Chapter 26 - Carbonyls And Carboxylic Acids Flashcards
Why are carbonyls more reactive that alkenes?
Because the C=C in alkenes is non-polar and the C=O in carbonyl did polar. Nucleophiles are attracted and attack the slightly positive carbon, adding across the double bond.
How do you reduce an aldehyde to a primary alcohol and a ketone to a secondary alcohol ?
In the presence of water and using NaBH4 as a catalyst.
How do you make a hydroxynitrile?
Reacting a carbonyl with hydrogen cyanide
Sodium cyanide (NaCN) and H2SO4 are used to provide hydrogen cyanide
Makes an OH group and a CN group
Why is the reaction of hydrogen cyanide with aldehydes useful?
Because it increases the length of the carbon chain
How do you test for a carbonyl?
If you add 2,4-DNP you get a yellow/orange ppt
Qualitative test to distinguish between ketones and aldehydes
Add Tollens’ reagent (silver nitrate and aqueous ammonia). Aldehyde will form a silver mirror wheras ketones will not.
Aldehyde is oxidised (to a carboxylic acid) meaning the Ag+ is reduced:
Ag+ (aq) + e- -> Ag(s) which is a silver mirror
Describe the mechanism for the reaction of NaBH4 and a carbonyl compound
1) Lone pair of electrons from hydride ion is attracted and donated to d+ carbon in C=O
2) Dative covalent bond formed between hydride ion and carbon atom in C=O
3) Pi-bond in C=O breaks by heterolytic fission forming a negatively charged intermediate
4) Oxygen atom of intermediate donates a lone pair of electrons to a hydrogen atom in a water molecule, forming an alcohol
Describe the mechanism of the reaction of NACN with a carbonyl
1) Lone pair of electrons from cyanide ion is attracted and donated to d+ carbon in C=O
2) Dative covalent bond formed between cyanide ion and carbon atom in C=O
3) Pi bond in C=O breaks by heterolytic fission forming a negatively charged intermediate
4) Intermediate is protonated by donating a line pair of electrons to a hydrogen ion (H+) forming a hydroxynitrile
What is a quantitative test to distinguish between an aldehyde and ketone?
After adding 2,4-DNP filter the ppt under pressure then allow it to crystallize and measure the meeting point. Compare the values to a database of derivative melting points.
What happens when you react a carboxylic acid with a metal oxide?
It forms a salt and water
e.g 2CH3COOH + CaO -> (CH3COO-)Ca2+ +H2O
What happens when you react a carboxylic acid with a 1)metal, 2)alkali or 3)carbonate?
1) Carboxylate salt and hydrogen. Metal disappears and effervescence
2) Carboxylate salt and water. May not see a reaction as they react to form an aqueous solution of the salt
3) Carboxylate salt, carbon dioxide and water. If carboxylic acid in excess the solid carbonate disappears.
Describe the solubility of carboxylic acids
They contain C=O and O-H which are polar and can form hydrogen bonds with water.
As number of carbons increases, solubility decreases because the non-polar carbon chain has a greater effect on the overall polarity.
What are the three ways of forming an Ester?
1) Alcohol and carboxylic acid -> Ester and water
2) Acyl chloride and alcohol -> Ester and hydrogen chloride
3) Acid anhydride and alcohol -> Ester and carboxylic acid
What is the catalyst needed to form an Ester from a carboxylic acid and an alcohol?
Small amount of concentrated H2SO4
Describe the acid hydrolysis of esters
Ester is heated under reflux with acid. Ester broken down by water and acid acts as a catalyst
Ester and water -> Carboxylic acid and alcohol
