Aldehydes and Ketones Module 6

Question 1

What type of compounds are aldehydes and Ketones?

Answer 1

Carbonyl compounds, they contain the functional group C=O

Question 2

Where do aldehydes have their carbonyl group, and how does their name end?

Answer 2

Have it at the end of the carbon chain, and name ends in “al”

Question 3

Where do ketones have their carbonyl group, and what does their name end in?

Answer 3

Have it in the middle of the carbon chain, name ends in “one”

Question 4

How do you make an aldehyde and then make a carboxylic acid?

Answer 4

Gently heat a primary alcohol with acidified pottassium dichromate in a distillation apparatus to form an aldehyde (orange to green)

Then if you reflux the aldehyde (or a primary alcohol), with pottassium dichromate you will form a carboxylic acid (orange to green)

Question 5

Describe the reaction mechanism of the reducing agent NaBH4, dissolved in water with methanol reducing an aldehyde to a primary alcohol, or a Ketone to a secondary alcohol and what type of reaction is it?

Answer 5

It’s a nucleophillic addition reaction

NaBH4 provides a H- ion with a lone pair, the carbonyls groups carbon will have a delta positive charge, and the oxygen will have a delta negative charge

H- ion will have a curly arrow going from the lone pair to the delta + carbon, and there will be a curly arrow going from the double bond in the carbonyl group to the delta negative oxygen

This will form a carbon bonded to the 2 other original elements, a hydrogen and a single bonded to a negative oxygen with a lone pair

Now draw a water molecule, with 2 delta positive hydrogens, and a delta negative oxygen

Draw a curly arrow going from lone pair on oxygen to a delta positive hydrogen, and a curly arrow going from the bond between the delta postive hydrogen, and the delta negative oxygen to the oxygen

This will form an OH- ion, and either a primary or secondary alcohol

Question 6

Describe how hydrogen cyanide will react with aldehydes or ketones to form hydroxynitriles and what type of reaction is it?

Answer 6

Nucleophillic reaction

Hydrogen cyanide is a weak acid so HCN will dissociate into H+ and CN- ions

Draw N triple bonded to C(-) and draw the aldehyde or ketone with a delta negative oxygen double bonded to the delta positive carbon

Draw a curly arrow going from the lone pair on Carbon to the delta positive carbon, and also a curly arrow from the double bond in the carbonyl group, to the delta negative oxygen

This will form a carbon bonded to it’s 2 original elements, a carbon triple bonded to a Nitrogen, and single bonded to an oxygen with a lone pair and a negative charge.

The lone pair on the oxygen will have a curly arrow going to the H+ remaining from the dissociation of the hydrogen cyanide, making it so the carbon is now bonded to an OH group, which is the hydroxynitrile

Question 7

How do you test for Aldehydes and Ketones, and also which specific one is?

Answer 7

When 2-4 DNP (brady’s reagent) is dissolved in methanol and sulfuric acid, it will react with carbonyl groups to form a bright orange precipitate

Can then determine then melting point of the orange precipitate and compare it to a database to find which exact carbonyl compound it is

Question 8

How do you test for aldehydes only?

Answer 8

Heat test substance with tollens reagent, in water bath (silver nitrate dissolved in aqueous ammonia)

If present aldehyde present it will be oxidised, and silver ions are reduced to form a silver mirror