Carbonyl Compounds Flashcards
What is the result of the great polarity of the carbonyl group?
Aldehydes and ketones tend to have higher mpts and bpts than halogenoalkanes and non-polar alkenes of similar Mr.
Smaller aldehydes + ketones tend to show good solubility in water. Why?
Partly due to their ability to form H bonds with water molecules and partly due to their ability to undergo complete hydration.
Why do their solubilities decrease rapidly down both homologous series?
Solubility decreases as the C chain length increases.
Long C chains will not penetrate the highly bonded water structure.
How are aldehydes made?
From partial oxidation of primary alcohols.
How are ketones made?
From oxidation of secondary alcohols.
Aldehydes and ketones undergo nucleophilic addition reactions. What does this term mean?
The nucleophile forms a dative covalent bond with the electropositive carbon centre.
What is the name of the reaction that occurs between 2,4-dinitrophenylhydrazine and carbonyl compounds?
Nucleophilic Condensation - addition followed by elimination of water.
What is the lab test for carbonyl compounds?
Using 2,4-dinitrophenylhydrazine - all aldehydes and ketones react to form brightly coloured instantaneous ppt in high yield.
Why do we use such a complicated reagent like 2,4-DNP for identification purposes?
1) Reagent reacts with all aldehydes + ketones.
2) High mass yields are obtained.
3) Product is easily purified by recrystallisation.
4) Products do not decompose at their mpts; enables exact identification to be made from published mpt tables.
How can we determine the mpt of a hydrazone derivative?
1) Compare mpt to those published in standard data table of hydrazone derivatives.
2) Identify carbonyl compound by comparison.
Why can aldehydes be oxidised?
Aldehydes possess α H atom; can be oxidised.
Why do ketones resist oxidation?
This would cause fragmentation of the molecule.
What can be observed when an aldehyde is added to ammoniacal silver nitrate solution [alkaline conditions]?
Silver mirror or grey black ppt.
What can be observed when an aldehyde is added to sodium potassium tartrate?
Brick red ppt.
How can aldehydes and ketones be reduced to the corresponding primary and secondary alcohols?
Use mild reducing agent, NaBH4 (aq) - affects reduction by transferring hydride ion H- to the carbon atom of the carbonyl group.
