Chapter 16

Aldehydes and Aldehydes and KetonesKetones

Structure

The functional group of an aldehydealdehyde is a carbonyl group bonded to a hydrogen atom

The functional group of a ketoneketone is a carbonyl group bonded to two carbons

CH3CHO

HCHO

CH3CCH3

O

Propanone(Acetone)

Ethanal(Acetaldehyde)

Methanal(Formaldehyde)

Nomenclature

IUPAC names for aldehydes to name an aldehyde, change the suffix -ee of the

parent alkane to -alal the aldehyde carbon must be carbon-1 for unsaturated aldehydesunsaturated aldehydes, indicate the presence of

a carbon-carbon double bond and an aldehyde by changing the ending of the parent alkane from -aneane to -enalenal; show the location of the carbon-carbon double bond by the number of its first carbon

Nomenclature

the IUPAC system retains common names for some aldehydes, including these three

3-Methylbutanal 2-Propenal(Acrolein)

Hexanal

12

34H

O

H

O1

23

45

6

123

H

O

CHO

H

OCHO

OCH3

OHtrans-3-Phenyl-2-propenal

(Cinnamaldehyde; inoil of cinnamon)

Benzaldehyde(in almonds)

Vanillin(from vanilla

beans)

Nomenclature

IUPAC names for ketones parent = longest chain that contains the carbonyl indicate with parent alkane as -oneone carbonyl carbon gets the smaller number the IUPAC retains the common name acetone for

2-propanone

O

Acetone 2-Methylcyclohexanone5-Methyl-3-hexanone

OO

12

34

56

12

Nomenclature To name an aldehyde or ketone that also

contains an -OH or -NH2 group give the carbonyl carbon the lower number indicate an -OH substituent by hydroxy-hydroxy-, and an -NH2

substituent by amino-amino- hydroxy and amino substituents are numbered and

alphabetized along with other substituents

O

H

OOH

NH2

3-Hydroxy-4-methylpentanal 3-Amino-4-ethyl-2-hexanone

1345 12346

Nomenclature

Common names derived from the common name of the

corresponding carboxylic acid; drop the word "acidacid" and change the suffix -icic or -oicoic to -aldehydealdehyde

name each alkyl or aryl group bonded to the carbonyl carbon as a separate word, followed by the word "ketoneketone”;

O

CH3CH

O

CH3COH

Acetaldehyde Acetic acid Ethyl isopropyl ketoneMethyl ethyl ketone

OO

Physical Properties

A C=O bond is polar, with oxygen bearing a partial negative charge and carbon bearing a partial positive charge

Physical Properties

in liquid aldehydes and ketones, the intermolecular attractions are polar

no hydrogen bonding is possible between aldehyde or ketone molecules

aldehydes and ketones have lower boiling points than alcohols and carboxylic acids, compounds in which there is hydrogen bonding between molecules

Physical Properties

formaldehyde, acetaldehyde, and acetone are infinitely soluble in water

aldehydes and ketones become less soluble in water as the hydrocarbon portion of the molecule increases in size,

CH3CH2CH2CH2CH3CH3CH2CH2CHO

CH3CH2CH2CH2OHCH3CH2COOH

CH3CH2COCH3

CH3CH2OCH2CH3pentanebutanal2-butanone1-butanolpropanoic acid

Name Structural FormulaMolecular

Weight (amu)

72727274

72

367680

117

141

bp(°C)

diethyl ether 74 34

Oxidation Aldehydes are oxidized to carboxylic acids by

a variety of oxidizing agents, including potassium dichromate

liquid aldehydes are sensitive to oxidation by O2

H

OK2Cr2O7

H2SO4OH

O

Hexanal Hexanoic acid

CH

O

O2

COH

O

Benzoic acidBenzaldehyde

+

Oxidation

Ketones resist oxidation by most oxidizing agents, including potassium dichromate and molecular oxygen

