Carbonyls- Aldehydes And Ketones Flashcards
What is an aldehyde?
Molecules with a carbonyl group (C=O) at the end of the carbon chain
Functional group CHO
Eg ethanal CH3CHO
What is a ketone?
A molecule with a C=O carbonyl group in the middle of the molecule
Eg propanone CH3COCH3
How are aldehydes formed?
Via the oxidation of primary alcohols (but not refluxed)
How are ketones formed?
Via the oxidation of secondary alcohols
What happens when you oxidise an aldehyde?
Aldehydes can be oxidised to form a carboxylic acid
Heat with acidified potassium dichromate and refluxed
What are some physical properties of carbonyls?
Cannot form intermolecular hydrogen bonds as all the hydrogen atoms are bonded to a carbon atom
Can form hydrogen bonds with water molecules making aldehydes and ketones soluble in polar solvents (short chain)
There are permanent dipole dipole interactions due to the bond polarity of the C=O bond
What addition reactions do carbonyls under go?
What other kind of reaction is it?
Due to carbonyls having a dipole they are susceptible to attack from nucleophiles (nucleophilic addition)
Also a reduction reaction
Aldehydes form a primary alcohol
Ketones form a secondary alcohol
What nucleophile is used to reduce aldehydes and ketones? And what is the source of these nucleophiles?
Reduced via hydride ions
Source from sodium tetrahydridoborate (III)
NaBH4
What is the purpose of using cyanide nucleophiles?
Enables us to increase carbon chain length
What is formed when we mix a carbonyl with a cyanide nucleophile?
Forms a hydroxynitrile
This uses NaCN or KCN as the source of cyanide ions and mixed with dilute acid as a source of H+ ions
Hydroxy nitriles have stereoisomers for all asymmetric ketones and all aldehydes other than methanal
Why is hydrogen cyanide not used?
This is a highly toxic gas and is not easily contained in a lab
What is the test for a carboxylic acid?
Add sodium carbonate and look for effervescent
If CO2 is produced then carboxylic acid is present
What tests are used to distinguish between aldehydes and ketones?
Fehlings
Tollens
What is the fehlings test?
Fehlings solution
produces a colour change from blue to brick red with aldehydes
Not result with tollens
What is the tollens test
When warmed with aldehyde they produce a silver mirror as the aldehydes become oxidised and silver ions a reduced to metallic silver
Ketones give no reaction to this test
