Aldehydes and Ketones Flashcards
Functional Group
Carbonyl
-C=O
Formation of Aldehydes
Alcohol dehyrogenation
Oxidation of a 1° R-X
Formula of Aldehydes
Cn H2n+1 CHO
Written in this format to distinguish from Alcohols
R – C = O
|
H
Formula of Ketones
O
||
R – C – R’
Rs mustn’t be an H
R’ means that the 2 Rs don’t have to be the same
1st member is propanone
Formation of Ketones
Oxidation of a 2° R-X
Can use relux as Ketones can’t be [O] further
Acidified CrO7 2-
Aldehyde - Suffix for naming
-al
Ketone - Suffix for naming
-one
B.P trends for Aldehydes / Ketones
b.p of (corresponding) alkane polar molecule)
As aldehyde gets larger, the effect of the polar C=O becomes less significant
H bonding conditions
H is bonded to F/O/N (E.N element)
Solubility of Aldehydes/Ketones
Small A/Ks are soluble in H2O
Larger A/Ks become less and less soluble in H2O and more soluble in non polar substances
(effect of polar C=O group)
Uses of aldehydes
Methanal in H2O = Formalin
Used to preserve biological specimens/organs/bodies
Natural products
Smell in almonds
Uses of ketones
Hormones
- testosterone
- progesterone
Propanone (Acetone)
- organic solvent
- nail polish remover
Reactions of Aldehydes to form Carboxylic Acids
Oxidation
a) with H+/Cr2O7 2-
(orange to green)
Cr3+
b) with H+/MnO4-
(purple to colourless)
if not acidified, brown ppt. of MnO2 will form instead of Mn2+
Mn7+ –> Mn2+
Distinguishing between Aldehydes and Ketones
- Tollen’s Reagent (Silver Mirror Test)
2. Fehling’s Solution
Fehling’s solution
alkaline solution (NaOH)
Cu2+ ions
(OX. Agent) Aldehyde + Cu2+ --> Carboxylic Acid + Cu2O
blue solution red ppt
Tollen’s reagent
Ammoniacal Silver Nitrate
(excess NH3)
Clean test tube - otherwise black Ag due to [O]
Aldehyde + Ag+ –> Carboxylic Acid + Ag
silver mirror on test tube
Reducing reagents for the reduction of Aldehydes and Ketones
Sodium tetrahydridoborate - NaBH4 (aq)
Lithium tetrahydrioaluminate - LiAlH4 (aq)
more powerful
Reaction of Aldehydes and Ketones with HCN - reactants
HCN is generated in situ
(poisonous gas)
NaCN/KCN + dil. H2SO4 generates HCN
==> NaCN/KCN act as a catalyst
Overall Reaction Mechanism for addition of HCN to Aldehydes
H H O:-
\ NaCN + H2SO4 \ /
C = O ——————-> C (oxoanion)
/ / \
R R CN
|
|
H OH
\ /
C (hydroxy nitrile)
/ \
H3C CN
Extra electrons from the O- of the oxoanion get a curly arrow pointing to the imaginary line between the H and the O
H is bonded to CN and the bond between these 2 goes to the CN
e.g. 2-hydroxypropanenitrile
pH of HCN
pH 4 - 5
Reduction Reaction of Aldehyde with H2 and Metal Catalyst
H H OH
\ Ni/Pt \ /
C = O + H2 ——————-> C
/ / \
R R H
Reduction Reaction of Aldehyde with H2 an Metal Catalyst
R R OH
\ Ni/Pt \ /
C = O + H2 ——————-> C
/ / \
R R H
Reduction reaction of Aldehydes and Ketones with NaBH4 and LiAlH4 conditions
NaBH4
- aqueous alkaline solution e.g NaOH
- warm with aldehyde/ketone
LiAlH4
- in dry ether
- otherwise LiAlH4 will react with H2O
- r.t
Type of reaction between carbonly compounds and HCN?
Nucleophilic Addition
C=O is being attacked by an nucleophile ( :CN - )
Nitrile group to Carboxylic Acid
Hydrolysis
–CN + H+ + H2O –> –COOH + NH4+
Nucleophilic Addition Reaction Mechanism - Cyanide ion and Carbonyl
Stage 1
- C = O is polarised
==> C is open to attack by a nucleophile
e.g. - CN:
Nucleophilic Addition Reaction Mechanism - Cyanide ion and Carbonyl
Stage 2
Intermediary compound formed is highly reactive
( -O: )
and reacts with an H+ ion
(from H2O/HCN/dilute acid)
Test to detect presence of carbonyl compounds?
Condensation Reaction with
2,4-DNPH
(2,4-Dinitrophenylhydrazine)
Carbonyl + 2,4-DNPH: what happens?
2 molecules join together and eliminate a small molecule
(here, H2O)
orange ppt.
Why doesn’t 2,4-DNPH react with amines, esters & carboxylic acids?
extra resonance structure delocalises some of the positive charge away from the carbonyl atom
onto the adjacent hetero-atom
==> the C atom is less prone to attack by the nucleophilic 2,4-DNPH
Observation for Tollen’s reagent and Aldehyde
Silver mirror formed in the inside of test tube
Equation for Tollen’s reagent and Aldehyde
Oxidation
RCHO + H2O —-> RCOOH + 2e- + 2H+
Reduction
e- + Ag+ —> Ag(s)
RCHO + 2Ag+ + H2O —-> RCOOH + 2Ag + 2H+
Observation for Fehling’s solution and Aldehyde
Blue solution to red ppt.
Equation for Fehling’s solution and Aldehyde
Oxidation
RCHO + H2O —-> RCOOH + 2e- + 2H+
Reduction
2e- + Cu2+ + 2OH- —> Cu2O(s) + H2O
