26- carbonyls and carboxylic acids

Question 1

Explain the oxidation of aldehydes

Answer 1

They can be oxidised to carboxylic acids when refluxed with acidified dichromate ions, usually as a mixture of sodium or potassium dichromate and dilute sulfuric acid.

Question 2

Why is the reactions of carbonyl compounds different to that of alkenes?

Answer 2

The C=C double bond in alkenes is non-polar, whereas the C=O bond in carbonyl compounds is polar. A C=O bond is made up of both a sigma and pi bond. Oxygen is more electronegative than carbon. The electon density in the double bond lies closer to the O atom. This makes the C end of the bond slightly + and the O end slightly -

Question 3

Explain the reaction of an aldehyde with NaBH4

Answer 3

NaBH4 is a reducing agent that is used to reduce aldehydes and ketones to alcohols. the aldehyde is usually heated with the NaBH4 reducing agent in an aq solution. Aldehydes are reduced to primary alcohols e.g. butanal + 2[H] -(NaHB4/H2O) butan-1-ol.

Question 4

Explain the reduction of a ketone

Answer 4

they are reduced to secondary alcohols by NaBH4. e.g. propanone + 2[H] - (NaBH4/H2O) propan-2-ol

Question 5

why cant hydrogen cyanide be used in an open laboratory? How is this combatted?

Answer 5

It is colourless and extremely poisonous, it boils slightly above room temperature. Sodium cyanide and sulfuric acid are used to provide the hydrogen cyanide in the reaction, however it is still potentially hazardous.

Question 6

Why is the reaction of carbonyl compounds with HCN particularly useful?

Answer 6

It means increasing the length of the carbon chain. The organic product formed from the reaction of an aldehyde with hydrogen cyanide contains two functional groups, a hydroxyl group and an nitride group, C triple bond N. These compounds are classed as hydroxynitriles.

Question 7

Explain the mechanism of a carbonyl compound with NaBH4.

Answer 7

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

Question 8

Explain the mechanism of nucleophilic addition of carbonyl compounds with NaCN/ H+.

Answer 8

1. The lone pair of electrons from the cyanide ion is attracted and donated to the delta positive carbon atom in the aldehyde or ketone C=O bond. The dative covalent bond forms.
2. the pi bond in the 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, to form the product.
4. The product is a hydroxynitrile.

Question 9

What is brady’s reagent used for, what does a positive test look like?

Answer 9

its used to detect the presence of the carbonyl functional group in both aldehydes and ketones. In the presence of a carbonyl functional group, a yellow or orange precipitate forms.

Question 10

How do you test for the carbonyl functional group with 2,4-DNP (Brady’s reagent)?

Answer 10

1. Add a 5cm depth of solution of 2,4-DNP to a clean test tube, this is in excess.
2. use a dropping pipette, add three drops of the unknown compound, leave to stand.
3. If no yellow crystals form, add a few drops of sulfuric acid.
4. A yellow/orange precipitate indicates the presence of an aldehyde or ketone.

Question 11

What reaction can be used to distinguish between an aldehyde and a ketone?

Answer 11

1. In a clean test tube, add a 3cm depth of aq silver nitrate.
2. Add aq sodium hydroxide to the silver nitrate until a brown precipitate of Ag2O is formed.
3. add dilute ammonia solution until the brown precipitate just dissolves to form a clear, colourless solution. This is Tollen’s reagent.
4. Pour 2 cm depth of the unknown solution into a clean test tube.
5. Add an equal depth of Tollen’s reagent.
6. Leave the test tube in a beaker of water at about 50 degrees for about 10-15 mins and then observe if any silver mirror is formed.
7. If the mirror forms, an aldehyde is present, and if there is no reaction observed, it is a ketone.

Question 12

Why does can the tollens reagent distinguish between an aldehyde and a ketone?

Answer 12

Tollen’s reagent contains Ag+ ions, which act as an oxidising agent in the presence of ammonia. In the reaction, silver ions are reduces to silver as the aldehyde is oxidised to a carboxylic acid.
Ag+ + e- = Ag(s)

Question 13

How can you identify an aldehyde or ketone by its melting point after carrying out the reaction with 2,4-DNP?

Answer 13

1. The impure yellow/orange solid is filtered to separate the solid precipitate from the solution.
2. The solid is then recrystalised to produce a pure sample of crystals.
3. The melting point of the purified 2,4 dinitrophenylhydrazone is measured and recorded.
4. The melting point is then compared to a database or data table of melting points to identify the original carbonyl compound.

Question 14

Explain the solubility of carboxylic acids.

Answer 14

The C=O and the O-H bonds in carboxylic acids are polar, allowing them to form hydrogen bonds with water molecules. Carboxylic acids with up to 4 C atoms are soluble in water. As the number of carbon atoms increases, the solubility decreases, as the non-polar carbon chain has a greater effect on the overall polarity of the molecule.
Dicarboxylic acids have 2 polar carboxyl groups to form hydrogen bonds, they are solids at room temp and dissolve readily in water.

Question 15

What is the strength of carboxylic acids?

Answer 15

They are classified as weak acids as they partially dissociate when dissolved in water.

Question 16

What type of reactions do carboxylic acids react in?

Answer 16

Redox reactions with metals, and neutralisation reactions with bases ( metal oxides, alkalis and carbonates). In these reactions, carboxylic acids form carboxylate salts. The carboxylate ion in the salt is named by changing the -ic acid ending of the carboxylic acid to -ate, carboxylate.

