Aldehydes and Ketones

Bettelheim, Brown, Campbell and Farrell

Chapter 17

Structure• Aldehyde:Aldehyde: carbonyl group bonded to a

hydrogen atom– in methanal, the simplest aldehyde, the

carbonyl group is bonded to two hydrogens– in other aldehydes, it is bonded to one

hydrogen and one carbon

• Ketone:Ketone: carbonyl group bonded to two carbons

CH3CHO

HCHO

CH3CCH3

O

Propanone(Acetone)

Ethanal(Acetaldehyde)

Methanal(Formaldehyde)

Nomenclature• IUPAC names for aldehydes

– Change suffix -ee of the parent alkane to -alal– Aldehyde group can only be at end of chain – Aldehyde group always C-1 so we don’t need to

number it.– Start counting chain from aldehyde end– Unsaturated aldehydesUnsaturated aldehydes show the carbon-carbon

double bond and an aldehyde by changing the ending of the parent alkane from -aneane to –eenalnal

– Show the location of the carbon-carbon double bond by the number of its first carbon

Nomenclature

– IUPAC system uses common names for some aldehydes, including

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

– Find longest chain that contains the carbonyl group– Show ketone by changing the -ee of the parent

alkane -oneone – Show position of carbonyl group with a number– Number the parent chain from the direction that

gives the carbonyl carbon the smaller number– IUPAC uses the common name acetone for 2-

propanoneO

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

– Number chain to give the carbonyl carbon the lower number

– Show 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

– Aldehyde: Common name of the corresponding carboxylic acid is modified– substitute the word -aldehydealdehyde and for the suffix -icic or -oicoic acidacid

– Ketone: 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

CH3CH2CCH3

O

CH3CHCH2CH2CH

NH2

O

Physical Properties• A C=O bond is polar, with oxygen bearing

a partial negative charge and carbon bearing a partial positive charge– Aldehydes and ketones are polar molecules

Physical Properties– Intermolecular attractions (dipole-dipole)

between partial positive and negative charges on different molecules

– NO hydrogen bonding is possible between aldehyde or ketone molecules

– Lower boiling points than alcohols and carboxylic acids (which can H-bond with each other)

– Small aldehydes and ketones are soluble in water (can form H-bonds WITH WATER)

Physical Properties

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 and must be protected from contact with air during storage

H

OK2Cr2O7

H2SO4OH

O

Hexanal Hexanoic acid

CH

O

O2

COH

O

Benzoic acidBenzaldehyde

+

Oxidation• Ketones are NOT oxidized by most

oxidizing agents, including potassium dichromate and molecular oxygen– Tollens’ reagent is specific for the oxidation of

aldehydes; if done properly, silver deposits on the walls of the container as a silver mirror

R-C-HO

2Ag(NH3)2+ 3OH-

R-C-O-O

2Ag 4NH3 2H2O

+ +

+ + +

Tollens'reagent

Carboxylicanion

Silvermirror

Aldehyde

Reduction

• C=O group of an aldehyde or ketone is reduced to an -CHOH group by hydrogen in the presence of a transition-metal catalyst– reduction of an aldehyde gives a primary alcohol– reduction a ketone gives a secondary alcohol

H2

transition metal catalyst+H

O

PentanalOH

1-Pentanol

H2

transition metal catalyst

+O

Cyclopentanone

OH

Cyclopentanol

Reduction• Most common laboratory reagent for the

reduction of an aldehyde or ketone is sodium borohydride, NaBHNaBH44

• Product is an alcohol

• NaBH4 does NOT add H atoms to C=C double bond (only to C=O)

• H2 does add H atoms to C=C (and to C=O)

Reduction

• Reduction by NaBH4 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

CH2=CHCCH3

O

H2 / Ni

CH2=CHCCH3

ONaBH4

CH2=CHCCH3

ONa2Cr2O7/H2SO4

or KMnO4

CH3CH2CH

OO2

CH3CH2CCH3

O[Ox]

Addition of Alcohols to Carbonyls• Addition of an alcohol to the carbonyl group

of an aldehyde or ketone forms a hemiacetalhemiacetal (a half-acetal)– Functional group of a hemiacetal is a carbon

bonded to one -OH group and one -OR group– 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 to Carbonyl– Hemiacetals are generally unstable and are only

minor components of most equilibrium mixtures– Major exception: When both the carbonyl group

and the hydroxyl group are in the same molecule and can form a cyclic hemiacetal with a 5- or 6-member ring

– Cyclic hemiacetals predominate

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

• Hemiacetal can react further with a second alcohol to form an acetalacetal plus water– Reaction is acid catalyzed– 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 in hemiacetal and acetal formation are reversible

– Le Chatelier's Principle applies to this equilibrium– Adding a large excess of alcohol or removing water will

shift the equilibrium to the right– Adding a large excess of water will shift the equilibrium

to the left

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

• A hydrogen atom bonded to an -carbon 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 will form an equilibrium with a constitutional isomer called an enolenol– The isomer will have a C=C double bond and

an alcohol group– The name “enol” is derived from the IUPAC

designation of it as both an alkene (-enen-) and an alcohol (-olol)

– Keto form generally predominates in a keto-enol equilibrium

Keto-Enol Tautomerism

• Keto form has C=O double bond

• Enol form has C=C double bond and –OH

CH3-C-CH3

OCH3-C=CH2

OH

Acetone(keto form)

Acetone(enol form)

Keto-Enol Tautomerism

– Draw structural formulas for the two enol forms for each ketone

(a)

(b)

O

O

Keto-Enol Tautomerism

– Draw structural formulas for the two enol forms for each ketone

– For compounds a & b, two possible isomers exist

(a)

(b)

O

O

OH OH

OH OH

Nucleic Acids

HN

NO

H

N

N

NH2

H

HN

N

H

CH3

Uracil (U)(in RNA)

Thymine (T)(DNA andsome RNA)

Cytosine (C)(DNA andsome RNA)

N

N

Pyrimidine

1

2

3

4

5

6

HN

N N

NO

HH2N

Guanine (G)(DNA and RNA)

N

N N

N

NH2

HAdenine (A)

(DNA and RNA)

N

N N

N

HPurine

1

2

3

4

56 7

8

9

O O

O O

HN

N NH

N

O

H2N

HN

N NH

N

O

H2N

Reactions of Aldehydes and Ketones

Oxidation (K2Cr2O7/H2SO4 OR KMnO4)

Aldehyde → Carboxylic acid

Ketone → No Rxn

Reactions of Aldehydes and Ketones

Reduction (H2/Ni,Pd, Pt OR NaBH4)

Aldehyde → 1o Alcohol

Ketone → 2o Alcohol

H2 adds to BOTH C=O and C=C bonds

NaBH4 adds ONLY to C=O bonds (NOT C=C)

Reactions of Aldehydes and Ketones

Addition of Alcohols

Aldehyde + ROH → Hemiacetal

Hemiacetal + ROH → Acetal

Summary: Aldehyde + 2 ROH → Acetal

Similar reactions with Ketones