THE REACTIONS
OF ALDEHYDES
AND KETONES
KNOCKHARDY PUBLISHING
2015
SPECIFICATIONS
CARBONYL COMPOUNDS - FORMULAE
Molecular
C 3H 6O
Structural
C2H5CHO
CH3COCH3
CH3
C2H5
C=O
C=O
H
Displayed
H
CH3
H
H
H
C
C
C
H
H
Skeletal
O
O
H
H
O
H
C
C
C
H
H
O
H
CARBONYL COMPOUNDS - NOMENCLATURE
Aldehydes
C2H5CHO
propanal
Ketones
CH3COCH3
propanone
CH3CH2COCH3
butanone
CH3COCH2CH2CH3
pentan-2-one
CH3CH2COCH2CH3
pentan-3-one
C6H5COCH3
phenylethanone
CARBONYL 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 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 the
difference in electronegativities
alkenes are non-polar and are attacked by electrophiles
undergoing electrophilic addition
Group
Bond
Polarity
Attacking species
Result
ALKENES
C=C
NON-POLAR
ELECTROPHILES
ADDITION
CARBONYLS
C=O
POLAR
NUCLEOPHILES
ADDITION
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
HIGHLY TOXIC
TAKE GREAT CARE
Reagent
potassium cyanide – followed by dilute acid
Conditions
reflux
Nucleophile
cyanide ion CN¯
Product(s)
hydroxynitrile (cyanohydrin)
Equation
CH3CHO
Notes
HCN is a weak acid and has difficulty dissociating into ions
+
HCN
HCN
——> CH3CH(OH)CN
2-hydroxypropanenitrile
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¯
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism
Nucleophilic addition
STEP 1
STEP 2
Step 1
CN¯ acts as a nucleophile and attacks the slightly positive C
One 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
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism
Nucleophilic addition
STEP 1
STEP 2
Step 1
CN¯ acts as a nucleophile and attacks the slightly positive C
One 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
CARBONYL 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 ADDITION
Watch out for the possibility of optical isomerism in hydroxynitriles
CN¯ attacks
from above
CN¯ attacks
from below
CARBONYL COMPOUNDS - REDUCTION WITH NaBH4
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]
CH3COCH3 + 2[H]
Notes
The water provides a proton
——>
——>
CH3CH2OH
CH3CHOHCH3
CARBONYL COMPOUNDS - REDUCTION WITH NaBH4
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)
Aldehyde
Primary alcohol
Water is added
CARBONYL COMPOUNDS - REDUCTION WITH NaBH4
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)
ANIMATED MECHANISM
THE TETRAHYDRIDOBORATE(III) ION BH4
H
H
B
atom has three
electrons to share
H
each atom needs one
electron to complete
its outer shell
H
boron shares all 3 electrons to
form 3 single covalent bonds
H
H
B
H
B
H
H
H
H
The hydride ion forms a
dative covalent bond
H
B
H
H
CARBONYL COMPOUNDS - REDUCTION WITH HYDROGEN
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
CH3COCH3 + H2
Note
——>
CH3CH2OH
——>
CH3CHOHCH3
Hydrogen also reduces C=C bonds
CH2 = CHCHO
+
2H2
——>
CH3CH2CH2OH
CARBONYL COMPOUNDS - REDUCTION
Introduction
Functional groups containing multiple bonds can be reduced
C=C
C=O
CN
Hydrogen
Reactions
is reduced to
is reduced to
is reduced to
CH-CH
CH-OH
CH-NH2
H•
H2
H+ (electrophile)
H¯ (nucleophile)
Hydrogen reduces C=C and C=O bonds
CH2 = CHCHO
+
4[H]
——>
CH3CH2CH2OH
Hydride ion H¯ reduces C=O bonds
CH2 = 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 - REDUCTION
Example
COMPOUND X
What are the products when Compound X is reduced?
H2
NaBH4
CARBONYL COMPOUNDS - REDUCTION
Example
What are the products when Compound X is reduced?
COMPOUND X
H2
NaBH4
C=O is polar so is attacked by the nucleophilic H¯
C=C is non-polar so is not attacked by the nucleophilic H¯
2,4-DINITROPHENYLHYDRAZINE
Structure
C6H3(NO2)2NHNH2
Use
reacts with carbonyl compounds (aldehydes and ketones)
used as a simple test for aldehydes and ketones
makes orange crystalline derivatives - 2,4-dinitrophenylhydrazones
derivatives have sharp, well-defined melting points
also used to characterise (identify) carbonyl compounds.
Identification / characterisation
A 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.
2,4-DINITROPHENYLHYDRAZINE C6H3(NO2)2NHNH2
The following structural isomers have similar boiling points because of similar
van der Waals forces and dipole-dipole interactions. They would be impossible
to identify with any precision using boiling point determination.
CHO
CHO
CHO
Cl
Cl
Cl
Boiling point
213°C
214°C
214°C
Melting point of
2,4-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.
THE CHEMISTRY
OF ALDEHYDES
AND KETONES
THE END
©2015 JONATHAN HOPTON & KNOCKHARDY PUBLISHING