Chapter 9 Chapter 9 Aldehydes and KetonesAldehydes and Ketones

StructureStructure

The functional group of an aldehydealdehyde is a carbonyl group bonded to a hydrogen atom.• In methanal, the simplest aldehyde

(formaldehyde), the carbonyl group is bonded to two hydrogens.

• In other aldehydes, it is bonded to one hydrogen and one carbon group.

The functional group of a ketoneketone is a carbonyl group bonded to two carbon groups.

O

C

R H

Aldehyde

O

C

R R

Ketone

NomenclatureNomenclatureIUPAC names for aldehydes:• The longest chain must have an aldehyde group (-

COH)• No need to use a number to indicate the aldehyde

group• To name an aldehyde, change the suffix -ee of the

parent alkane to -alal.• For unsaturated aldehydesunsaturated aldehydes, indicate the presence

of a carbon-carbon double bond by changing the ending of the parent alkane from -aneane to -enalenal. Numbering the carbon chain begins with the aldehyde carbonyl carbon.

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

ExamplesExamples Name the following molecules

Draw a line-angle for◦ 5-isopropyl-2,5-dimethyl-heptanal

◦ 2-propenal

H

O

Cl

O

H

NomenclatureNomenclature• The IUPAC system retains common names for some

aldehydes, including these three.

NomenclatureNomenclatureIUPAC names for ketones.

• The parent alkane is the longest chain that contains the carbonyl group.

• Indicate the presence of the carbonyl group by changing the -aneane of the parent alkane -oneone.

• Number the parent chain from the direction that gives the carbonyl carbon the smaller number.

• The IUPAC retains the common name acetone for 2-propanone.

ExamplesExamples Name the following molecules

Draw line-angle for these o-Ethylbenzylaldehyde

3,3-dimethylcylohexanone

O O O

NomenclatureNomenclatureTo name an aldehyde or ketone that also contains an -OH (hydroxyl) or -NH2 (amino) group:

• Number the parent chain to give the carbonyl carbon the lower number.

• Indicate an -OH substituent by hydroxy-hydroxy-, and an -NH2 substituent by aminoamino--.

• Hydroxyl and amino substituents are numbered and alphabetized along with other substituents.

NomenclatureNomenclatureCommon names

The common name for an aldehyde is 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”. Alkyl or aryl groups are generally listed in order of increasing molecular weight.

ExamplesExamples Name the following compounds

O

OH

CHO

NH2

ExamplesExamples Draw the structure for each of the following compounds:

5-aminobenzaldehyde

2,4-pentadione

Physical PropertiesPhysical PropertiesA C=O bond is polar, with oxygen bearing a partial negative charge and carbon bearing a partial positive charge.• Therefore, aldehydes and ketones are polar molecules.• Figure 9.1 The polarity of a carbonyl group.

Physical PropertiesPhysical Properties• In liquid aldehydes and ketones, there are weak

intermolecular attractions between the partial positive charge on the carbonyl carbon of one molecule and the partial negative charge on the carbonyl oxygen of another molecule.

• 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. See the table on the next screen.

Physical PropertiesPhysical Properties• Table 9.1 Boiling Points for Six Compounds of

Comparable Molecular Weight.

• 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.

OxidationOxidation• Aldehydes are oxidized to carboxylic acids by a variety

of oxidizing agents, including potassium dichromate.

R C

O

H[ O ]

R C

O

OHoxidizing agent

K2Cr2O7/H2SO4

KMnO4

Ag2O

OxidationOxidation Liquid aldehydes are so sensitive to oxidation by O2 in

the air that they must be protected from contact with air during storage.

OxidationOxidation◦ Ketones resist oxidation 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

O

H + 2Ag(NH3)2+ + 3 -OH R C

O

O- + 2Ag + 4NH3 + 2H2Ocarboxylateanion

silver miror

Tollen'sreagentAldyhyde

ExamplesExamplesCHO

HO

+ Ag2O

Hexanal + O2

3-Phenylpropanal + Ag(NH3)2+

ReductionReduction• The carbonyl 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.

pentanal pentanol

cyclopentanone cyclopentanol

ReductionReductionThe most common laboratory reagent for the reduction of an aldehyde or ketone is sodium borohydride, NaBHNaBH44.

