Chapter 26 - Compounds Containing the Carbonyl Group

Question 1

What can aldehydes form when oxidised?

Answer 1

Carboxylic acids

Question 2

Can ketones be further oxidised?

Answer 2

Only very rarely, in the presence of a very strong oxidising agent as it would require the breaking of a C-C bond to form a carboxylic acid

Question 3

What can aldehydes and ketones be reduced to?

Answer 3

Primary and secondary alcohols, but also hydroxynitriles by cyanides

Question 4

What is the mechanism of a reaction between a ketone or aldehyde and sodium tetrahydridoborate (NBH4)?

Answer 4

The NBH4 is a source of negative hydrogen ions (hydride ions), which attack the partially positive carbon as the double bond goes to the oxygen to make a negatively charged oxygen ion on the carbonyl carbon, which has now gained a hydrogen atom. This oxygen ion will react with a proton in solution to form an alcohol group. This reaction takes place in aqueous solution

Question 5

What cyanide compound is used to form hydroxynitriles?

Answer 5

KCN, because it is fairly readily available, and it is significantly less toxic than HCN. Furthermore, KCN releases a lot more free CN- groups than HCN does. However in using KCN you need to add a dilute acid to provide a source of H+ ions

Question 6

What type of compound do aldehydes and unsymmetrical ketones form when they react to form a hydroxynitrile?

Answer 6

An enantiomer, because the old carbonyl carbon becomes a chiral carbon. This is because the carbonyl carbon was in a trigonal planar shape, allowing the cyanide ion to attack from either the bottom or the top

Question 7

Although carboxylic acids are weak acids, what are they strong enough to liberate?

Answer 7

Carbon dioxide from carbonates (this leads to their test being sodium hydrogen carbonate

Question 8

What are important uses of esters?

Answer 8

Solvents, plasticisers, perfumes and food flavourings

Question 9

What is an ester formed from, and under what conditions?

Answer 9

Esters are formed from an alcohol and a carboxylic acid, and usually require the presence of a strong acid, such as sulfuric acid to act as a catalyst

Question 10

What are triglycerides an example of?

Answer 10

Esters of the molecule propane-1,2,3-triol, or glycerol

Question 11

What can triglycerides be turned into and what is their usage?

Answer 11

They can be hydolysed in alkaline conditions to regenerate the glycerol and also give soaps (or salts of long chain carboxylic acids)

Question 12

What is biodiesel?

Answer 12

A mixture of methyl esters and long chain carboxylic acids

Question 13

How do you form biodiesels?

Answer 13

By recting methanol with vegetable oils in the presence of a catalyst

Question 14

What is the general formula of an acyl group?

Answer 14

RCOZ (where Z is another, typically non-carbon, group

Question 15

What are the three acid derivatives formed from carboxylic acids?

Answer 15

Esters (RCOOR’), acid chlorides (RCOCl) and acid anhydrides (RCOOCR’)

Question 16

What is acylation and how does it occur?

Answer 16

Acylation is when a nucleophile attacks the C on the acyl group due to its partially positive charge. The nucleophile replaces the Z group and therefore becomes acylated due to its gain of an acyl group

Question 17

What is the structure of an acid anhydride?

Answer 17

It is 2 carbonyl carbon which are both connected to the same carbon through single bonds

Question 18

What is the structure of an acid chloride?

Answer 18

It is a carbonyl carbon with an R group and a Cl and then end of the chain

Question 19

Which are the two strongest “leaving” or “Z” groups, and why?

Answer 19

Acid chlorides and then acid anhydrides are the best leaving groups, because they are able to attract the electron more strongly towards its nucleus and would form somewhat stable negative ions, making the partial positive charge on the carbonyl carbon stronger

Question 20

What are the strongest nucleophiles, in order?

Answer 20

primary amines (NH2R) > ammonia (NH3) > Alcohols (ROH) > Water (H2O)

Question 21

In which mechanism do acyl groups react with nucleophiles?

Answer 21

Through addition-elimination

Question 22

How do addition - elimination reactions take place?

Answer 22

In step one, the nucleophile uses its lone pair to bond to the carbonyl carbon, and the double bond on the oxygen breaks to form a negatively charged, single bond oxygen. In step 2 the oxygen ion will reform the double bond whilst the C - Cl bond is broken to the chlorine. In step 3, the positively charged nucleophile will also break its bond to hydrogen to form a proton and a neutrally charged molecule

Question 23

Why, despite ethanoyl chloride being more reactive, is ethanoic anhydride typically used in addition-elimination reactions instead?

Answer 23

They are cheaper to produce
The by product of ethanoic acid is safer than hydrogen chloride
It doesn’t react with water as readily
it is less corrosive

Question 24

What is the structure of amides?

Answer 24

O = C - N