Carbonyl compounds- ketones and aldehydes

Question 1

What is the function group of aldehydes?

Answer 1

-CHO

Question 2

Where is the functional group of aldehydes normally found?

Answer 2

At the end of the carbon chain/ end of molecule.

Question 3

What is the name of an aldehyde with 5 carbons?

Answer 3

Pentanal.

Question 4

What is the functional group of a ketone?

Answer 4

C=O

Question 5

Where is the functional group of a ketone found?

Answer 5

It is found on any inner carbon and never at the end of a molecule.

Question 6

What is the name of a ketone with 6 carbons?

Answer 6

Hexanone.

Question 7

What is included in the name of a ketone?

Answer 7

A number is included to indicate where the C=O functional group is found. For example, in butan-2-one, the C=O group is found on the second carbon.

Question 8

Generally, what occurs when aldehydes are oxidised?

Answer 8

When aldehydes are oxidised, they form carboxylic acids.

Question 9

Generally, what occurs when ketones are oxidised?

Answer 9

Ketones cannot oxidise, so they have no reaction and hence show no change in observation.

Question 10

How can we distinguish between aldehydes and ketones?

Answer 10

As aldehydes oxidise and show an observation, we can use this observation as a sign of the presence of an aldehyde. Ketones do not oxidise, so show no observation change.

Question 11

An example of an oxidising agent is Tollen’s reagent. What is observed when aldehydes are oxidised with this?

Answer 11

Tollen’s reagent is ammoniacal silver nitrate. When aldehydes are oxidised with this, a silver mirror (precipitate) is formed on the reaction vessel. The mixture should be warm.

Question 12

Another oxidising agent is acidified potassium dichromate. What is observed when this

Answer 12

Acidified potassium dichromate must react with aldehydes under reflux conditions. It is observed that the mixture turns from orange to green.

Question 13

What is the purpose of reacting something under reflux conditions?

Answer 13

In reflux, the apparatus is a round-bottom flask and connected is a Liebig condenser, that is vertically upright. The top of the Liebig condenser is open (prevents he condenser from exploding and shattering from pressure). When the mixture is heated, it starts to evaporate. Heating the mixture provides a faster reaction rate. The mixture starts to heat up and evaporate, then reaching the condenser, it condenses and turns to a liquid, returning to the flask. This allows something to constantly react, without anything escaping as a gas.

Question 14

In the reduction of an aldehydes what is formed? In the reduction of a ketone what is formed?

Answer 14

When an aldehyde is reduced, a primary alcohol is formed. When a ketone is reduced, a secondary alcohol is formed.

Question 15

What reducing agent is used to reduce aldehydes and ketones?

Answer 15

Sodium borohydride, NaBH4. After add dilute HCl to finish.

Question 16

What reducing agent is used to reduce carboxylic acids?

Answer 16

Lithium aluminium hydride. LiAlH4.

Question 17

What mechanism occurs in the reduction of aldehydes and ketones? Why does this occur?

Answer 17

Nucleophilic addition.
In nucleophilic addition, a nucleophile (electron pair donor) attacks an electron deficient substance.
In a carbonyl compound, the C=O bond is polar, where carbon is delta positive and hydrogen is delta negative. Hence, here the carbon is electron deficient. This means that the carbon is exposed to attack from nucleophiles. When sodium borohydride is added to a ketone or aldehyde, the NaBH4 gives a source of hydride ions (negative hydrogen ions) which acts as nucleophiles. This initiates the mechanism.

Question 18

What reagent is used to test for a C=O group in carbonyl groups?

Answer 18

Brady’s reagent which is 2,4-dinitrophenylhydraze (2,4-DNP). When Brady’s reagent reacts with ketones and aldehydes, a bright orange precipitate is formed. Carboxylic acids do not react with Brady’s reagent.

Question 19

What are hydroxy nitriles?

Answer 19

Hydroxy nitriles are compounds containing both -OH and -CN groups.

Question 20

How are hydroxy nitriles formed from carbonyl compounds?

Answer 20

React carbonyl compounds react with potassium cyanide (or hydrogen cyanide) to form hydroxy nitriles.

Question 21

How is potassium cyanide used in everyday life?

Answer 21

Organic synthesis.
Electroplating.
Gold mining.

Question 22

What mechanism is undergone when potassium cyanide reacts with a carbonyl compound?

Answer 22

Nucleophilic addition.

Question 23

What is step 1 of nucleophilic addition mechanism when potassium cyanide reacts with a carbonyl compound?

Answer 23

Formation of nucleophile
After KCN is added to solution containing carbonyl compound, dilute HCl is added. The HCl dissolves the KCN and causes it to dissociate into K+ and CN-. Hence, the nucleophile is formed- CN-.

Question 24

What occurs in step 2 of the nucleophilic addition mechanism when KCN reacts with a carbonyl compound?

Answer 24

The CN- nucleophile attacks the electron deficient carbon in the C=O bond. The CN donates its electrons to the carbon and forms a bond with the carbon. The carbon now has too many bonds, so carbon breaks one of its bonds with the oxygen via heterolytic bond fission. The oxygen has now received a pair of electrons and now has a fully negative charge. This negative charge attracts positive hydrogen ions in solution (from the HCl acid) and forms a bond with a hydrogen. This creates a hydroxyl group. Now the compound has a CN group and OH group.

Question 25

What are the risks of using KCN? Why not use HCN? (when reacting with carbonyl compounds to form hydroxy nitriles)

Answer 25

KCN is an irritant if inhaled or ingested. When KCN reacts with moisture, it forms toxic hydrogen cyanide.
If we use HCN, there is already a supply of hydrogen ions, hence, an acid is not necessary. However, HCN has higher risks than KCN as well as being harder to maintain and control.