Module 6 - Carbonyls Flashcards
What is the test and results for aldehydes
Tollens reagent
Silver mirror/ precipitate produced
What is the test and result for ketones
2,4-DNP
Yellow/orange/red precipitate
What is the test and observations for carboxylic acids
Sodium carbonate
Fizzing/effervescence/ bubbling
Suggest the chemical test to distinguish an aldehyde from two esters
Reagent = tollens reagent
Observation = silver mirror
Suggest the test to identify esters
H+/CrOO7^2- (acidified potassium dichromate
Orange to green (with CH3OH)
What is the carbonyl group
C=O
What type of intermolecular forces do molecules with the carbonyl group have?
Why?
Permanent dipole-dipole due to the polar C=O bind
(O is delta -ive)
How soluble are carbonyls in water?
What influences solubility?
Soluble - form hydrogen bonds between water molecules and oxygen of C=O bind
As chain length increases, solubility decreases
What is the name of the product when HCN is added to a carbonyl group
Hydroxynitriles
What is Fehling’s solution?
What colour is it?
Copper complex ions
Blue
What happens when an aldehyde is added to Fehling’s solution
Reduced to Cu+ ions —> colour change to red brick ppt
What happens when a ketone is added to Fehling’s solution
No visible change —> stays blue
How do you test for a carbonyl compound
Use Brady’s reagent - 2,4-DNP
If a carbonyl is present - an orange ppt is formed
What is tollen’s reagent
Silver complex in colourless solution
What happens when an aldehyde is added to tollens reagent
Silver mirror forms as Ag+ reduced to Ag (s)
What is a reducing agent for aldehydes sand ketones?
What ions does this release?
NaBH4 (sodium tetrahydridooborate (III))
Releases an H- ion
Why do aldehydes react with tollens reagent and not ketones
Because aldehydes are oxidised to form carboxylic acids and ketones cannot be further oxidised
Describe how you would make tollens reagent and carry out the test for aldehydes in the lab
Explain what happens to both the tollens reagent and the aldehyde
Silver nitrate + ammonia (warm/heat)
Silver mirror forms
Explanation :
Silver ions reduced Ag+ +e- = Ag
Aldehyde oxidised to carboxylic acids
Why nucleophilic addition reactions important in organic synthesis
Provides a method for increasing length of carbon chain
Describe how the use of 2-4 DNP allows a student to confirm the identity of a sample (either aldehyde or ketone)
Add 2,4-DNP, yellow precipitate formed
Recrystallise
Measure melting point
Compare to a database to identify derivative (and hence original carbonyl)
Use ideas about the bonding in alkenes and carbonyl compounds to discuss the similarities and differences in chemical properties
Similarities:
Both take part in addition reactions with Br2/HBr/H2
Addition reactions occur because Pi bond can break
Differences:
Alkenes attacked by electrophiles
Because C=C is an area of high electron density
Carbonyls attacked by nucleophiles
Because C=O is delta positive (due to differences in electro negativity)
Aldehydes can be oxidised, alkenes cannot be easily
Alkenes take part in a wider range of addition reactions than carbonyls
Pi bond is weak in alkene, stronger in carbonyl
Suggest why esters have a lower boiling point than butanoic acid
Because methyl butanoate has permanent dipole-dipole intermolecular forces and butanoic acid has hydrogen which is stronger than permanent dipole-dipole forces
