6.1.2: Carbonyl compounds

Question 1

Carbonyl group

Answer 1

C=O

Question 2

Physical properties of carbonyl compounds

Answer 2

• Permanent dipole (C = δ+, O = δ-)
• London forces between molecules
⟶ Cannot hydrogen bond (because O not bonded to H)

Question 3

Carbonyls have higher/lower boiling points than alkanes because?

Answer 3

Higher bp that alkanes

• Due to permanent dipole on the C=O

Question 4

Carbonyls are soluble in polar/non-polar solvents?

Answer 4

soluble in POLAR solvents e.g. water

∴ insoluble in organic solvents

Question 5

How can carbonyls be synthesised?

Answer 5

• By oxidation of alcohols
⟶ Partial oxidation of 1° alcohol = aldehyde
⟶ Oxidation of 2° alcohol = ketone

Question 6

Oxidising agent is oxidation of alcohols

Answer 6

Acidified potassium dichromate (K₂Cr₂O₇/H₂SO₄)

Question 7

Redox equation

Answer 7

14H⁺ + Cr₂O₇²⁻ + 6e- ⟶ 2Cr³⁺ + 7H₂O

Question 8

Aldehyde functional group

Answer 8

R–C = O

–H

Question 9

Ketone functional group

Answer 9

R–C = O

–R

Question 10

Carbonyls readily undergo what type of reaction

Answer 10

Nucleophilic addition

Question 11

How to get alcohol from aldehyde or ketone?

Answer 11

• Reduce aldehyde/ketone

* Mild reducing agent = NaBH₄ (sodium borohydride)

Question 12

Tests for aldehydes and ketones

Answer 12

• Brady’s reagent (2,4-dinitrophenylhydrazine)

* Tollen’s (silver mirror) test

Question 13

Describe Brady’s reagent test (to detect presence of carbonyl group)

Answer 13

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

Question 14

How could you use the Brady’s reagent test to identify which ketone/aldehyde is present?

Answer 14

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

Question 15

Describe Tollen’s regent test

Answer 15

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

Question 16

Describe how to make Tollen’s reagent

Answer 16

Make fresh from aqueous silver nitrate, aqueous sodium hydroxide and ammonia solution

Question 17

Tollen’s reagent is also known as

Answer 17

ammoniacal silver nitrate

Question 18

Test to confirm the presence of a carbonyl group

Answer 18

Brady’s reagent

Question 19

Test to confirm the presence of an aldehyde

Answer 19

Tollen’s reagent

Question 20

Test for OH group

Answer 20

• Add PCl₅

* White fuming clouds of HCl will form

Question 21

Describe reaction between NaBH₄ (mild reducing agent) and aldehyde/ketone

Answer 21

• 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

Question 22

Describe nucleophilic addition mechanism

Answer 22

• 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