Carbonyl compounds (aldehydes and ketones) (A-level organic chemistry)

Carbonyl compounds

These are compounds that contain carbonyl group

Aldehydes

These are carbonyl compounds with the following structure.

Nomenclature of aldehydes

1. Aldehyde are named by replacing the final “e” of the names of corresponding alkanes with “al.”
2. Since the aldehyde group must be at the end of the chain its position in not indicated.
3. When other substituents are present, the carbonyl carbon is assumed to occupy position one.

Example

HCHO                        Methanal (40% solution is called formalin) or formaldehyde

CH₃CHO                                        Ethanal or Acetaldehyde

CH₃CH₂CHO                                Propanal or propionaldehyde

CH₃CH₂CH₂CHO                        Butanal or butyraldenhyde

CH₃CH₂CH₂CH₂CHO               Pentanal or veraldehyde             

Nomenclature of ketones

1. Their names end in suffix “one”
2. The position of the ketone group (-CO-) is given the lowest number.

CH₃COCH₃                                           Propanone

CH₃CH₂COCH₃                                    Butanone

Preparation of carbonyl compounds

(a) Oxidation of alcohol with acidified potassium dichromate, sodium dichromate or potassium permanganate. Primary alcohols give aldehydes whereas secondary alcohols give ketones.

(b) Oxidation of alcohols using pyridiumchlorochromate [PCC, {C₅H₅NH⁺}Cl.CrO₃] and pyridinium dichromate [ PDC, {C₅H₅NH⁺}₂Cl.Cr₂O₇^2-] are currently the reagents of choice, particularly for oxidation of α,β- unsaturated primary and secondary alcohols to give aldehydes and ketones respectively.

(c) Hydrolysis of gem dihalide, (RCHCl₂, R₂CCl₂)

(d)  Reduction of acid chloride by hydrogen on palladium which is supported on barium sulphate or by using LiAlH(OBu^t).

(e) Cleavage of 1,2-diol using either lead tetraacetate or sodium metaperiodate

Example

(f) Preparation of ethanal

(i) By oxidation of ethane with palladium chloride in water

CH₂=CH₂ + H₂O + PdCl₂         →     CH3CHO

(ii) Passing ethyne through dil sulphuric acid in presence of mercury sulphate as a catalyst.

(iii) Oxidation ethanol

Physical properties

(a) The polar carbonyl group makes carbonyl compounds polar and therefore, the have melting and boiling points than non-polar compounds with similar molecular mass.

(b) Lower members are soluble in water but the solubility decreases with the increase in chain length.

Chemical properties

The structure of carbonyl compound is sp2 hybridized

As a result of the partial positive charge, the carbonyl carbon is subjected to nucleophilic attack by a large number of nucleophiles.

This results in nucleophilic addition reaction a cross the carbon-oxygen double bond. The nucleophilic addition to the carbon-oxygen double bond, can be regarded to occur in two possible ways generally.

(i) In presence of a strong nucleophile.

(ii) Weak nucleophile, the carbon oxygen bond has to be protonated first.

Generally, aldehydes undergo nucleophilic addition reaction more readily than ketones

Reasons

(i) Steric effect: the bulkiness of the alkyl groups attached to the carbonyl carbon hinders the approach of nucleophile.

(ii) Inductive effect: the positive inductive effect of the alkyl groups attached to the carbonyl carbon in ketones, reduce the positivity of the carbonyl carbon thus rendering it less reactive towards nucleophiles.

NB. Presence of an electron withdrawing group on the alkyl carbon make the carbonyl carbon more reactive towards nucleophilic addition reaction. For example, CCl₃CHO is more reactive than CH₃CHO.

General scheme of nucleophilic reaction to carbonyl compounds

(a) Reaction of carbonyl compounds with aldehydes and ketones.

The reaction for addition of HCN is catalyzed by NaOH or KOH

Mechanism

This reaction can be used to increase the carbon chain by one carbon atom

Exercise

Complete and write a mechanism

CH₃COCH₃ + KCN   →           

(b) Addition of sodium hydrogen sulphite, NaHSO₃.

Example

Mechanism

The reaction can be used to purify carbonyl compound since the products formed are solid. After crystallization, the can be redissolved.

(c) Addition of alcohols

1mole of aldehyde + 1mole of alcohol in presence of an acid the product is hemiacetal

1mole of aldehyde + 2moles of alcohol in presence of an acid is acetal.

The reaction is used to protect the aldehyde group in chemical synthesis.

