Chapter 17 - Carbonyl Compounds Flashcards
Why do aldehydes and ketones have higher boiling points than corresponding non-polar hydrocarbons?
The carbonyl group is polar, thus pd-pd interactions exist between molecules in aldehyde and ketones on top of dispersion forces, which require more energy to overcome as compared to only dispersion forces present in non-polar alkanes.
Why do aldehydes and ketones have lower boiling points than corresponding carboxylic acids and alcohols?
Alcohols and carboxylic acids can form hydrogen bonds between molecules, which are stronger and require more energy to overcome than pd-pd interactions in aldehydes and ketones.
Why are aldehydes and ketones soluble in water?
They can form hydrogen bonds with water molecules.
Why are aldehydes and ketones with more than 5 carbons unable to dissolve in water? (2)
1) As the number of carbon atoms in the alkyl chain increases, the strength of dispersion forces between molecules increase. Energy released from hydrogen bonding formed between carbonyl group and water is less able to overcome the increasingly stronger dispersion forces and hydrogen bonding in water.
2) hydrogen bonding with water is disrupted by large no-polar alkyl chain
How can aldehydes be prepared? (1)
Oxidation of primary alcohols (Hydroxy compounds)
How can ketone be prepared? (2)
1) Oxidation of secondary alcohols (Hydroxy compounds)
2) Oxidative cleavage of substituted alkenes (Alkenes)
Why can aldehydes and ketones undergo nucleophilic addition?
- The carbonyl carbon is sp2 hybridised. Carbonyl compounds are therefore planar about the carbonyl carbon. The unhybridised p orbital of carbon overlaps with a p orbital of oxygen to form a carbon-oxygen double bond. (C=O double bond made up of one sigma and one pi bond). This double bond can react to saturate the double bond by addition.
- however, the oxygen is much more electronegative than the carbon atom and will attract electron density in the double bond towards itself. The carbonyl carbon thus carries a partial positive charge and can be attacked by nucleophiles.
Describe the general mechanism of nucleophilic addition of carbonyls. (2 steps)
Step 1: a nucleophile attacks the electron-deficient carbonyl carbon, breaking the pi bond of the C=O group to form an alkoxide ion.
Step 2: The tetrahedral intermediate is protonated by water or acid to give an alcohol as the final product.
Why are aldehydes more reactive than ketones in nucleophilic addition reactions? (2)
1) Steric reason: the presence of two relatively large substituents in ketones versus only one large substituent in aldehydes means that attacking nucleophiles are able to approach the carbonyl carbon in aldehydes with less steric hindrance than in ketones.
2) Electronic reason: Aldehydes have one electron donating alkyl group while ketones have two. Hence, the carbonyl carbon of aldehydes has a higher partial positive charge compared to that of ketones, making it more susceptible towards nucleophilic attack.
What are the reagents and conditions needed for aldehydes/ketones to react with HCN via nucleophilic addition? (2)
HCN with trace KCN, cold to ensure HCN remains liquid. (KCN is a catalyst)
How can the rate of reaction of the nucleophilic addition of aldehydes/ketones to HCN be increased? (2)
1) Add a trace amount of KCN as catalyst. Since HCN is a weak acid that partially ionises in water to give CN-, the reaction is very slow due to the low concentration of CN-. KCN dissolves completely in water to provide free CN- ions, which can be regenerated at the end of the reaction.
2) add a small amount of base. This will neutralise H3O+ ions, and by LCP, will shift position of equilibrium to the right and increase concentration of CN-, increasing the rate of reaction.
Describe the general mechanism of a condensation reaction of carbonyl compounds. (2 steps)
Step 1: Addition of nucleophile containing 2 H atoms on a nitrogen atom onto carbonyl compound
Step 2: Elimination of a water molecule
What are the reagents and conditions needed for condensation reaction with 2,4-DNPH and carbonyl compounds, and what is it used to distinguish?
Reagents and conditions: 2,4-DNPH, room temperature
Observation: orange precipitate
Test for: presence of aldehydes and ketones
What are the reagents and conditions needed for reduction of carbonyl compounds and what is formed?
Reagents and conditions: LiAlH4 in dry ether, room temperature
Product: Primary/secondary alcohol respectively
Compare reducing agents LIAlH4 and NaBH4. (What makes both a good nucleophile, why LiAlH4 is better, why NABH4 is preferred)
1) Both have hydrogen atoms covalently bonded to Al and B respectively, and because hydrogen is more negative than electronegative than aluminium and boron, it will bear the negative charge, making it a good nucleophile.
2) Al is less electronegative than B, thus more of the negative charge will be borne by the hydrogen atom. Thus, LiAlH4 is a much stronger reducing agent and much less selective than NaBH4.
3) Due to the hazardous nature of LiAlH4 and ether, which forms highly explosive mixtures in air, the use of NABH4 is usually preferred.
