Carbonyl chemistry - MrFisherChemistry

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Carbonyl chemistry
-Production of carbonyl compounds
-Oxidation and Reduction reactions
-Further oxidation reactions.
Formation of carbonyl compounds
• Starting with an alcohol, react with an
oxidising agent (such as acidified potassium
dichromate)
• Propan-1-ol + [O]  Propanal +water
The carbonyl functional group
The bond is polar due to the difference in electronegativity.
PLANAR
WITH
BOND
ANGLES
OF 120°
ORBITAL
OVERLAP
NEW
ORBITAL
Aldehydes vs. Ketones
• Aldehydes have the carbonyl group at the end
of a carbon chain (joined to only one carbon)
H
CH3
C=O
H
C=O
H
• Ketones have the carbonyl group bonded to
two carbons.
CH3
C2H5
C=O
CH3
C=O
CH3
Naming aldehydes/ketones
Aldehydes
C2H5CHO
propanal
Ketones
CH3COCH3
propanone
CH3CH2COCH3
butanone
CH3COCH2CH2CH3
pentan-2-one
CH3CH2COCH2CH3
pentan-3-one
C6H5COCH3
phenylethanone
Identification of carbonyls
• IR spectroscopy shows a strong peak around
1400-1600 cm-1
Oxidation of carbonyl
• Reacting an aldehyde with an oxidising agent
(acidified dichromate ions) produces a
carboxylic acid.
• Propanal + [O]  Propanoic acid +water
Reduction of carbonyls
• Carbonyl groups can also be reduced.
• Reduction agent used is Sodium
tetrahydroborate(III) (also called Sodium
Borohydride)
• This is classed as a weak reducing agent.
Reduction of carbonyls
• Propanal is reduced to a primary alcohol by
sodium borohydride.
• Propanal + 2[H]  Propan-1-ol
Reduction continued
• NaBH4 reacts with ketones also.
• Propanone + 2[H]  propan-2-ol
Reduction explained
• The BH4 ion is acting as a source of hydride
ions (:H-)
» Hydride ions being a negatively charged hydrogen ion
containing a lone pair of electrons
• The mechanism is an example of nucleophilic
addition.
– H- ion is attracted to the δ+C of the carbonyl,
forming a new C-H bond
– The resulting O- ion forms a dative covalent bond
using the H+ from a water molecule
Reduction reaction mechanism
• Propanal + 2[H]  propan-1-ol
Carbonyl questions
• Write equations + mechanisms for the
reduction of:
– Butanal
– 2-methylhexan-3-one
• Define the term nucleophile
• Show the mechanism for the reaction
between butanone and NaBH4
Carbonyl chemistry 2
chemical tests on carbonyls
-How to detect a carbonyl group
-Aldehyde or ketone?
ILPAC experiment 8.4
• Using the instruction sheets, follow the
method for experiment 8.4 A and 8.4 B
• DO NOT DO EXPERIMENT C
• Make full and detailed notes on what you
observe during the reactions.
Condensation reactions
• We can detect the presence of a carbonyl
group using 2,4-dinitrophenylhydrazine
(abbreviated to 2,4-DNP or 2,4-DNPH).
• 2,4-DNPH is also
called Brady’s reagent when
prepared with methanol and
sulfuric acid.
Using 2,4-DNPH
• When Brady’s reagent is added to an aldehyde
or ketone, a yellow or orange precipitate is
formed.
• The precipitate, called a 2,4dinitrophenylhydrazone derivative confirms
the presence of a carbonyl functional group.
Aldehyde or ketone
• Aldehydes and ketones can be distinguished
from each other by using Tollen’s reagent
Ammoniacal silver nitrate
• Tollen’s reagent is a weak oxidising agent, the
carbonyl group is oxidised to form a carboxylic
acid.
Tollen’s reagent.
• Silver ions are reduced to silver metal
• Ag+(aq) + e-  Ag(s)
• Aldehyde + [O]  Carboxylic acid
Carbonyl questions 2
• The carbonyl compounds CH3COCH3 and
CH3CH2CHO are structural isomers.
– Name these compounds
– State the reagents used and observations made to
prove the presence of a carbonyl group in these
compounds
– State the reagents and observations made to
distinguish between the two chemicals.
Carbonyl chemistry 3
Identifying a carbonyl compound
-Determining experimentally an
unknown carbonyl compound
ILPAC 8.5
• Using the instruction sheets given last lesson,
carry out experiment 8.5, parts A and B
• Complete a detailed method for identifying a
carbonyl compound
– Purifying by recrystallisation -> melting point
determination -> compare melting point to known
values.
Carbonyl questions 3
• Explain how you would use the solid obtained
from the reaction of an aldehyde with 2,4DNPH to prove the original aldehyde was
butanal.
• The reaction of 2,4-DNPH with ethanal gives
water as a product. Suggest the type of
reaction taking place.

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