Carbonyles and Carboxylic Acids

Question 1

What is the carbonyl functional group?

Answer 1

C=O

Question 2

Where is the carbonyl functional group found in an aldehyde? How is it written in the structural formula?

Answer 2

At the end of the carbon chain. It is written as CHO.

Question 3

Where is the carbonyl functional group found in a ketone? How is it written in the structural formula?

Answer 3

It is joined to two carbon atoms in the carbon chain. In its structural formula it is written as CO.

Question 4

What is the suffix of an aldehyde?

Answer 4

-al

Question 5

What is the suffix of a ketone?

Answer 5

-one

Question 6

Describe the process of oxidising an aldehyde. Include reagents and conditions as well as the final product formed.

Answer 6

Heated under reflux with acidified potassium dichromate ions (Mixture of sodium or potassium dichromate and dilute sulfuric acid). The product is a carboxylic acid.

Question 7

What symbol can represent an oxidising agent in an equation?

Answer 7

[O]

Question 8

What is the reactivity of aldehydes and ketones influenced by?

Answer 8

The carbon-oxygen double bond

Question 9

Briefly explain how the double bond is formed.

Answer 9

The double bond is made up of a sigma bond and a pi bond. The pi bond is above and below the plane of the carbon and oxygen atoms, formed by the sideways overlap of p-orbitals.

Question 10

Why don’t carbonyl compounds react in the same way as alkenes?

Answer 10

There is a significant difference in the nature of the double bond. The C=C in alkenes is non-polar whereas the C=O bond in carbonyl compounds is polar.

Question 11

Explain what causes the difference in the double bond between carbonyl compounds and alkenes?

Answer 11

Oxygen is more electronegative than carbon so the electron density lies closer to the oxygen atom than the carbon. This makes the carbon atom slightly positive and the oxygen slightly negative.

Question 12

Why can aldehydes and ketones react with nucleophiles? What type of reaction is this?

Answer 12

Due to the polarity of the C=O double bond. A nucleophile is attracted to and attacks the slightly positive carbon atom resulting in addition across the double bond. This is electrophilic addition.

Question 13

What is sodium tetrahydridiborate (NaBH4) used for? What conditions are required?

Answer 13

It’s used as a reducing agent to reduce aldehydes and ketones to alcohols. The aldehyde and ketone are warmed with the reducing agent in aqueous solution.

Question 14

What type of alcohols are aldehydes reduced to?

Answer 14

Primary alcohols.

Question 15

What type of alcohols are ketones reduced to?

Answer 15

Secondary alcohols.

Question 16

What symbol can represent a reducing agent in an equation?

Answer 16

[H]

Question 17

How do carbonyl compounds react with HCN?

Answer 17

In an addition reaction. HCN adds across the double bond.

Question 18

What is HCN? What problems may we encounter with using this? How can they be overcome?

Answer 18

Hydrogen Cyanide is an extremely poisonous, colourless liquid. It boils only slightly above room temperature which makes it hazardous for use in the lab. Sodium cyanide and H2SO4 might be used instead.

Question 19

Why is the reaction with HCN important?

Answer 19

It increases the length of the carbon chain.

Question 20

What is the name given to the product of the reaction with HCN?

Answer 20

Hydroxynitriles

Question 21

Draw the mechanism for the reaction with NaBH4

Answer 21

(see paper) Check for all curly arrows, partial charges and lone pairs of electrons.

Question 22

Explain the mechanism for the reaction with NaBH4

Answer 22

Question 23

Draw the mechanism for the reaction of propanal with sodium cyanide in the presence of acid.

Answer 23

Question 24

What is 2,4-dinitrophenylhydrazine used for?

Answer 24

Detecting the presence of a carbonyl functional group in aldehydes and keytones.

Question 25

For a positive result, what colour change would you expect to see with 2,4-DNP?

Answer 25

A yellow or orange precipitate is formed.

Question 26

Why can’t 2,4-DNP be used as a solid? How to we overcome this?

Answer 26

Solid 2,4-DNP is hazardous because friction or a sudden blow can cause it to explode. Instead it is usually dissolved in methanol and sulfuric acid.

Question 27

What reagent is required to distinguish between an aldehyde and a ketone? What is a positive result?

Answer 27

Tollens’ reagent- a solution of silver nitrate in aqueous ammonia. In the presence of an aldehyde group, a silver mirror is formed.

Question 28

Describe how you use Tollens’ reagent to test for an aldehyde.

Answer 28

1. Add 3cm depth of aqueous silver nitrate AgNO3 to a test tube
2. Add aqueous sodium hydroxide to the silver nitrate until a brown precipitate of silver oxide is formed
3. Add dilute ammonia solution until the brown precipitate just dissolves. This is Tollens’ reagent.
4. Pour unknown solution in to a test tube.
5. Add equal volume of the Tollens’ reagent
6. Leave to stand in warm water and observe for a silver mirror.

Question 29

Write the oxidation and reduction equations for the reaction between an aldehyde and silver ions.

Answer 29

Question 30

How could you identify an aldehyde or ketone by melting point?

Answer 30

1. Filter off the 2,4-dinitrophenylhydrazone precipitate
2. Recrystallise the solid to form pure crystals
3. Measure and record the melting point
4. Compare the meling point to a table or data book