THE CHEMISTRY
OF ALDEHYDES
AND KETONES
ALDEHYDES & KETONES
CONTENTS
• Prior knowledge
• Structural differences
• Nomenclature
• Preparation
• Identification
• Oxidation
• Nucleophilic addition
• Reduction
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
O
H
H
O
H
C
C
C
H
H
H
CARBONYL COMPOUNDS - NOMENCLATURE
Aldehydes
C2H5CHO
propanal
Ketones
CH3COCH3
propanone
CH3CH2COCH3
butanone
CH3COCH2CH2CH3
pentan-2-one
CH3CH2COCH2CH3
pentan-3-one
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
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
ALDEHYDES
easily oxidised to acids
RCHO(l) + [O] ——> RCOOH(l)
CH3CHO(l) + [O] ——> CH3COOH(l)
KETONES
are not readily oxidised
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Nucleophilic addition:
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 - REDUCTION WITH NaBH4
Reagent
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
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 the products from the reduction of carbonyl compounds
CAN YOU DO ALL OF THESE?
YES
NO
THE CHEMISTRY
OF ALDEHYDES
AND KETONES
THE END