18.5 Side Chain Reactions of Benzenes | Organic Chemistry

Chad's Prep
13 Mar 202116:56
EducationalLearning
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TLDRThis lesson delves into the side chain reactions of benzene, focusing on the oxidation and reduction of substituents attached to the benzene ring. It begins with a reminder of the oxidation of primary alcohols using chromic acid or potassium permanganate, emphasizing the importance of the benzylic carbon in these reactions. The video explains that primary alcohols attached to the benzene ring can be oxidized to carboxylic acids, while quaternary benzylic carbons are resistant to oxidation. The discussion then shifts to benzylic bromination, highlighting the use of N-bromo-succinimide to introduce a bromine as a good leaving group, which opens up a range of further reactions including SN2 and E2. The Clemenson and Wolf-Kishner reductions are introduced as methods to deoxygenate ketones and aldehydes to alkanes, with the former being performed under acidic conditions and the latter under basic conditions. The video also touches on the reduction of nitro groups to amines using specific metals and acid, and the general reduction capabilities of catalytic hydrogenation. The lesson concludes with a call to action for viewers to like, share, and explore additional study materials for organic chemistry.

Takeaways
  • 🔬 The focus of the lesson is on side chain reactions of benzene, specifically looking at oxidation and reduction of substituents attached to the benzene ring.
  • ⚛️ Side chain oxidation involves the benzylic carbon and can lead to the formation of carboxylic acids, with the oxidation extent depending on the type of carbon (primary, secondary, or tertiary).
  • 🔄 Chromic acid and potassium permanganate are common oxidizing agents used in side chain oxidation, with the potential to oxidize primary alcohols to carboxylic acids.
  • 🚫 Quaternary benzylic carbons do not undergo oxidation due to the lack of hydrogens attached to the carbon.
  • 🌿 Benzylic bromination is a key reaction where a bromine atom substitutes a hydrogen on the benzylic carbon, creating a good leaving group for further reactions.
  • 🔬 Side chain reductions such as Clemenson and Wolff-Kishner reductions are methods to reduce ketones or aldehydes to alkanes, with Clemenson reduction occurring under acidic conditions and Wolff-Kishner under basic.
  • ⚠️ The choice between Clemenson and Wolff-Kishner reductions can be significant when the molecule contains alkenes or alkyl halides, as these functional groups react differently with the reagents used in each reduction method.
  • 🌟 Catalytic hydrogenation is a versatile reduction technique that can reduce a variety of functional groups including nitro, alkene, and carbonyl groups.
  • ➡️ Nitro groups on benzene can be selectively reduced to amines using metals like zinc, iron, or tin with acid, followed by neutralization with a base.
  • 📚 The lesson is part of an organic chemistry playlist released weekly, and students are encouraged to subscribe to the channel for updates.
  • 📘 For additional resources, including study guides and practice problems, students are directed to the instructor's premium course on chadsprep.com.
Q & A

  • What is the focus of this lesson on side chain reactions of benzene?

    -The focus of this lesson is on side chain oxidation and reduction for different substituents attached to the benzene ring, as well as a review of benzylic bromination and the implications of having a good leaving group on the benzylic carbon.

  • What are the two common oxidizing agents mentioned for the oxidation of primary alcohols?

    -The two common oxidizing agents mentioned are chromic acid (sodium or potassium dichromate with sulfuric acid) and potassium permanganate.

  • What is the role of the benzylic carbon in side chain oxidation?

    -The benzylic carbon is the primary site of interest in side chain oxidation. The oxidation process and the resulting products depend on the nature of the benzylic carbon, with primary and secondary alcohols being oxidizable, while quaternary carbons are not.

  • What happens when a benzylic carbon is quaternary in side chain oxidation?

    -When the benzylic carbon is quaternary, no oxidation takes place because it lacks hydrogens necessary for the oxidation process.

  • What is the significance of benzylic bromination in organic chemistry?

    -Benzylic bromination is significant because it introduces a bromine atom, which is a good leaving group, at the benzylic carbon. This allows for further reactions such as nucleophilic substitution (SN2), elimination (E2), and other reactions that can be useful in organic synthesis.

  • What are the Clemenson and Wolff-Kishner reductions?

    -The Clemenson reduction is the reduction of a ketone or aldehyde to an alkane using a zinc amalgam and aqueous hydrochloric acid. The Wolff-Kishner reduction is a similar process that uses hydrazine under highly basic conditions with potassium hydroxide and heat.

  • Why is the choice between Clemenson and Wolff-Kishner reduction important when there is an alkene or alkyne present in the molecule?

    -The choice is important because the Clemenson reduction can lead to addition across alkenes and alkynes due to the presence of hydrohalic acids, while the Wolff-Kishner reduction, being under basic conditions, does not cause such addition reactions.

  • What is the difference in the reaction of an alkyl halide with the Clemenson and Wolff-Kishner reductions?

