3.3.8 Aldehydes & ketones

What is a carbonyl group?

C=O

What is the functional group & general formula for an aldehyde?

RCHO (C double bonded to O, single bond to H & R)

What is the functional group for a ketone?

RCOR’ (C double bonded to O)

How do you name aldehydes?

-al suffix (C=O is on the end of a chain)

How do you name ketones?

-one suffix (designate number for which carbon C=O is on)

What kind of intermolecular forces do molecules with the carbonyl group have & why?

Permanent dipole-dipole forces, due to the polar C=O bond (C is δ⁺ & O is δ⁻)

Are molecules with a carbonyl group soluble in water & what influences solubility?

• Yes, they form hydrogen bonds between water molecules & the oxygen of C=O

• As the carbon chain length increases, solubility decreases

Which bond in carbonyl compounds is usually involved in reactions & why?

C=O, due to the polarity of the bond (large difference in electronegativity between carbon & oxygen)

What is the strongest bond in carbonyl compounds?

C=O

Draw & outline a mechanism for the nucleophilic addition of HCN to a carbonyl compound

Why is the addition of HCN important?

It increases the carbon chain length by one carbon atom, which is useful as it forms a nitrile that can be further converted into carboxylic acids or amines

Will the product of HCN added to a carbonyl compound have optical isomers & why?

Yes:

• in the aldehyde/ketone, the carbonyl compound is planar, so the :CN⁻ can attack from either above or below, forming two enantiomers

What is the name of the product when HCN is added to a carbonyl compound?

Hydroxynitriles (contain OH & CN groups)

What is Fehling’s solution & what colour is it?

Copper complex ions (blue)

What happens when an aldehyde is warmed with Fehling’s solution?

• Reduced to Cu⁺ ions

• There is a colour change from a blue solution to a brick red precipitate

What happens when a ketone is added to Fehling’s solution?

No reaction (Fehling’s solution remains a blue solution)

What is Tollens’ reagent?

Silver complex ions (colourless solution)

What happens when an aldehyde is warmed with Tollens’ reagent?

Silver mirror forms, as Ag⁺ is reduced to Ag(s)

What happens when a ketone is added to Tollens’ reagent?

No reaction (Tollens’ reagent remains colourless)

What is another oxidising agent for alcohols/aldehydes & what is the colour change?

• Acidified potassium dichromate (K₂Cr₂O₇)

• Colour change from orange to green

What alcohols can be oxidised to by acidified potassium dichromate?

• Primary (1°) alcohols → aldehydes (distillation) → carboxylic acids

  • or primary (1°) alcohols can be oxidised straight to carboxylic acids under reflux

• Secondary (2°) alcohols → ketones

Why can’t tertiary (3°) alcohols be oxidised by acidified potassium dichromate?

• The oxidation of alcohols requires the removal of a hydrogen atom from the carbon bonded to the –OH group

• Tertiary alcohols don’t have a hydrogen atom on that carbon, as it is bonded to 3 alkyl groups instead

• Without this hydrogen, a carbonyl (C=O) group cannot form, so oxidation cannot proceed under these conditions

What is a reducing agent for aldehydes/ketones & what ions does this release in solution?

• Sodium borohydride in aqueous solution (NaBH₄)

• Releases a hydride ion, which acts as a nucleophile (H^-)

What is a nucleophile?

An electron-pair donor

What can aldehydes & ketones be reduced to?

• Aldehydes: reduced to primary alcohols

• Ketones: reduced to secondary alcohols

Draw & name the mechanism for the reduction of an aldehyde

Nucleophilic addition

What are the hazards of using potassium cyanide (KCN) ?

• KCN is an irritant & can be very dangerous if inhaled or ingested

• KCN forms hydrogen cyanide (toxic gas) when reacted with moisture

How can you reduce the risk associated with potassium cyanide (KCN) ?

• Wear a lab coat to prevent clothing contamination

• Wear safety goggles at all times

• Use a fume cupboard to prevent exposure to toxic fumes

• Wear gloves while handling

Write 2 equations for the reduction of pentan-2-one & 3-methylbutanal

Pentan-2-one:

• CH₃CH₃COCH₂CH₃ + 2[H] → CH₃CH₂CH(OH)CH₂CH₃

3-methylbutanal:

• CH₃CH(CH₃)CH₂CHO + 2[H] → CH₃CH(CH₃₎CH₂CH₂OH

What are the functional groups of hydroxynitriles?

Hydroxyl group (OH) & a nitrile group (CN)

Name this molecule

2-hydroxy-3-methylbutanenitrile

How are hydroxynitriles produced?

Nucleophilic addition reaction: reacting aldehydes/ketones with KCN & a HCl catalyst

What is the overall reaction for when aldehydes & ketones react with KCN to make hydroxynitriles?