Carbonyls

Question 1

What are aldehydes?

Answer 1

The carbonyl C=O group is at the end of the carbon chain.
The carbon atom is attached to one or two hydrogen atoms.
Structural formula is CHO.

Question 2

What are ketones?

Answer 2

The carbonyl C=O functional group is joined to two carbon atoms in the carbon chain (middle).
Structural formula is CO.

Question 3

How are carbonyl compounds named?

Answer 3

-al for an aldehyde.
-one for a ketone, ketones need numbering.

Question 4

What is the oxidation of aldehydes?

Answer 4

Aldehydes are oxidised to carboxylic acids when refluxed with acidified potassium dichromate (VI) ions, K2Cr2O7, and dilute H2SO4.
Ketones do not undergo oxidation, so this lack of reactivity is used to distinguish between this and aldehydes.

Question 5

What is the C=O bond of carbonyls?

Answer 5

This is responsible for the reactivity of the ketones and aldehydes.
It is made up of a σ-bond and π-bond.
(Draw diagram)
The π bond is above and below the plane of the carbon and oxygen atoms, formed by the sideways overlap of p oribtals.

Question 6

Why do carbonyls not react in the same way to alkenes?

Answer 6

The C=C bond in alkenes is non-polar.
The C=O bond in carbonyls is polar.
Oxygen is more electronegative than carbon, so the electron density is closer to the oxygen than carbon atom.
This makes the carbon slightly positive and oxygen slightly negative.

Question 7

What reactions do carbonyls and alkenes undergo?

Answer 7

Due to the polarity of the C=O bond, aldehydes and ketones react with some nucleophiles.
A nucleophile is attracted to and attacks the slightly positive carbon atom resulting in addition across the C=O bond.
Carbonyls - nucleophilic addition, alkenes - electrophilic addition.

Question 8

What is the reduction of aldehydes?

Answer 8

Aldehydes are reduced to primary alcohols by NaBH4.
Usually warmed in aqueous solution.
CH3CH2CHO + 2[H] —> CH3CH2CHOH
Make sure to include 2 of the reducing agent [H].

Question 9

What is the reduction of ketones?

Answer 9

Ketones are reduced to secondary alcohols by NaBH4.
Usually warmed in aqueous solution.
CH3COCH3 + 2[H] —> CH3CHOHCH3
Make sure to include 2 of the reducing agent [H].

Question 10

What is the mechanism for reduction of a carbonyl by nucleophilic addition?

Answer 10

(Draw)
NaBH4 contains the hydride ion :H-, which acts as the nucleophile.
The lone pair of electrons from :H- is attracted and donated to the S+ C atom in the C=O bond.
The π-bond in the C=O bond breaks by heterolytic fission, forming a negative intermediate.
The oxygen atom of the intermediate donates a lone pair to a hydrogen atom in a water molecule.
The intermediate has been protonated to form an alcohol.

Question 11

What is hydrogen cyanide?

Answer 11

Hydrogen cyanide (HCN) adds across the C=O bond of aldehydes and ketones.
HCN is a colourless, poisonous liquid that boils at room temperature, so cannot be used safely.
Sodium cyanide (NaCN) and sulfuric acid are used to provide the hydrogen cyanide, but are still hazardous.

Question 12

What is the reaction of carbonyl compounds with HCN?

Answer 12

This reaction increases the length of the carbon chain.
CH3CH2CHO + HCN –H2SO4/NaCN–> CH3CH2CHOHCN
The organic product formed contains a hydroxyl group -OH and a nitrile group C≡N, classified as hydroxynitriles.

Question 13

What is the mechanism for the reaction of carbonyls with HCN by nucleophilic addition?

Answer 13

(Draw)
The cyanide ion :CN- attacks the electron deficient carbon atom in the carbonyl.
The lone pair of electrons from :CN- is attracted and donated to the S+ carbon atom in the C=O bond, a dative covalent bond forms.
The π-bond in the C=O bond breaks by heterolytic fission, forming a negative intermediate.
The intermediate is protonated by donating a lone pair of electrons to a hydrogen ion to form the product, a hydroxynitrile.

