6.2 Carbonyls and Carboxylic Acids Flashcards
What is the functional group of aldehydes and ketones ?
C=O
At the end of the chain in aldehydes but in the middle of the chain in ketones.
Oxidation of aldehydes and ketones
Aldehydes can be oxidised to carboxylic acids under reflux with acidified dichromate. Ketones do not undergo oxidation due to their lack of reactivity.
What is the reactivity of aldehydes and ketones influenced by ?
Influenced by the nature of the C=O double bond, which is polar bond as oxygen is more electronegative than carbon so the electron density lies closer to the oxygen than the carbon.
Reducing an aldehyde and ketones
NaBH4 is used as a reducing agent to reduce aldehydes and ketones to alcohols. They are usually warmed with NaBH4 in an aqueous solution. Aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols.
Reaction of carbonyls with HCN
It increases the length of the carbon chain through an addition reaction.
Eg. propanal + HCN = (OH and CN groups) known as a hydroxynitrile.
What is the mechanism for carbonyl compounds ?
Nucleophilic addition. The carbon in the C=O bond is electron deficient and attracts nucleophiles. Aldehydes and ketones react to form alcohols.
Nucleophilic Substitution of carbonyls
Lone pair of electrons is attracted to carbon atom.
Dative bond forms between H and C.
C=O bond breaks by heterolytic fission.
Oxygen donates lone pair of electrons to a hydrogen atom and the intermediate has been pronated to form an alcohol.
What’s used to detect a carbonyl functional group ?
Bradys reagent, which forms a yellow or orange ppt in the presence of a carbonyl group.
What is used to distinguish between an aldehyde and a ketone ?
After using Brady’s reagent to identify th carbonyl group, Tollen’s reagent is used. If an aldehyde is present, a silver mirror is produced, but if it is a ketone, no reaction is observed.
Tollen’s reagent and aldehydes
Contains Ag+ ions that act as an oxidising agent in the presence of ammonia. Silver ions are reduced to silver as the aldehyde is oxidised to a carboxylic acid.
What is the carboxylic acid functional group ?
COOH
Are carboxylic acids soluble ?
C=O and O-H bonds are polar allowing them to form hydrogen bonds with water. However, as the number of carbon atoms increase, the solubility decreases as the non-polar chain has greater effect on the overall polarity of the molecule.
Are carboxylic acids strong or weak acids ?
Weak acids, as they partially dissociate some of its H+ ions.
Carboxylic acids and metals
React to form H2 gas ad carboxylate salt. Metal disappears as effervescence is produced, forming hydrogen gas.
Carboxylic acid with oxides
Forms salt and water.
Carboxylic acid with alkalis
Forms aslt and water
Carboxylic acids with carbonates
Forms salt, water and carbon dioxide.
How do you test for the carboxyl group ?
Through neutralisation reaction of carboxylic acids with carbonates. Carboxylic acids are the only common organic compounds to react with carbonates.
What is a derivative of a carboxylic acid ?
Is a compound that can be hydrolysed to form the parent carboxylic acid. They have a common sequence of atoms in their structure, known as acyl group. Esters, acyl chlorides, acyl anhydrides and amides are all derivatives of carboxylic acids.
What is an ester ?
Named after the parent carboxylic acid. Alkyl chain is attached to the oxygen atom of the COO group.
R - C = O
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O-CH3
What is an acyl chloride ?
Derivative of parent carboxylic acid. Add oyl chloride at the end.
What is an acid anhydride ?
Formed by the removal of water from 2 carboxylic acid molecules. Add anhydride as a suffix.
What is esterification ?
Reaction of an alcohol with a carboxylic acid to form an ester. Alcohol is warmed with a carboxylic acid with a small amount of conc H2O4, which acts as a catalyst.
Acid hydrolysis of an ester ?
Ester is heated under reflux with dilute aqueous acid. Ester is broken down by water, with the acid acting as a catalyst. The products are a carboxylic acid and an alcohol.
Alkaline hydrolysis of an ester ?
Also known as saponification and is an irreversible process. Ester is heated under reflux with aqueous hydroxide ions. Forms a carboxylate ion and an alcohol.
How to prepare acyl chlorides ?
Prepared directly from a carboxylic acid with thionyl chloride SOCl2. Other products, SO2 and HCl, are evolved as gases, leaving just acyl chloride. Carried out in a fume cupboard as the products are harmful.
Reactions of acyl chlorides ?
React with nucleophiles by losing the chloride ion whilst retaining the C=O double bond.
Acyl chlorides and alcohols
React with alcohols to form esters.
Acyl chlorides and phenol
Carboxylic acids are not reactive enough to form esters with phenols. Acyl chlorides and acid anhydrides are more reaction so can. They produce phenyl esters and neither reaction needs an acid catalyst.
Acyl chlorides with water
Violent reaction takes place with evolution of dense steamy hydrogen chloride fumes. Carboxylic acid is formed.
Acyl chlorides with ammonia and amines
Ammonia and amines act as nucleophiles by donating the lone pair of electrons on the nitrogen atom to an electron-deficient species. Their reaction with acyl chlorides form amides. Ammonia reacts with acyl chlorides to form a primary amide (one C atom). Primary amine reacts with acyl chloride, forming a secondary amide (2 C atoms).
Reactions of acid anhydrides
React in similar way to acyl chlorides, but are less reactive and are useful when acyl chloride may be too reactive.
