THE REACTIONSTHE REACTIONSOF ALDEHYDES OF ALDEHYDES AND KETONESAND KETONES

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SPECIFICATIONSSPECIFICATIONS

CARBONYL COMPOUNDS - FORMULAECARBONYL COMPOUNDS - FORMULAE

Molecular C3H6O

Structural C2H5CHO CH3COCH3

Displayed

Skeletal

C = OH

C2H5

C = OCH3

CH3

H C C C H

H O H

H H

H C C C O

H H H

H H

O O

CARBONYL COMPOUNDS - NOMENCLATURECARBONYL COMPOUNDS - NOMENCLATURE

Aldehydes C2H5CHO propanal

Ketones CH3COCH3 propanone

CH3CH2COCH3 butanone

CH3COCH2CH2CH3 pentan-2-one

CH3CH2COCH2CH3 pentan-3-one

C6H5COCH3 phenylethanone

CARBONYL COMPOUNDS - CHEMICAL PROPERTIESCARBONYL COMPOUNDS - CHEMICAL PROPERTIES

OXIDATION

• provides a way of differentiating between aldehydes and ketones• mild oxidising agents are best• aldehydes are easier to oxidise

• powerful oxidising agents oxidise ketones to a mixture of carboxylic acids

ALDEHYDES easily oxidised to acids

RCHO(l) + [O] ——> RCOOH(l)CH3CHO(l) + [O] ——> CH3COOH(l)

KETONES oxidised under vigorous conditions to acids with fewer carbons

C2H5COCH2CH3(l) + 3 [O] ——> C2H5COOH(l) + CH3COOH(l)

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION

Mechanism occurs with both aldehydes and ketones

involves addition to the C=O double bond

unlike alkenes, they are attacked by nucleophiles

attack is at the positive carbon centre due to thedifference in electronegativities

alkenes are non-polar and are attacked by electrophiles

undergoing electrophilic addition

C=C ELECTROPHILESALKENES

CARBONYLS

NON-POLAR

C=O POLAR NUCLEOPHILES

ADDITION

ADDITION

Bond Attacking speciesGroup Polarity Result

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION

Reagent potassium cyanide – followed by dilute acid

Conditions reflux

Nucleophile cyanide ion CN¯

Product(s) hydroxynitrile (cyanohydrin)

Equation CH3CHO + HCN ——> CH3CH(OH)CN

2-hydroxypropanenitrile

Notes HCN is a weak acid and has difficulty dissociating into ions

HCN H+ + CN¯

Using ionic KCN produces more of the nucleophilic CN¯

Alternative reagent: HCN catalysed by alkali which shifts the above equilibrium in favour of CN¯

HIGHLY TOXICTAKE GREAT CARE

HIGHLY TOXICTAKE GREAT CARE

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION

Mechanism Nucleophilic addition

Step 1 CN¯ acts as a nucleophile and attacks the slightly positive COne of the C=O bonds breaks; a pair of electrons goes onto the O

Step 2 A pair of electrons is used to form a bond with H+

Overall, there has been addition of HCN

STEP 2STEP 1

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION

Mechanism Nucleophilic addition

Step 1 CN¯ acts as a nucleophile and attacks the slightly positive COne of the C=O bonds breaks; a pair of electrons goes onto the O

Step 2 A pair of electrons is used to form a bond with H+

Overall, there has been addition of HCN

STEP 2STEP 1

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION

Watch out for the possibility of optical isomerism in hydroxynitriles

CN¯ attacks from above

CN¯ attacks from below

Both optical isomers are produced in a racemic mixture

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION

Watch out for the possibility of optical isomerism in hydroxynitriles

CN¯ attacksfrom above

CN¯ attacksfrom below

Reagent sodium tetrahydridoborate(III) (sodium borohydride), NaBH4

Conditions aqueous or alcoholic solution

Mechanism Nucleophilic addition (also reduction as it is addition of H¯)

Nucleophile H¯ (hydride ion)

Product(s) Alcohols Aldehydes are REDUCED to primary (1°) alcohols. Ketones are REDUCED to secondary (2°) alcohols.