• This reagent contains 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.

H3C C CH2CH3

O

2-butanone

NaBH4

CH3OHH3C C

HCH2CH3

OH

2 butanol

ReductionReduction

ReductionReduction

•Reduction by NaBH4 does not affect a carbon-carbon double bond or an aromatic ring.

Cinnamaldehyde Cinnamyl alcohol

ReductionReduction• In biological systems, the agent for the reduction of

aldehydes and ketones is the reduced form of nicotinamide adenine dinucleotide, abbreviated NADH

• This reducing agent, like NaBH4, delivers a hydride ion to the carbonyl carbon of the aldehyde or ketone.

• Reduction of pyruvate, the end product of glycolysis, by NADH gives lactate.

Pyruvate Lactate

ExamplesExamples What alcohol is obtained from the reduction of the

following compound 2-methylpropanal with NaBH4/H+

Determine whether the starting material is an aldehyde or a ketone from the production of 2,6-heptanol with H2/metal catalyst

Addition of AlcoholsAddition of AlcoholsAddition 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.

Addition of AlcoholAddition of Alcohol

H3CC

H

O

+ ROHH+

H3C OR

H

OH

an aldehydea hemiacetal

+ ROHH+

H3C OR

H

OR

an acetal

H3CC

H

O

+ ROHH+

H3C OR

H

OH

an aldehydea hemiacetal

Further Addition of Alcohol

Addition of AlcoholsAddition of Alcohols

H3CC

CH3

O

+ ROHH+

H3C OR

CH3

OH

a ketonea hemiketal

H3CC

CH3

O

+ ROHH+

H3C OR

CH3

OH

an ketonea hemiketal

+ ROHH+

H3C OR

CH3

OR

a ketal

Further Addition of Alcohol

Addition of AlcoholsAddition of Alcohols• Hemiacetals are generally unstable and are only minor

components of an equilibrium mixture except in one very important type of molecule.

• When a hydroxyl group is part of the same molecule that contains the carbonyl group and a five- or six-membered ring can form, the compound exists almost entirely in a cyclic hemiacetal form.

Addition of AlcoholAddition of AlcoholFormation of acetal using a diol

as the alcohol gives a cyclic acetal

C

O

H+

H2C CH2

OHOH

H

O O

Benzylaldehyde ethylene acetalBenzaldehyde

Addition of AlcoholsAddition of Alcohols• All steps in hemiacetal and acetal formation are

reversible.• As with any other equilibrium, we can drive it in either

direction by using Le Chatelier's principle.• To drive it to the right, we either use a large excess of

alcohol or remove water from the equilibrium mixture

Addition of AlcoholAddition of Alcohol To drive it to the left, we use a large excess of water.

OH

OCH2CH3

H2O excess

H+ O

Ketonehemiacetal

+ HOCH2CH3

ExampleExample Show the reaction of benzaldehyde with one molecule

of methanol to form a hemiacetal and then with a second of methanol to form an acetal

ExamplesExamples Draw the structures of the aldehyde or ketones and

alcohols formed when these acetals are treated with aqueous acid and hydrolyzed.

OCH3

OCH3

OCH3

O O

Keto-Enol TautomerismKeto-Enol Tautomerism

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

Keto-Enol TautomerismKeto-Enol Tautomerism An aldehyde or ketone that has a hydrogen on an a-carbon is in equilibrium with a constitutional isomer called an enoenoll.◦ The name “enol” is derived from the IUPAC

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

◦ In a keto-enol equilibrium, the keto form generally predominates.

RH2CC

O

RCRHC

OH

R

Keto tautomer enol tautomer

Keto-Enol TautomerismKeto-Enol Tautomerism• Example:Example: Draw structural formulas for the two enol

forms for each ketone.