Aldehydes and ketones

Chapter 15

The carbonyl group• Aldehydes and ketones are among the first examples of compounds that

possess a C-O double bond that we’ve seen (oxidation of alcohols section, Ch-14).

• This group is called a carbonyl group, and it has very different chemical properties than a C-C double bond in alkenes:

• Because oxygen is more electronegative than carbon, the bond is polar.• Bond angles are about 120o around the carbon atom (see VSEPR theory).

+ C O

The carbonyl group

• The local geometry around the carbonyl group is trigonal planar. The rest of the molecule doesn’t have to be planar:

H

H

H CH 3CH 3

CH 3CH 3

CH 3

C

C

O

C

C

Local trigonal planar geometry

Compounds containing the carbonyl group

• The following classes of organic compounds involve the carbonyl group:– Aldehydes have a H-atom or a carbon substituent (alkyl,

cycloalkyl, aromatic) bound to a CHO group (carbonyl group bound to a H-atom):

C H

O

R

C H

O

C H

O

CH 3C H

O

H

General formula for aldehyde:

Compounds containing the carbonyl group

• Ketones have two carbon substituents (akyl, cycloalkyl, aromatic and not necessarily the same)

R'C

O

R

CH2CH3 C H

C H 3

C H 3

C

O

C

O

CH 3C

O

CH 3

General formula for ketones:

Compounds containing the carbonyl group

• Carboxylic acids have an OH (hydroxyl) group bound to the carbonyl carbon, in addition to either a H-atom or a carbon group (alkyl, cycloalkyl, aromatic):

C O H

O

R

C O H

O

C O H

O

H C H

C H 3

C H 3 C H 2 C O H

O

C O H

O

CH 3

General formula for carboxylic acids:

Compounds containing the carbonyl group

• Esters have a carbonyl group singly bound to an oxygen, which in turn is bound to a carbon group (alkyl, cycloalkyl, or aromatic). The other bond to the carbonyl is either to a H-atom or another carbon group:

R'OC

O

R

C H 2

C H 3

C H

C H 3

OC

O

C H 2 C H 3OC

O

C H 3O C H 3C

O

H

General formula for an ester:

Compounds containing the carbonyl group• Amides are the first nitrogen-containing organic compounds we’ve seen.

In these compounds, the carbonyl group is bound to a nitrogen (an amino group), in addition to either a H-atom or a carbon group (alkyl, cycloalkyl, aromatic). The R’ and R” groups of the amino group may either be H or carbon groups:

R"

R'

C N

O

R

C NC N C H 2C H 3 C H 2 C N H 2

O

C H 3

HO

C H 3

C H 3

C H 3O

H

General formula for an amide:

Aldehyde and ketone functional group

• As we saw, alcohols can be used to create aldehydes and ketones. Oxidation of a primary alcohol yields an aldehyde:

• And oxidation of a secondary alcohol yields a ketone:

[O]CH 2CH 3 C H

O

CH 2CH 3 C

H

H

O H

[O]CH 2CH 3 C CH 3

O

CH 2CH 3 C

H

CH 3

O H

Aldehyde and ketone functional group

• Cyclic aldehydes are not possible, because in order for the carbonyl group to be part of the ring structure, two bonds to carbon groups would be required.

• Aldehydes may incorporate ring structures, but not be part of the ring.• Also, note that cyclic ketones aren’t heterocyclic compounds.

a cyclic ketone

a cyclic diketone

an aldehyde incorporatinga cyclic compound

C H

O

O

O

O

C H

C H 3

O

H

C H 3

C H

O

C H 2C H 2C H

O

C H 2C H 3

Nomenclature for aldehydes• IUPAC rules:

– Select as the parent chain the longest continuous chain that includes the carbonyl carbon

– Name the parent chain by changing the corresponding alkane name (ending with “e”) to an ending with “al”

– Number the parent chain assuming the carbonyl carbon is C-1– Identify substituents on the parent chain as before, at the beginning

of the compound’s name.

Propanal

4-Methylpentanal2-Ethylpentanal

Nomenclature for aldehydes

• For aldehydes having short carbon chains, the following common names are usually encountered:

• The following aromatic aldehyde is called benzaldehyde:

Formaldehyde(Methanal)

Acetaldehyde(Ethanal)

Propionaldehyde(Propanal)

Butyraldehyde(Butanal)

C H

O

C H 2C H 3C H

O

C H 3C H

O

C H 2C H 2C H 3CH H

O

C

H

O

4-Bromobenzaldehyde 4-Hydroxy-2-methylbenzaldehyde

OH

C H 3

C

H

O

Br

C

H

ODerivatives:

IUPAC

Benzaldehyde

Nomenclature for ketones• IUPAC:

– Select as the parent chain the longest continuous chain that involves the carbonyl carbon

