Aldehydes and Ketones Flashcards Preview

Chemistry Unit 3 > Aldehydes and Ketones > Flashcards

Flashcards in Aldehydes and Ketones Deck (19):
1

Why do aldehydes and ketones have a higher boiling point than alkanes of similar formula mass?

Because the presence of the polar carbonyl group allows permanent dipole-permanent dipole reactions to be set-up between their molecules. Ref to van der vaals in alkanes.

2

Why do aldehydes and ketones have a lower boiling point than alcohols?

Because they only have permanent dipole - permanent dipole attractions which are weaker than the hydrogen bonds between the alcohol molecules.

3

Can aldehydes and ketones dissolve in water?

They are generally insoluble in water but lower members can form hydrogen bonds with water so can dissolve to some extent

4

What is used to reduce aldehydes and ketones back to alcohols?

Lithium aluminium hydride dissolved in ether.

5

How do aldehydes react with hydrogen cyanide?

They react by nucleophilic addition to form cyanohydrins.

6

Why isn't hydrogen cyanide used directly?

Very poisonous gas

7

What is used as the reagent instead of hydrogen cyanide?

Acidified potassium cyanide.

8

What is the action of the nucleophile cyanide on the aldehyde?

It attacks the carbon atom of the carbonyl group since it has a slight positive charge.

9

What is the structure of a cyanohydrin?

CN
:
R---C---OH

10

How can a cyanohydrin be hydrolysed to a hydrocarboxylic acid?

Refluxing them with dilute hydrochloric acid.

11

Draw the structure of a hydroxycarboxylic acid

COOH
:
R---C---OH

12

What is formed during a nucleophilic addition-elimination reaction with hydrazine?

A hydrazone and water

13

Which is more reactive when under nucleophilic attack?

Aldehydes

14

Why are ketones less reactive than aldehydes?

Ketones have two alkyl groups next to the slightly positive carbon atom of the carbonyl group. Alkyl groups are electron donating meaning the partial positive charge of this carbon atom smaller in a ketone making it less susceptible to an attack by a nucleophile. The two ally groups of a keno tend to hinder the nucleophiles access to the carbon and this further reduces its activity.

15

What is the oxidation and reduction between aldehydes and fehlings solution?

Copper(II) ions are reduced to copper (I) ions.
The aldehyde is oxidised to a carboxylic acid.

16

What is the reaction between an aldehyde and tollen's reagent?

The aldehyde is oxidised and the silver (I) ions are reduced to silver atoms

17

What is Brady's reagent?

A solution of 2,4-dinitrophenylhydrazine

18

What happens when an aldehyde or ketone reacts with Brady's reagent?

An orange yellow precipitate is formed, indicating the presence of the carbonyl group. A nucleophilic addition- elimination reaction takes place.The precipitate that is formed is called a 2,4-dinitrophenylhydrazone.

19

How can an unknown ketone or aldehyde become identified?

By preparing its 2,4-dinitrophenylhydrazone derivative. The melting point of the derivative is determined and compared with the melting points of the 2,4-dinitrophenylhydrazones of known compounds.