Carbonyl Chemistry

Question 1

why are aldehydes and ketones also known as carbonyls

Answer 1

Aldehydes and ketones contain the carbonyl functional group, C=O

Question 2

difference between aldehyde and ketone

Answer 2

aldehyde: carbonyl group always has a H bonded to it

ketone: carbonyl group will be bonded to 2 alkyl groups

Question 3

formula for aldehyde

Answer 3

RCHO (CnH2nO)

Question 4

formula for ketone

Answer 4

R–C = O–R

CnH2nO

Question 5

why do aldehydes and ketone have dipoles within their structure

Answer 5

due to the electronegative oxygen of the carbonyl group this means that aldehydes and ketones have permanent dipole-dipole interactions and London forces between molecules

Question 6

why is melting and boiling points of aldehydes and ketones lower than corresponding alcohol

Answer 6

There are no polar bonds in aldehydes or ketones that can result in a δ+ hydrogen for hydrogen bonding, e.g. O-H, N-H therefore, the melting and boiling points of aldehydes and ketones are lower than their corresponding alcohol.

Question 7

why are smaller aldehydes and ketones able to hydrogen bond with water

Answer 7

The δ– oxygen atom from the carbonyl uses its lone pairs to form hydrogen bonds with the δ+ hydrogen from water

Question 8

why cant larger aldehydes and ketone hydrogen bond with water

Answer 8

Larger aldehydes and ketones have longer hydrocarbon chains

These hydrocarbon chains can disrupt the hydrogen bonding within water but cannot form hydrogen bonds themselves

Question 9

why wont a carbonyl compound dissolve in water if it is large enough

Answer 9

In order to dissolve, the strength of the potential hydrogen bonding of the carbonyl with water must be higher than the combined strength of the intermolecular forces of the carbonyl and the hydrogen bonding of water

Question 10

what reactions allow you to distinguish between an aldehyde and a ketone

Answer 10

Oxidation - with acidified potassium dichromate(VI) solution, Tollens’ reagent or Fehling’s / Benedict’s solution

Reduction - with lithium tetrahydridoaluminate / lithium aluminium hydride in dry ether

Nucleophilic addition - with HCN or acidified KCN

Non-specific carbonyl testing
- 2,4-dinitrophenylhydrazine (2,4-DNPH) - to identify a carbonyl compound
Iodoform test - iodine in the presence of alkali

Question 11

how can aldehydes be oxidised to form carboxylic acids

Answer 11

using acidified potassium dichromate(VI) solution / H2SO4

Question 12

why can oxidation of aldehydes using potassium dichromate(VI) be a way to distinguish between aldehyde and a ketone

Answer 12

Ketones are very resistant to being oxidised

This is because ketones do not have a readily available hydrogen atom, like aldehydes do

An extremely strong oxidising agent would be needed for oxidation of a ketone to take place

The aldehyde would be oxidised, and you would see an orange to green colour change

The ketone would not be oxidised, so you would see no colour change

Question 13

what does tollens’ reagent contain and how is that complex ion formed

Answer 13

silver(I) complex ion [Ag(NH3)2]+

when aqueous ammonia is added to a solution of silver nitrate

Question 14

what happens when tollens’ reagent is warmed with an aldehyde

Answer 14

an aldehyde will become oxidised

As the aldehyde is oxidised, it causes the [Ag(NH3)2]+ ions to become reduced to solid metallic silver, Ag
This is why a positive test result is called a “silver mirror”

The Ag+ ions in Tollens’ reagent are oxidising agents, oxidising the aldehyde to a carboxylic acid and getting reduced themselves to silver atoms

Question 15

why can oxidation of aldehydes using tollens’ reagent be a way to distinguish between aldehyde and a ketone

Answer 15

When Tollens’ reagent is gently warmed with a ketone, no silver mirror will be seen, as the ketone cannot be oxidised by Tollens’ reagent, so no reaction takes place

When Tollens’ reagent is gently warmed with an aldehyde, the silver mirror is formed

Question 16

what does Fehling’s solution contain

Answer 16

copper(II) ions dissolved in sodium hydroxide, which act as the oxidising agent

Question 17

how does Benedicts solution and Fehlings differ

Answer 17

Benedict’s solution is exactly the same as Fehling’s solution but the copper(II) ions are dissolved in sodium carbonate

Question 18

what is produced when Fehling’s solution is reacted with aldehyde

Answer 18

oxidised to a carboxylic acid, the blue Cu2+ ions are reduced to Cu+ ions and a brick red precipitate is formed

The brick red precipitate is copper(I) oxide

Question 19

what happens if ketone is warmed with Fehling’s solution

Answer 19

no reaction takes place as the ketone will not be oxidised, so the solution will remain blue

Question 20

what are Aldehydes and ketones reduced to

Answer 20

Aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols

This is done with lithium tetrahydridoaluminate / lithium aluminium hydride, LiAlH4, in dry ether

Question 21

explain nucleophilic addition of carbonyl compounds

Answer 21

The carbonyl group -C=O, in aldehydes and ketones is polarised

The oxygen atom is more electronegative than carbon drawing electron density towards itself

This leaves the carbon atom slightly positively charged and the oxygen atom slightly negatively charged

The carbonyl carbon is therefore susceptible to attack by a nucleophile, such as the cyanide ion

Question 22

explain the addition of HCN to carbonyl compounds (nucleophilic addition)

Answer 22

In step 1, the cyanide ion attacks the carbonyl carbon to form a negatively charged intermediate

In step 2, the negatively charged oxygen atom in the reactive intermediate quickly reacts with aqueous H+ (either from HCN, water or dilute acid) to form 2-hydroxynitrile compounds,

Question 23

why is the reaction of HCN with carbonyl compounds important in organic synthesis

Answer 23

because it adds a carbon atom to the chain, increasing the chain length
The products of the reaction are hydroxynitriles

Question 24

how does 2,4-dinitrophenylhydrazine detect the presence of carbonyl compounds

Answer 24

The carbonyl group of aldehydes and ketones undergoes a condensation reaction with 2,4 dinitrophenylhydrazine and a small molecule (such as H2O or HCl) is eliminated

The product formed when 2,4-DNPH is added to a solution that contains an aldehyde or ketone is a deep-orange precipitate which can be purified by recrystallisation

The melting point of the formed precipitate can then be measured and compared to literature values to find out which specific aldehyde or ketone had reacted with 2,4-DNPH

Question 25

why is 2,4-DNPH a useful test

Answer 25

as other carbonyl compounds such as carboxylic acids and esters do not give a positive result (deep-orange precipitate)

Question 26

what does iodoform / tri-iodomethane form with methyl ketone and explain the procedure

Answer 26

Tri-iodomethane (also called iodoform) forms a yellow precipitate with methyl ketones

The reagent is heated with an alkaline solution of iodine

This reaction involves a halogenation and hydrolysis step
In the halogenation step, all three H-atoms in the -CH3 (methyl) group are replaced with iodine atoms, forming a -CI3 group

The intermediate compound is hydrolysed by an alkaline solution to form a sodium salt (RCO2- Na+) and a yellow precipitate of CHI3
(check savemyexams for pic)

Question 27

what are methyl ketones

Answer 27

compounds that have a CH3CO-group

Question 28

difference between amide and amine

Answer 28

amide has a carbon that has a double bond to oxygen and connected to NH2

amine has a carbon that is connected to only NH2 and hydrogens