CONTENTS

1
ALDEHYDES KETONES

• CONTENTS
• Prior knowledge
• Bonding in carbonyl compounds
• Structural differences
• Nomenclature
• Preparation
• Identification
• Oxidation
• Nucleophilic addition
• Reduction
• 2,4-dinitrophenylhydrazine

2
ALDEHYDES 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

3
CARBONYL COMPOUNDS - BONDING
Bonding the carbon is sp2 hybridised and three
sigma (s) bonds are planar
PLANAR WITH BOND ANGLES OF 120
4
CARBONYL 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
P ORBITAL
PLANAR WITH BOND ANGLES OF 120
5
CARBONYL 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
(p) bond
P ORBITAL
PLANAR WITH BOND ANGLES OF 120
6
CARBONYL 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
(p) bond
P ORBITAL
ORBITAL OVERLAP
PLANAR WITH BOND ANGLES OF 120
7
CARBONYL 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
(p) bond
P ORBITAL
ORBITAL OVERLAP
PLANAR WITH BOND ANGLES OF 120
NEW ORBITAL
8
CARBONYL 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
(p) bond as oxygen is more
electronegative than carbon the bond is polar
P ORBITAL
ORBITAL OVERLAP
PLANAR WITH BOND ANGLES OF 120
NEW ORBITAL
9
CARBONYL COMPOUNDS - STRUCTURE
Structure carbonyl groups consists of a
carbon-oxygen double bond the bond is polar
due to the difference in electronegativity Di
fference ALDEHYDES - at least one H attached to
the carbonyl group
10
CARBONYL COMPOUNDS - STRUCTURE
Structure carbonyl groups consists of a
carbon-oxygen double bond the bond is polar
due to the difference in electronegativity Di
fference ALDEHYDES - at least one H attached to
the carbonyl group KETONES - two
carbons attached to the carbonyl group
11
CARBONYL COMPOUNDS - FORMULAE
Molecular C3H6O Structural C2H5CHO CH3COCH3
Displayed Skeletal
12
CARBONYL COMPOUNDS - NOMENCLATURE
Aldehydes C2H5CHO propanal Ketones CH3COCH3
propanone CH3CH2COCH3 butanone CH3COCH2
CH2CH3 pentan-2-one CH3CH2COCH2CH3 pentan-3
-one C6H5COCH3 phenylethanone
13
CARBONYL COMPOUNDS - FORMATION
ALDEHYDES Oxidation of primary (1)
alcohols RCH2OH O gt RCHO
H2O beware of further oxidation RCHO O
gt RCOOH Reduction of carboxylic acids
RCOOH H gt RCHO H2O
KETONES Oxidation of secondary (2)
alcohols RCHOHR O gt RCOR
H2O
14
CARBONYL 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 difference between an aldehyde or a
ketone. To narrow it down you must do a second
test.
15
CARBONYL COMPOUNDS - IDENTIFICATION
Differentiation to distinguish aldehydes from
ketones, use a mild oxidising agent Tollens Re
agent 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 gt
Ag(s) the test is known as THE SILVER MIRROR
TEST
16
CARBONYL COMPOUNDS - IDENTIFICATION
Differentiation to distinguish aldehydes from
ketones, use a mild oxidising agent Tollens Re
agent 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 gt
Ag(s) the test is known as THE SILVER MIRROR
TEST Fehlings Solution 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
Tollens Reagent or Fehlings Solution
17
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 gt
RCOOH(l) CH3CHO(l) O gt
CH3COOH(l) KETONES oxidised under vigorous
conditions to acids with fewer carbons C2H5COC
H2CH3(l) 3 O gt C2H5COOH(l)
CH3COOH(l)
18
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism occurs with both aldehydes and
ketones involves addition to the CO 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
Bond
Attacking species
Group
Polarity
Result
CC
ELECTROPHILES
ALKENES
NON-POLAR
ADDITION
CARBONYLS
CO
POLAR
NUCLEOPHILES
ADDITION
19
CARBONYL 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) Equa
tion CH3CHO HCN gt
CH3CH(OH)CN 2-hydroxypropanenitri
le Notes HCN is a weak acid and has
difficulty dissociating into ions HCN
H CN the reaction is catalysed by
alkali which helps produce more of the
