6.1.2 Carbonyl compounds

Question 1

What are carbonyls

Answer 1

the carbonyls are compounds that contain the carbonyl functional group of C=O.
This primarily refers to aldehydes and ketones but also carboxylic acids, acyl chlorides and esters

Question 2

what is an aldehyde

Answer 2

• In aldehydes the C=O is found at the end of a carbon chain.
• It has a structural formula of CHO
• The simplest aldehyde is HCHO also known as formaldehyde which is used to preserve biological specimen

Question 3

what is a ketone

Answer 3

• In ketones the C=O is joined to 2 carbons in the carbon chain.
• It has a structural formula of CO
• The simplest ketone is CH₃COCH₃ also known as acetone which is used as nail polish remover

Question 4

what is the suffix for aldehyde

Answer 4

-al

enal or anal depending on if it is alkane or alkene

Question 5

what is the suffix for ketone

Answer 5

-one

enone or anone depending on if it is alkane or alkene

Question 6

where should numbering be used for aldehydes and ketones

Answer 6

in an aldehyde the carbon atom of the carbonyl group is designated one so a number is not required, however in a ketone the carbonyl compound needs to be numbered.

Question 7

what is the cause of the reactivity of ketones and aldehydes

Answer 7

the polarity of the C=O dipole
Cᵟ⁺=Oᵟ⁻

Question 8

what reactions do aldehydes and ketones undergo

Answer 8

• oxidation
• nucleophilic addition (reduction)

Question 9

How are aldehydes formed

Answer 9

through the oxidation of primary alcohols using distillation with acidified dichromate (Cr₂O₇²⁻/ H⁺)

Question 10

How is acidified dichromate (Cr₂O₇²⁻/ H⁺) represented in an equation

Answer 10

It is an oxidising agent
[O]

Question 11

what happened to aldehydes is you reflux them with acidified dichromate (Cr₂O₇²⁻/ H⁺)

Answer 11

after forming an aldehyde, aldehydes can be oxidised further to give carboxylic acids

Question 12

What are the ways to oxidise an aldehyde to a carboxylic acid

Answer 12

• reflux with acidified dichromate (Cr₂O₇²⁻/ H⁺)
• react with Tollen’s reagent

Question 13

How are ketones formed

Answer 13

through the oxidation of secondary alcohols under reflux with acidified dichromate (Cr₂O₇²⁻/ H⁺)

Question 14

What makes aldehydes and ketones reactive

Answer 14

The polarity of the C=O bond. Oxygen is more electronegative than carbon. The electron density in the double bond lies closer to the oxygen atom than the carbon. This makes the carbon end of the double bond slightly positive and the oxygen slightly negative.

Question 15

Describe aldehydes and ketones undergoing nucleophilic addition

Answer 15

Due to this aldehydes and ketones react with some nucleophiles. The nucleophile is attracted to and attacks the slightly positive carbon atom resulting in addition across the C=O bond. This forms C-O⁻.

Question 16

Describe the C=O

Answer 16

• is polar due to the difference in electronegativity of oxygen and carbon
• the double bond is made up of both a 𝝈-bond and a π-bond

Question 17

what reagents are requred to reduce an aldehyde and the same for a ketone

Answer 17

both require slightly heated conditions and NaBH₄/ H₂O.
This reduces the aldehyde into a primary alcohol and the ketone into a secondary alcohol.
The NaBH₄/ H₂O can be represented with [H] as it is a reducing agent and this aids balancing of the equation

Question 18

describe the reaction of ketones and aldehydes with HCN

Answer 18

• hydrogen cyanide (HCN) adds across the C=O bond.
• HCN is a colourless extremely poisonous liquid that boils just above room temperature.
• The reaction provides a means of increasing the carbon chain.
• The reagents involved are NaCN and H₂SO₄ which make HCN in situ
• It produces hydroxynitrile

Question 19

describe the mechanism for the reaction of ketones and aldehydes with NaBH₄

Answer 19

• The NaBH₄ can be considered to have the H⁻ ion (hydride ion) which acts as the nucleophile
• The lone pair of electrons on the H⁻ is attracted and donated to the δ+ carbon atom of in the aldehyde or ketone from the C=O. This forms a dative covalent bond.
• The π-bond in the C=O bond breaks by heterolytic fission, forming a negatively charged intermediate.
• The oxygen atom of the intermediate donates a lone pair of electrons to a hydrogen atom in a water molecule. The intermediate then becomes an alcohol.

