3.8 Aldehydes and Ketones

Carbonyls: Aldehydes and Ketones

.

Carbonyls are compounds with a C=O bond.They can be either aldehydes or ketones

If the C=O is on the end of thechain with an H attached it is analdehyde.The name will end in –al

CH3CHO ethanalCH3COCH3

If the C=O is in the middle ofthe chain it is a ketoneThe name will end in -one

propanone

Solubility in water

The smaller carbonyls are solublein water because they can formhydrogen bonds with water.

C

CH3

CH3

OO H

H

Intermolecular forces in carbonylsPure carbonyls cannot hydrogen bond, butbond instead by permanent dipole forces.

Reactions of carbonylsIn comparison to the C=C bond inalkenes, the C=O is stronger and doesnot undergo addition reactions easily.

The C=O bond is polarised becauseO is more electronegative thancarbon. The positive carbon atomattracts nucleophiles. This is in contrast to the electrophiles

that are attracted to the C=C .

Oxidation Reactions

Primary alcohol

Tertiary alcoholsSecondary alcohol

aldehydes carboxylic acidketones

do not oxidise

Potassium dichromate K2Cr2O7 isan oxidising agent that causesalcohols and aldehydes tooxidise.

Key point: Aldehydescan be oxidised tocarboxylic acids, butketones cannot beoxidised.

Reaction: aldehyde carboxylic acidReagent: potassium dichromate (VI) solution anddilute sulfuric acid.Conditions: heat under reflux

Oxidation of Aldehydes

RCHO + [O] RCO2H

+ [O]CO

C

H

H

C

HH

H

H

C C

O

O HH

C

H

H

H

H

Full equation for oxidation3CH3CHO + Cr2O7

2- + 8H+ 3 CH3CO2H + 4H2O + 2Cr3+

Observation: the orange dichromate ion(Cr2O7

2-) reduces to the green Cr 3+ ion

Aldehydes can also be oxidised using Fehling’ssolution or Tollen’s reagent. These are used astests for the presence of aldehyde groups

Tollen’s Reagent

CH3CHO + 2Ag+ + H2O CH3COOH + 2Ag + 2H+

Reagent: Fehling’s solution containing blue Cu 2+ ions.Conditions: heat gentlyReaction: aldehydes only are oxidised by Fehling’s

Solution into a carboxylic acid. The copper (II) ionsare reduced to copper(I) oxide . .

Observation: Aldehydes :Blue Cu 2+ ions in solutionchange to a red precipitate of Cu2O. Ketones donot react.

Fehling’s solution

CH3CHO + 2Cu2+ + 2H2O CH3COOH + Cu2O + 4H+

Reagent: Tollen’s reagent formed by mixingaqueous ammonia and silver nitrate. Theactive substance is the complex ion of[Ag(NH3)2]+ .

Conditions: heat gentlyReaction: aldehydes only are oxidised by

Tollen’s reagent into a carboxylic acid. Thesilver(I) ions are reduced to silver atoms

Observation: with aldehydes, a silver mirror formscoating the inside of the test tube. Ketonesresult in no change.

1

CH3

C

CH3

O

δ+

δ-

nucleophile

C

O

C

H

H

H

H

C C CO

HH

H

HH

H

δ+ δ-δ-

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C

H

CH3 CH3

O

2

Reduction of carbonylsReducing agents such as NaBH4 (sodium tetrahydridoborate)or LiAlH4 (lithium tetrahydridoaluminate) will reduce carbonylsto alcohols.

Aldehydes will be reduced to primary alcohols Ketones will be reduced to secondary alcohols.

propanone

C C CO

HH

H

HH

HC

H

H

C

H

C

H

H

H OH

H+ 2[H] CO

C

H

H

C

HH

H

H

+ 2[H] C O H

H

H

C

H

H

C

H

H

Hpropanal Propan-1-ol

Propan-2-ol

Reagents: NaBH4 In aqueous ethanolConditions: Room temperature and pressure

NaBH4 contain a source ofnucleophilic hydride ions (:H-)which are attracted to the positivecarbon in the C=O bond.

Nucleophilic Addition Mechanism H+ from water or weak acid

Catalytic HydrogenationCarbonyls can also be reduced using catalytic hydrogenation

Reagent: hydrogen and nickel catalyst

Conditions: high pressure

CH3CHO + H2 CH3CH2OHExample Equations

CH3COCH3 + H2 CH3CH(OH)CH3

Addition of hydrogen cyanide to carbonyls to form hydroxynitriles

Reaction: carbonyl hydroxynitrileReagent: sodium cyanide (NaCN) and dilute sulfuricacid.Conditions: Room temperature and pressureMechanism: nucleophilic addition

NC C

R

H

OH

hydroxynitrile

The NaCN supplies thenucleophilic CN- ions. TheH2SO4 acid supplies H+ ionsneeded in second step of themechanism

CH3COCH3+ HCN CH3C(OH)(CN)CH3

CH3CHO + HCN CH3CH(OH)CN

When naming hydroxy nitrilesthe CN becomes part of themain chain and carbon no 12-hydroxy-2-methylpropanenitrile

2-hydroxypropanenitrile

Nucleophilic Addition MechanismH+ from sulfuric acid

NC C

CH3

CH3

OH

NC C

CH3

H

OH

We could use HCN for this reaction but itis a toxic gas that is difficult to contain.KCN/NaCN are still, however, toxic,because of the cyanide ion.

CH3

CCH3

O

:H-

δ+

δ-

: H+

C

H

CH3 CH3

O H-

CH3

CCH3

O

:CN-

δ+

δ-

: H+-

C

CN

CH3 CH3

O

C

CN

CH3 CH3

O H

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CH3C

H

O

3

:CN--NC: C

CH3

O

H

C

NC CH3

OH

H

C

CH3 CN

OH

HThere is an equalchance of eitherenantiomer formingso a racemate forms.No optical activity isseen.

Nucleophilic addition of HCN to aldehydesand ketones (unsymmetrical) when thetrigonal planar carbonyl is approached fromboth sides by the HCN attacking species:results in the formation of a racemate.

Mechanism for the reaction (drawn the same for both enantiomers)

:CN-

δ+

δ-

: H+-

C

CN

CH3 H

O

C

CN

CH3 H

O H

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