Question 17

What happens when a carboxylic acid is reacted with metals?

Answer 17

Aq solutions of carboxylic acids react with metals in a redox reaction to form hydrogen gas and the carboxylate salt. In this reaction, you would observe the metal disappearing and effervescence as H2 gas is evolved.

Question 18

Explain the reaction of a carboxylic acid with metal oxides.

Answer 18

Carboxylic acids react with metal oxides to form salt and water. e.g. ethanoic acid + calcium oxide = calcium ethanoate and water.

Question 19

Explain the reaction of a carboxylic acid and an alkali

Answer 19

They react with alkalis to form a salt and water. There may not be a visible reaction as an aq salt is formed. e.g. ethanoic acid reacts with sodium hydroxide to form sodium ethanoate and water.

Question 20

Explain the reaction of a carboxylic acid and a carbonate.

Answer 20

When a carbonate is added, CO2 gas is evolved. If the carboxylic acid is in excess, a solid carbonate will disappear. E.g. ethanoic acid + aq sodium carbonate = sodium ethanoate, water and CO2 gas. Bubbling will be seen

Question 21

How do you test for the carboxyl group?

Answer 21

The neutralisation reaction of carboxylic acids with carbonates, allows a carboxylic acid to be distinguished from any other organic compounds. Carboxylic acids are the only common organic compounds that are sufficiently acidic to react with carbonates. This is especially useful for distinguishing carboxylic acids from phenols. Phenols aren’t acidic enough to react with carbonates.

Question 22

What is a derivative of a carboxylic acid?

Answer 22

Its a compound that can be hydrolysed to form the parent carboxylic acid. They have a common sequence of atoms in their structure, known as an acyl group.

Question 23

How do you name an ester?

Answer 23

Named after the carboxylic acid parent chain from which it is derived. You have to remove the -oic acid suffix from the parent carboxylic acid and replace with the -oate. The alkyl chain attached to the O atom of the COO group is then added as the first word in the name. e.g. methyl ethanoate.

Question 24

How is an acyl chloride named?

Answer 24

Named after the carboxylic acid parent from which it is derived. Remove the -oic acid suffix from the parent carboxylic acid and replace with -oyl chloride e.g. Propanoyl chloride.

Question 25

How is an acid anhydride formed?

Answer 25

It is formed by the removal of water from two carboxylic acid molecules.

Question 26

What is esterification?

Answer 26

The reaction of an alcohol with a carboxylic acid to form an ester. An alcohol is warmed with a carboxylic acid and a small amount of conc H2SO4, which acts as a catalyst.

Question 27

Explain the process of making ethyl propanoate.

Answer 27

1. Pour equal amounts of ethanol and propanoic acid into a boiling tube
2. Add a few drops of conc H2SO4.
3. Place in a beaker of hot water at about 80 degrees for about 5 mins.
4. pour contents of boiling tube into a beaker containing aq sodium carbonate ( sodium carbonate is a weak base and neutralises the acids in the mixture. this removes the smell of any carboxylic acid that hasn’t reacted, making it easier to smell the ester. There will be a few oily drops of the ester on the top of the water.

Question 28

How do you carry out the acid hydrolysis of an ester?

Answer 28

The ester is heated under reflux with dilute aq acid. The ester is broken down by water, with the acid acting as a catalyst.
The products are a carboxylic acid and an alcohol.

Question 29

What is alkaline hydrolysis?

Answer 29

It is also known as saponification and is irreversible. The ester is heated under reflux with aq hydroxide ions. E.g. if methyl ethanoate is hydrolysed, it forms a carboxylate ion and an alcohol. If sodium hydroxide is the alkali, the salt sodium ethanoate is formed.

Question 30

How do you prepare an acyl chloride?

Answer 30

From their parent carboxylic acid by a reaction with thionyl chloride SOCl2.Other products of this reaction, SO2 and HCL are evolved as gases, leaving just the acyl chloride. The reaction should be carried out in a fume cupboard as the products are harmful.

Question 31

Explain the reaction of acyl chlorides with alcohols

Answer 31

acyl chlorides react with alcohols to form esters. The reaction of ethanoyl chloride and propan-1-ol produces propyl ethanoate and hydrogen chloride.

Question 32

Explain the reaction of an acyl chloride and a phenol.

Answer 32

produces a phenyl ester and doesn’t need an acid catalyst. e.g. ethanoyl chloride + phenol makes phenyl ethanoate and hydrogen chloride.

Question 33

What happens when an acyl chloride reacts with water?

Answer 33

When water is added to an acyl chloride, a violent reaction takes place, with the evolution of dense, steamy hydrogen chloride fumes. A carboxylic acid is formed. The reaction of water with ethanoyl chloride forms ethanoic acid and HCl.

Question 34

What happens when an acyl chloride reacts with ammonia and amines?

Answer 34

Ammonia and amines can act as nucleophiles by donating the lone pair of electrons on the nitrogen atom to and electron deficient species. Their reaction with acyl chlorides forms amides.
Ammonia reacts with acyl chlorides, forming a primary amide. In the primary amide, the nitrogen atom is attached to one carbon.
A primary amine reacts with an acyl chloride in the same way as ammonia to form a secondary amide.

Question 35

How do the reactions of acid anhydrides and acyl chlorides compare?

Answer 35

they react in similar ways with alcohols, phenols, water, ammonia and amines. Theyre less reactive than acyl chlorides and are useful for some laboratory preparations where acyl chlorides may be too reactive.