Aldehydes and ketones

When does nucleophilic addition occur?

• When there is a reaction of an aldehyde or ketone

• Both have carbonyl group c=o

What happens in a nucleophilic addition mechanism and why?

• A positive seeking species (nucleophile) joins on to the carbonyl carbon

• The carbon in the carbonyl group is partially positive as the O is more electronegative

• Nucleophile attacks partially positive carbon

2 examples of nucleophilic addition

• Reduction of aldehydes and ketones to form alcohols

Aldehyde→ Primary alcohol

Ketone→ Secondary alcohol

• Preparation of hydroxynitriles

What is required in the reduction of aldehydes/ketones to form alcohols?

• Aqueous NaBH₄ (Sodium borohydride)

• Which is the reducing agent

What is required to prepare hydroxynitriles?

• KCN (aq)

• Dilute acid (for H+)

What is NaBH₄

• A reducing agent

• Ionic compound

• When it is dissolved in water, to make an aqueous solution it is a source of H^- - hydride ion

Mechanism for the reduction of an aldehyde (ethanal) and explain it

• Partially positive carbon attracts the hydride ion

• Hydride ion attacks the partially positive carbon and uses its lone pair of electrons to form a new bond between carbon and hydride ion

• Bond between C=O partially breaks→ 2 of the 4 electrons are pulled by the O away from the carbon

• Negatively charged oxygen uses its lone pair to form a new bond with H+ (which typically comes from the solvent the sodium borohydride was dissolved in)

• Producing an alcohol - ethanol

Write the equation for ethanal being reduced to ethanol

• 2 before the H because the aldehyde (or ketone) will be gaining 2 hydrogens

• 1 from the nucleophile and 1 from the solvent.

How do you name a hydroxynitrile?

• Nitrile functional group takes priority

• So carbon attached to the N is always treated as carbon 1

• Check notes for further guidance

What is KCN and what happens when u make it an aqueous solution?

• Ionic compound

• When dissolved in a solvent (water) it will fully dissociate into CN^- and K⁺

Draw the mechanism for ethanal being used to make a hydroxynitrile

• ( H+ Comes from dilute acid)

Write the equation of ethanal becoming a hydroxynitrile

• (Don’t worry about state symbols)

What does the solubility of organic compounds like ketones and aldehydes depend on?

• Molecular size

• Larger carbon chain= Less soluble

• E.g ethanal has a smaller carbon chain so its likely to be soluble

Risk assessment for producing hydroxynitriles: Hazard is KCN - why is it dangerous and give a control measure?

• KCN is toxic and produces HCN which is also toxic

• Should be handled and stored carefully

• Do the reaction in a fume cupboard to prevent HCN from entering the room

Risk assessment for producing hydroxy nitriles: Organic products are the hazard- why are they dangerous and give a control measure

• Flammable

• Heat in water bath and don’t expose it to a naked flame (if you wish to speed up the reaction)

What can be used instead of KCN when preparing hydroxynitriles?

• HCN

• In this case the mechanism will look the same but the reaction equation will be slightly different

Write an equation for ethanol reacting with HCN to produce a hydroxynitrile and explain why the equation is different?

• No acid will be added when using HCN as HCN can be the source for both cyanide ion and H+

• Because it is weakly acidic and dissociates into H⁺ and CN ^-

• So in the equation there is not H+ over the arrow and no K+

Why is KCN preferred over HCN?

• KCN is an ionic compound so it completely/ readily dissociates in water and produces a higher cyanide ion concentration than HCN

• HCN is also a gas so its harder to handle and store.

What happens when you carry out nucleophilic addition(to produce hydroxynitriles) reactions on aldehydes and unsymmetrical ketones?

• You produce a mixture of enantiomers

Even though nucleophilic addition produces 2 enantiomers, why is there no optical activity?

• The product is a racemate

• Meaning there’s equal amounts of the 2 enantiomers

• The rotation of the plane polarised light clockwise and anticlockwise will cancel out

• Thus no optical activity

Why is the racemic mixture produced?

• Molecules with a carbonyl group are planar in the region around the carbonyl carbon

• Nucleophile can attack from either side with equal probability

• Producing equal amounts of the 2 enantiomers