Carbonyl Chemistry Flashcards
why are aldehydes and ketones also known as carbonyls
Aldehydes and ketones contain the carbonyl functional group, C=O
difference between aldehyde and ketone
aldehyde: carbonyl group always has a H bonded to it
ketone: carbonyl group will be bonded to 2 alkyl groups
formula for aldehyde
RCHO (CnH2nO)
formula for ketone
R–C = O–R
CnH2nO
why do aldehydes and ketone have dipoles within their structure
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
why is melting and boiling points of aldehydes and ketones lower than corresponding alcohol
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.
why are smaller aldehydes and ketones able to hydrogen bond with water
The δ– oxygen atom from the carbonyl uses its lone pairs to form hydrogen bonds with the δ+ hydrogen from water
why cant larger aldehydes and ketone hydrogen bond with water
Larger aldehydes and ketones have longer hydrocarbon chains
These hydrocarbon chains can disrupt the hydrogen bonding within water but cannot form hydrogen bonds themselves
why wont a carbonyl compound dissolve in water if it is large enough
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
what reactions allow you to distinguish between an aldehyde and a ketone
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
how can aldehydes be oxidised to form carboxylic acids
using acidified potassium dichromate(VI) solution / H2SO4
why can oxidation of aldehydes using potassium dichromate(VI) be a way to distinguish between aldehyde and a ketone
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
what does tollens’ reagent contain and how is that complex ion formed
silver(I) complex ion [Ag(NH3)2]+
when aqueous ammonia is added to a solution of silver nitrate
what happens when tollens’ reagent is warmed with an aldehyde
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
why can oxidation of aldehydes using tollens’ reagent be a way to distinguish between aldehyde and a ketone
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
what does Fehling’s solution contain
copper(II) ions dissolved in sodium hydroxide, which act as the oxidising agent
how does Benedicts solution and Fehlings differ
Benedict’s solution is exactly the same as Fehling’s solution but the copper(II) ions are dissolved in sodium carbonate
what is produced when Fehling’s solution is reacted with aldehyde
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
what happens if ketone is warmed with Fehling’s solution
no reaction takes place as the ketone will not be oxidised, so the solution will remain blue
what are Aldehydes and ketones reduced to
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
explain nucleophilic addition of carbonyl compounds
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
explain the addition of HCN to carbonyl compounds (nucleophilic addition)
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,
why is the reaction of HCN with carbonyl compounds important in organic synthesis
because it adds a carbon atom to the chain, increasing the chain length
The products of the reaction are hydroxynitriles
how does 2,4-dinitrophenylhydrazine detect the presence of carbonyl compounds
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
why is 2,4-DNPH a useful test
as other carbonyl compounds such as carboxylic acids and esters do not give a positive result (deep-orange precipitate)
what does iodoform / tri-iodomethane form with methyl ketone and explain the procedure
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)
what are methyl ketones
compounds that have a CH3CO-group
difference between amide and amine
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
