Carbonyl Compounds Flashcards
(23 cards)
Describe the hybridisation in the CO functional group.
The carbonyl carbon is sp2 hybridised and lies on the same plane as the three atoms directly attached to it (ie trigonal planar arrangement of atoms)
The bond angle is approximately 120 degrees.
Why do carbonyl compounds have higher boiling points than their corresponding alkanes/alkenes with similar Mr?
More energy is needed to overcome the stronger pd-pd interactions between carbonyl molecules than the weaker id-id interactions between alkane/alkene molecules.
Why are carbonyl compounds with a smaller number of C atoms completely miscible with water?
Carbonyl compounds can form hydrogen bonds with water molecules.
Why does the solubility of carbonyl compounds in water decrease with an increasing number of carbon atoms?
The larger non-polar alkyl/aryl group makes the molecule less soluble. The pd-pd interactions between the akyl/aryl groups and water molecules do not release sufficient amounts of energy to overcome the strong hydrogen bonds between the water molecules. In addition, the larger alkyl group also hinders the formation of hydrogen bonds between the carbonyl functional group and water molecules.
State the reagents and conditions to form aldehydes from primary alcohols and any observations seen.
Reaction: Oxidation of primary alcohols
Reagents and conditions: K2Cr2O7(aq), H2SO4(aq), immediate distill
Observations: Orange K2Cr2O7 solution turns green.
State the reagents and conditions to form ketones from secondary alcohols and any observations seen.
Reaction: Oxidation of secondary alcohols
Reagents and conditions: K2Cr2O7(aq), H2SO4(aq),heat OR KMnO4(aq), H2SO4(aq), heat
Observations: Orange K2Cr2O7 solution turns green. OR Purple KMnO4 solution is decolourised
State the reagents and conditions of the oxidation of carbonyl compounds and any observations seen.
Reaction: Oxidation of aldehydes to form carboxylic acids
Reagents and conditions: K2Cr2O7(aq), H2SO4(aq),heat OR KMnO4(aq), H2SO4(aq), heat
Observations: Orange K2Cr2O7 solution turns green. OR Purple KMnO4 solution is decolourised
Side note: Ketones are inert to oxidation
State the reagents and conditions of the reduction of aldehydes.
Reaction: Reduction of aldehydes to form primary alcohol
Reagents and conditions:
1. LiAlH4, dry ether r.t OR NaBH4, ethanol, r,t
2. H2(g) Ni/Pd/Pt, r.t
State the reagents and conditions of the reduction of ketones.
Reaction: Reduction of ketones to form secondary alcohol
Reagents and conditions:
1. LiAlH4, dry ether r.t OR NaBH4, ethanol, r,t
2. H2(g) Ni/Pd/Pt, r.t
Why can’t LiAlH4 and NaNH4 reduce alkenes and alkynes?
Both LiAlH4 and NaBH4 provide a negative H nucleophiles which attack electron-deficient C atoms of the reactant molecule. The C-C double and triple bonds in alkenes and alkynes are electron-rich but non-polar and will not react with these reagents.
Why is LiAlH4 a stronger reducing agent than NaBH4?
Because Al is less electronegative than B and H, so H- is more readily produced.
Why do carbonyl compounds undergo nucleophilic addition reactions?
Due to the highly electronegative O atom and the contribution of the dipolar resonance structure, the C-O double bond of the carbonyl group is highly polar. The partially positive carbonyl C atom acts as an electron-deficient site, which will be attacked by a nucleophile. Since the C of the C-O double bond is unsaturated, it can undergo nucleophilic addition.
Even though carbonyls and alkenes are unsaturated, they behave differently towards nucleophilic reagents. Why?
The carbonyl carbon is electron-deficient as it is bonded to an electronegative oxygen atom. Hence, the carbonyl is more susceptible to nucleophilic attack by nucleophilic reagents.
The C-C double bond of the alkene is electron-rich, hence it is not susceptible to nucleophilic attack.
State the reagent and conditions needed for the reaction of carbonyl compounds with hydrogen cyanide.
Reaction: Nucleophilic addition of HCN
Reagents and conditions: HCN, trace amount of NaCN, Cold OR HCN, trace amount of NaOH, Cold
Why is the cold condition needed for the nucleophilic addition of HCN?
HCN is a volatile gas and there should be no heating to prevent the poisonous HCN from escaping into the environment. Hence, cold condition is needed.
What is the role of HCN in the nucleophilic addition of HCN to carbonyl compounds?
HCN acts as a Bronsted-Lowry acid (or proton donor) or a Lewis acid in Step 2 of the mechanism, protonating the intermediate anion.
Why is a racemic mixture produced when an aldehyde or an unsymmetrical ketone undergoes nucleophilic addition with HCN?
There is equal probability of CN- nucleophile to attack either side of the trigonal planar C-O functional group, producing a racemic mixture.
Why do aldehydes undergo nucleophilic addition more rapidly than ketones?
a) Electron factor
Ketone has one more electron-donating alkyl group, which makes the carbonyl carbon in ketones less electron-deficient.
b) Steric factor
Ketone has one more bulky alkyl/aryl group attached to the carbonyl carbon. This hinders the approach of nucleophiles to the carbonyl carbon of ketones.
Why is nucleophilic addition with HCN useful in organic synthesis?
The reaction of carbonyl compound with HCN is a step up reaction (increases the number of carbon atoms in the molecule by one)
Furthermore, cyanohydrins are useful organic intermediates as they can be hydrolysed to give 2-hydroxycarboxylic acids or reduced to give primary amines.
What is an identification test for carbonyl compounds in general?
Reaction with 2,4-dinitrophenylhydrazine (2,4-DNPH). r.t
Observation: Orange ppt
Reaction: Condensation reaction
What is an identification test for aldehydes (aliphatic and aromatic)?
Reaction with Tollens’ Reagent (Al[NH3]20+(aq), heat
Observation: silver mirror (grey/black ppt of Ag)
Reaction: Oxidation
What is an identification test for aliphatic aldehydes?
Reaction with Fehling’s solution, heat
Observation: Brick red ppt of Cu2O
Reaction: Oxidation
Side note:
Fehling’s solution is deep blue in colour
What is an identification test for -COCH3 group in carbonyl compounds?
Tri-iodomethane(Iodoform) Test
Reagents and conditions: I2 in NaOH(aq), heat
Observations: Brown I2(aq) is decolourised Pale yellow ppt of tri-iodomethane, CHI3
Reaction: Oxidation
