Aldehydes And Ketones
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What makes a carbonyl an aldehyde
If the C=O is on the end of the chain with an H attached it is an aldehyde. The name will end in –al
What makes a carbonyl a ketone
If the C=O is in the middle of the chain it is a ketone The name will end in -one
What is a carbonyl
Carbonyls are compounds with a C=O bond. They can be either aldehydes or ketone
Intermolecular forces in carbonyls
Pure carbonyls cannot hydrogen bond to themselves, but are attracted instead by permanent dipole forces
Carbonyls solubility in water
The smaller carbonyls are soluble in water because they can form hydrogen bonds with water.
Reactions of carbonyls
The C=O bond is polarised because O is more electronegative than carbon. The positive carbon atom attracts nucleophile In comparison to the C=C bond in alkenes, the C=O is stronger and does not undergo addition reactions easily. This is in contrast to the electrophiles that are attracted to the C=C .
What is the oxidising agent for carbonyls
Potassium dichromate K2Cr2O7 is an oxidising agent that causes alcohols and aldehydes to oxidise Aldehydes can be oxidised to carboxylic acids, but ketones cannot be oxidised.
Products of oxidation reactions with alcohols
Primary alcohol —> aldehydes —> carboxylic acid Secondary alcohol —> ketones Tertiary alcohols do not oxidis
Oxidation of aldehydes
Reaction: aldehyde —> carboxylic acid Reagent: potassium dichromate (VI) solution and dilute sulfuric acid. Conditions: heat under reflux RCHO + [O] —> RCOOH
Full equation for oxidation
3CH3CHO + Cr2O7 2- + 8H+ —> 3 CH3CO2H + 4H2O + 2Cr3+ Observation: the orange dichromate ion (Cr2O7 2-) reduces to the green Cr 3+ ion
Reduction of carbonyls reagents and conditions
Reagents: NaBH4 In aqueous ethanol Conditions: Room temperature and pressure Reducing agents such as NaBH4 (sodium tetrahydridoborate) or LiAlH4 (lithium tetrahydridoaluminate) will reduce carbonyls to alcohols.
What are aldehydes reduced to
Aldehydes will be reduced to primary alcohols Propanal + [H] —> propan1ol
What will ketones be reduced to
Secondary alcohols Propanone —> propan2ol
What reducing agents will reduce carbonyls to alcohols
Reducing agents such as NaBH4 (sodium tetrahydridoborate) or LiAlH4 (lithium tetrahydridoaluminate) will reduce carbonyls to alcohols.
Why does nucleophilic addition happen when reducing carbonyls
NaBH4 contain a source of nucleophilic hydride ions (:H-) which are attracted to the positive carbon in the C=O bond.
What is catalytic hydrogenation?
Carbonyls can also be reduced using catalytic hydrogenation Reagent: hydrogen and nickel catalyst Conditions: high pressure
Example equations for catalytic hydrogenation
CH3CHO + H2 —> CH3CH2OH CH3COCH3 + H2 —> CH3CH(OH)CH3
Addition of hydrogen cyanide to carbonyls to form hydroxynitriles
Reaction: carbonyl —> hydroxynitrile Reagent: potassium cyanide (KCN) and dilute sulfuric acid. Conditions: Room temperature and pressure Mechanism: nucleophilic addition
Why is the addition of hydrogen cyanide to carbonyls to form hydroxynitriles nucelophilic ddition
The KCN supplies the nucleophilic CN- ions. The H2SO4 acid supplies H+ ions needed in second step of the mechanism
What happens when naming hydroxynitriles
When naming hydroxy nitriles the CN becomes part of the main chain and carbon n1
What is the advantage of using KCN or NaCN over HCN
We could use HCN for this reaction but it is a toxic gas that is difficult to contain. KCN/NaCN are still, however, toxic, because of the cyanide ion. Another advantage of using KCN or NaCN is that there will be a higher concentration of the CN- ion as these compounds will completely ionise. HCN is a weak acid an will only partially ionise
Example equations of addition of hydrogen cyanide to carbonyls to form hydroxynitriles
CH3COCH3 + HCN —> CH3C(OH)(CN)CH3 2-hydroxy-2-methylpropanenitrile CH3CHO + HCN —> CH3CH(OH)CN 2-hydroxypropanenitrile
Shape of the nucleophilic addition of hcn to aldehydes and ketones
Nucleophilic addition of HCN to aldehydes and ketones (unsymmetrical) when the planar carbonyl group is approached equally from both sides by the HCN attacking species: results in the formation of a racemate
Three unlabelled bottles are known to contain either propan-1-ol, propanal, or propanone. A sample of each liquid is warmed with a few drops of Fehling’s solution. Identify the liquid that reacts with Fehling’s solution and give the expected observation. Suggest a further simple test-tube reaction that can be used to distinguish between the remaining two liquids. Give the expected observation with the liquid that reacts.
Liquid that reacts with Fehling’s solution - propanal Observation - blue to red ppt Further test - acidified potassium dichromate Observation - propan1ol oraneg to green