the chemistry of aldehydes and ketones a guide for a level students knockhardy publishing 2008...
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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°
CARBONYL COMPOUNDS - BONDINGCARBONYL COMPOUNDS - BONDING
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
CARBONYL COMPOUNDS - BONDINGCARBONYL COMPOUNDS - BONDING
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
CARBONYL COMPOUNDS - BONDINGCARBONYL COMPOUNDS - BONDING
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
ORBITAL OVERLAP
CARBONYL COMPOUNDS - BONDINGCARBONYL COMPOUNDS - BONDING
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
ORBITAL OVERLAP
NEW ORBITAL
CARBONYL COMPOUNDS - BONDINGCARBONYL COMPOUNDS - BONDING
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
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.
CARBONYL COMPOUNDS - IDENTIFICATIONCARBONYL COMPOUNDS - IDENTIFICATION
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
CARBONYL COMPOUNDS - IDENTIFICATIONCARBONYL COMPOUNDS - IDENTIFICATION
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
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
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¯
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 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
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
ANIMATED MECHANISM
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
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Watch out for the possibility of optical isomerism in hydroxynitriles
CN¯ attacksfrom above
CN¯ attacksfrom below
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
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
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)
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
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
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
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)
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.
CHO CHO CHO
Cl
Cl
Cl
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
CHO CHO CHO
Cl
Cl
Cl
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
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
©©2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING