6.1.7: Reactions of carbonyl compounds

Question 1

What is a carbonyl functional group?

Answer 1

C=O

• If the functional group is on the end carbon atom, an aldehyde is formed.
• If the functional group is not at the end of a carbon chain, then a ketone is formed.

Question 2

What do aldehydes form when they become oxidised?

Answer 2

A carboxylic acid

Question 3

What are the reagents needed for the oxidation of an aldehyde and what do these reagents react in situ to form?

Answer 3

• Potassium dichromate, K2Cr2O7
• Sulfuric acid H2S04
• Reagents react in situ to form the oxidising species Cr2O7-, and H+.

Question 4

What is the balanced ionic equation for the oxidation of ethanal to ethanoic acid?

Answer 4

3CH3CHO(l) +Cr2O7-(aq) +8H+(aq) –>

3CH3COOH(aq) + 2Cr3+(aq) + 4H2O(l)

Question 5

What is the simplified balanced symbol equation for the oxidation of ethanal to ethanoic acid once the oxidising agent has been simplified to [O]?

Answer 5

CH3CHO + [O] –>CH3COOH

Question 6

In a laboratory, how is an aldehyde oxidised to a carboxylic acid and what colour change will be observed?

Answer 6

• The reaction mixture is heated gently under reflux.
• As the reaction proceeds, a colour change is observed, from orange to green, due to the oxidation state of chromium changing.

Question 7

What makes carbonyl compounds susceptible to nucleophilic attack?

Answer 7

• They have a dipole in the C=O functional group.

- This makes them susceptible to attack on the delta -ve carbon atom.

Question 8

What happens when A nucleophile donates a lone pair of electrons to the electron deficient carbon in a carbonyl compound?

Answer 8

• Simultaneously, the pi-bond in the C=O breaks and a reactive intermediate is formed.
• The extra electron pair is is equally quickly donated to the neighbouring atom to form an alcohol group and the stable product.

Question 9

Draw a general mechanism for nucleophilic addition on a carbonyl compound.

Answer 9

Check in book

Question 10

What is sodium tetrahydridoborate(III) more commonly known as?

Answer 10

sodium borohydride

formula: NaBH4

Question 11

What is sodium tetrahydridoborate(III) and what is it used for?

Answer 11

• A reducing agent commonly used in organic synthesis.

- This compound is made of a BH4- ion, which acts as a source of hyride ions, H-.

Question 12

What is the hydride ion from sodium tetrahydridoborate(III) involved in?

Answer 12

The hyride ion is commonly used in the electrophilic addition and reduction of carbonyl compounds to alcohols.

Question 13

What do aldehydes reduce to?

Answer 13

primary alcohols

Question 14

What do ketones reduce to?

Answer 14

secondary alcohols

Question 15

What is the simplified balanced chemical equation for the reduction of pentan-2-one, using [H] as the reducing agent?

Answer 15

CH3COC3H7 + 2[H] –> CH3CH(OH)C3H7

Question 16

Draw the mechanism for the reduction of an aldehyde (propanal) by nycleophlic addition.

Answer 16

Check in book

Question 17

What is hydrogen cyanide?

Answer 17

-A weak acid that will partially ionise in solution.

Question 18

What happens when hydrogen cyanide partially ionises in water and what is the balanced symbol equation for this?

Answer 18

• A cyanide nucleophile with a negative charge on the carbon atom is formed.
• HCN + H2O CN- + H3O+

Question 19

What are other sources of cyanide (not hydrogen cyanide)?

Answer 19

sodium cyanide, NaCN

Question 20

The cyanide cannot react directly with carbonyl compounds. However, what happens when the reaction is acidified?

Answer 20

The carbonyl functional group becomes more reactive as polarity of the C=O bond increases.

Question 21

Draw the mechanism for the reaction of hydrogen cyanide with propanone.
In the second step, where could the hydrogen also be obtained from?

Answer 21

• See book for mechanism

- Another small molecule in the reaction mixture such as ehanol.

Question 22

What does the addition of a cyanide to an organic molecule allow?

Answer 22

-The addition of cyanide allows more carbon atoms to be added to the organic molecule.

Question 23

Which important bi-product is produced as a result of the reaction to add more carbon atoms to an organic molecule?

Answer 23

-In this reaction, hydroxynitrile is formed, which is an important chemical used in many industrial processes. `

Question 24

Why is the reaction between hydrogen cyanide and propanone completed in solution with an acidity no lower than pH4?

Answer 24

Because the addition of acid drives the cyanide equilibrium to the left and reduces the amount of nucleophiles.

Question 25

What is a nucleophile?

Answer 25

A species attracted to an electron-deficient part of a molecule, where it donates a pair of electrons to make an new covalent bond.