6B - Carbonyls

Question 1

What is the carbonyl functional group?

Answer 1

C = O

Question 2

Name the 2 main important carbonyl compounds that you need to know

Answer 2

Aldehydes and ketones

Question 3

How do you test for the presence of a carbonyl group?

• Method (5)
• Any hazards?

Answer 3

2,4-DNPH (Brady’s reagent - but don’t call it this in the exam):

• Gives a result = Aldehyde or ketone
• Solid 2,4-DNP is VERY hazardous and friction can cause it to explode ∴ it’s dissolved into H₂SO_2(aq) to form the orange solution of 2,4-DNPH
• 2,4-DNPH is corrosive and toxic by skin contact

Method:

1. 5cm³ depth of 2,4-DNPH into a clean test tube
2. Use dropping pipette, add 5 drops of unknown into the test tube
3. Stir w/ glass rod
4. Usually takes approx 1 min at room temp. If no crystals form add a few drops of H₂SO_4(aq)
5. Positive test:
   
   • Orange solution → formation of orange precipitate (needle like)

Question 4

How do you distinguish between an aldehyde and ketone? (6)

Include oxidation and reduction equations (2)

Answer 4

Tollens’ Reagent:

1. Make Tollens’ reagent:
   
   1. Add 5cm³ depth of AgNO_3(aq) into test tube
   2. Add dilute NaOH_(aq) until brown precipitate forms then just dissolves to give a colourless solution - This is the Tollens’ Reagent
2. Actual Tollens’ reagent test:
   
   1. 1cm³ depth of unknown into a clean test tube
   2. Equal volume of freshly prepared Tollens’ reagent
   3. Put test tube into hot water bath (kettle + beaker)
      
      • Approx 50ºC for 10-15 mins
   4. Positive test for aldehyde = silver mirror forms

RED: Ag⁺_(aq) + e^- → Ag_(s)

OXI: R-CHO + 2OH^-_(aq) → R-COOH + 2H₂O

Question 5

How do you identidy aldehydes and ketones by melting point? (7)

Answer 5

Orange precipitate formed from 2,4-DNPH test can be analysed

1. React unknown with 2,4-DNPH to form the hydrazone
2. Impure orange precipitate is filtered off from solution. Wash with distilled water
3. Redissolve crystals in a minimum volume of hot solvent, then cool on ice to recrystallise
4. Fiter again, wash again with distilled water
5. Dry sample (pure)
6. Melt pure sample
7. Compare melting point to database + match up with compound

Question 6

Name the different reactions that carbonyl compounds undergo?

Answer 6

• Oxidation (aldehydes only)
• Nucleophilic addition:
  
  • Form alcohols with NaBH₄
  • Form hydroxynitriles with HCN (NaCN / H⁺)
• Nucleophilic substitution of HCN
• Acid / alkaline hydrolysis

Question 7

Define nucleophile

Answer 7

Nucleophile = An atom or group of atoms which is attracted to an electron deficient centre or atom, where it donates a pair of electrons to form a new covalent bond

Question 8

Why do carbonyl compounds undergo nucleophilic addition reactions and doesn’t behave like an alkene?

Answer 8

• C = O is polar where as C = C isn’t
• Oxygen is much more electronegative than carbon ∴ the electron density of the π bond is distorted towards the oxygen atom in the C = O bond

Question 9

What is the full name of NaBH₄?

Answer 9

Sodium tetrahydridoboreate (III)

Question 10

NaBH₄ Reducing aldehydes:

• General equation
• Use the example of propanal to illustrate the mechanism

Answer 10

Question 11

NaBH₄ Reducing ketones:

• General equation
• Use the example of propanone to illustrate the mechanism

Answer 11

Question 12

HCN + Aldehydes:

• Reagents
• General equation
• Illustrate mechanism using the example: CH₃CH₂CHO + HCN
• What acts as the nucleophile?

Answer 12

• R = HCN (provided by H₂SO₄ and NaCN)
• Nucleophile = ^-CN ion

Question 13

HCN + Ketones:

• Reagents
• General equation
• Illustrate mechanism using the example: CH₃COCH₃ + HCN
• What acts as the nucleophile?

Answer 13

• R = HCN (Provided by H₂SO₄ and NaCN)
• Nucleophile = CN^-
• Both HCN and CN^- have a lone pair meaning theoretically they can both act as nucleophiles BUT N is very electronegative ∴ attracts its lone pair very strongly ∴ awful nucleophile
• Lone pair on carbon is not as strongly attacted to carbon compared to the lone pair on nitrogen to nitrogen ∴ this lone pair gives up more easily ∴ CN^- is a better nucleophile

Question 14

Why must cyanide reactions must be carried out in aqueous conditions?

Answer 14

Cyanide is toxic otherwise!

Question 15

List 3 ways to extend the carbon chain?

Answer 15

• Acid hydrolysis of nitriles
• Reduction of nitriles
• Nucleophilic substitution of CN^-

Question 16

Acid hydrolysis of nitriles to extend the carbon chain:

• Reagents
• Conditions
• General equation

Answer 16

• R = Nitrile
• C = Heat with dilute aqueous acid e.g. HCl_(aq)

Question 17

Reduction of nitriles to extend the carbon chain:

• Reagents
• Conditions
• General equation

Answer 17

• R = Nitrile
• C = Nickle catalyst

Question 18

Why is nucleophilic substitution of CN^- on haloalkanes useful in organic synthesis?

Answer 18

• Method of extending carbon chain
• Nucleophilic addition of CN^- always has a OH group on the carbon that CN is added onto. Don’t always want a OH groups there and it’s difficult to get rid of
• This method doesn’t result in any OH group being present!

Question 19

Nucleophilic substitution of CN^-on haloalkanes to extend the carbon chain:

• Why is it useful?
• Reagents
• Conditions
• Nucleophile = ?
• Illustrate the mechanism using the example: CH₃CH₂CH₂Cl

Answer 19

• Useful because no OH group is found on carbon that CN is added on to
• R = Haloalkane, CN^- ions (provided by NaCN or KCN)
• Conditions = Cyanide ions dissolved in ethanol
• Nucleophile = CN^-

Question 20

Describe a series of test to help distinguish between:

• Aldehyde
• Carboxylic acid
• Ester
• Ketone
• Primary alcohol

Answer 20