Electrophilic substitution reactions of benzene with mechanism.

The most important/common reactions of benzene are electrophilic substitution reactions. In electrophilic substitution reactions an electrophile attacks the benzene and substitutes one of the hydrogen atoms of benzene ring to give substituted product.

Electrophiles : (Electron lovers) . Electrophiles are electron deficient species which attack on electron rich centre during chemical reactions. These are either a positively charged species or the neutral species having electron deficient centre. Eg. Cl+, CH3+, SO3, AlCl3, BF3, etc.

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Contents [hide]

• General mechanism of electrophilic substitution reactions
• Halogenation of benzene
  • Mechanism of halogenation of benzene
• Nitration of benzene
  • Mechanism of nitration of benzene
• Sulphonation of benzene
  • Mechanism of sulphonation of benzene
• Friedel – Craft’s reaction of benzene
  • Friedel Craft’s alkylation reaction
  • Mechanism of Friedel-Craft’s alkylation reaction
  • Friedel craft’s acylation reaction
  • Mechanism of Friedel-Craft’s acylation reaction:
• References

General mechanism of electrophilic substitution reactions

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Four types of electrophilic substitution reactions of benzene are described below with their mechanism.

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Halogenation of benzene

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Benzene reacts with halogen in presence of a Lewis acid such as FeCl3 to give halobenzene. For example, chlorine reacts with benzene in presence of ferric chloride or AlCl3 as catalyst to give chlorobenzene.

Similarly, benzene reacts with bromine in presence of ferric bromide as catalyst to give bromobenzene.

Mechanism of halogenation of benzene

This reaction involves the following steps:

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Nitration of benzene

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When benzene is heated with conc. HNO3 in the presence of conc. H2SO4 at about 600C gives nitrobenzene.

Mechanism of nitration of benzene

This reaction involves the following steps:

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Sulphonation of benzene

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When benzene is heated with conc. H2SO4 , benzene sulphonic acid is formed.

Mechanism of sulphonation of benzene

This reaction involves the following steps:

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Friedel – Craft’s reaction of benzene

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Friedel- Craft’s reaction is of two types- alkylation and acylation.

Friedel Craft’s alkylation reaction

Introduction of an alkyl group ( – R ) in the benzene ring by treating benzene with an alkyl halide (R-Cl or R-Br) in the presence of anhydrous AlCl3 is known as Friedel- Craft’s alkylation reaction. Eg.

Mechanism of Friedel-Craft’s alkylation reaction

This reaction involves the following steps:

Friedel craft’s acylation reaction

Introduction of an acyl group (i.e.keto group) (  ) in the benzene ring by treating benzene with an acylating agent like acid chloride (RCOCl) or acid anhydride in the presence of anhydrous AlCl3 is known as Friedel- Craft’s acylation reaction. Eg.

Similarly, benzene when treated with benzoyl chloride in the presence of anhydrous AlCl3 gives benzophenone. This reaction is also called benzoylation reaction.

Mechanism of Friedel-Craft’s acylation reaction:

This reaction involves the following steps:

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References

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• Bahl, B.S., A., Advanced Organic Chemistry, S. Chand and company Ltd, New Delhi, 1992.
• Finar, I. L., Organic Chemistry, Vol. I and Vol. II, Prentice Hall, London, 1995.
• Ghosh, S.K., Advanced General Organic Chemistry, Second Edition, New Central Book Agency Pvt. Ltd., Kolkatta, 2007.
• Sharma, M.L., Chaudary, P.N., A Text Book of B.Sc. Chemistry, Second edition (Volume one and two), Ekta Books, Kathmandu, 2004.
• Sthapit, M.K., Pradhananga, R.R., Foundations of Chemistry, Vol 1 and 2, Fourth edition, Taleju Prakashan, 2005.
• https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Arenes/Reactivity_of_Arenes/Substitution_Reactions_of_Benzene_Derivatives
• https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/friedel-crafts-reaction
• https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Reactions/Substitution_Reactions/Electrophilic_Substitution_Reactions/The_Halogenation_of_Benzene.

 

Aromatic Aldehydes and Ketones – Preparation and Properties

Contents 

• What are aromatic aldehydes and ketones?
• Preparation of benzaldehyde and acetophenone
• Properties of benzaldehyde
• REFERENCES

What are aromatic aldehydes and ketones?

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Aromatic aldehydes are the compounds in which –CHO group is bonded directly to an aromatic ring. Eg.

Aromatic ketones are the compounds in which carbonyl group is bonded with either both aryl group or aryl and alkyl group. Eg.

Note: The compound in which carbonyl group is not bonded directly to the benzene ring are considered as arly substituted aliphatic aldehydes. Eg.

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Preparation of benzaldehyde and acetophenone

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Preparation of benzaldehyde:

1. From toluene: Benzaldehyde is prepared by oxidation of toluene with cerium oxide (CeO2) in the presence of conc. H2SO4.

2. From Rosenmund’s reduction : Benzaldehyde is obtained by reducing benzoyl chloride with hydrogen in the presence of Pd catalyst deposited in BaSO4.

Preparation of acetophenone from benzene:

Acetophenone is prepared by the treatment of benzene with acetyl chloride in the presence of anhydrous AlCl3.

