Carbonyls

Identify the geometry around the carbon atom in the C=O bond of the aldehyde and ketone functional group

Trigonal planar

Explain the solubility of carbonyl compounds with water

They can form hydrogen bonds with water. The y are very soluble in water, especially with shorter chains

How can aldehydes and ketones be formed from alcohols

• (Partially) oxidise a primary alcohol to form an aldehyde - fractional distillation

• Oxidise a secondary alcohol to form a ketone

Identify the oxidising agent used to obtain an aldehyde or ketone

Potassium or sodium dichromate (VI) in sulphuric acid

State the observation made and the species responsible when the oxidising agent is used to oxidise an alcohol

Orange solution (Cr2O72-) to green solution (Cr3+)

What is formed in the oxidation of alcohols

A carbonyl and a water molecule

Why can’t ketones be oxidised further

The carbon in the C=O functional group have no other hydrogen atoms bonded

What is Tollen’s Reagent

Ammonical silver nitrate [Ag(NH3)2]+

What happens when aldehydes are heated with Tollens’ Reagent

The aldehyde is oxidised to a salt of carboxylic acid and the silver ions are reduced to form a silver mirror

Explain why salts of carboxylic acids form in the reaction with Tollens’ reagent instead of protonated carboxylic acids

Tollens’ reagent solution is basic due to ammonia so a deprotonated form of the carboxylic acid will form

What happens when aldehydes are heated with Fehling’s solution

Aldehydes reduce blue solution containing Cu2+ to Cu+ to form red precipitate of copper (I) oxide (Cu2O)

What is formed in the reduction of carbonyls

Alcohols

What is the reducing agent used to reduce carbonyls

Lithium tetrahydridoaluminate (III) (LiAlH4) in a dry ether solvent [H]

How many [H] is needed in the reduction equation forming alcohols from carbonyls

2[H]

By what mechanism are carbonyls reduced

Nucleophilic addition - a nucleophile attacks the partial positive charge on the carbon caused by the electronegative oxygen

What is the nucleophile in a nucleophilic addition reaction

The H- from the LiAlH4

What is the purpose of the H+ ions in nucleophilic addition

• In the protonation step, it gets attacked by the lone pairs on the oxygen atom which is acting as a nucleophile

• They come from an acid during the work-up of the product

Outline the mechanism for the nucleophilic addition for the reduction of ethanal

Outline the mechanism for the reduction of propanone using the full structure of the AlH4- ion

State one alternative that can be used in the protonation step of the reduction of carbonyls and why

A water molecule due to the δ+ charge on the hydrogen from its dipole

Outline the mechanism for the reaction between ethanal and KCN in the presence of HCN

Explain why the product of the reaction between ethanal and KCN in HCN is optically inactive

• The CN- nucleophile can attack the carbon atom in the C=O bond from either face with equal probability.

• Forms a racemic mixture with both enantiomers forming in 1:1

• Equal rotation of plane polarised light clockwise and anticlockwise so cancel each other out

Why is KCN in HCN or KCN in acid a more preferable reaction mixture to pure HCN

Pure HCN is an extremely toxic gas - KCN is a toxic solid so is easier to handle

What can be used to test for an aldehyde or ketone containing a CH3COR group

An iodoform reaction

How do you carry out the iodoform reaction

Iodine solution is added to a small amount of aldehyde or ketone, followed by just enough sodium hydroxide solution to remove the colour of the iodine

Explain why the product of the reaction between propanone and KCN in HCN is optically inactive

There is no chiral carbon so not optically active

State the observation made and the species responsible when aldehydes and ketones containing a CH3COR group are reacted with iodine in presence of sodium hydroxide

Straw yellow precipitate of CHI3 (s)

What does the first stage of the iodoform reaction involve

• the substitution of all three hydrogens in the methyl group by iodine atoms

• The presence of hydroxide ions is important for the reaction to happen

Write an equation for the first step of the reaction of the reaction of iodine with propanone in the presence of hydroxide ions

CH3COCH3 +3I2 + 3OH- → CI3COCH3 + 3I- + 3H2O

What is involved in the second stage of the iodoform reaction

The bond between the Cl3 and the rest of the molecule is broken to produce triiodomethane (iodoform) and the salt carboxylic acid

Write an equation for the second stage of the reaction between iodine with propanone in the presence of hydroxide ions

CI3COCH3 +OH- → CHI3 + CHCOO-

Write the overall equation for the reaction of iodine with propanone in the presence of hydroxide ions

CH3COCH3 + 3I2 + 4OH- → CHI3 + CH3COO- + 3I- +3H2O

What is compound is used to test for the carbonyl function

• Brady’s Reagent

• 2,4 - DNPH

• 2,4 - dinitrophenylhydrazine

State the observation made when aldehydes and ketones are reacted with 2,4-DNPH

Orange solution forms yellow precipitate

Which is more reactive between aldehydes and ketones and why

• The more polar the C=O bond, the more susceptible the carbon atom in the C=O bond in a carbonyl compound is to attack from a nucleophile.

• Ketones contain two alkyl groups with positive inductive effects (electron pushing)

• Aldehydes contain only one only one alkyl group

• More positive inductive effect means less δ+ the carbon atom so less polar the C=O and less susceptible to nucleophilic attack

What effect do chlorines have on the reactivity of aldehydes

• As the number of adjacent chlorine atoms increases, the magnitude of the negative inductive effect (electron pulling) increases caused by the electronegative chlorines

• Electron density pulled away from the C=O bond making it more polar so more susceptible for nucleophilic attack so more reactive