Transcript Carbonyls - wellswaysciences

Week 3
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Recognise and name aldehydes and ketones.
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Week 3
The carbonyl functional group
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Week 3
Formation of the C=O group π-bond
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Naming aldehydes
• See p. 20 A2 text book.
• In an aldehyde the carbon atom of the
carbonyl group is joined to at least 1 hydrogen
atom.
• Aldehydes end in ‘al’.
• The parent chain is the longest unbranched
carbon chain present in the molecule.
• The carbonyl C atom is always the 1 carbon in
the chain.
Week 3
2-methylpentanal
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Naming ketones
• See p. 21 A2 text book.
• In ketones the carbonyl group is in the middle
of a chain with a C atom on either side.
• Ketones end in ‘one’.
• The parent chain is the longest chain
containing the carbonyl group.
• The carbonyl is numbered with the smallest
possible number.
Week 3
The structure of 2-methylpentan-3-one
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Aromatic aldehydes and ketones
• These contain the carbonyl functional group
and a ring.
• The simplest aromatic aldehyde is?
• The simplest aromatic ketone is?
• Aromatic carbonyls contribute to the flavours
and smells of many stone fruits, plums,
peaches, cherries etc.
• Cinnamaldehyde is the characteristic smell
and flavour of cinnamon.
• Phenylethanone resembles strawberries etc.
Week 3
Structures of benzaldehyde and phenylethanone
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Week 3
Structure of cinnamaldehyde
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Physical Properties of Carbonyl
Compounds
• Aliphatic carbonyl compounds are generally
less pleasant, particularly aldehydes which are
described as lachrymatory.
• E.g. Heptan-2-one is responsible for the
flavour of blue cheese.
• Lower members of both carbonyl series are
soluble in water.
• Solubility decreases with increasing RMM.
• Volatility decreases with increasing RMM.
Volatility
Compound
BPoC
Ethanal
20
Propane
-42
Ethanol
78
Week 3
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Describe the oxidation of primary alcohols to form aldehydes and carboxylic
acids.
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Describe the oxidation of secondary alcohols to form ketones.
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Describe the oxidation of aldehydes to form carboxylic acids.
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Week 3
Ethanol oxidised to ethanal, and finally to ethanoic acid
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Week 3
Propan-2-ol can be oxidised to propanone
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Week 3
Oxidation of an aldehyde to a carboxylic acid
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Oxidation
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Oxidation is:
Gain of oxygen
Loss of hydrogen
Loss of electrons
In the oxidation of alcohols or aldehydes, ion
electron equations can be written which are
more informative than the [O] symbol.
• See p. 23 in A2 text.
Week 3
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Describe the reduction of carbonyl compounds to form alcohols.
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Outline the mechanism for nucleophilic addition reactions of aldehydes and
ketones with hydrides.
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Reduction of carbonyl compounds
• Aldehydes and ketones are made by the
OXIDATION of alcohols.
• It follows that with a suitable reducing agent
carbonyl compounds should be reduced back
to alcohols.
• NaBH4 is used with warming and water or
ethanol as a solvent.
• The reaction can be considered as the
addition of hydrogen across the double bond,
though the actual reaction is nucleophilic
addition using H- as the nucleophile.
Week 3
Reduction of an aldehyde produces a primary alcohol
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Week 3
The primary alcohol propan-1-ol and the aldehyde propanal
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Week 3
Reduction of a ketone produces a secondary alcohol
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Week 3
NaBH4 readily generates hydride ions
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Week 3
Reduction of an aldehyde by nucleophilic addition
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Week 3
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Describe the use of 2,4-dinitrophenylhydrazine to detect a carbonyl group
and to identify a carbonyl compound.
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Describe the use of Tollens’ reagent to detect the presence of an aldehyde
group.
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Test for the Presence of a Carbonyl
Group
• 2,4-dinitrophenylhydrazine will give an orange
precipitate with any carbonyl compound (not
acids, esters etc).
• The reaction is an addition/elimination
reaction or condensation reaction which
involves addition across the double bond
followed by loss of water.
• 2,4-DNP or Brady’s reagent is used because
the big molecule gives better precipitates of
the hydrazone derivative with distinct and
diagnostic melting points.
Test for the Presence of a Carbonyl
Group
• If the hydrazone derivative is recrystallised
from hot ethanol the melting point
determination can be used to identify an
unknown carbonyl compound.
Week 3
Reaction of propanal with 2,4-dinitrophenylhydrazine
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Aldehyde or Ketone?
• Aldehydes are oxidised to carboxylic acids
with mild oxidising agents but ketones are
unaffected.
• This can be used to distinguish between them.
• Tollen’s Reagent – ammoniacal silver nitrate,
will oxidise an aldehyde when left to stand in
mildly warm,not hot water to a silver mirror.
• The oxidation of the aldehyde is shown after
this but the oxidising agent – Ag+ ions are
reduced to metallic silver.
• Ag+(aq) + e- → Ag(s)
Week 3
Oxidation of an aldehyde using Tollens’ reagent
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