Transcript CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION

THE CHEMISTRY
OF ALDEHYDES
AND KETONES
A guide for A level students
KNOCKHARDY PUBLISHING
KNOCKHARDY PUBLISHING
ALDEHYDES & KETONES
INTRODUCTION
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ALDEHYDES & KETONES
CONTENTS
• Prior knowledge
• Bonding in carbonyl compounds
• Structural differences
• Nomenclature
• Preparation
• Identification
• Oxidation
• Nucleophilic addition
• Reduction
• 2,4-dinitrophenylhydrazine
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
CARBONYL COMPOUNDS - BONDING
Bonding
PLANAR
WITH
BOND
ANGLES
OF 120°
the carbon is sp2 hybridised and three sigma (s) bonds are planar
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°
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°
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°
ORBITAL
OVERLAP
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°
ORBITAL
OVERLAP
NEW
ORBITAL
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°
ORBITAL
OVERLAP
NEW
ORBITAL
as oxygen is more electronegative than carbon the bond is polar
CARBONYL COMPOUNDS - STRUCTURE
Structure
carbonyl groups consists of
a carbon-oxygen double bond
the bond is polar due to the
difference in electronegativity
Difference
ALDEHYDES - at least one H attached to the carbonyl group
H
CH3
C=O
H
C=O
H
CARBONYL COMPOUNDS - STRUCTURE
Structure
carbonyl groups consists of
a carbon-oxygen double bond
the bond is polar due to the
difference in electronegativity
Difference
ALDEHYDES - at least one H attached to the carbonyl group
H
CH3
C=O
H
C=O
H
KETONES - two carbons attached to the carbonyl group
CH3
C2H5
C=O
CH3
C=O
CH3
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 - FORMATION
ALDEHYDES
Oxidation of
primary (1°) alcohols
RCH2OH + [O] ——> RCHO + H2O
beware of further oxidation
RCHO + [O] ——> RCOOH
Reduction of
carboxylic acids
RCOOH + [H] ——> RCHO + H2O
KETONES
Oxidation of
secondary (2°) alcohols
RCHOHR + [O] ——> RCOR
+ H2O
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.
CARBONYL COMPOUNDS - IDENTIFICATION
Differentiation
Tollen’s
Reagent
to distinguish aldehydes from ketones, use a mild oxidising 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¯ ——> Ag(s)
the test is known as THE SILVER MIRROR TEST
CARBONYL COMPOUNDS - IDENTIFICATION
Differentiation
Tollen’s
Reagent
Fehling’s
Solution
to distinguish aldehydes from ketones, use a mild oxidising 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¯ ——> Ag(s)
the test is known as THE SILVER MIRROR TEST
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 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
Reagent
hydrogen cyanide - HCN (in the presence of KCN)
Conditions
reflux in alkaline solution
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¯
the reaction is catalysed by alkali which helps
produce more of the nucleophilic CN¯
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism
Nucleophilic addition
STEP 1
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
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
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
ANIMATED MECHANISM
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Watch out for the possibility of optical isomerism in hydroxynitriles
CN¯ attacks from above
CN¯ attacks from below
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Watch out for the possibility of optical isomerism in hydroxynitriles
CN¯ attacks
from above
CN¯ attacks
from below
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
ANIMATED MECHANISM
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
Question
NaBH4 doesn’t reduce C=C bonds. WHY?
CH2 = CHCHO
+
——>
——>
2[H]
CH3CH2OH
CH3CHOHCH3
———>
CH2 = CHCH2OH
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.
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
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THE CHEMISTRY
OF ALDEHYDES
AND KETONES
THE END
© 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING