6.1.2 Carbonyls

Question 1

Functional groups in carbonyls

Answer 1

• Aldehyde -CHO
• Ketone
  C=O
• The C with the double bond O is electron deficient so attracts nucleophiles

Question 2

Shape around the carbon

Answer 2

• bond angle 120
• trigonal planar
• 3 bonding pairs of e- which repel equally

Question 3

Why is the carbonyl bond polar?

Answer 3

• oxygen is more electronegative than carbon

Question 4

How are sigma bonds made?

Answer 4

• orbital are adjacent C=O atoms overlap directly head on between the atoms

Question 5

How are pi bonds made?

Answer 5

• P orbitals on the adjacent C=O atoms overlap sideways above and below the atoms

Question 6

Making carbonyls by partial oxidation of alcohols

• oxidising agent
• colour change
• products
• equation
• apparatus

Answer 6

• Acidified dichromate Cr2O7 2-/H+ or Na2Cr2O7 in H2SO4 [O]
• orange to green Cr2O7 2- (aq) -> Cr3+ (aq)
• Oxidise primary alcohol to make aldehyde and H2O using distillation
• oxidise secondary alcohols to ketones to make ketone and H2O using distillation
• the aldehyde has a lower boiling point than the water and the alcohol because the aldehyde has weaker permanent dipoles whilst water and alcohols have H bonds which are the strongest IMF and need the most energy to overcome

Question 7

Complete oxidation of primary alcohol using reflux

Answer 7

Makes carboxylic acid and water
- use 2[O]

• reflux is the continuous evaporation and condensation of a reaction mixture, allowing the chemicals to react completely without escaping

Question 8

Oxidation of aldehydes

Answer 8

• aldehydes oxidised into carboxylic acids
• with Tollens (mild) - silver mirror made
• or with acidified dichromate (strong) - orange to green colour change

Question 9

Reduction definition

Answer 9

The gain of hydrogen, the loss of oxygen

Question 10

Reduction of aldehydes and ketones

Answer 10

• mechanism is nucleophilic addition
• reducing agent in NaBH4 in water 2[H]
• aldehydes become primary alcohols
• ketones become secondary alcohols
• for every C=O present in a molecule, you need 2 moles of [H]

Question 11

Testing for a carbonyl bond

Answer 11

• Add a few drops of 2,4 DNPH (Brady’s)
• Orange yellow crystalline solid forms (hydrazone precipitate - must refer by this name at least once)
• Indicating presence of C=O bond

Question 12

Finding exact identity of carbonyl

Answer 12

• Purify the hydrazone precipitate by filtering and recrystallisation
• determine the unique melting point, and compare in a database