Chapter 26 - Carbonyls & Carboxyllic Acids

Carbonyl group (C=O)

At the end of the carbon chain CHO

Methanal (HCHO) Common name is formaldehyde and it’s used in solution to preserve biological specimens

Found in the in the middle of the compound joined to two carbon atoms either side

Propanone (CH3COCH3) Acetone is the common name and used as an important industrial solvent as well as in nail-varnish removers

As aldehydes can be further oxidised whereas ketones cannot

Pi bond above and below the plane of the carbon and oxygen, formed by the sideways overlap of p-orbitals

• the C=C in alkenes is non-polar • the C=O in carbonyl compounds is polar Alkenes undergo electrophilic addition while carbonyl compounds undergo nucleophilic addition

As oxygen is more electronegative than carbon, the electron density in the double bond lies closer to the O so dipoles form Nucleophiles are attracted to the slightly positive carbon resulting in nucleophillic addition

Warmed with the aldehyde or ketone and as a reducing agent to reduce them to alcohols

Primary alcohols

[H]

Butanal + 2[H] -> butan -1- ol The 2 on the reducing agent is just there to balance the hydrogens

Propanone + 2[H] -> propan-2-ol

It is a colourless, extremely poisonous liquid that boils slightly above temperature so sodium cyanide and sulfuric acid are used

It provides a means of increasing the carbon chain length

Contains two functional groups: a hydroxyl group (-OH), and a nitrile group (C triple bond N) Classifies as hydroxynitriles

Look at notes sections: 1. Lone pair of electrons from the hydride ion is attracted and donated to the slightly positive carbon atom in the aldehyde or ketone C=O double bond 2. A dative covalent bond is formed between the hydride ion and the carbon atom of the C=O double bond 3. The pi bond in the C=O double bond breaks by heterocyclic 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 5. The intermediate has then been protonated to form an alcohol

• The lone pair on the cyanide ion is attracted and donated to the slightly positive carbon in the aldehyde or ketone C=O double bond so a dative covalent bond forms • The pi bond in the C=O breaks by heterolytic fission forming a negatively charged intermediate donates • The intermediate is protonated by donating a lone pair of electrons to a hydrogen ion, to form the product • Product is a hydroxynitrile

Using Brady’s Reagent (2,4-DNP) to detect the presence of the functional group in both aldehydes and ketones In the presence a yellow or orange precipitate (2,4-dinitrophenylhyrdazine) is produced

Used dissolved in methanol and sulfuric acid as a pale orange solution

• Add 5cm depth of a solution of 2,4-dinitrophenylhyrdazine to a clean test tube in excess • Using a dropping pipette, add three drops of the unknown compound and leave to stand • If no cyrstals form, add a few drops of sulfuric acid • A yellow/orange precipitate indicates the presence of an aldehyde or ketone

• In a clean test tube, add 3cm depth of aqueous silver nitrate • Add aqueous sodium hydroxide to the silver nitrate until a brown precipitate of silver oxide is formed • Add dilute ammonia solution until the brown precipitate just dissolved to form a clear colourless solution which is Tollens’ reagent

• Pour the unknown solution into a clean test tube • Add an equal volume of Tollens’ reagent • Leave the test tube to warm in a beaker of water at around 50 degrees for 10-15 minutes and then observe for a silver mirror to indicate an aldehyde