Question 20

describe the mechanism for the reaction of aldehydes and ketones with NaCN/H⁺

Answer 20

• the CN⁻ ion is formed in situ and attacks the electron deficient carbon atom in the aldehyde or ketone. The lone pair of electrons from the cyanide ion is attracted and donated to the δ+ carbon atom from the C=O forming a dative covalent bond.
• The π-bond in the C=O bond breaks by heterolytic fission, forming a negatively charged intermediate.
• The intermediate is protonated by donating a lone pair of electrons to a hydrogen ion to form the hydroxynitrile product

Question 21

define wet test

Answer 21

‘wet tests’ are chemical reactions with characteristic, observable outcomes which can be used to identify the functional groups in an unknown substance.

Question 22

Give the wet tests for carbonyl compounds (aldehydes and ketones)

Answer 22

• 2,4-DNPH (otherwise known as Brady’s reagent)
• Tollen’s reagent (ammoniacal silver nitrate)
• acidified dichromate (Cr₂O₇²⁻/ H⁺)

Question 23

Difference between 2,4-DNPH and brady’s reagent

Answer 23

They can be used interchangably here.
2,4-DNPH is dissolved in methanol and sulfuric acid which forms a pale orange solution which is known as Brady’s reagent

Question 24

why is Brady’s reagent used over solid 2,4-DNPH

Answer 24

solid 2,4-DNPH can be hazardous as friction or a sudden blow can cause it to explode.

Question 25

describe how to test for the carbonyl group in aldehydes and ketones using 2,4-DNPH

Answer 25

1. add 5cm depth of a solution of 2,4-DNPH in a test tube - this will be in excess.
2. Using a dropping pipette, add 3 drops of the unknown compound and leave to stand
3. if no crystals form add a few drops of H₂SO₄
4. A orange precipitate forms which can be used to identify the original aldehyde or ketone.

Question 26

breifly outline 2,4-DNPH (brady’s reagent) as a wet test for carbonyls

Answer 26

2,4-DNPH reacts with aldehydes and ketones to give an orange precipitate. If then recrystallised its melting point can be compare to known values as a way of identifying the original carbonyl compound

Question 26

describe how 2,4-DNPH can be used to identify an aldehyde or ketone

Answer 26

1. The impure orange precipitate is filtered to seperate the precipitate from the solution.
2. The precipitate is recrystallised to produce a pure sample of crystals
3. The melting point of the purified crystals can be measured, recorded and compared to known values to identigy the original aldehyde/ ketone.

Question 27

what wet tests can be used to distinguish between aldehydes and ketones

Answer 27

• recrystallisation of orange precipitate from reaction with 2,4-DNPH
• Tollen’s reagent

Question 28

what does 2,4-DNPH test for

Answer 28

Aldehydes and ketones

Question 29

what does tollen’s reagent test for

Answer 29

aldehydes

Question 30

describe how to produce Tollen’s reagent

Answer 30

Due to its short shelf-life Tollen’s reagent should be made immediately before the experiment:
- Add 3cm depth of AgNO₃(aq) to a test tube.
- To the same test tube add aqueous sodium hydroxide to the silver nitrate until a brown precipitate of silver oxide Ag₂O is formed.
- Add dilute ammonium solution until the brown precipitate just dissolves to form a clear colourless solution - Tollen’s reagent

Question 31

describe how Tollen’s reagent can be used to test for aldehydes

Answer 31

1. pour 2cm depth of the unknown solution into a test tube
2. add an equal volume of Tollen’s reagent
3. leave the test tube to stand in a beaker of warm water at 50℃ for 10-15 minutes.
4. If the unknown solution contained an aldehyde functional group a silver mirror will form.

Question 32

how is tollen’s reagent written in an equation

Answer 32

As it oxidises it can be represented using [O]

Question 33

briefly outline Tollen’s reagent as a wet test for carbonyl compounds

Answer 33

Tollen’s reagent reacts with aldehydes to give a silver mirror.
Silver ions are reduced from Ag⁺ to metallic silver and the aldehyde is oxidised to COOH

Question 34

briefly outline acidified dichromate as a wet test for carbonyl compounds

Answer 34

acidified dichromate (Cr₂O₇²⁻/ H⁺) oxidises aldehydes to carboxylic acids but doesnt oxidise ketones further.
The colour changes from orange to green.
BUT acidified dichromate (Cr₂O₇²⁻/ H⁺) also oxidises primary and secondary alcohols