nucleophilic CN
20
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism Nucleophilic addition Step
1 CN acts as a nucleophile and attacks the
slightly positive C One of the CO bonds
breaks a pair of electrons goes onto the O
STEP 1
21
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism Nucleophilic addition Step
1 CN acts as a nucleophile and attacks the
slightly positive C One of the CO 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 2
STEP 1
22
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism Nucleophilic addition Step
1 CN acts as a nucleophile and attacks the
slightly positive C One of the CO 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 2
STEP 1
23
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism Nucleophilic addition Step
1 CN acts as a nucleophile and attacks the
slightly positive C One of the CO 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 2
STEP 1
24
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
ANIMATED MECHANISM
25
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Watch out for the possibility of optical
isomerism in hydroxynitriles CN attacks from
above
CN attacks from below
26
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Watch out for the possibility of optical
isomerism in hydroxynitriles CN attacks from
above
CN attacks from below
27
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
ANIMATED MECHANISM
28
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) Alco
hols Aldehydes are REDUCED to primary (1)
alcohols. Ketones are REDUCED to secondary
(2) alcohols. Equation(s) CH3CHO 2H
gt CH3CH2OH CH3COCH3 2H gt
CH3CHOHCH3 Notes The water provides a
proton Question NaBH4 doesnt reduce CC bonds.
WHY? CH2 CHCHO 2H
gt CH2 CHCH2OH
29
CARBONYL COMPOUNDS - REDUCTION WITH HYDROGEN
Reagent hydrogen Conditions catalyst -
nickel or platinum Reaction type
Hydrogenation, reduction Product(s) Alcohols Ald
ehydes are REDUCED to primary (1)
alcohols. Ketones are REDUCED to secondary
(2) alcohols. Equation(s) CH3CHO H2
gt CH3CH2OH CH3COCH3 H2 gt
CH3CHOHCH3 Note Hydrogen also reduces CC
bonds CH2 CHCHO 2H2 gt
CH3CH2CH2OH
30
CARBONYL COMPOUNDS - REDUCTION
Introduction Functional groups containing
multiple bonds can be reduced CC is reduced
to CH-CH CO is reduced to CH-OH C?N is
reduced to CH-NH2 Hydrogen H H2 H
(electrophile) H (nucleophile) Reactions
Hydrogen reduces CC and CO bonds CH2
CHCHO 4H gt CH3CH2CH2OH Hyd
ride ion H reduces CO bonds CH2 CHCHO
2H gt CH2CHCH2OH Explanation CO
is polar so is attacked by the nucleophilic
H CC is non-polar so is not attacked by the
nucleophilic H
31
CARBONYL COMPOUNDS - REDUCTION
Example What are the products when Compound X is
reduced?
COMPOUND X
H2
NaBH4
32
CARBONYL COMPOUNDS - REDUCTION
Example What are the products when Compound X is
reduced?
COMPOUND X
H2
NaBH4
CO is polar so is attacked by the nucleophilic
H CC is non-polar so is not attacked by the
nucleophilic H
33
2,4-DINITROPHENYLHYDRAZINE
Structure 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.
C6H3(NO2)2NHNH2
34
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. Boili
ng point 213C 214C
214C Melting point of 2,4-dnph
derivative 209C
248C 265C By forming the
2,4-dinitrophenylhydrazone derivative and taking
its melting point, it will be easier to identify
the unknown original carbonyl compound.
35
REVISION CHECK
What should you be able to do?
Recall the structure of and bonding in the
carbonyl group Explain the difference in
structure between aldehydes and ketones Recall
the different response to oxidation of aldehydes
and ketones Recall and understand the mechanism
of nucleophilic addition Recall the products from
the reduction of carbonyl compounds
CAN YOU DO ALL OF THESE? YES NO