C26 - Carbonyls and Carboxylic Acids Flashcards
What happens when aldehydes are oxidised?
They form carboxylic acids (e.g. when refluxed with acidified dichromate ions).
Ketones will not undergo oxidation.
How does the reaction of carbonyl compounds differ to the type of reactions alkenes undergo?
Due to the polarity of the C=O bond in carbonyls, aldehydes and ketones can react with nucelophiles. Thus they can undergo nucleophilic addition.
Alkenes, with non polar C=C bonds, react with electrophiles via electrophilic addition.
What is NaBH4?
Sodium tetrahydridoborate.
It’s a reducing agent to reduce aldehydes and ketones to alcohols (under warm conditions).
What’s sodium tetrahydridoborate?
NaBH4
It’s a reducing agent to reduce aldehydes and ketones to alcohols (under warm conditions).
What happens when aldehydes and ketones are reduced?
Under warm conditions, using reducing agent NaBH4, aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols.
How do carbonyls react with HCN, hydrogen cyanide?
HCN is extremely poisonous and has a low boiling point so it must be carried out in a fume cupboard.
It also can’t be used directly (due to being poisonous) so sodium cyanide and sulphuric acid are used to produce the HCN.
It can result in the length of the carbon chain increasing.
What happens when carbonyls react with HCN, hydrogen cyanide?
The H and C≡N bond across the C=O.
HCN is extremely poisonous and has a low boiling point so it must be carried out in a fume cupboard.
It also can’t be used directly so sodium cyanide and sulphuric acid are used to produce the HCN.
It can result in the length of the carbon chain increasing.
What is used to produce HCN, hydrogen cyanide, in reactions with carbonyls?
(Conditions)
Sodium cyanide and sulphuric acid.
The reaction is carried out in a fume cupboard.
What is produced when hydrogen cyanide reacts specifically with aldehydes?
Hydroxynitriles (or cyanohydrins)
They contain hydroxyl and nitrile functional groups.
What happens in the mechanism for the reaction with NaBH4?
Nucleophilic addition
1) The lone pair of electrons from the hydride ion ( :H- ) is attracted and donated to the (+) carbon atom in the carbonyl C=O double bond.
2) A dative covalent bond is formed between the hydride ion and carbon atom.
3) The pi bond in the C=O bond breaks by 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 becomes pronated into an alcohol.
What acts as the nucleophile in the reaction of NaBH4 with carbonyls?
A hydride ion
:H -
What happens in the mechanism for the reaction with HCN?
Nucleophilic addition
1) The lone pair of electrons from the cyanide ion ( :CN- ) is attracted and donated to the (+) carbon atom in the carbonyl C=O double bond.
2) A dative covalent bond is formed between the hydride ion and carbon atom.
3) The pi bond in the C=O bond breaks by 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 becomes pronated into a hydroxynitrile.
What acts as the nucleophile in the reaction of hydrogen cyanide with carbonyls?
A cyanide ion
:C≡N -
What is Brady’s reagent?
(A.k.a 2,4-dinitrophenylhydrazine OR 2,4-DNP)
It’s used to detect the presence of the carbonyl functional group. (Detects both aldehydes and ketones).
What is 2,4-dinitrophenylhydrazine / 2,4-DNP?
Brady’s reagent
It’s used to detect the presence of the carbonyl functional group. (Detects both aldehydes and ketones).
How is 2,4-DNP / Brady’s reagent used?
When added to the unknown compound, if produces a yellow/orange precipitate, a carbonyl is present.
What is used to distinguish between aldehydes and ketones?
Tollens’ reagent
It will oxidise an aldehyde into a carboxylic acid and form a silver mirror (as silver ions get reduced).
Ketones won’t react as they won’t further oxidise.
What happens when Tollens’ reagent is added to carbonyls?
It contains silver ions which act as an oxidising agent (in the presence of ammonia).
It will oxidise an aldehyde into a carboxylic acid and form a silver mirror (as silver ions get reduced).
Ag+ + e- -> Ag
Ketones won’t react as they won’t further oxidise.
How soluble are carboxylic acids?
The C=O and O-H bonds are polar, allowing them to form hydrogen bonds with water molecules.
As the number of carbon atoms increases, solubility decreases as the non-polar carbon chain has a greater effect on the overall polarity of the molecule.
(Carboxylic acids with up to 4 C atoms are soluble in water)
How acidic are carboxylic acids?
They’re classified as weak acids.
More acidic than phenols and alcohols
How do carboxylic acids react with metals and bases?
They undergo redox reactions with metals and neutralisation reactions with bases.
This forms carboxylate salts.
What is the test for carboxyl groups?
React with carbonates.
Carboxylic acids are the only common organic compound acidic enough to react with carbonates. (E.g. phenols not acidic enough to react with carbonates)
What are the (4) carboxylic derivatives?
Esters
Acyl chlorides
Acid anhydrides
Amides
What are esters?
A carboxylic derivative.
To name an ester, the -oic acid suffix is replaced with -oate.
-COO-
