Chemistry A2 Chapter 26 - Carbonyls and Carboxylic Acids

Question 1

What functional group do ketones and aldehydes both have?

Answer 1

Carbonyl - A carbon which has a double bond to an oxygen

Question 2

Where is the carbonyl functional group found in an aldehyde?

Answer 2

At the end of a carbon chain. The carbon atom is attached to one oxygen (double bond) and hydrogen (single bond)

Question 3

Where is the carbonyl functional group found on a ketone?

Answer 3

In the middle of the chain

Question 4

What are the suffixes for aldehydes and ketones?

Answer 4

Aldehyde -al
Ketone -one

Question 5

What are aldehydes oxidised to, how, and what is the catalyst?

Answer 5

Carboxylic acids
When refluxed
With acidified potassium dichromate and dilute sulfuric acid

Question 6

What are ketones oxidised to?

Answer 6

They can’t be oxidised!

Question 7

What is the difference in the double bonds in an alkene and a carbonyl compound?

Answer 7

Alkene - Carbon-carbon double bond is non-polar
Carbonyl compound - Carbon-oxygen double bond is polar

Question 8

Define the term nucleophile

Answer 8

A nucleophile is attracted to and attacks the slightly positive carbon atom resulting in addition across the double C-O bond (electron pair donator)

Question 9

What is the mechanism by which an aldehyde/ketone reacts with a nucleophile?

Answer 9

Nucleophillic addition

Question 10

What substance is a reducing agent for aldehydes and ketones?

Answer 10

NaBH4 - reduces aldehydes and ketones to alcohols

Question 11

What are aldehydes reduced to?

Answer 11

Primary alcohols by NaBH4

Question 12

What are ketones reduced to?

Answer 12

Secondary alcohols by NaBH4

Question 13

Why is the reaction of hydrogen cyanide useful?

Answer 13

It provides a mean of increasing the length of the carbon chain

Question 14

Why is hydrogen cyanide dangerous?

Answer 14

It is a colourless, extremely poisonous liquid and boils slightly above room temperature so cannot be used safely in an open laboratory

Question 15

What two substances react to produce hydrogen cyanide?

Answer 15

Sodium cyanide and sulfuric acid

Question 16

Describe a nucleophillic addition reaction with NaBH4 in 4 steps (NaBH4 contains the hydride ion)

Answer 16

1. The lone pair of electrons from the hydride ion is attracted to and donates its electron pair to the delta positive carbon in the aldehyde or ketones double C-O bond
2. A dative covalent bond is formed between the hydride ion and the carbon atom
3. The pi-bond in the double C-O bond breaks by heterolytic fission, forming a negatively charged intermediate
4. The oxygen atom of the intermediate donates a lone pair of electrons to a delta positive hydrogen atom in a water molecule. The intermediate has then been protonated to form an alcohol

Question 17

Describe the mechanism for a aldehyde or ketone reacting with NaCN/H+ (the cyanide ion attacks the electron deficient carbon in the aldehyde or ketone)

Answer 17

1. The lone pair of electrons from the cyanide ion is attracted to and donates its electron pair to the delta positive carbon atom in the aldehyde or ketone. A dative covalent bond forms
2. The pi-bond in the double C-O bond breaks by heterolytic fission, forming a negatively charged intermediate
3. The intermediate is protonated by donating a lone pair of electrons to a hydrogen ion
4. The product formed is a hydroxynitrile

Question 18

How is the presence of a carbonyl functional group detected using a chemical test?

Answer 18

2,4-DNP, also known as Brady’s reagent, is used to detect the functional group. In the presence of a carbonyl function group, a yellow or orange precipitate called 2,4-dinitrophenylhydrazone is produced

Question 19

How do we distinguish between aldehydes and ketones?

Answer 19

Once a compound has been identified as containing a carbonyl group using 2,4-DNP, a fresh sample of the compound can have Tollen’s reagent added (a solution of silver nitrate in aqueous ammonia) to it. In the presence of an aldehyde, a silver mirror is produced, if nothing occurs then the compound is a ketone

Question 20

Why does a ketone not produce a silver mirror?

Answer 20

Tollens’ reagent contains silver ions which act as an oxidising agent in the presence of ammonia. Ketones cannot be oxidised and therefore do not produce a silver mirror