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Chapter 14: Carbonyl Compounds

Chapter 14

Carbonyl Compounds

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As the carbonyl group is polar, both aldehydes and ketones are polar simple covalent molecules. However, they are unable to form intermolecular hydrogen bonds as they do not have a H atom bonded to a F/O/N atom. The table below highlights some of the important physical properties of carbonyl compounds:

Boiling point Solubility
(i) Both aldehydes and ketones have higher boiling points than alkanes with similar number of electrons

  • - More energy required to overcome the stronger permanent dipole permanent dipole attractions between the carbonyl molecules as compared to the instantaneous dipole induced dipole attractions between alkane molecules

(ii) Aldehydes and ketones have lower boiling points than alcohols or carboxylic acids with similar number of electrons

  • - More energy is required to overcome the stronger hydrogen bonds between
    alcohol or carboxylic acid molecules as compared to the pd-pd attractions between the carbonyl molecules
In water:
(i) Soluble in water (smaller aldehydes and ketones)

  • - Energy released from the formation of hydrogen bonds between carbonyl compounds and water is sufficient to overcome the permanent dipole permanent dipole interactions between carbonyl compound molecules and hydrogen bonds between water molecules

(ii) As the hydrocarbon length increases, solubilities of aldehydes and ketones decreases

  • - As hydrocarbon length increases, the molecule becomes more non-polar and the instantaneous dipole induced dipole interactions become more significant
  • - Energy released from hydrogen bonding between water and carbonyl molecule is insufficient to overcome the id-id interactions between the large hydrocarbon chain and the hydrogen bonds between water molecules
In organic solvent
(i) Generally soluble

  • - Energy released from id-id interactions between the carbonyl and solvent is sufficient to overcome the pd-pd interactions between the carbonyl molecule and the id-id interactions between the solvent molecule

The most common chemical reaction undertaken by carbonyl compounds is the nucleophilic addition reactions. A typical nucleophilic reaction is the addition of the HCN molecule to the carbonyl functional groups.

Here are some of the Commonly Asked Questions:

1. Why do carbonyl compounds attract nucleophiles?
Answer : The C atom of the -C=O group (the carbonyl carbon) bears the partial positive charge as it is bonded to a more electronegative oxygen atom. As a result, electron rich nucleophiles are attracted to this electron-deficient site.

2. Why do carbonyl compounds undergo addition reactions?
Answer : Simply, there is a C=O bond that is unsaturated.

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Now, let’s take a look at the Nucleophilic Addition mechanism below:

REAGENTS and CONDITIONS:

HCN + trace NaOH (aq) / trace NaCN (aq), cold

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Note: the role of HCN in the reaction is a bronsted acid while the role of NaCN in the reaction is to act as a catalyst by providing CN⁻ nucleophile initially.

Checklist when drawing nucleophilic addition mechanism

  • Name of the mechanism (nucleophilic addition)
  • Steps in sequential order (generation of nucleophile, nucleophilic attack on carbonyl carbon, regeneration of nucleophile)
  • Curved arrows showing movement of electrons (from nucleophile to carbonyl C, from C=O bond to O, from O⁻ to H in HCN, from H-C bond to C in HCN)
  • Charges and lone pair on intermediates and nucleophiles
  • Labels (fast, slow)

Just like in SN1 mechanism, the compound attacked by the nucleophile is trigonal planar in shape. Hence, in nucleophilic addition, the CN - nucleophile has an equal chance of attacking the trigonal planar C=O group from above or below the plane. If the resulting molecule is chiral, a racemic mixture is produced and the product is optically inactive.

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Between aldehydes and ketones, aldehydes are generally more reactive than ketones to nucleophilic attack. The table below highlights two main reasons.

Steric Factor Electronic Factor
Bulky hydrocarbon groups increase steric hindrance about the carbonyl carbon, hindering the approach of the attacking nucleophile hence ketones are less reactive than aldehydes as they are less susceptible to attack by nucleophiles Electron donating alkyl or aryl groups reduce the partial positive charge (electron deficiency) on the carbonyl carbon and hence decreases the susceptibility of the carbonyl carbon to nucleophilic attack, making ketones less reactive than aldehydes

As mentioned in previous chapters of organic chemistry, distinguishing tests make up an important bulk of concepts. In the chapter of carbonyl compounds, students are introduced to many different types of distinguishing tests to identify various compounds. Hence, the table below would provide a concise summary of the different tests that students are expected to familiarise with.

