate no reaction, for the following. 1. Propanone in the Tollens test 2. Pentanal in the Benedict’s test 70 70

Learning Check The conversion of 2-pentanone to 2-pentanol is a(n): Hydration Dehydration Hydrolysis Oxidation Reduction 71 71

Learning CheckAnswers The conversion of 2-pentanone to 2-pentanol is a(n): Hydration Dehydration Hydrolysis Oxidation Reduction 2-Pentanone is a ketone and 2-pentanol is an alcohol. The conversion of a ketone to an alcohol is a reduction. 72 72