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Aldehydes and Ketones Chapter 23

Aldehydes and Ketones Chapter 23

Hein * Best * Pattison * Arena

Colleen KelleyChemistry DepartmentPima Community College

© John Wiley and Sons, Inc.

Version 1.0

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Chapter Outline23.1 Structures of Aldehydes and Ketones

23.2 Naming Aldehydes and Ketones

23.3 Bonding and Physical Properties

23.4 Chemical and Physical Properties of Aldehydes and Ketones

23.5 Common Aldehydes and Ketones

23.6 Condensation Polymers

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Structures of Structures of Aldehydes & KetonesAldehydes & Ketones

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

RC

H

O

ArC

H

O

aldehydes

RC

R

O

ArC

R

O

ketones

ArC

Ar

O

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• The general formula for the saturated homologous series of aldehydes and ketones is:

CnH2nO

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

CHO

H3CC

H

O

is equivalent to CH3CHO

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

CO

H3CC

CH3

O

is equivalent to CH3COCH3

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Naming Naming Aldehydes & KetonesAldehydes & Ketones

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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.

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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.

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Naming Aldehydes

H3CC

H

O

ethanal

HC

CH2CH2CHCH2CH3

O

CH34-methyhexanal

2 3 4 5 61

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Common Names for Aldehydes

HC

H

O

formaldehydeH

CCH3

O

acetaldehyde

CH

O

benzaldehyde

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Dialdehydes

• In dialdehydes, the suffix –dial is added to the corresponding hydrocarbon name.

HCCH2CH2CH

O O

butanedial

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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.

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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.

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Naming Ketones

H3CC

CH3

O

propanone

H3CH2CC

CH2CH2CHCH2CH3

O

CH36-methyl-3-octanone

23 4 51

H3CC

CH2CH2CH3

O

2-petanone

4 51 23

6 7 8

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Common Names for Ketones

H3CC

CH3

O

propanoneacetone

H3CC

CH2CH3

O

butanonemethyl ethyl ketone, MEK

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Bonding and Bonding and Physical PropertiesPhysical Properties

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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.

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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 + -

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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.

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Chemical Properties Chemical Properties of Aldehydes & of Aldehydes &

KetonesKetones

Chemical Properties Chemical Properties of Aldehydes & of Aldehydes &

KetonesKetones

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Reactions of Aldehydes & Ketones

• Oxidation– aldehydes only

• Reduction– aldehydes and ketones

• Addition– aldehydes and ketones

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Oxidation of Aldehydes• Aldehydes are easily oxidized to carboxylic

acids by a variety of oxidizing agents, including (under some conditions) oxygen of the air.

RC

H

O

+ Cr2O72- + 8 H+

3

RC

OH

O3

+ 3 Cr3+ + 4H2O

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Tollens test

• The Tollens test (silver-mirror test) for aldehydes is based on the ability of silver ions to oxidize aldehydes.

RC

H

O

+ 2 Ag+

RC

O-NH4+

O

NH3H2O

+ 2 Ag (s)

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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.

RC

H

O

+ 2 Cu+2

RC

O-Na+

O

NaOHH2O

+ 2 Cu2O (s)

blue brick red

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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.

RC

R

O

+ 2 Cu+2 NaOHH2O

no reaction

RC

R

O

+ 2 Ag+ NH3H2O

no reaction

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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.

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Reduction of Aldehydes & Ketones

• Aldehydes and ketones are easily reduced to alcohols.

RC

H

O

RC

R

O

H2/Niheat

H2/Niheat

RCH2OH

primary alcohol

RCHR

OH

secondaryalcohol

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Addition Reactions of Aldehydes & Ketones

• Common addition reactions:– Addition of alcohols

• hemiacetal, hemiketal, acetal, ketal

– Addition of hydrogen cyanide (HCN)• cyanohydrin

– Aldol Condensation (self-addition)

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Addition of Alcohols

• Aldehydes react with alcohols in the presence of a trace of acid to form hemiacetals:

HC

CH2CH3

O

propanal

+ CH3OHH+

OCH3

HC CH2CH3

OH

1-methoxy-1-propanol(propionaldehyde methyl hemiacetal)

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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:

+ CH3OHdry HCl

OCH3

HC CH2CH3

OH

OCH3

HC CH2CH3

OCH3

+H2O

1,1-dimethoxypropane(propionaldehyde dimethyl acetal)

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Addition of Alcohols to Aldehydes and Ketones

OH

OR'C

RH

OH

OR'C

RR

OR'

OR'C

RH

OR'

OR'C

RR

hemiacetal hemiketal acetal ketal

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Addition of Hydrogen Cyanide• The addition of HCN to aldehydes and ketones

forms a class of compounds known as cyanohydrins:

HC

CH3

O

acetaldehydeCN

HC CH3

OH + HCN HO

-

acetaldehyde cyanohydrin

H3CC

CH3

O

acetoneCN

H3CC CH3

OH + HCN HO

-

acetonecyanohydrin

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Aldol Condensation (Self-Addition)

• In a carbonyl compound, the carbon atoms are labeled alpha (), beta (), gamma (), delta (), and so on, according to their positions with respect to the carbonyl group.

-C-C-C-C=O

• The hydrogen atoms attached to the -carbon have the unique ability to be more easily released as protons than other hydrogens within the molecule.

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Aldol Condensation (Self-Addition)• An aldehyde or ketone that contains -

hydrogens may add to itself or to another -hydrogen containing aldehyde or ketone.

HC

CH3

O

H2CC

H

O

HC

C

OH

H

H

HdiluteNaOH

H3CCH

OH

aldol(3-hydroxybutanal)

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Common Aldehydes & Common Aldehydes & KetonesKetones

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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

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Acetaldehyde (Ethanal)

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

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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.

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Condensation PolymersCondensation Polymers

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Phenol-Formaldehyde Polymers (Bakelite)

• Each formaldehyde molecule reacts with two phenol molecules to eliminate water. The polymer is then formed. OH OH

HC

H

O

+ +

OH OH

H2C

+ H2O

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Phenol-Formaldehyde Polymers (Bakelite)

• Polymers of this type are still used, especially in electrical equipment, because of the insulating and fire-resistant properties.

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