To distinguish Reagent and conditions Observation
Carbonyl compounds from other functional groups 2,4-DNPH, warm Carbonyl compounds: orange ppt formed

Non-carbonyl compounds: no orange ppt formed

Aldehydes from ketones KMnO4 (aq), H2SO4 (aq), heat Aldehydes: Purple KMnO4 is decolourised Ketones: Purple solution remains purple
K2Cr2O7 (aq), H2SO4 (aq),heat Aldehydes: Orange K2Cr2O7 turns green Ketones: Orange solution remains orange
Tollens’ reagent, warm Aldehydes: silver mirror is formed Ketones: No silver mirror formed
Aliphatic aldehydes from aromatic aldehydes Fehling’s solution, warm Aliphatic aldehydes: reddish brown ppt is formed

Aromatic aldehydes: no reddish brown ppt seen

Methyl carbonyl compounds (and methyl alcohol compounds as mentioned previously in hydroxy compounds chapter) I₂ (aq), NaOH (aq), warm Methyl carbonyls (and methyl alcohols) : yellow ppt is formed

Others: No yellow ppt observed

Test Yourself (Part I):

Suggest a structure for each of the isomers, A,B and C of the compound C3H6O2 , based on the following reactions, Explain which functional groups in each molecule are taking part in the reaction.

  • A gives tri-iodomethane and reduces Fehling’s solution
  • B gives tri-iodomethane but does not reduce Fehling’s solution
  • C does not give tri-iodomethane but does reduce Fehling’s solution

Solution:

1. A gives tri-iodomethane and reduces Fehling’s solution

The methyl group gives tri-iodomethane while the aldehyde group reduces Fehling’s solution.

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2. B gives tri-iodomethane but does not reduce Fehling’s solution

The methyl group gives tri-iodomethane while the ketone group is unable to reduce Fehling’s solution.

3. C does not give tri-iodomethane but does reduce Fehling’s solution

There is no methyl group to give tri-iodomethane and the ketone group is unable to reduce Fehling’s solution.

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Test Yourself (Part II):

Quadratic acid, used medically in the treatment of wart, is an unusual organic compound with molecular formula, C4H2O4 and has the following structure:

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  • Suggest what you would see when quadratic acid reacts with 2,4 dinitrophenylhydrazine. Write a balanced equation for the reaction.
  • Describe a test to distinguish between quadratic acid and CHOCH2CH2CHO.

Solution:

1. Orange precipitate

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Note: Since there is presence of 2 carbonyl groups in the organic compound C4H2O4 , two 2,4-dinitrophenylhydrazine molecules will react with C4H2O4

2. Reagents and Conditions: Tollens’ reagent, warm

Observations:
Quadratic acid: No silver mirror
CHOCH2CH2CHO: Silver mirror formed

Organic chemistry questions most notoriously come in the form of elucidation questions. But not to fear, they are conquerable.

Let’s walk through a sample elucidation question

Worked Example 1

Treating B with NaBH4 in methanol produces compound C, C3H8O2. C consists of a 50:50 mixture of two isomers, both of which give the same compound D, C3H6O, on passing their vapours over hot Al2O3. D gives no reaction with 2,4- dinitrophenylhydrazine but decolourises aqueous bromine.

Identify C and D.

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Solution:

Observation Type of reaction Functional Group present
Reacts with NaBH4 to produce C, which consists of 50:50 mixture of 2 isomers Reduction Diol present in C

Chiral carbon atom present in C

Aldehydes from ketones KMnO4 (aq), H2SO4 (aq), heat Aldehydes: Purple KMnO4 is decolourised Ketones: Purple solution remains purple
Reacts with Al2O3 Elimination Carbon carbon double bond present in D
No reaction with 2,4-DNPH Absence of condensation Absence of carbonyl functional group in D
Aqueous bromine decolourised Electrophilic addition Carbon carbon double bond present in D

Tip: Always list out the observations (stated in the question) and deductions (type of reaction, functional group present, etc) from the information provided neatly in a table form.

Answers C

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Answers D

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