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.