Aldehydes & Ketones Flashcards
-> Describe the electronegativity in carbonyls:
-> The C = O is polarised because the Oxygen is more electronegative than Carbon. As a result, the partially positive Carbon attracts nucleophiles.
->Why do carbonyls not undergo addition reactions easilty?
-> Because the C = O bond is stronger than C = C bond in alkenes
-> When is it a ketone?
-> Naming?
-> When the C = O bond is in the middle of the chain.
-> Name ends in -one
-> What are carbonyls?
-> Compounds with a C = O bond
-> How is the solubility of carbonyls?
-> smaller carbonyls are soluble in water as they can form Hydrogen Bonds.
-> Oxidation Reactions of aldehydes and ketones:
- Oxidising Reagent:
- Reactions:
->Potassium Dichromate (K2Cr2O7)
=> 1* alcohol -> aldehydes -> carboxylic acid
=> 2* alcohol -> ketones -> don’t oxidise further
=> 3* -> don’t oxidise because the carbon atom attached to the -OH group does not have any other hydrogen atoms.
-> When is a compound an aldehyde?
-> Naming?
-> The C = O bond is at the end of the chain with an H attached.
-> The name ends in -al.
-> What intermolecular forces are present in carbonyls?
-> Attracted by permanent dipole forces.
-> Describe the oxidation of aldehydes:
- How is Oxidation represented?
- reaction?
- reagent?
- conditions?
- Equation:
- Observations?
-> aldehyde to carboxylic acid
-> potassium dichromate (VI) solution and dilute sulfuric acid
-> Heat under reflux
->3CH3CHO + Cr2O7^2- + 8H+ -> 3 CH3CO2H + 4H2O + 2Cr^3+
-> Orange dichromate gets reduced to green Cr^3+ ion.
Tests to confirm the presence of Aldehydes?
- Reagent
- Condition
- Reaction
- Observation
- Equation
- Tollens’ Reagent Test:
- Tollens’ reagent -> mix aqueous ammonia and silver nitrate -> forming [Ag(NH3)2]+
- Gentle heating
- only aldehydes oxidised to carboxylic acids and silver ions reduced to silver atoms.
- a silver mirror forms when it is a positive reaction.
- CH3CHO + 2Ag+ + H2O -> CH3COOH + 2Ag + 2H+
- Fehling’s solution Test:
- Fehling’s solution + blue Cu^2+ ions
- gentle heating
- aldehydes oxidised to carboxylic acids and copper(II) reduced to copper (I)
- Blue solution -> red ppt of Cu2O
- CH3CHO + 2Cu^2+ + 2H2O -> CH3COOH + Cu2O + 4H+
-> How do you reduce carbonyls to alcohol?
- Reducing reagents:
- Conditions:
- Reaction Name and Draw Mechanism:
-> NaBH4 in aqueous ethanol
-> Room temperature + Pressure
=> aldehydes -> 1* alcohols
=> ketones -> 2* alcohols
- reduction reaction:
1. aldehyde -> 2[H] -> 1* alcohol
2. ketone -> 2[H] -> 2* alcohol
-> Nucleophilic Addition Mechanism
- aldehyde - partially +ve on C and partially -ve on O
- Hydride ion (H-) , which are the nucleophiles from NaBH4 with one lone pair attacks Carbon with partial +ve charge
- C = O double bond breaks to a single bond and arrow goes from double bond to Oxygen.
- Oxygen has 1 lone pair and a negative charge and this lone pair donates electrons to H+ from water or ethanol (weak acid) -> O-H bond forms.
-> Describe catalytic Hydrogenation:
- Reagents:
- Conditions:
- Examples of equations:
- a way of reducing carbonyls using catalyst
- Hydrogen and Nickel catalyst
- High Pressure
- CH3CHO + H2 -> CH3CH2OH
- CH3COCH3 + H2 -> CH3CH(OH)CH3
-> Describe the addition of HCN to carbonyls -> hydroxynitriles
- Reaction:
- Reagent:
- Conditions:
- Mechanism:
-> carbonyl -> hydroxynitrile
-> sodium cyanide + dilute Sulfuric Acid
-> room temperature and pressure
-> Nucleophilic Addition
- The sodium cyanide provides the nucleophile CN- and these attack the carbonyl group’s partially positive carbon.
- The bond between C = O breaks to a single bond and the arrow goes from the double bond to the partially negative oxygen, whilst the CN bonds with the Carbon atom.
- The oxygen with a lone pair and negative charge donates its electrons to the H+ from the sulfuric acid.
- This then forms the hydroxynitrile.
-> How do you name hydroxynitriles?
-> when there is a OH with CN then the -OH is referred as hydroxyl group
-> the CN becomes part of the carbon chain when naming.
-> the N part of the chain is referred as a nitrile
-> Why do you use KCN or NaCN instead of HCN?
-> HCN - can be used, but it is a toxic gas that is difficult to contain.
-> KCN/ NaCN are still toxic because of the Cyanide ion
-> KCN/ NaCN give a higher concentration of CN- ions as these compounds completely ionise.
-> HCN - weak acid so partially ionises
