Chemistry A2 Chapter 26 - Carbonyls and Carboxylic Acids Flashcards
What functional group do ketones and aldehydes both have?
Carbonyl - A carbon which has a double bond to an oxygen
Where is the carbonyl functional group found in an aldehyde?
At the end of a carbon chain. The carbon atom is attached to one oxygen (double bond) and hydrogen (single bond)
Where is the carbonyl functional group found on a ketone?
In the middle of the chain
What are the suffixes for aldehydes and ketones?
Aldehyde -al
Ketone -one
What are aldehydes oxidised to, how, and what is the catalyst?
Carboxylic acids
When refluxed
With acidified potassium dichromate and dilute sulfuric acid
What are ketones oxidised to?
They can’t be oxidised!
What is the difference in the double bonds in an alkene and a carbonyl compound?
Alkene - Carbon-carbon double bond is non-polar
Carbonyl compound - Carbon-oxygen double bond is polar
Define the term nucleophile
A nucleophile is attracted to and attacks the slightly positive carbon atom resulting in addition across the double C-O bond (electron pair donator)
What is the mechanism by which an aldehyde/ketone reacts with a nucleophile?
Nucleophillic addition
What substance is a reducing agent for aldehydes and ketones?
NaBH4 - reduces aldehydes and ketones to alcohols
What are aldehydes reduced to?
Primary alcohols by NaBH4
What are ketones reduced to?
Secondary alcohols by NaBH4
Why is the reaction of hydrogen cyanide useful?
It provides a mean of increasing the length of the carbon chain
Why is hydrogen cyanide dangerous?
It is a colourless, extremely poisonous liquid and boils slightly above room temperature so cannot be used safely in an open laboratory
What two substances react to produce hydrogen cyanide?
Sodium cyanide and sulfuric acid
Describe a nucleophillic addition reaction with NaBH4 in 4 steps (NaBH4 contains the hydride ion)
- The lone pair of electrons from the hydride ion is attracted to and donates its electron pair to the delta positive carbon in the aldehyde or ketones double C-O bond
- A dative covalent bond is formed between the hydride ion and the carbon atom
- The pi-bond in the double C-O bond breaks by heterolytic fission, forming a negatively charged intermediate
- The oxygen atom of the intermediate donates a lone pair of electrons to a delta positive hydrogen atom in a water molecule. The intermediate has then been protonated to form an alcohol
Describe the mechanism for a aldehyde or ketone reacting with NaCN/H+ (the cyanide ion attacks the electron deficient carbon in the aldehyde or ketone)
- The lone pair of electrons from the cyanide ion is attracted to and donates its electron pair to the delta positive carbon atom in the aldehyde or ketone. A dative covalent bond forms
- The pi-bond in the double C-O bond breaks by heterolytic fission, forming a negatively charged intermediate
- The intermediate is protonated by donating a lone pair of electrons to a hydrogen ion
- The product formed is a hydroxynitrile
How is the presence of a carbonyl functional group detected using a chemical test?
2,4-DNP, also known as Brady’s reagent, is used to detect the functional group. In the presence of a carbonyl function group, a yellow or orange precipitate called 2,4-dinitrophenylhydrazone is produced
How do we distinguish between aldehydes and ketones?
Once a compound has been identified as containing a carbonyl group using 2,4-DNP, a fresh sample of the compound can have Tollen’s reagent added (a solution of silver nitrate in aqueous ammonia) to it. In the presence of an aldehyde, a silver mirror is produced, if nothing occurs then the compound is a ketone
Why does a ketone not produce a silver mirror?
Tollens’ reagent contains silver ions which act as an oxidising agent in the presence of ammonia. Ketones cannot be oxidised and therefore do not produce a silver mirror
