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]  Propan al +water

The carbonyl functional group

PLANAR WITH BOND ANGLES OF 120 °

The bond is polar due to the difference in electronegativity.

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 CH 3 C = O C = O H H

• Ketones have the carbonyl group bonded to two carbons.

CH 3 C 2 H 5 C = O C = O CH 3 CH 3

Aldehydes

Naming aldehydes/ketones

C 2 H 5 CHO propan al Ketones CH 3 CO CH 3 CH 3 CH 2 CO CH 3 propan butan one one CH 3 CO CH 2 CH 2 CH 3 pentan-2 one CH 3 CH 2 CO CH 2 CH 3 pentan-3 one C 6 H 5 CO CH 3 phenylethan one

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 • NaBH 4 reacts with ketones also.

• Propanone + 2[H]  propan-2-ol

Reduction explained • • The BH 4 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 NaBH 4

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 2,4 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

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 CH 3 COCH 3 CH 3 CH 2 CHO are structural isomers.

– Name these compounds and – 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 2,4 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.