Chapter 26 - Carbonyls and Carboxylic acids Flashcards
What is the carbonyl functional group?
C=O
What is the difference between an aldehyde and a ketone?
Aldehyde group written as CHO, attached to end of carbon chain
Ketone group written as CO, attached to two carbon atoms in the carbon chain
What oxidation reactions do aldehydes and ketones undergo?
Aldehydes oxidised to carboxylic acids when refluxed with acidified K2Cr2O7.
Ketones do not undergo oxidation reactions
Why do aldehydes and ketones react with nucleophiles?
Due to the polarity of the C=O bond (O is more electronegative, making it slightly negative).
What is a nucleophile?
An electron donor
How is an aldehyde reduced, what is the product?
Aldehydes are reduced to primary alcohols by NaBH4
NUCLEOPHILIC ADDITION
How is a ketone reduced, what is the product?
Ketones are reduced to secondary alcohols by NaBH4
NUCLEOPHILIC ADDITION
How do carbonyl compounds react with HCN?
HCN adds across the C=O bond to form a hydroxynitrile
NUCLEOPHILIC ADDITION
What happens in nucleophilic addition?
Nucleophile attracted to delta positive C atom, and a dative covalent bond is formed.
The pi bond in the C=O double bond breaks by heterolytic fission to form a negatively charge intermediate. The oxygen atom of the intermediate donates a lone pair of electrons to a H atom in a water molecule. Intermediate has been protonated to form an alcohol.
What is the mechanism for the reaction with NaCN/H+?
Lone pair from CN- ion forms dative covalent bond with the delta positive C atom. Pi bond in C=O double bond breaks by heterolytic fission to form negatively charged intermediate. Intermediate protonated by donating lone pair to H+ ion to form hydroxynitrile
How can you test for the presence of the carbonyl functional group in both aldehydes and ketones?
2,4-DNP
In the presence of a carbonyl group, a yellow/orange precipitate forms
What test can you use to distinguish between aldehydes and ketones?
Use Tollen’s reagent - solution of silver nitrate in aqueous ammonia. In presence of aldehyde group, a silver mirror is produced
Why does Tollen’s create a silver mirror for aldehydes/not react with ketones?
Silver ions reduced to silver as aldehyde is oxidised to a carboxylic acid.
Ketones cannot be oxidised
How can the precipitate formed by the 2,4 - DNP test be analysed to identify the carbonyl compound?
Impure solid filtered to separate solid precip from solution. Solid then recrystallised to produce a pure sample of crystals. The melting point is then measured and recorded and compared to a database
What is the solubility of the carboxylic acids?
C=O and O-H bonds are polar allowing carboxylic acids to form hydrogen bonds with water molecules. Carboxylic acids with up to 4 C atoms are soluble in water - as chain length increases, solubility decreases
What is the strength of carboxylic acids?
They are weak acids, which only partially dissociate
How do carboxylic acids react with metals, what type of reaction is this?
Form hydrogen gas and carboxylate salt
REDOX
How do carboxylic acids react with metal oxides, alkalis and carbonates - what type of reaction are these?
Metal oxides and alkalis - salt + water
Carbonates - salt + water + carbon dioxide
NEUTRALISATION
How can you test for the carboxyl group?
Only common organic compound sufficiently acidic to react with carbonates (produce CO2)
What is an ester’s functional group?
COO
What is an acyl chloride’s functional group?
COCl
What is an acid anhydride’s functional group?
(CO)2O
What happens in esterification?
Reaction of an alcohol with a carboxylic acid to form an ester
Alcohol warmed with carboxylic acid and a small amount of conc H2SO4
What is hydrolysis?
The chemical breakdown of a compound in the presence of water or in aqueous solution
What happens in acid hydrolysis?
Ester heated under reflux with dilute aq acid. Ester broken down by water, with acid acting as a catalyst. REVERSIBLE REACTION
What are the products of acid hydrolysis?
A carboxylic acid and a base
What happens in alkaline hydrolysis?
Ester heated under reflux with aqueous hydroxide ions. IRREVERSIBLE REACTION
What are the products of alkaline hydrolysis?
A carboxylate ion and an alcohol. Salt can form if NaOH used
How can acyl chlorides be prepared?
Reaction between carboxylic acid and thionyl chloride, SOCl2. (Other products are SO2 and HCl)
What happens when an acyl chloride reacts with an alcohol?
Ester formed + HCl
What happens when an acyl chloride reacts with water?
Violent reaction takes place with evolution of dense steamy HCl fumes. Carboxylic acid formed
What happens when an acyl chloride reacts with ammonia or amines?
Ammonia and amines acts as nucleophiles to form amides.
Ammonia + acyl chloride = primary amide
Primary amide + acyl chloride = secondary amide
What happens when an acyl chloride reacts with phenol?
Forms an ester
(carboxylic acids are unable to do this as they are not reactive enough)
How are the reactions of acid anhydrides similar/different to acyl chlorides?
React in similar way with alcohols, phenols, water, ammonia and amines
but less reactive than acyl chlorides
How are acid anhydrides formed?
removal of water from two carboxylic acid molecules
