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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)
• Ketones have the carbonyl group bonded to two carbons.
C = O
H
CH3
C = O
H
H
C = O
CH3
CH3
C = O
C2H5
CH3
Naming aldehydes/ketonesAldehydes 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 2chemical 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 2,4-dinitrophenylhydrazone 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
• Tollen’s reagent is a weak oxidising agent, the carbonyl group is oxidised to form a carboxylic acid.
Ammoniacal silver nitrate
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 3Identifying 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 2,4-DNPH 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.