Question 14

What is the trick for finding the origin of hydroxynitriles?

Answer 14

If it forms a cross shape, it was a ketone.
If it is triangular, it was an aldehyde.

Question 15

How are carbonyl compounds detected?

Answer 15

A solution of 2,4-dinitrophenylhydrazine (2,4-DNPH) or brady’s reagent will form a yellow or orange precipitate in the presence of an aldehyde or ketone.
It is usually dissolved in methanol and sulfuric acid as a pale orange solution called brady’s reagent.
The mixture can then be recrystallised and melting points compared to known values.

Question 16

How are aldehydes and ketones distinguished from one another?

Answer 16

Tollen’s reagent - a solution of silver nitrate in aqueous ammonia, will produce a silver mirror in the presence of an aldehyde.
It contains Ag+ ions, which act as an oxidising agent in the presence of ammonia, to form a carboxylic acid from an aldehyde.

Question 17

What is the practical method for using Tollen’s reagent?

Answer 17

To form Tollen’s: Add AgNO3, then add aq NaOH until a brown precipitate of silver oxide is formed.
Add dilute ammonia solution to form a clear colourless solution.
To test for aldehyde: Add the substance to tollens and heat to see if a silver mirror forms.

Question 18

What is a carboxylic acid?

Answer 18

An organic acid with the COOH functional group.
Found in medicines, fruit juices, vinegar, and rhubarb leaves.
Carboxylic acids and its derivatives are used in organic synthesis as starting materials or intermediates of more useful compounds.
Aspirin is synthesised from salicylic acid.

Question 19

How are carboxylic acids named?

Answer 19

The suffix -oic acid is added to the stem of the longest carbon chain.
The carboxyl carbon atom is always carbon-1.

Question 20

What is the solubility of carboxylic acids?

Answer 20

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

Question 21

What is the strength of carboxylic acids?

Answer 21

Carboxylic acids are weak acids.
When dissolved in water, they only partially dissociate.
HCOOH <—> H+ + HCOO-

Question 22

What are redox reactions of carboxylic acids with metals?

Answer 22

Aqueous solutions of carboxylic acids react with metals in redox to form hydrogen gas and the carboxylate salt.
Observe the metal disappearing and effervescence.
2CH3CH2COOH(aq) + Mg(s) –> (CH3CH2COO-)2Mg2+(aq) + H2(g)

Question 23

What is the reaction of carboxylic acids with metal oxides (base)?

Answer 23

Carboxylic acid + metal oxide –> Salt + water
2CH3COOH(aq) + CaO(s) –> (CH3COO)2Ca(aq) + H2O(l)

Question 24

What is the reaction of carboxylic acids with alkalis?

Answer 24

Carboxylic acid + alkali –> Salt + water
Might not observe any reaction as two solutions react together to form an aqueous solution of the salt.
CH3COOH(aq) + NaOH(aq) –> CH3COONa(aq) + H2O (l)

Question 25

What is the reaction of carboxylic acids with carbonates (base)?

Answer 25

Carbonate + carboxylic acid --> CO2 + salt + H2O If the carboxylic acid is in excess a solid carbonate would disappear, bubbles form. 2CH3COOH(aq) + Na2CO3(aq) --> 2CH3COONa(aq) + H2O (l) + CO2(g)

Question 26

What is the test for carboxylic acid?

Answer 26

The neutralisation reaction of carboxylic acids with carbonates, forms bubbles. Carboxylic acids are the only organic compounds acidic enough to react with carbonates. Useful to distinguish from phenols, which are not acidic enough.

Question 27

What is a derivative of a carboxylic acid?

Answer 27

A compound that can be hydrolysed to form the parent carboxylic acid. They have a common sequence of atoms in their structure - an acyl group. RC=O(R) Esters, acyl chlorides, acid anhydrides and amides.

Question 28

What is the reactivity of carboxylic acid derivatives?

Answer 28

Acyl chloride Acid anhydride Ester Amide Carboxylic acid

Question 29

What is an ester?

