Aldehydes and Ketones

  • Created by: Hindleyc
  • Created on: 04-12-18 14:53
What are carbonyls?
Either aldehydes or ketones- compounds with a C=O bond
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When is it an aldehyde?
When the C=O is on the end of the chain with an H attached -anal
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When is it a ketone?
When the C=O is in the chain -one
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What intermolecular forces are in carbonyls?
pure carbonyls cannot hydrogen bond but bond instead by permanent dipole forces
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What is their solubility like?
Smaller carbonyls are soluble in water because they can form hydrogen bonds with water
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What is the C=O bond? why?
Polarised because O is more electronegative than carbon.
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What does this positive carbon do?
Attract nucleophiles
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What about in comparison to the C=C bond in alkenes?
C=O is stronger and doesn't undergo reactions easily
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What is this in contrast do?
Electrophiles that are attracted to the C=C
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What is the oxidising agent that causes alcohols and aldehydes to oxidise?
Potassium dichromate - K2Cr2O7
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What does a primary alcohol oxidise to? then?
Aldehyde, Carboxylic acid
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Secondary
Ketones
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Tertiary
Dont oxidise
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What is the key point from these reactions
Aldehydes can be oxidised to carboxylic acids but ketones can't be oxidised
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Reagents and conditions for oxidation of aldehyde to carboxylic acid
Potassium dichromate solution and dilute sulphuric acid, heat under reflux
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Observation
Orange dichromate ion (Cr2O7^2-) reduced to the green Cr 3+ ion
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General equation
RCHO+ [O] = RCO2H
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How else can aldehydes be oxidised- what is this used for?
Using Feelings or Tollens reagent- test for presence of aldehyde group
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Active substance in Tollens
Complex ion [Ag(NH3)2]+
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How is it formed?
Mixing aq ammonia and silver nitrate
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Condition?
heat gently
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Reaction
Aldehydes only are oxidised by TR into a carboxylic acid - silver ions reduce to silver atoms
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Observation
Aldehyde: Silver mirror forms coating inside of test tube
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Ketones?
results in no change
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Reagent for Fehlings test?
FS containing blue Cu2+ ions
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Conditions?
Heat gently
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Reaction?
Aldehydes are oxidised by feelings into a carboxylic acid so copper ions reduced to copper oxide
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Observation
Blue Cu^2+ ions in solution change to a red precipitate of Cu2O. Ketones don't react
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What can carbonyls do?
Be reduced
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Using what?
Reducing agents such as NaBH4 (sodium tetrahydridoborate) or LiAlH4 (Lithium tetrahydridoaluminate) will reduce carbonyls to alcohols
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Reagents
NaBH4 in aqueous ethanol
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Conditions?
Room temperature and pressure
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What will aldehydes by reduced to?
Primary alcohols
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What will ketones be reduced to?
Secondary alcohol
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Nucleophilic addition steps. C=O
The oxygen is slightly negative and Carbon slightly positive
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what next
H- ion attracted with lone pair of electrons.bonds to carbon but carbon has too many bonds so breaks the double bond. electrons to oxygen atom
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What next
Lone pair of electrons on negative oxygen attracted to H+ bond
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Where is the H+ from
water or weak acid
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What does NaBH4 contain
source of nucleophilic hydride ions ( :H-) which are attracted to the +ve carbon in C=O bond
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Overall eqn reductant?
[H]
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What can happen because its planar?
can attack either side
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What does the additions of hydrogen cyanide to carbonyls form?
Hydroxynitriles
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Reaction?
Carbonyl to hydroxynitrile
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Reagent
Sodium cyanide (NaCN) and dilute sulphuric acid
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Conditions?
Room temp and pressure
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Mechanism
Nucleophilic addition
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What the NaCN supply
Nucleophilic CN- ions
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What does the H2SO4 supply?
H+ ions needed in second step of mechanism
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FG?
NC-C-OH
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When naming?
CN becomes part of main chain and carbon no 1
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What could we use for this rxn? why not? what's used instead?
HCN but is a toxic gas that is difficult to contain. instead KCN/NaCN - still toxic because of cyanide ion
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Steps of nucleophilic addition of KCN in dilute acid
1. nucleophile attacks slightly positive C forming bond so ejects- one of covalent bonds (pair of electrons) move onto O atom in intermediate stage
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2
oxygen -ve charge as gained electrons so within solution H+ so proton forms bond with H ion
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What do aldehydes and unsymmetrical ketones form?
Mix of enantiomers when they react with KCN followed by dilute acid
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What is KCN?
Toxic
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Other cards in this set

Card 2

Front

When is it an aldehyde?

Back

When the C=O is on the end of the chain with an H attached -anal

Card 3

Front

When is it a ketone?

Back

Preview of the front of card 3

Card 4

Front

What intermolecular forces are in carbonyls?

Back

Preview of the front of card 4

Card 5

Front

What is their solubility like?

Back

Preview of the front of card 5
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