Aldehydes and Ketones Flashcards
What is the general formula for aldehydes and ketones
CnH2nO
What are aldehydes and ketones functional group and label the polarity
CHO - Aldehydes
CO - Ketones
How are aldehydes and ketones named?
Aldehydes will always be on carbon 1, Ketones are names so that the C=O will have the lowest number possible
What state are short chain aldehydes and ketones
Liquids, the permanent dipole of the C=O causes permanent dipole dipole attractions which require more energy to break than Van der Waals alone
Are they miscible or immiscible
miscible, C=O allows hydrogen bonding with water.
Which of the two can undergo mild oxidation?
Aldehydes can undergo mild oxidation but ketones cannot. Both can be reduced
How are aldehydes formed? and draw out the example with ethanol.
Aldehydes are formed from the oxidation of Primary (1º) Alcohols. Prolonged oxidation forms a carboxylic acid.
Example in book
How are ketones formed? and draw out the example propan-2-ol
Ketones are formed from the oxidation of Secondary (2º) alcohols.
How do you common mild oxidise an aldehyde/ketone
Mix the aldehyde/ketone with the oxidising mixture and warm the mixture in a water bath. The aldehyde will show the expected change but the ketone will not.
What are the three mild oxidising agents?
- Acidified Potassium Dichromate (VI) solution
- Tollens’ reagent
- Fehling’s solution
Write out the equation for the reduction of acidified potassium dichromate solution and state the colour change
Orange solution → Green solution
Cr2O7^2- + 14H+ + 6e- → 2Cr3+ + 7H2O
Tollens reagent
Ammoniacal solution of silver (I) nitrate
- Tollens reagent is a colourless solution that only reacts with aldehydes
- The Ag+ ions are reduced to Ag, Ag+ +e- → Ag
- The solution changes from colourless to silver mirror if the silver ions are reduced by the oxidation of an aldehyde
Fehlings solution
- Fehlings solution is blue and only reacts with aldehydes
- The copper (II) ions are reduced to Copper (II) oxide which appears as a red precipitate, Cu2+ + e- → Cu+
- Solution changes from blue to red precipitate
- Full eqn: 2Cu2+ + 2OH- + 2e- → Cu2O + H2O
What reactions can aldehydes and ketones undergo?
React with hydrogen cyanide to for cyanohydrins, and with 2,4-dinitrophenylhydrazine to form 2,4-dinitrophenylhydrazones
How is the reaction of the aldehyde / ketone with hydrogen cyanide carried out?
HCN is a toxic gas, any reaction is carried out using a solution of sodium or potassium cyanide salt with sulfuric acid
What is the general eqn for HCN with an aldehyde/ketone?
Aldehyde/Ketone + HCN → Cyanohydrin
Give the reaction and mechanism for the reaction of propanone with HCN
in book - Nucleophilic substitution
What happens when ethanal reacts with HCN?
2-hydroxypropanenitrile is formed which exhibits optical isomerism and has an asymmetric centre. The product is optically inactive as it is a racemic mixture. Equal amounts of dextro and laevorotation
what will all aldehydes and ketones produce when reacted with HCN and what is the exception?
All will produce a racemic mixture except methanal as it does not have a chiral centre.
Draw the structure of 2,4-dinitrophenylhydrazine
in book
Draw the reaction of propanone and ethanal with 2,4-dinitrophenylhydrazine and what is formed?
in book
ethanal/propanone - 2,4-dinitrophenylhydrazone
How can this be used to compare aldehydes and ketones ?
The 2,4-dinitrophenylhydrazone that is fomed is a yellow solid that can be purified and its melting point determined. Comparison of the melting point against a table of known values could identify the aldehyde / ketone.
What are all derivatives of 2,4-dinitrophenylhydrazine?
Yellow solids
What does the NO2 group and the size of the nolecule mean
Permanent dipole-dipole forces of attraction and strong van der waal forces means the molecule has a high mp
What is the solution of 2,4-dinitrophenylhydrazine often referred to as?
Bradys reagent
How do you prepare a pure sample of a 2,4- dinitrophenylhydrazone derivative?
- Mix the aldehyde/ketone with 2,4-dinitrophenylhydrazine solution
- A yellow precipitate will form (if not warm and add dilute sulfuric acid)
- Suction filter to obtain the solid
- Dissolve the solid in a minimum volume of hot ethanol
- Filter through fluted filter paper while hot to remove any impurities. The fluted filter paper prevents precipitation of the solid on the cold filter funnel
- Allow the solution to cool and crystallise
- Suction filter to obtain the solid crystals and wash with a little cold water
- Dry the crystals between two sheets of filter paper
- Determine the melting point of the solid to check purity
How do you determine the melting point of a solid?
Seal a capillary tube at one end and tap some of the sample in. Place in melting point apparatus and heat to 15ºC of the melting point and then heat slowly. Time to start and finish melting recorded. Pure = sharp melting point, Impure = melts over a range.
What can aldehydes and ketones be reduced to?
Aldehydes can be reduced to Primary (1º) Alcohols, Ketones can be reduced to Secondary (2º) alcohols.
What carries out the reduction?
Lithal - LiAlH4, represented by [H]
Lithium Tetrahydridoaluminate (III) dissolved in dry ether
The reaction is heated under reflux
Draw the reaction of ethanal to ethanol using lithal
in book