    -With the Wolff-Kishner reduction, the presence of a strong base and nucleophile (potassium hydroxide) can lead to SN2 substitution of the halide, while the Clemenson reduction, lacking a strong nucleophile, will not react with the alkyl halide.

  • How does catalytic hydrogenation with H2 and a palladium catalyst (H2/Pd) affect a benzylic ketone?

    -Catalytic hydrogenation with H2/Pd can completely deoxygenate a benzylic ketone, reducing it to an alkane, which is functionally equivalent to the Clemenson or Wolff-Kishner reductions.

  • What is the specific reduction method for a nitro group to an amine without affecting other functional groups like alkenes?

    -The specific reduction of a nitro group to an amine without affecting alkenes can be achieved using one of three metals: zinc, iron, or tin, with acid (usually HCl), followed by neutralization with hydroxide to deprotonate the formed amine.

  • Why is it necessary to neutralize the amine formed during the specific reduction of a nitro group?

    -Neutralization is necessary because the amine forms under acidic conditions and gets protonated. Adding hydroxide deprotonates the amine, converting it back to its conjugate base, which is the desired product.

Outlines
00:00
🔍 Benzene Side Chain Reactions Overview

This paragraph introduces the focus on side chain reactions of benzene, contrasting with the previous lesson on pi electron reduction. It discusses the oxidation and reduction of substituents attached to the benzene ring, benzylic bromination, and the importance of a good leaving group on the benzylic carbon. The lesson is part of an organic chemistry series released weekly throughout the school year, and viewers are encouraged to subscribe for updates. The paragraph also reviews the oxidation of primary alcohols using chromic acid or potassium permanganate, highlighting the relevance to side chain oxidation and the role of the benzylic carbon in these reactions.

05:01
🌟 Side Chain Oxidation and Reduction Reactions

The second paragraph delves into side chain oxidation, emphasizing the importance of the benzylic carbon and its hydrogen content in determining the possibility of oxidation. It explains that a quaternary benzylic carbon will not undergo oxidation. The paragraph also covers the oxidation of carbon side chains, leading to carboxylic acids, and the cleaving of carbon-carbon bonds. It transitions into a review of benzylic bromination with N-bromo-succinimide, discussing the stability of the radical intermediate and the subsequent reactions that can occur due to the presence of a good leaving group, such as SN1, SN2, E1, and E2 reactions. The paragraph concludes with an introduction to side chain reductions, specifically the Clemenson and Wolf-Kishner reductions, and their conditions and outcomes.

10:02
🔬 Clemenson and Wolf-Kishner Reductions

This paragraph provides a detailed comparison between the Clemenson and Wolf-Kishner reductions, both of which achieve complete deoxygenation of ketones and aldehydes to alkanes. It outlines the reagents and conditions for each reduction method, noting that the Clemenson reduction occurs under acidic conditions with a zinc amalgam and hydrochloric acid, while the Wolf-Kishner reduction takes place under basic conditions with hydrazine and potassium hydroxide. The paragraph also discusses the synthetic utility of these reactions, particularly in avoiding carbocation rearrangements in Friedel-Crafts alkylation reactions. Additionally, it highlights situations where the choice between the two reduction methods can be significant, such as when an alkene or alkyl halide is present in the molecule.

15:02
⚖️ Selective Reductions and Nitro Group Reduction

The final paragraph focuses on the selective reduction of the nitro group to an amine using specific metals like zinc, iron, or tin with acid, followed by neutralization with hydroxide. It contrasts this specific reduction with the broader effects of catalytic hydrogenation, which can reduce a variety of functional groups, including alkenes, alkynes, and ketones. The paragraph also mentions a special case where catalytic hydrogenation on the benzylic carbon results in deoxygenation, similar to Clemenson or Wolf-Kishner reductions. It concludes with a call to action for viewers to like, share, and check out additional resources for further study and practice.

