6.1.2: Carbonyl compounds Flashcards
Carbonyl group
C=O
Physical properties of carbonyl compounds
• Permanent dipole (C = δ+, O = δ-)
• London forces between molecules
⟶ Cannot hydrogen bond (because O not bonded to H)
Carbonyls have higher/lower boiling points than alkanes because?
Higher bp that alkanes
• Due to permanent dipole on the C=O
Carbonyls are soluble in polar/non-polar solvents?
soluble in POLAR solvents e.g. water
∴ insoluble in organic solvents
How can carbonyls be synthesised?
• By oxidation of alcohols
⟶ Partial oxidation of 1° alcohol = aldehyde
⟶ Oxidation of 2° alcohol = ketone
Oxidising agent is oxidation of alcohols
Acidified potassium dichromate (K₂Cr₂O₇/H₂SO₄)
Redox equation
14H⁺ + Cr₂O₇²⁻ + 6e- ⟶ 2Cr³⁺ + 7H₂O
Aldehyde functional group
R–C = O
–H
Ketone functional group
R–C = O
–R
Carbonyls readily undergo what type of reaction
Nucleophilic addition
How to get alcohol from aldehyde or ketone?
- Reduce aldehyde/ketone
* Mild reducing agent = NaBH₄ (sodium borohydride)
Tests for aldehydes and ketones
- Brady’s reagent (2,4-dinitrophenylhydrazine)
* Tollen’s (silver mirror) test
Describe Brady’s reagent test (to detect presence of carbonyl group)
1) Add an excess of 2,4-dinitrophenylhydrydrazine (2,4-DNPH) to a few drops of the unknown substance
2) Formation of yellow-orange precipitate confirms presence of carbonyl group
How could you use the Brady’s reagent test to identify which ketone/aldehyde is present?
1) Excess of 2,4-DNPH with few drops of unknown
2) Filter the impure precipitate (the 2,4-DNPH derivative)
3) Recrystallise the solid to produce a pure sample of crystals
4) Measure the melting point of the crystals and compare to a data book. The aldehyde/ketone with a melting point closest to the crystals is likely the one present
Describe Tollen’s regent test
1) Combine equal volumes of unknown and Tollen’s reagent
2) Warm and swirl test tube containing mix
3) Silver mirror forms with aldehyde, NOT with ketone
Describe how to make Tollen’s reagent
Make fresh from aqueous silver nitrate, aqueous sodium hydroxide and ammonia solution
Tollen’s reagent is also known as
ammoniacal silver nitrate
Test to confirm the presence of a carbonyl group
Brady’s reagent
Test to confirm the presence of an aldehyde
Tollen’s reagent
Test for OH group
- Add PCl₅
* White fuming clouds of HCl will form
Describe reaction between NaBH₄ (mild reducing agent) and aldehyde/ketone
- Nucleophilic addition
- The hydride (H⁻) ions from the NaBH₄ are the nucleophilic
- Nucleophilic attack at the δ⁺ of the carbon of the carbonyl
- Intermediate picks up H⁺ from solvent H₂O molecule
Describe nucleophilic addition mechanism
- Hydride ion provided by NaBH₄ reducing agent has a lone pair of electrons
- The C=O in …. is polar, leading to the formation of a δ⁺ on the carbon
- Hydride ion is attracted to this δ⁺, donating its lone pair of electrons to the relevant carbon atom