Reduction

The carbonyl group of an aldehyde or ketone can be reduced to an -CHOH group by hydrogen in the presence of a metal catalyst

H2

transition metal catalyst+H

O

PentanalOH

1-Pentanol

H2

transition metal catalyst

+O

Cyclopentanone

OH

Cyclopentanol

Reduction

The most common laboratory reagent for the reduction of an aldehyde or ketone is sodium borohydride, NaBHNaBH44 hydrogen in the form of hydride ion, H:H:--

in a reduction by sodium borohydride, hydride ion adds to the partially positive carbonyl carbon which leaves a negative charge on the carbonyl oxygen

reaction of this intermediate with aqueous acid gives the alcohol

Reduction

does not affect a carbon-carbon double bond

HCO

1. NaBH4

2. H2O

CH2OH

Cinnamaldehyde Cinnamyl alcohol

O NaBH4O-

HH3O+ O-H

H

H - C O H C O - H3O+

H C O-H: +

: : :: :

::

Hydrideion

Reduction

In biological systems, the agent for the reduction of aldehydes and ketones is NADH (Section 26.3) this reducing agent also delivers a hydride ion reduction of pyruvate, the end product of glycolysis,

by NADH gives lactate

CH3-C-COO-O

NADH CH3-C-COO-

H

O-

H3O+

CH3-C-COO-

H

O-H

Pyruvate Lactate

Addition of Alcohols

Addition of a molecule of alcohol to the carbonyl group of an aldehyde or ketone forms a hemiacetalhemiacetal (a half-acetal) the functional group of a hemiacetal is a carbon

bonded to one -OH group and one -OR group in forming a hemiacetal, H of the alcohol adds to

the carbonyl oxygen and OR adds to the carbonyl carbon

CH

OO-CH2CH3

HC OCH2CH3

H

O-H+

Benzaldehyde Ethanol A hemiacetal

Addition of Alcohols

hemiacetals are generally unstable and are only minor components of an equilibrium mixture

If a five- or six-membered ring can form, the compound exists almost entirely in a cyclic hemiacetal form

H

O

O-HC

O O

H

H

O O-H

H

4-Hydroxypentanal A cyclic hemiacetal

123

45

1345

redraw to show the -OH and -CHO close

to each other2

Addition of Alcohols

A hemiacetal can react further with an alcohol to form an acetalacetal plus water this reaction is acid catalyzed the functional group of an acetal is a carbon

bonded to two -OR groups

C OCH2CH3

H

O-HOCH2CH3

H H+

C OCH2CH3

H

OCH2CH3

H2O

A hemiacetal(from benzaldehyde)

Ethanol

+ +

An acetal

Addition of Alcohols

all steps are reversibleLe Chatelier's principle

to drive it to the right, we either use a large excess of alcohol or remove water from the equilibrium mixture

to drive it to the left, we use a large excess of water

OCH2CH3

O-HOCH2CH3

HH+

OCH2CH3

OCH2CH3H2O

An acetalA hemiacetal(from cyclohexanone)

Ethanol

+ +

Keto-Enol Tautomerism

A carbon atom adjacent to a carbonyl group is called an -carbon-carbon, and a hydrogen atom bonded to it is called an -hydrogen-hydrogen

-carbons

-hydrogens

CH3-C-CH2-CH3

O

Keto-Enol Tautomerism

A carbonyl compound that has a hydrogen on an -carbon is in equilibrium with a constitutional isomer called an enolenol

in a keto-enol equilibrium, the keto form generally predominates

CH3-C-CH3

OCH3-C=CH2

OH

Acetone(keto form)

Acetone(enol form)

Keto-Enol Tautomerism

example:example: draw structural formulas for the two enol forms for each ketone

(a)

(b)

O

O

Keto-Enol Tautomerism

example:example: draw structural formulas for the two enol forms for each ketone

solution:solution:

(a)

(b)

O

O

OH OH

OH OH

End End Chapter 17Chapter 17

Aldehydes and Ketones