Chapter 26 - Compounds Containing the Carbonyl Group Flashcards
What can aldehydes form when oxidised?
Carboxylic acids
Can ketones be further oxidised?
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
What can aldehydes and ketones be reduced to?
Primary and secondary alcohols, but also hydroxynitriles by cyanides
What is the mechanism of a reaction between a ketone or aldehyde and sodium tetrahydridoborate (NBH4)?
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
What cyanide compound is used to form hydroxynitriles?
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
What type of compound do aldehydes and unsymmetrical ketones form when they react to form a hydroxynitrile?
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
Although carboxylic acids are weak acids, what are they strong enough to liberate?
Carbon dioxide from carbonates (this leads to their test being sodium hydrogen carbonate
What are important uses of esters?
Solvents, plasticisers, perfumes and food flavourings
What is an ester formed from, and under what conditions?
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
What are triglycerides an example of?
Esters of the molecule propane-1,2,3-triol, or glycerol
What can triglycerides be turned into and what is their usage?
They can be hydolysed in alkaline conditions to regenerate the glycerol and also give soaps (or salts of long chain carboxylic acids)
What is biodiesel?
A mixture of methyl esters and long chain carboxylic acids
How do you form biodiesels?
By recting methanol with vegetable oils in the presence of a catalyst
What is the general formula of an acyl group?
RCOZ (where Z is another, typically non-carbon, group
What are the three acid derivatives formed from carboxylic acids?
Esters (RCOOR’), acid chlorides (RCOCl) and acid anhydrides (RCOOCR’)
What is acylation and how does it occur?
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
What is the structure of an acid anhydride?
It is 2 carbonyl carbon which are both connected to the same carbon through single bonds
What is the structure of an acid chloride?
It is a carbonyl carbon with an R group and a Cl and then end of the chain
Which are the two strongest “leaving” or “Z” groups, and why?
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
What are the strongest nucleophiles, in order?
primary amines (NH2R) > ammonia (NH3) > Alcohols (ROH) > Water (H2O)
In which mechanism do acyl groups react with nucleophiles?
Through addition-elimination
How do addition - elimination reactions take place?
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
Why, despite ethanoyl chloride being more reactive, is ethanoic anhydride typically used in addition-elimination reactions instead?
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
What is the structure of amides?
O = C - N
