Carbonyls - Organic Chem II Flashcards
What are carbonyl compounds function group?
C=O
What is the difference between aldehydes and ketones structurally?
Aldehydes have the C=O group at the end of the carbon chain
Ketones have the C=O group in the middle of the carbon chain
What is the general formula of carbonyl compounds?
CnH2nO
How many sigma and pi bonds does the C=O in a carbonyl compound consist of?
One each
- The electron density is not distributed evenly between the 2 atoms. There is greater electron density over the more electronegative oxygen atom
Why do carbonyl compounds have lower bpt than alcohols/carboxylic acids of similar Mr?
Because they cannot form hydrogen bonds. Carbonyl compounds are polar molecules because they have the polar C=O, however, they don’t have a H atom attached to an oxygen atom and so they cannot form intermolecular hydrogen bonds
Why do carbonyl compounds have higher bpt than alkanes of similar Mr?
Because more energy is needed to overcome the dipole-dipole forces and London forces between molecules of carbonyl compounds than is needed to overcome the London forces between alkane molecules
Why are smaller carbonyl compounds soluble in water?
Because, although they cannot form hydrogen bonds with each other they can form hydrogen bonds with water molecules
What do carbonyl compounds smell like?
Both homologous series contain many compounds with strong odours. The aldehydes in particular are responsible for imparting the characteristic smells and flavours to a variety of foods
What is usually the oxidation agent?
Potassium dichromate (VI) acidified with dilute sulfuric acid
Orange -> green
What is a primary alcohol oxidised to form?
An aldehyde. If an excess of the oxidising agent is used, then the aldehyde is oxidised to a carboxylic acid
What can the oxidation of primary alcohols be controlled by?
If an alcohol is oxidised using distillation apparatus, then any aldehyde formed will distil off immediately from the reaction mixture
If an alcohol is oxidised by heating under reflux, then the aldehyde cannot escape and will be oxidised to form a carboxylic acid
What is a secondary alcohol oxidised to form?
Ketone
What is the test for carbonyl compounds?
React with 2,4-dinitrophenylhydrazine (2,4-DNPH/Brady’s reagent) to produce a yellow/orange precipitate
What reaction is this? (test for carbonyls)
Condensation
What happens to the precipitate next? (carbonyls test)
Can be filtered and the solid can be purified using recrystallisation. The purified solid should have a sharp melting point, and the identity of the aldehyde or ketone used to make the yellow or orange precipitate can be determined by comparing melting point with data base values.
Why can aldehydes be oxidised but ketones cannot?
Aldehydes are reducing agents because they have a H atom directly attached to the C=O
What are the two different mild oxidising agents that could be used?
Tollens’ reagent (ammoniacal silver nitrate)
Fehling’s solution
What are the observations? (Tollens reagent and Fehlings)
Tollens’ reagent (ammonical silver nitrate) - a silver mirror is formed (grey ppt) for aldehyde
Fehling’s solution - brick red ppt for aldehyde
What are the products of oxidised methyl ketones in alkaline conditions?
Pale yellow precipitate of triiodomethane (iodoform) which has a characteristic antiseptic smell, and the sodium salt of the carboxylic acid (having one fewer carbon atoms than the original ketone)
What are the reagents and conditions needed? (iodoform test)
Add iodine solution followed by sodium hydroxide solution dropwise until the colour of the iodine just disappears and a straw coloured solution remains. Warm in a beaker of hot water
Give the equation for the iodoform test: (positive result)
Used to test for a methyl ketone or a methyl secondary alcohol (+ethanal + ethanol)
CH3CH3CO + 3I2 + 4NaOH -> CHI3 + CH3COO-Na+ + 3NaI + 3H2O
Why are carbonyl compounds attacked by nucleophiles?
Nucleophiles are attracted to positive centres i.e. species with either a full positive charge or a partial positive charge
Which carbonyl compounds would you expect to be more reactive towards nucleophiles?
Aldehydes will be more reactive as they only have one electron releasing alkyl group to negate the C δ+
What reagents and conditions are needed for the reaction with hydrogen cyanide?
Reagents and conditions: HCN (aq) and KCN(aq)
An aqueous solution of sodium or potassium cyanide hydrogen cyanide
What does the reaction between a carbonyl and hydrogen cyanide look like? What is this reaction called?
Nucleophilic Addition
How is this reaction useful?
The reaction with hydrogen cyanide is useful synthetically as a method of introducing an extra carbon atom into a molecule
The product (a nitrile) can be hydrolysed to a hydrocarboxylic acid
Why is the product a racemic mixture?
The starting aldehyde is a trigonal planar molecule so the nucleophile can approach either from above or below the plane of the molecule with equal probability. This means that the product is a racemic mixture of both isomers.
The mechanism for the reaction of HCN with ketones is the same as with aldehydes.
Why is the reagent for this reaction a mixture of both KCN and HCN?
HCN is a weak acid:
Only a very small proportion of HCN molecules dissociate in aqueous solution. However, the rds relies on the CN-. It needs to be present in high concentration in order for the rate of reaction to be significant. Therefore, KCN(aq) is also added as KCN is ionic, so it fully dissociated in aqueous solution
KCN(aq) → K+(aq) + CN- (aq)
What are the reagents and conditions for reduction?
LiALH4 in dry ether at room temperature
What are the different reactions for the reduction of carbonyl compounds?
The reaction is vigorous and LiAlH4 reduces all compounds containing the C=O group
- RCHO + 2[H] → RCH2OH (primary alcohol)
- RCOR’ + 2[H] → RCH(OH)R’ (secondary alcohol)
- RCOOR’ + 4[H] → RCH2OH + R’OH
- RCOOH + 4[H] → RCH2OH + H2O
What is the alternative reducing agent?
NaBH4 in aqueous solution at room temperature
What is the alternative reducing agent?
NaBH4 in aqueous solution at room temperature