Answer 29

To name, remove the -oic acid suffix and replace with -oate. The alkyl chain attached to the oxygen atom of the COO group is added as the first word. E.g. Methyl ethanoate. Sweet smelling liquids, used in perfumes, and flavourings.

Question 30

What is esterification?

Answer 30

The reaction of an alcohol with a carboxylic acid to form an ester. Warmed alcohol + carboxylic acid, with a small amount of concentrated H2SO4 as a catalyst. Forms water as a by-product. Propanoic acid + ethanol --> ethyl propanoate + water.

Question 31

What is acid hydrolysis of esters?

Answer 31

The ester is heated under reflux with dilute aqueous acid. The ester is broken down by water, with the acid acting as a catalyst. Forms carboxylic acid and alcohol, and is reversible.

Question 32

What is alkaline hydrolysis of esters?

Answer 32

Saponification, irreversible. Ester is heated under reflux with aqueous hydroxide ions. Forms a carboxylate ion (or salt if using NaOH) and an alcohol. CH3COOCH3 + NaOH --> CH3COONa + CH3OH Methyl ethanoate + sodium hydroxide --> sodium ethanoate + methanol

Question 33

What are acyl chlorides?

Answer 33

To name, remove the -oic acid suffix from the parent carboxylic acid and replace with -oyl chloride. Propanoic acid --> propanoyl chloride

Question 34

What is the preparation of acyl chlorides?

Answer 34

Carboxylic acid + thionyl chloride, SOCl2. SO2 and HCl products are evolved as gases, leaving only the acyl chloride. It should be done in a fume cupboard as the products are harmful.

Question 35

What are the reactions of acyl chlorides?

Answer 35

They are very reactive and very useful in organic synthesis as they are easily converted into carboxylic acid derivatives, with good yields. Acyl chlorides react with nucleophiles by losing the chloride ion whilst retaining the C=O bond.

Question 36

What is the reaction of acyl chlorides with alcohols?

Answer 36

Acyl chlorides + alcohol --> ester + HCl Ethanoyl chloride + propan-1-ol --> propyl ethanoate + Hydrogen chloride

Question 37

What is the reaction of acyl chlorides with phenols?

Answer 37

Carboxylic acids are not reactive enough to form esters with phenols, but acyl chlorides and acid anhydrides are, to produce phenyl esters, without acid catalysts. Ethanoyl chloride + phenol --> Phenyl ethanoate + HCl

Question 38

What is the reaction of acyl chlorides with water?

Answer 38

A violent reaction occurs with the evolution of dense steamy HCl fumes, to form a carboxylic acid. Ethanoyl chloride + H2O --> Ethanoic acid + HCl

Question 39

What is the reaction of acyl chlorides with ammonia?

Answer 39

Ammonia acts as a nucleophile by donating the lone pair of electrons on the nitrogen atom to an electron deficient species, to form amides. Ammonia and acyl chlorides forms a primary amide. The Nitrogen atom is attached to one carbon atom. Ethanoyl chloride + 2NH3 --> Ethanamide + NH4Cl (ammonium chloride)

Question 40

What is the reaction of acyl chlorides with amines?

Answer 40

Amines act as a nucleophile by donating the lone pair of electrons on the nitrogen atom to an electron deficient species, to form amides. A primary amine reacts with acyl chloride to form a secondary amide. The nitrogen atom is attached to 2 carbon atoms. Ethanoyl chloride + methylamine (2CH3NH2) --> N-methylethanoamide + CH3NH3Cl (methylammonium chloride)

Question 41

What is an acid anhydride?

Answer 41

It is formed by the removal of water from two carboxylic acid molecules. 2ethanoic acid --> ethanoic anhydride + water

Question 42

What are the reactions of acid anhydrides?

Answer 42

React similarly to acyl chlorides with alcohols, phenols, water, ammonia and amines. They are less reactive than acyl chlorides and are useful for lab prep where acyl chlorides would be too reactive. 2(CH3CO)2O + C6H5OH --> CH3COOC6H5 + CH3COOH ethanoic anhydride + phenol --> phenyl ethanoate + ethanoic acid