Aldehydes and Ketones
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What is a primary alcohol oxidised to?
An aldehyde
What is a secondary alcohol oxidised to?
A ketone
Can an aldehyde be oxidised? If so, what to?
Yes. It can be oxidised to a carboxylic acid
Can a ketone be oxidised? If so, what to?
No, it cannot be oxidised further.
Acidified potassium dichromate is added to a substance. The colour changes from orange to green. What type of substance is it?
An aldehyde
There are three tests to distinguish between an aldehyde and a ketone. What are they, and what are the changes you’d see?
Acidified potassium dichromate + heat under reflux - Ald orange to green, Ket nvc Tollen’s reagent + gently heat - Ald colourless to silver ppt, Ket nvc Fehling’s soln. + gently heat - Ald blue to red ppt, Ket nvc
Give a balanced equation for the reduction of ethanal
CH3COH + 2[H] –> CH3CH2OH
Carbonyls can be reduced using NaBH4 in aqueous ethanol. They can also be reduced using another method. State the name of this other method, the reagents used, and the conditions.
Catalytic Hydrogenation. Reagents: hydrogen and nickel catalyst Conditions: high pressure
Hydroxynitriles can be formed from aldehydes and ketones. Give the mechanism, the reagents and the conditions.
Mechanism: Nucleophilic addition Reagent: sodium cyanide (NaCN) and dilute H2SO4 Conditions: room temp. and pressure
CH3COCH3 is reacted with NaCN and another reagent under room temp. and pressure. Name the other reagent and the product formed.
Dilute H2SO4 2-hydroxy-2-methylpropanenitrile
Nucleophilic addition of HCN to aldehydes and ketones results in the formation of a racemate. Why?
The middle step of the reaction is a trigonal planar carbonyl (2D). The H+ can attack from either above or below, resulting in two different enantiomers. Because the carbonyl has equal chance of being
Give two methods of reduction
NaBH4 in aq ethanol Catalytic hydrogenation
What is a suitable oxidising agent?
Acidified potassium dichromate
The reduction of an aldehyde or ketone involves what type of reaction mechanism?
Nucleophilic addition
Describe and explain the nucleophilic addition of a nucleophile to propanal
The C=O is polar, with O being more electronegative and so being more -ve The -ve nucleophile is attracted to the slightly +ve C The C can only have 4 bonds, so breaks the double bond, allowing Nu to join O takes the electrons, making it negative, which allows it to attract an electrophile
Why will the nucleophilic addition reaction of carbonyl compounds (eg propanal) produce a racemic mixture?
It is a trigonal planar, meaning the attacking species is equally likely to attack from the top or the bottom
Name a mechanism for the reaction of an aldehyde with HCN
Nucleophilic addition
Some were asked to distinguish between a ketone and an aldehyde. One suggested looking at the optical activity. Why would this not work?
The ketone is not optically active An aldehyde can be optically active, but is formed in a racemic mixture, and so any effects will be cancelled out
Someone wants to use acidified potassium dichromate to distinguish between an aldehyde and a ketone. What are the conditions?
Heat under reflux
What test produces a silver mirror effect when an aldehyde is present?
Tollen’s reagent
Someone wants to use Tollen’s reagent to distinguish between an aldehyde and a ketone. What else would they have to do to the substance? (conditions)
Heat gently
What colour change is observed when Tollen’s reagent is added to an aldehyde?
Colourless to silver precipitate
What colour change is observed when Tollen’s reagent is added to a ketone?
No visible change
Someone wants to use Fehling’s solution to distinguish between an aldehyde and a ketone. What else would they have to do to the substance? (conditions)
Gently heat