Carbonyls

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Topic 8a – Carbonyl compounds
Revision Notes

1.Introduction

• Aldehydes and ketones are carbonyl compounds
• They contain the carbonyl group C=O
• The functional group in aldehydes is –CHO on the end of a chain e.g. ethanal CH3CHO • The functional group in ketones is C=O not at the end of a chain e.g. propanone CH3COCH3

2.AS Recap

• Primary alcohols are oxidised by acidified potassium dichromate. An aldehyde is produced first and this can be further oxidised to a carboxylic acid. To get the aldehyde, it must be distilled off as it is formed. To get the acid, heat under reflux. • Secondary alcohols are oxidised to ketones by acidified K2Cr2O7. Colour change is orange to green. • The C=O bond in aldehydes, ketones, carboxylic acids and esters can be identified by infrared spectroscopy. It produces a large peak around 1700 cm-1.

3.Reduction of Aldehydes and Ketones

Reduction, here, means addition of hydrogen.

a)Reduction using NaBH4

A specific reducing agent for aldehydes and ketones is sodium borohydride, NaBH4. In equations the reducing agent is represented by [H].

Aldehydes are reduced to primary alcohols by NaBH4 e.g.

CH3CHO + 2[H] ( CH3CH2OH

Ketones are reduced to secondary alcohols by NaBH4 e.g.

CH3COCH3 + 2[H] ( CH3CH(OH)CH3

Other points to note about this reaction are:
• It is an addition reaction (there is only one product) • The mechanism is called nucleophilic addition
• The nucleophile is H- which is provided by NaBH4

[pic]

b)Comparison with hydrogen gas

NaBH4 will reduce C=O double bonds but it will not reduce C=C double bonds

e.g.CH2=CH-CHO + 2[H] ( CH2=CH-CH2OH

To reduce both C=O and C=C use H2 with Ni catalyst

e.g.CH2=CH-CHO +2H2 ( CH3CH2CH2OH

4.Reaction with 2,4-dinitrophenylhydrazine (2,4-DNPH)

• Aldehydes and ketones react with 2,4-DNPH to give an orange precipitate (solid) • This is used a test for the presence of an aldehyde or ketone • The particular aldehyde or ketone can be identified by purifying (recrystallisation) the precipitate, measuring its melting point and comparing this with the melting points of known compounds (from a data book).

5.Reaction with Tollen’s Reagent

• Tollen’s reagent is ammoniacal silver nitrate.
• It is a mild oxidising agent that is used to distinguish between aldehydes and ketones. The compound to be tested is warmed with Tollen’s reagent. • Aldehydes are oxidised by Tollen’s reagent which is reduced to silver metal

CH3CHO + [O] (CH3COOH Oxidation
Ag+(aq) + e- (Ag(s)Reduction
silver mirror on inside of test tube

• Ketones do not react with Tollen’s reagent because they are not easily oxidised.

Topic 8b – Carboxylic acids and esters
Revision Notes

1.Carboxylic acids

• Carboxylic acids contain the functional group –COOH on the end of a chain. • They are weak acids (H+ donors). The acidic H is in the –COOH group e.g.

CH3COOH ( CH3COO- + H+(note – reversible reaction so ( not ()

• They are soluble in water because they can hydrogen bond to water molecules

a)Reactions of carboxylic acids

As they are acids they will react with metals, carbonates and bases e.g.

CH3COOH + Na ( CH3COONa + ½H2Fizzing seen
Ethanoic acidsodium ethanoateSodium dissolves

2CH3COOH + Na2CO3 ( 2CH3COONa + H2O + CO2Fizzing seenCarbonate dissolves

CH3COOH + NaOH ( CH3COONa + H2O

2.Esters

• Esters contain the functional group –COOR on the end of a chain • Making esters is called esterification
• Esters can be made in two ways: carboxylic acid + alcohol or acid anhydride + alcohol • Esters are sweet smelling...
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