Aldehydes And Ketones Flashcards
Aldehyde general formula
R-C-H
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O
Ketone General formula
R-C-R’
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O
R and R’ are alkyl groups
Functional group
-C=O
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Carbonyl group
Nomenclature
Suffice for aldehydes= al
Suffix for ketones = one
For ketones The position of the carbonyl group is specified by inserting the number of its carbon atom from the nearer end of the chain
Why be a butanone and not butan-2-one
Can only be attached to carbon – two. If on 1/4 it would be an aldehyde and 2,3 are the same carbon atom.
Why is there no such compound as ethanone?
There will always be a H atom attach to C atom with C=O on it and therefore we have an ethanal. the first Keytone is propanone
What intermolecular forces are between aldehydes and ketones?
The molecules are polar, so dipole dipole forces as well as Vanderwall forces exist between them.
What has higher boiling points alkanes and Alkenes or ketones and aldehydes?
Alkanes and Alkenes or nonpolar with just Vandevelde forces between the molecules, so their intermolecular forces are weaker and therefore boiling points lower than those of all the aldehydes and ketones with the same number of electrons in the molecule.
Why is intermolecular hydrogen bonding not possible In carbonara compounds?
Neither aldehydes or ketones have a hydrogen atom that is sufficiently delta plus. Therefore they have boiling points that are lower than those of alcohols, which do form into molecular hydrogen bonds. ( ethanal and ethanol 20° 78°).
What results in an increase in melting points and boiling points and aldehydes and ketones?
Increasing relative molecular mass resulting in increased melting points and boiling points (more electrons – stronger Vandevelde forces).
Solubility
The lower members of both series are soluble in water. This solubility is due to hydrogen bonding between Delta negative oxygen in the carbonyl compound and the Delta positive hydrogen in a water molecule.
Hydrogen bonds formed between the oxygen atom of the carbonyl group and the hydrogen atoms of water molecules.
How to prepare aldehydes and ketones?
In the Labatory by oxidation of alcohols using a solution of potassium (or sodium) dichromate and dilute sulfuric acid.
Aldeydes prepared
By the oxidation of primary alcohols. To prevent further oxidation to a carboxylic acid the aldehydes distilled off immediately as it is formed. It has a lower boiling point than the alcohol
Ketones firmed
The oxidation of secondary alcohols. Heat the mixture under reflux.
Observation in Preparation of aldehydes and ketones
The observation is the same – the orange colour of potassium dichromate (six) changes to green because chromium (three) ions are formed.
What makes a carbon atom more susceptible to nucleophilic attack?
Oxygen is more electronegative than carbon. Both aldehydes and ketones undergo nuclear fillet addition.
Why are ketones less able to undergo nucleophilic addition?
Less positive and blocked by groups as alcohol groups are electron donating.
Addition of hydrogen cyanide
Addition of hydrogen cyanide, generated by the action of die loot sulphuric acid on an alkali metal cyanide at about 10–20°
Naming form addition of HCN
The CN group has a higher priority than the OH group so that all H is considered a substituent called hydroxy
What is the nuclear file and the addition of hydrogen cyanide
CN minus
The lone pair of electrons on the carbon atom in the cyanide Ion attacked the Delta plus carbon atom on the aldehyde or Keytone. The intermediate form reacts with hydrogen iron present in the solution to form the product
Reduction
Aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols
Suitable reducing agent
Lithium aluminium hydride (with all), and dry ether (heat under reflux).
LiAlH4
Carboxylic acids are reduced to??
Primary alcohols
It is not possible to reduce carboxylic acids and produce an aldehyde directly.
Why does reaction with compounds that contain the dash and H to group produced a mixture of two geometric isomers?
The nitrogen atom has a lone pair of electrons, the bond angles around the nitrogen in the product are 120°. This means that in this type of reaction aldehydes (other than methanol) and asymmetric ketones produced a mixture of two geometric isomers.
Reaction with two, four – dinitrophenyl hydrazine
Two, four – DNP is dissolved in aqueous methanol and made slightly acidic with sulphuric acid forming Bradys reagent. The carbon compound is an added on the picture is warmed.
Use of 2,4 DNP
As a method for identifying aldehydes and ketones. The 2–4 – dinitrophenyl hydrazone formed is a yellow – orange solid that can be purified and its melting point determent. Comparison of the melting point with a table of no values could identify the aldehyde or Keytone.
Advantage of using a Buchner funnel
Rapid filtration and added advantage that the flow of air pass the crystals – solid helps to dry them out.
Why is a heated Buckner funnel and flask used in purification by recrystallisation
To prevent clean of the solution on the crystallisation of the solute
The soluble impurities
Present in smaller quantities and remain in solution at the lower temperature
Choose a solvent in which
The product ready dissolves when hot but hardly does so when cold. Then impurities must either not dissolve or remain dissolved st low temperatures.
Most pure solids have characteristic
Sharp melting points. A pure organic side melts over a degree or two but as an impure solid may melt over a range of five or more degrees Celsius below the pure melting temp
If not sharp constant melting pint
Recrystallisation is usually repeat it.
Preparation
L
Purification
L
Mention pint determination
L
How to distinguish between aldehydes and ketones
The relative ease with which all the highs are oxidised (number of reactions)
Potassium dichromate and die loot sulphuric acid test
Makes a few centimetres cubed of the compound to be tested with acidified potassium dichromate (six) solution into a test tube and warm in a water bath.
Potassium dichromate and dilute sulphuric acid Observation with an aldehyde
The compound turns the orange dichromate to a green coloured solution. Change in smell
Potassium dichromate and dilute sulphuric acid Observation with a ketone
The orange dichromate solution stays orange
Tollens reagent test
Makes a few centimetre cubed of the compound to be tested with ammonia and silver nitrate solution in a test tube and warm in a water bath.
Tollens reagent Observation with an aldehyde
The colourless solution changes to give a silver mirror
Tollens reagent Observation with a ketone
The solution remains colourless
Fehlings solution test
Add a few centimetres cubed of the compound to be tested to a few centimetres cubed of feeling solution into a test tube and warm in a water bath.
Fehlings solution. Observation with an aldehyde
The blue solution changes to give a rare precipitate
Fehlings solution Observation with the Keytone
No reaction remains blue.
Reduction of feeling solution
Cu2+ + e- = Cu+