Example

Mechanism

Note that

(i) Formation of acetals is sensitive to steric hindrances, i.e. depends on the size of groups attached to carbonyl carbon and the size of alcohol. Simple compounds give up to 80% yield, but the yield decrease with the increase in the size of the groups.

(ii) Reaction is reversible; therefore, it’s necessary to reduce the concentration of the acid to minimize the reversibility of the reaction.

(iii) Ketals are not prepared by direct reaction between ketones and alcohol. This is because the equilibrium of the reaction lies mainly to the left. In this case orthoformate is used.

2. Addition of ammonia derivatives

Carbonyl compounds react with compounds of the general formula H₂N-Y with elimination of water. The reaction is catalysed by acids. A reaction in which two molecules combine with elimination of small molecules e.g. water is called condensation reaction.

General equation

The products of these condensation reactions, i.e. oxime and hydrazones are orange crystalline solids with sharp melting points. Thus, they can be used to characterize carbonyl compounds. The most commonly used ammonia derivatives to characterize carbonyl compounds is 2,4-dinitrophenyl hydrazine (Brady’s reagent). Reaction of carbonyl compounds with this reagent produces orange colored crystalline solids.

3. Reduction of carbonyl compounds

Reduction of carbonyl compounds to alcohols

Aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols

  Reducing agents include

(a) H₂/catalyst (Ni, Pt, Pd): the disadvantage with this reagent is that it reduces double bonds when present.

mechanism

(ii) Reduction of carbonyl compounds to methylene. i.e. 

(a) Clemmensen’s reduction: it is useful for compounds that are stable under acidic conditions

It is carried out by refluxing a ketone with hydrochloric acid containing amalgamated zinc. Zinc and hydrochloric acid also reduces nitro groups to amines.

Example

(b) Hong-Mislon modification of Walf-Kishner reaction: a carbonyl compound is heated in presence of high boiling polar solvent, e.g. ethane-1,2-diol with hydrazide + KOH

4. Polymerization of aldehyde

Aldehyde(methanal) polymerizes, mainly under basic conditions.

Paraformaldehyde is a useful form for transportation of methanal

5. Reaction of Grignard’s reagents

Reaction of carbonyl compounds with Grignard’s reagents produces all the three types of alcohols. Thus this is an important reaction for the preparation of alcohols.

6. Aldol condensations

Carbonyl compounds which contain at least one alpha hydrogen, react in presence of alkali to form hydroxyl carbonyl compounds called Aldol

(i) Two molecules of ethanal combine to form 3-hydroxybutanal

Crotonaldehyde is used to calibrate spectrometer because it’s absorbance is known

mechanism

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(ii)

 

mechanism

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7. Cannizzaro reaction

This is a reaction between sodium hydroxide solution and aldehydes with no α-hydrogen

It’s  a self-oxidation – reduction reaction

Examples

             Mechanism

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8. Iodoform reactions

Iodine in presence of sodium hydroxide solution react with carbonyl compounds with structure   CH₃CO -R    to form a yellow ppt.                                           

       NB

(i) the reaction is useful when identifying compound with the group CH₃CO -R

(ii)  Ethanal is the only aldehyde that gives a positive iodoform test

 (iii) All ketone with the structure RCOCH₃ give positive iodoform test

(iv) Secondary alcohols of the group RCHOHCH₃ give positive iodoform test

(v) Ethanol is the only primary alcohol that gives a positive iodoform test.

Exercise

Give one reagent that can be used to distinguish between the following pairs of substances. In each case state what would be observed the reagent is treated separately with the reagent you have mentioned.

(i) CH₃OH  and CH₃CH₂OH

(ii) HCHO and CH₃CHO

(iii) CH₃COCH₂CH₂CH₃  and CH₃CH₂COCH₂CH₃

9. Oxidation of carbonyl compounds

Aldehydes are easily oxidized to carboxylic acid. The oxidizing agent, normally used are K₂Cr₂O₇/H⁺, Na₂Cr₂O₇/H⁺, KMnO₄/H⁺.

Example

Ketones are not oxidized under mild condition

Distinguishing between aldehydes and ketone

Reagent	Observation
	Aldehydes	Ketones
Fehling’s solution	Brown ppt	No observable change
Tollen’s reangent or ammoniacal silver nitrate	Black ppt or silver mirror	No observable change
Acidified potassium dichromate	Orange solution turns green	No observable change

Exercise

Give one reagent that can be used to distinguish between the following pairs of substances. In each case state what would be observed the reagent is treated separately with the reagent you have mentioned.

• CH₃CH₂CHO and CH₃COCH₃

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Dr. Bbosa Science