– Name the parent chain by removing the “e” from the corresponding alkane name and adding “one”

– Number the chain to give the carbonyl group the lowest numbering. The number goes before the parent chain name

– Determine the number and location of substituents and number them accordingly

– For cyclic ketones, the carbonyl carbon is C-1 and the name begins with “cyclo”

3-Hexanone

4-Methyl-2-hexanone3-Bromo-2-butanone

2-Methylcyclopentanone

CH 3 O

C H

C H 3

Br

C

O

C H 3

C H 2 C H 3

C H C C H 3

O

C H 2C H 3C H 2 C H 3C

O

C H 2C H 2C H 3

Nomenclature for ketones

• The common system of naming ketones is similar to what we saw for ethers:

Ethyl propyl ketone

Isobutyl methyl ketone 1-Bromoethyl methyl ketone

C H

C H 3

C C H 3

O

C H 2C H 3C H 2 C H 3C

O

C H 2C H 2C H 3 C H

C H 3

Br

C

O

C H 3

Isomerism for aldehydes and ketones

• Aldehydes and ketones that have a given number of carbon atoms are functional group isomers. (This is the third group of compounds we have seen that have this relationship; others were alcohols/ethers and thiols/thioethers)

Propanone Propanal

C3H6O

C H 2

C H 3C

O

H

C

C H 3

O

C H 3

a ketone an aldehyde

Isomerism for aldehydes and ketones

• Positional isomers are possible for ketones (but not aldehydes)

• And skeletal isomers are possible for both

3-Pentanone

C5H10O

2-Pentanone

C H 2

C H 3C H 2

C

O

C H 3C H 2

C H 3

C H 2

C H 3 C

O

C5H10O

2-Pentanone3-Methyl-2-butanone

C H

C H 3

C H 3C

O

C H 3

C H 2

C H 3C H 2

C

O

C H 3

Common aldehydes and ketones

• Aldehydes are often recognizable by their “sweet” smells:

Vanillin(vanilla flavoring)

Benzaldehyde(almond flavoring)

Cinnamaldehyde(cinnamon flavoring)

C

O

HC H

C HC

H

O

O

C H 3

OH

C

H

O

Common aldehydes and ketones

• Some ketones (e.g. acetone) have a “sweet” smell also). Other examples are:

2-Heptanone(clove flavoring)

Butanedione(butter flavoring) Carvone

(spearmint flavoring)

C H 3

O

C H 3 C H 2

C

C H 3

O

C

C H 3

O

C H 3

C

O

C

C H 3(CH2)4

Naturally occurring aldehydes and ketones

• A wide variety of biologically relevant molecules possess aldehyde and/or ketone functional groups:

Testosterone

Progesterone

CortisoneO H

O

C H 3

C H 3

O

O

C H 3

O

C H 3

C H 3

O

O HC H 3

C H 3

O

CH2OH

D-Glucose

CH2OHC

H

C

H

C

H

C

H

O HO H

O H

O H

C

H

O

Physical properties of aldehydes and ketones

• Neither aldehydes nor ketones possess the ability to H-bond with other molecules like themselves. Consequently, boiling points for aldehydes and ketones are lower than for alcohols of similar molar mass.

• The C-O double bond in these molecules is polar, so dipole-dipole forces do exist. As a result, their boiling points tend to be higher than for alkanes of similar molar mass.

+ C O

Physical properties of aldehydes and ketones

Physical properties of aldehydes and ketones

• Water molecules can interact (H-bond) with the non-bonding pairs of the carbonyl group oxygen atom, enabling aldehydes and ketones that have small carbon chain components to be water-soluble.

• As we saw for alcohols, the greater the carbon chain length, the lower the water-solubility (makes the molecule less polar)

H-bond

..

....

..

H

OH

OC

Physical properties of aldehydes and ketones

Physical properties of aldehydes and ketones

Comparing an aldehyde and a ketone of a given number of C-atoms, the ketoneis generally more soluble. Why?

Preparation of aldehydes and ketones

• We saw already (in Ch-14) how alcohols can be oxidized to form aldehydes and ketones.

• Primary (1o) alcohols are oxidized to aldehydes (and subsequently to carboxylic acids)

• Secondary (2o) alcohols are oxidized to ketones

1o alcohol

[O]

[O]

2o alcohol

aldehyde

ketone

H

O

CR

H

H

O H

CR

O

CR

H

O H

CR R'R'

[O] = KMnO4 or K2Cr2O7

Oxidation and reduction of aldehydes and ketones

• Aldehydes can be oxidized easily to carboxylic acids

• Ketones are resistant to oxidation.