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Properties of benzaldehyde

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1. Cannizzaro’s reaction:

Aldehydes which do not contain α-hydrogen like HCHO, C6H5CHO,etc. undergo self oxidation and reduction on treatment with conc. alkali. In this reaction one molecule is oxidized to carboxylic acid and other molecule is reduced to alcohol. Thus, a mixture of an alcohol and a salt of carboxylic acid is formed by Cannizzaro’s reaction.

2. Perkin’s (condensation) reaction:

The condensation of an aromatic aldehyde with an acid anhydride in the presence of sodium or potassium salt of the same acid to produce α,β-unsaturated acid is known as the Perkin’s condensation.

3. Benzoin condensation reaction:

Benzaldehyde when heated with alcoholic solution of potassium cyanide, undergoes self condensation between two molecules to form an α-hydroxy ketone known as benzoin. This reaction is called benzoin condensation reaction. Eg.

4. Electrophilic substitution reaction:

-CHO group is electron withdrawing group. It withdraws ∏- electrons from benzene ring, decreasing electron density of aromatic ring.

Benzaldehyde is resonance hybrid of following resonance structures:

From the above resonating structures, it is clear that electron density is comparatively high at meta position. Thus incoming electrophile attacks at meta position to give meta substituted product. Thus –CHO is a meta directing group.

Similarly, acetophenone also undergoes electrophilic substitution reaction at meta position.

• Other reactions of aromatic aldehydes and ketones are similar to the reactions of aliphatic aldehydes and ketones.

REFERENCES

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• Ghosh, S.K., Advanced General Organic Chemistry, Second Edition, New Central Book Agency Pvt. Ltd., Kolkatta, 2007.
• Morrison, R.T. , Boyd, R.N., Organic Chemistry, Sixth edition, Prentice-Hall of India Pvt. Ltd., 2008.
• https://www.snapsolve.com/class11/chemistry/cbse-1100177433
• https://chemicalnote.com/aldehydes-and-ketones-carbonyl-compounds-preparation-and-properties/

 

Aldol condensation and Crossed aldol condensation reaction- Definition, Examples, Mechanism and Uses.

Contents [hide]

• Aldol condensation reaction
• Mechanism of aldol condensation reaction
• Dehydration of aldol products
• Uses of aldol condensation in synthesis
• Crossed aldol condensation reaction
• Mechanism of crossed-aldol condensation reaction
• References

Aldol condensation reaction

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Aldol condensation reaction is one of the important reactions of carbonyl compounds (i.e. aldehydes and ketones)

Condensation between two molecules of aldehydes or ketones having at least one α – hydrogen atom in presence of dilute alkali to form β-hydroxy aldehyde or β-hydroxy ketone is known as aldol condensation reaction. Examples:

Aldehydes and ketones which do not contain any α – hydrogen atom such as HCHO, (CH3)3CCHO, C6H5CHO, etc. do not undergo aldol condensation reaction.

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Mechanism of aldol condensation reaction

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Taking acetaldehyde (ethanal) as an example, aldol condensation involves following steps:

Step-I :

In this step, hydroxide ion from alkali removes a proton from the α – carbon of one molecule of ethanal to give a carbanion (i.e. enolate ion).

Step-II :

In this step, there is nucleophilic addition of enolate ion to the carbonyl carbon of second molecule of ethanal to produce an alkoxide ion.

Step-III :

In this step, the alkoxide ion takes up a proton from water to form β-hydroxy aldehyde (aldol).

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Dehydration of aldol products

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The product of aldol condensation on heating with dilute acids undergo dehydration to form α, β-unsaturated aldehydes or ketones. Examples:

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Uses of aldol condensation in synthesis

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Aldol products contain two functional groups (-OH and –CHO) and hence they can be used in a number of reactions to give various products. For example:

1. Catalytic hydrogenation of α, β-unsaturated aldehydes or ketones, obtained from dehydration of aldol products gives saturated alcohols.

2. Reduction of α, β-unsaturated aldehydes or ketones by LiAlH4 gives unsaturated alcohol.

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Crossed aldol condensation reaction

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• Aldol condensation between two different carbonyl compounds is called a crossed aldol condensation.
• Crossed aldol condensation is not useful in laboratories if both of the carbonyl compounds have α-hydrogen, because a mixture of products are formed.
• However, it is useful when one of the carbonyl compounds does not have α-hydrogen and therefore cannot undergo self condensation. For example: benzaldehyde can be used with other aldehydes and ketones containing α-hydrogen.

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Mechanism of crossed-aldol condensation reaction

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[same as that of aldol condensation reaction]

Taking acetaldehyde (ethanal) and benzaldehyde as examples of aldehydes, crossed aldol condensation involves following steps:

Step-I :

In this step, hydroxide ion from alkali removes a proton from the α – carbon of ethanal to give a carbanion (i.e. enolate ion).

Step-II :

In this step, there is nucleophilic addition of enolate ion to the carbonyl carbon of benzaldehyde to produce an alkoxide ion.

Step-III :

In this step, the alkoxide ion takes up a proton from water to form β-hydroxy aldehyde (aldol).

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References

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• Finar, I. L., Organic Chemistry, Vol. I and Vol. II, Prentice Hall, London, 1995.
• Ghosh, S.K., Advanced General Organic Chemistry, Second Edition, New Central Book Agency Pvt. Ltd., Kolkatta, 2007.
• Morrison, R.T. , Boyd, R.N., Organic Chemistry, Sixth edition, Prentice-Hall of India Pvt. Ltd., 2008.
• March, j., Advanced Organic Chemistry, Fourth edition, Wiley Eastern Ltd. India, 2005.