8: Aldehydes and Ketones Flashcards
What is the difference between an aldehyde and a ketone?
Aldehyde groups are found at the end of the carbon chain ( C=O H)
Ketones are found in the middle of the carbon chain (C=O)
Primary alcohols can be oxidised into what type of compound?
Aldehyde
After primary alcohols have been oxidised into aldehydes, what can the aldehydes be oxidised into?
Carboxylic Acids
Secondary alcohols can be oxidised into what type of compound?
Ketone
What can tertiary alcohols be oxidised into?
Nothing.
Why is oxidation of aldehydes into carboxylic acids easy?
Aldehydes have a hydrogen attached to the carbonyl group. This means the hydrogen can be lost easily, allowing for oxidation.
Why are ketones not oxidised as easily?
To oxidise a ketone, you need to break the C=O bond, which is more difficult.
Name 2 reagents that can be used to distinguish aldehydes and ketones.
Tollens Reagent
Fehling’s Solution
Describe how to test for aldehydes using Tollens reagent.
Heat a test tube containing your solution with Tollens.
If aldehydes are present, a silver mirror forms.
What is contained in a solution of Tollens?
Name the complex ion.
Ammoniacal silver nitrate [Ag(NH3)2]
Describe the reduction/oxidation that takes place in a positive test for aldehydes using Tollens.
The aldehyde is oxidised (loss of hydrogen)
The diamine silver ions in the Tollens are reduced, forming silver and ammonia.
What happens when Tollens is added to a ketone?
Nothing, ketones don’t react with Tollens reagent.
Describe how to test for aldehydes using Fehling’s solution.
Heat the Fehling’s with the unknown solution .
If aldehydes are present, copper (II) ions are reduced to form a brick-red precipitate of copper (I) oxide.
Describe what you would see if you added ketones to Fehling’s solution.
Nothing, ketones don’t react with Fehling’s solution.
Name the compound used to reverse the oxidation of primary/secondary alcohols (reduction).
NaBH4
Give a general equation for the reduction of an aldehyde to a primary alcohol using a reducing agent (such as NaBH4)
RCOH + 2[H] -> RCH2OH
Give a general equation for the reduction of a ketone to a secondary alcohol using a reducing agent (such as NaBH4)
RC=OR + 2[H] -> RCHROH
Name the mechanism for aldehydes/ketones turning into alcohols.
Nucleophilic addition
During reduction of aldehydes/ketones into alcohols, where does the nucleophile come from?
Name the nucleophile.
H- ions come from the reducing agent and act as a nucleophile.
On an aldehyde, where does the H- ion attack and why?
H- ions attack the carbon. The C=O bond is polar, so the carbon is slightly positive and the oxygen is slightly negative.
How does the H- ion form a bond with the carbon?
The H- donates is lone pair of electrons forming a bond with the carbon.
Why does the addition of H- cause one of the carbon oxygen bond to break?
Where do electrons from the C=O bond go to?
Addition of the H- causes 5 bonds to form on the carbon atom. This can’t happen, so one of the bonds needs to break. This allows the carbon to have 4 bonds.
A lone pair of electrons is forced from the C=O bond onto the oxygen.
The lone pair on the oxygen is then forced onto where?
Where does this molecule come from?
H+ ion, which comes from water or a weak acid.
At the end of nucleophilic addition for aldehyde, what is produced?
Primary alcohol.
At the end of nucleophilic addition for ketone, what is produced?
Secondary alcohol.
What type of isomerism can occur during the formation of hydroxynitriles?
Optical isomers.
Name the mechanism for how hydroxynitriles are formed.
Nucleophilic addition.
Give the formula for a nitrile group.
C triple bond N
Give the overall equation for making hydroxynitriles by reacting aldehydes or ketones with HCN
Aldehyde: RCHO + HCN -> RCH(OH)CN
Ketone: RCOR’ + HCN -> RC(OH)R’CN
If C=O and C=C bonds are planar, how does this affect the attack of nucleophiles?
How does the attack affect which isomers are produced?
Nucleophiles attack from either above or below the plane of the bond.
Attack from either side of these bonds produces different enantiomers.
Why is a racemic mixture likey to form during the production of hydroxynitriles?
Attack of the nucleophile on either side of the planar bond is equally likely.
What happens during reflux?
The reaction mixture is vapourised. After it condenses, it drips back into the reaction vessel to prevent loss of reactants.
Ethanol is oxidised to ethanoic acid in reflux. Explain how and why.
- Reaction mixture heated for a prolonged time period.
- Some vapours produced escape. But they recondense and therefore drip back into the reaction mixture so are not lost.
- This ensures that any ethanol/ethanol lost initially in evaporation can be oxidised.
Write a half equation for the oxidation of ethanol into ethanoic acid.
CH3CH2OH + H2O -> CH3COOH + 4e- + 4H+
The boiling points of the organic compounds in a reaction mixture are shown in the following table.
Compound ethanol ethanal ethanoic acid
Boiling point / °C 78 21 118
Use these data to describe how you would obtain a sample of ethanal from a mixture of these three compounds. Include in your answer a description of the apparatus you would use and how you would minimise the loss of ethanal. Your description of the apparatus can be either a description in words or a labelled sketch.
Mixture heated in a suitable flask
With still head containing a thermometer
Water cooled condenser connected to the still head and suitable cooled collecting vessel
Collect sample at the boiling point of ethanal
Cooled collection vessel necessary to reduce evaporation of ethanal
Why is it possible to separate ethanal, ethanol and ethanoic acid using distillation? Explain in terms of structure and bonding.
Ethanal does not have hydrogen bonds (the strongest IMF) because the oxygen is not bonded to any hydrogen. It only has VDW and permanent dipole dipole forces which are weaker. Carboxylic acids and ethanol do have hydrogen bonds, but carboxylic acids have more hydrogen bonds, so have the highest boiling point. As their boiling points are very different to each other, they can be collected at different points in distillation.
State a hazard associate with the use of KCN but explain why its used instead of HCN when forming hydroxynitriles.
Hazard - Toxic
Why KCN used - HCN is too weak so the CN ion does not dissociate as well as in KCN.