Mindmap
Keywords
💡Benzene
Benzene is an organic chemical compound with the molecular formula C6H6. It is a hydrocarbon and the simplest aromatic ring, consisting of six carbon atoms joined in a planar ring with alternating single and double bonds. In the video, benzene serves as the central structure to which various substituents can be attached, and reactions such as side chain oxidation and reduction are discussed in relation to it.
💡Side Chain Oxidation
Side chain oxidation refers to the chemical process where the substituent attached to the benzene ring is oxidized. The video explains that this process focuses on the benzylic carbon, which must have at least one hydrogen to undergo oxidation. The oxidation can lead to the formation of carboxylic acids, and the video distinguishes between primary, secondary, and tertiary alcohols in this context.
💡Benzylic Bromination
Benzylic bromination is a chemical reaction where a bromine atom is added to a benzylic carbon, which is a carbon atom directly attached to a benzene ring. The video mentions the use of N-bromo-succinimide (NBS) in this reaction, resulting in a stable radical intermediate on the benzylic carbon. This reaction is significant as bromine acts as a good leaving group, which can be replaced in subsequent reactions.
💡Clemenson Reduction
The Clemenson reduction is a chemical reaction that reduces a ketone or aldehyde to an alkane by complete deoxygenation. The video explains that this reduction is applicable to benzylic ketones or aldehydes, and it involves the use of a zinc amalgam and aqueous hydrochloric acid. The Clemenson reduction is highlighted as a method to avoid carbocation rearrangements that can occur in Friedel-Crafts alkylation.
💡Wolff-Kishner Reduction
The Wolff-Kishner reduction is another method for reducing ketones or aldehydes to alkanes, but it operates under basic conditions using hydrazine and potassium hydroxide. The video points out that while both the Clemenson and Wolff-Kishner reductions achieve the same end product, the Wolff-Kishner reduction could lead to additional reactions with alkenes or alkyl halides present in the molecule.
💡Alkene Addition Reactions
Alkene addition reactions involve the reaction of alkenes with other molecules to form new compounds, often increasing the number of atoms in the molecule. The video discusses how alkenes can undergo Markovnikov addition when treated with hydrohalic acids like HCl, which is a consideration when choosing between the Clemenson and Wolff-Kishner reductions.
💡Nitro Reduction
Nitro reduction is the chemical process of converting a nitro group (-NO2) into an amine group (-NH2). The video describes the use of catalytic hydrogenation with a palladium catalyst (H2/Pd) for this purpose. It's also mentioned that specific metals like zinc, iron, or tin with acid can be used for a more selective reduction of the nitro group without affecting other functional groups like alkenes.
💡Carbocation Rearrangements
Carbocation rearrangements are a type of isomerization reaction that can occur during the formation of carbocations, where the positive charge moves to a more stable position. The video discusses how these rearrangements can be problematic in Friedel-Crafts alkylation, leading to less desirable products, but can be avoided using Friedel-Crafts acylation followed by reduction.
💡Friedel-Crafts Acylation
Friedel-Crafts acylation is a chemical reaction where an acyl group is added to an aromatic ring in the presence of a Lewis acid catalyst. The video mentions that this reaction results in the formation of a ketone at the benzylic position and is favored over Friedel-Crafts alkylation to avoid carbocation rearrangements.
💡Quaternary Carbon
A quaternary carbon is a carbon atom that is bonded to four other carbon atoms with no hydrogen atoms attached. The video explains that benzylic carbons that are quaternary do not undergo oxidation, which is a key consideration when discussing side chain oxidation reactions.
💡Chiral Center
A chiral center is a carbon atom that is bonded to four different groups, giving rise to stereoisomers. The video discusses the formation of a chiral center during an SN2 reaction with a benzylic halide and the potential for obtaining a racemic mixture as a result of backside attack by a nucleophile.
Highlights

Side chain reactions of benzene are the focus of the lesson, including oxidation and reduction of substituents attached to the benzene ring.

Benzylic bromination is discussed, emphasizing the creation of a good leaving group on the benzylic carbon.

Oxidation of primary alcohols using chromic acid or potassium permanganate is reviewed, noting the potential for two-step oxidation to carboxylic acids.

The importance of the benzylic carbon in side chain oxidation is emphasized, with oxidation not occurring on quaternary carbons.

Side chain oxidation can lead to the cleavage of all carbon-carbon bonds except the one to the benzene ring.

Benzylic bromination with N-bromo-succinimide results in a stable radical intermediate, allowing for further reactions like SN1, SN2, and E2.

Clemenson and Wolff-Kishner reductions are introduced for the reduction of ketones or aldehydes to alkanes, with different reagents and conditions.

The Clemenson reduction uses zinc amalgam and hydrochloric acid, while Wolff-Kishner reduction uses hydrazine under basic conditions.

Friedel-Crafts acylation is highlighted as a method to avoid carbocation rearrangements and produce unrearranged products.

The reduction of nitro groups to amines using specific metals and acid, followed by neutralization, is explained.

Catalytic hydrogenation with H2 and a palladium catalyst is shown to reduce a variety of functional groups, including nitro to amine.

The specificity of nitro group reduction without affecting alkenes is demonstrated using iron.

A reminder that the choice between Clemenson and Wolff-Kishner reductions can be crucial depending on the presence of alkenes or alkyl halides in the molecule.

The potential side reactions with alkenes or alkyl halides during reduction, and the conditions under which they occur, are discussed.

The impact of reaction conditions on the outcome of reductions, such as the use of acidic versus basic conditions, is highlighted.

The lesson provides a comprehensive overview of organic chemistry reactions involving benzene side chains, including oxidation, bromination, and reduction.

Practical applications of the discussed reactions in synthesis are explored, emphasizing their utility in creating specific products.

The importance of understanding the mechanism and conditions of each reaction for successful synthesis is stressed throughout the lesson.

Transcripts

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