Carbonyl Compounds Flashcards
What is the carbonyl group
C=O
Why is carbonyl polarised
Oxygen is more electronegative than carbon so electrons are more attracted towards the oxygen end of the bond
Why part of the carbonyl reacts with nucleophiles
The carbon
Which part of the carbonyl reacts with electrophiles
The oxygen
Describe aldehydes
Carbonyl group with one hydrogen and one alkyl or aryl (R-CO-H). Names end in -al or -aldehyde.
What are ketones
Carbonyl group with two alkyl or aryl substituents (R-CO-R). Names end in -one
Why are aldehydes usually more reactive than ketones
Inductive effect and steric hindrance
Why does inductive effect make aldehydes more reactive than ketones
The alkyl groups are electron donating (relative to the H), reduce the +ve charge on the carbonyl carbon and make it less attractive to nucleophiles
Why does steric hindrance make aldehydes more reactive than ketones
Alkyl groups get in the way of the approaching reagent. Aldehydes have a small hydrogen atom which does not get in the way of the nucleophile, whereas ketones have two substituents which repel the nucleophile
What happens with addition of water (hydration) of carbonyls
A reversible reaction. Double bond of oxygen to carbon breaks and two OH groups form and bond to carbon. However equilibrium usually favours the starting material
Describe nucleophilic addition of alcohols
Alcohols added to carbonyls is a reversible reaction. The first product is a hemiacetal (formed under an acid or base catalyst). Under an acid catalyst only the hemiacetal reacts to give an acetal.
Describe the nucleophilic addition of alcohols to carbonyls to form hemiacetals and acetals
Ketone + OH group from alcohol (with acid or base catalyst) -> hemiacetal (OH group and O with an open bond) (with an acid catalyst) -> acetal (no more OH group, two O groups with open bonds) and water
What is the significance of the formation of an acetal from the nucleophilic addition of alcohols to carbonyls being reversible
To make an acetal you add excess alcohol and remove water (distill it off). To hydrolyse an acetal you add dilute acid (large excess of water). Le Chatelier’s principle
What is addition of hydrogen (reduction) to carbonyls used for
To make alcohols from aldehydes or ketones.
How is the addition of hydrogen to carbonyls carried out
Done by adding H- (hydride) from reagent like LiAIH4 which gives electrons to oxygen of carbonyl making it -ve. Then adding H+ (from acid) to the reaction flask after which forms OH and so alcohol
What happens during oxidation of carbonyls
Requires an oxidising agent which becomes reduced during the reaction. Aldehyde oxidised to form OH on the H group (carboxylic acid)
What can be used to test for aldehydes
Fehling’s reagent (CU2+ in aqueous sodium tartrate) and Benedict’s reagent (Cu2+ in aqueous sodium citrate) give a reddy born precipitate of Cu2) when in the presence of an aldehyde
What does Tollen’s reagent give when in the presence of an aldehyde
A silver mirror (as is Ag+ in aqueous ammonia)
What does nucleophilic addition of primary amines with aldehydes or ketones produce
An imine
What happens when you add an amine to an aldehyde or ketone
Produces an imine and water. Imine has a C=N bond and the N is bonded to an R group.
What does the fact that the nucleophilic addition of primary amines and carbonyls is reversible mean
Imines are easily hydrolysed. Have to distil off water to shift equilibrium
What reactions do hydrazines undergo
Nucleophilic addition to form hydrozones and water. Hydrozones are reasonably stable towards hydrolysis
When is glucose a hemiacetal
When it is in its cyclic form
How does glucose form a hemiacetal
Undergoes intramolecular hemiacetal formation to form a ring. Glucose exists predominantly in its cyclic form
