THE CHEMISTRY OF ALDEHYDES AND KETONES A guide for A level students KNOCKHARDY PUBLISHING 2008...

THE CHEMISTRYTHE CHEMISTRYOF ALDEHYDES OF ALDEHYDES AND KETONESAND KETONES

A guide for A level studentsA guide for A level students

KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING2008 2008

SPECIFICATIONSSPECIFICATIONS

INTRODUCTION

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ALDEHYDES & KETONESALDEHYDES & KETONESKNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING

CONTENTS

• Prior knowledge

• Bonding in carbonyl compounds

• Structural differences

• Nomenclature

• Preparation

• Identification

• Oxidation

• Nucleophilic addition

• Reduction

• 2,4-dinitrophenylhydrazine

ALDEHYDES & KETONESALDEHYDES & KETONES

Before you start it would be helpful to…

• know the functional groups found in organic chemistry

• know the arrangement of bonds around carbon atoms

• recall and explain the polarity of covalent bonds

ALDEHYDES & KETONESALDEHYDES & KETONES

CARBONYL COMPOUNDS - BONDINGCARBONYL COMPOUNDS - BONDING

Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar

PLANAR WITH BOND

ANGLES OF 120°


Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar the unhybridised 2p orbital of carbon is at 90° to these

PLANAR WITH BOND

ANGLES OF 120°

P ORBITAL


Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar the unhybridised 2p orbital of carbon is at 90° to these it overlaps with a 2p orbital of oxygen to form a pi () bond

PLANAR WITH BOND

ANGLES OF 120°

P ORBITAL



PLANAR WITH BOND

ANGLES OF 120°

P ORBITAL

ORBITAL OVERLAP



PLANAR WITH BOND

ANGLES OF 120°

P ORBITAL

ORBITAL OVERLAP

NEW ORBITAL



as oxygen is more electronegative than carbon the bond is polar

PLANAR WITH BOND

ANGLES OF 120°

P ORBITAL

ORBITAL OVERLAP

NEW ORBITAL

CARBONYL COMPOUNDS - STRUCTURECARBONYL COMPOUNDS - STRUCTURE

Structure carbonyl groups consists ofa carbon-oxygen double bond

the bond is polar due to thedifference in electronegativity

Difference ALDEHYDES - at least one H attached to the carbonyl group

C = OH

CH3

C = OH

H

CARBONYL COMPOUNDS - STRUCTURECARBONYL COMPOUNDS - STRUCTURE

Structure carbonyl groups consists ofa carbon-oxygen double bond

the bond is polar due to thedifference in electronegativity

Difference ALDEHYDES - at least one H attached to the carbonyl group

KETONES - two carbons attached to the carbonyl group

C = OH

CH3

C = OH

H

C = OCH3

CH3

C = OC2H5

CH3

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 - FORMATIONCARBONYL COMPOUNDS - FORMATION

ALDEHYDES Oxidation ofprimary (1°) alcohols RCH2OH + [O] ——> RCHO + H2O

beware of further oxidation RCHO + [O] ——> RCOOH

Reduction ofcarboxylic acids RCOOH + [H] ——> RCHO + H2O

CARBONYL COMPOUNDS - FORMATIONCARBONYL COMPOUNDS - FORMATION

ALDEHYDES Oxidation ofprimary (1°) alcohols RCH2OH + [O] ——> RCHO + H2O

beware of further oxidation RCHO + [O] ——> RCOOH

Reduction ofcarboxylic acids RCOOH + [H] ——> RCHO + H2O

KETONES

Oxidation ofsecondary (2°) alcohols RCHOHR + [O] ——> RCOR + H2O

CARBONYL COMPOUNDS - IDENTIFICATIONCARBONYL COMPOUNDS - IDENTIFICATION

Method 1 strong peak around 1400-1600 cm-1 in the infra red spectrum

Method 2 formation of an orange precipitate with 2,4-dinitrophenylhydrazine

Although these methods identify a carbonyl group, they cannot tell the differencebetween an aldehyde or a ketone. To narrow it down you must do a second test.


Differentiation to distinguish aldehydes from ketones, use a mild oxidising agent

Tollen’sReagent ammoniacal silver nitrate mild oxidising agent which will oxidise aldehydes but not

ketones contains the diammine silver(I) ion - [Ag(NH3)2 ]+

the silver(I) ion is reduced to silver Ag+(aq) + e¯ ——> Ag(s)

the test is known as THE SILVER MIRROR TEST


Differentiation to distinguish aldehydes from ketones, use a mild oxidising agent

Tollen’sReagent ammoniacal silver nitrate mild oxidising agent which will oxidise aldehydes but not

ketones contains the diammine silver(I) ion - [Ag(NH3)2 ]+

the silver(I) ion is reduced to silver Ag+(aq) + e¯ ——> Ag(s)

the test is known as THE SILVER MIRROR TEST

Fehling’sSolution contains a copper(II) complex ion giving a blue solution on warming, it will oxidise aliphatic (but not aromatic) aldehydes the copper(II) is reduced to copper(I)

a red precipitate of copper(I) oxide, Cu2O, is formed

The silver mirror test is the better alternative as it works with all aldehydes

Ketones do not react with Tollen’s Reagent or Fehling’s Solution

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)

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


Reagent hydrogen cyanide - HCN (in the presence of KCN)

Conditions reflux in alkaline solution

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¯

the reaction is catalysed by alkali which helpsproduce more of the nucleophilic CN¯


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 1


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


ANIMATED MECHANISM


Watch out for the possibility of optical isomerism in hydroxynitriles

CN¯ attacks from above

CN¯ attacks from below


Watch out for the possibility of optical isomerism in hydroxynitriles

CN¯ attacksfrom above

CN¯ attacksfrom below


ANIMATED MECHANISM TO SHOWHOW DIFFERENT ISOMERS ARE FORMED

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





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






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

Question NaBH4 doesn’t reduce C=C bonds. WHY?

CH2 = CHCHO + 2[H] ———> CH2 = CHCH2OH







Water is added






Water is added

Primary alcoholPrimary alcoholAldehydeAldehyde






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)


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¯


Example What are the products when Compound X is reduced?

NaBH4

H2COMPOUND X


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.

CHO CHO CHO

Cl

Cl

Cl



Boiling point 213°C 214°C 214°C

CHO CHO CHO

Cl

Cl

Cl



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

©©2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING

THE CHEMISTRY OF ALDEHYDES AND KETONES A guide for A level students KNOCKHARDY PUBLISHING 2008...

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