Equation(s) CH3CHO + 2[H] ——> CH3CH2OH

CH3COCH3 + 2[H] ——> CH3CHOHCH3

Notes The water provides a proton

CARBONYL COMPOUNDS - REDUCTION WITH CARBONYL COMPOUNDS - REDUCTION WITH NaBHNaBH44

Reagent sodium tetrahydridoborate(III) (sodium borohydride), NaBH4

Conditions aqueous or alcoholic solution

Mechanism Nucleophilic addition (also reduction as it is addition of H¯)

Nucleophile H¯ (hydride ion)

CARBONYL COMPOUNDS - REDUCTION WITH CARBONYL COMPOUNDS - REDUCTION WITH NaBHNaBH44

Water is added

Primary alcoholPrimary alcoholAldehydeAldehyde

Reagent sodium tetrahydridoborate(III) (sodium borohydride), NaBH4

Conditions aqueous or alcoholic solution

Mechanism Nucleophilic addition (also reduction as it is addition of H¯)

Nucleophile H¯ (hydride ion)

CARBONYL COMPOUNDS - REDUCTION WITH CARBONYL COMPOUNDS - REDUCTION WITH NaBHNaBH44

ANIMATED MECHANISM

THE TETRAHYDRIDOBORATE(III) ION THE TETRAHYDRIDOBORATE(III) ION BHBH44

B H HH

H

H

H

H

each atom needs one electron to complete

its outer shell atom has three

electrons to share boron shares all 3 electrons to form 3 single covalent bonds

B

H

H

H

The hydride ion forms a dative covalent bond

B

H

H

H

B H

CARBONYL COMPOUNDS - REDUCTION WITH CARBONYL COMPOUNDS - REDUCTION WITH HYDROGENHYDROGEN

Reagent hydrogen

Conditions catalyst - nickel or platinum

Reaction type Hydrogenation, reduction

Product(s) Alcohols Aldehydes are REDUCED to primary (1°) alcohols.Ketones are REDUCED to secondary (2°) alcohols.

Equation(s) CH3CHO + H2 ——> CH3CH2OH

CH3COCH3 + H2 ——> CH3CHOHCH3

Note Hydrogen also reduces C=C bonds

CH2 = CHCHO + 2H2 ——> CH3CH2CH2OH

CARBONYL COMPOUNDS - REDUCTIONCARBONYL COMPOUNDS - REDUCTION

Introduction Functional groups containing multiple bonds can be reduced

C=C is reduced to CH-CHC=O is reduced to CH-OHCN is reduced to CH-NH2

Hydrogen H• H2

H+ (electrophile) H¯ (nucleophile)

Reactions Hydrogen reduces C=C and C=O bonds

CH2 = CHCHO + 4[H] ——> CH3CH2CH2OH

Hydride ion H¯ reduces C=O bondsCH2 = CHCHO + 2[H] ——> CH2=CHCH2OH

Explanation C=O is polar so is attacked by the nucleophilic H¯C=C is non-polar so is not attacked by the nucleophilic H¯

CARBONYL COMPOUNDS - REDUCTIONCARBONYL COMPOUNDS - REDUCTION

Example What are the products when Compound X is reduced?

NaBH4

H2COMPOUND X

CARBONYL COMPOUNDS - REDUCTIONCARBONYL COMPOUNDS - REDUCTION

Example What are the products when Compound X is reduced?

C=O is polar so is attacked by the nucleophilic H¯

C=C is non-polar so is not attacked by the nucleophilic H¯

NaBH4

H2COMPOUND X

2,4-DINITROPHENYLHYDRAZINE2,4-DINITROPHENYLHYDRAZINE

Structure

Use reacts with carbonyl compounds (aldehydes and ketones)used as a simple test for aldehydes and ketonesmakes orange crystalline derivatives - 2,4-dinitrophenylhydrazonesderivatives have sharp, well-defined melting pointsalso used to characterise (identify) carbonyl compounds.

Identification / characterisationA simple way of characterising a compound (finding out what it is) is to

measure the melting point of a solid or the boiling point of a liquid.

C6H3(NO2)2NHNH2

2,4-DINITROPHENYLHYDRAZINE C2,4-DINITROPHENYLHYDRAZINE C66HH33(NO(NO22))22NHNHNHNH22

The following structural isomers have similar boiling points because of similarvan der Waals forces and dipole-dipole interactions. They would be impossibleto identify with any precision using boiling point determination.

Boiling point 213°C 214°C 214°C

Melting point of2,4-dnph derivative 209°C 248°C 265°C

By forming the 2,4-dinitrophenylhydrazone derivative and taking its melting point,it will be easier to identify the unknown original carbonyl compound.

CHO CHO CHO

Cl

Cl

Cl

THE CHEMISTRYTHE CHEMISTRYOF ALDEHYDES OF ALDEHYDES AND KETONESAND KETONES

THE ENDTHE END

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