Oxidation reactions

aldehyde

[O]

[O]

ketone

no reaction

carboxylic acid

O

CR

O H

O

CRH

O

CR

R'

Oxidation and reduction of aldehydes and ketones

• There are several tests that have been developed to determine the presence of aldehydes, based on their oxidation to carboxylic acids:– Tollen’s test

– Benedict’s test

Oxidation reactions

Ag+

NH3, H2Oheat

Ag

silver metalcarboxylic acidaldehyde

H

O

CR O H

O

CR+ +

Cu2+ Cu2O

reddish solidcarboxylic acidaldehyde

H

O

CR O H

O

CR ++

Oxidation and reduction of aldehydes and ketones

• Both aldehydes and ketones are easily reduced to alcohols with H2 in the presence of a catalyst (Ni, Pt, Cu).

Reduction reactions

Reactions of aldehydes and ketones with alcohols

• When aldehydes and ketones react with alcohols in the presence of an acid, the resulting product is called a hemiacetal. Hemiacetals can further react with alcohols to form acetals:

aldehyde or ketone + alcohol hemiacetal

hemiacetal + alcohol acetal

acidcatalyst

acidcatalyst

Hemiacetals

Reactions of aldehydes and ketones with alcohols

hemiacetal

aldehyde alcohol

hemiacetal

ketone alcohol

R

O

C

H

OOR

HO

CR O

HO

C

O

H

R

HO

CR

H

O

HO

C

H

O

R H

O

C

R'

R"

R'

R"R"

R'

R'R'R'

+

+

A hemiacetal is an organic compound that possesses a carbon atom that is bound toan OH (hydroxy) group and an OR (alkoxy) group

Reactions of aldehydes and ketones with alcohols

• Hemiacetal formation can also involve a carbonyl group and OH group on the same molecule. Here is an important process which involves this reaction:

See this again in Ch-18

Hemiacetals

Cyclic hemiacetals are stable,unlike non-cyclic hemiacetals

Reactions of aldehydes and ketones with alcohols

• Hemiacetals can be converted to acetals in the presence of an alcohol and a catalytic amount of acid:

Acetals

H2O

acetal

H2O

acetalhemiacetal(derived from

a ketone)

hemiacetal(derived froman aldehyde)

HO

R C O

O

R C O

O

R

H

C OR

H

OC

HO

HO

O

H

R

HO

C

R'

R" R"' OH

R'

R"'

R"

R'

R"

R" OH

R"

R'R' +

++

+

Reactions of aldehydes and ketones with alcohols

• Indicate whether each of the following structures is a hemiacetal, acetal, or neither:

OH

C H 2C H 3

OH

C H 2

C H 3

C

OH

O

C H 3

C C H 3C H 3

C

C H 3

C H 3O

C H 3

C HC H 3

C H 3

O

Reactions of aldehydes and ketones with alcohols

• Acetals can be isolated and used in subsequent chemical reactions. (Hemiacetals are less stable and generally can’t be isolated.)

• If an acetal is treated with acid in the presence of water, a hydrolysis reaction occurs

Acetals

Hydrolysis reaction: a reaction of a compound with water in which the compound splits intotwo or more fragments.

H2O

acidcatalyst

acetal (derivedfrom an aldehyde)

acetal (derivedfrom a ketone)

ketone

aldehyde alcohol 1 alcohol 2

H2O

acidcatalyst

alcohol 2alcohol 1

H

O

R C O O HO H

O HO H

O

CR

O

R C O

H

O

CR

R"'

R" R"R'

R"'R"R'

R"'

R'

R" + +

+

+

++

Reactions of aldehydes and ketones with alcohols

• Draw the aldehyde/ketone and alcohols that will result when the acetals below are treated with acid/H2O:

aldehyde/ketone alcohols

C H

C H 3

C H 2C H 3

CC H 3

C H 2C H 3 O

O

C H 3

C H 2C H 3

OCH 3

C

C HC H 3

C H 3

O

H

O C H 3

C

C H 2 C H 3O

C H 3C H 2C H 3

Sulfur-containing carbonyl groups• Sulfur analogues of aldehydes and ketones are known. The sulfur

atom can either replace the carbon or the oxygen of the carbonyl group.

• In the first case, the resulting compounds are called thioaldehydes or thioketones, and these are generally unstable:

• In the second case, sulfoxides result:

a thioaldehyde a thioketone

R'C

S

RC H

S

R

a sulfoxide

R'S

O

R

Sulfur-containing carbonyl groups

• The best known example of a sulfoxide is Dimethyl sulfoxide (DMSO), which is a sulfur analogue of acetone:

• DMSO is an excellent solvent; it can dissolve a wide variety of polar and non-polar substances.

DMSO acetone

C C H 3

O

CH 3S C H 3

O

CH 3

-

.

.

.

... .

.

..

.

.. .

C H 3

O

SCH 3 C H 3

O

SCH 3

+