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Questions and Answers
What is the primary characteristic that defines aromatic compounds?
What is the primary characteristic that defines aromatic compounds?
- They solely consist of carbon and hydrogen atoms.
- They readily undergo addition reactions like alkenes.
- They are exclusively liquids at room temperature.
- They contain rings with delocalized π electrons. (correct)
Which statement accurately describes the geometry and bond angles in a benzene molecule?
Which statement accurately describes the geometry and bond angles in a benzene molecule?
- Tetrahedral geometry with bond angles of 109.5°.
- Bent geometry with bond angles of 104.5°.
- Linear geometry with bond angles of 180°.
- Trigonal planar geometry with bond angles of 120°. (correct)
Why is benzene resistant to undergoing typical addition reactions that alkenes undergo?
Why is benzene resistant to undergoing typical addition reactions that alkenes undergo?
- The delocalized π electron system in benzene provides extra stability. (correct)
- The sigma (σ) bonds in benzene are too strong to be broken.
- Benzene is saturated, so it cannot undergo addition reactions.
- Benzene lacks pi (π) bonds, making it less reactive.
What occurs during the nitration of benzene?
What occurs during the nitration of benzene?
Which of the following is a necessary condition for Friedel-Crafts alkylation to occur?
Which of the following is a necessary condition for Friedel-Crafts alkylation to occur?
What is the role of $FeBr_3$ in the bromination of benzene?
What is the role of $FeBr_3$ in the bromination of benzene?
What is the product when benzene reacts with hydrogen gas ($H_2$) in the presence of a nickel (Ni) catalyst at 200°C?
What is the product when benzene reacts with hydrogen gas ($H_2$) in the presence of a nickel (Ni) catalyst at 200°C?
When naming arenes with multiple substituents, what determines the numbering of the ring?
When naming arenes with multiple substituents, what determines the numbering of the ring?
Which of the following substituents is considered a 2,4-directing group?
Which of the following substituents is considered a 2,4-directing group?
What is the effect of an activating group on the rate of electrophilic substitution in a benzene ring?
What is the effect of an activating group on the rate of electrophilic substitution in a benzene ring?
What is the major difference between the reaction conditions for benzene and methylbenzene in nitration?
What is the major difference between the reaction conditions for benzene and methylbenzene in nitration?
What type of reaction occurs when methylbenzene reacts with chlorine in the presence of UV light?
What type of reaction occurs when methylbenzene reacts with chlorine in the presence of UV light?
What product is formed when alkylbenzenes are treated with hot acidified potassium permanganate ($KMnO_4$)?
What product is formed when alkylbenzenes are treated with hot acidified potassium permanganate ($KMnO_4$)?
During the halogenation of methylbenzene with $AlCl_3$ as a catalyst, where does the chlorine primarily attach?
During the halogenation of methylbenzene with $AlCl_3$ as a catalyst, where does the chlorine primarily attach?
Why do arenes not typically undergo addition reactions?
Why do arenes not typically undergo addition reactions?
What is the role of concentrated sulfuric acid in the nitration of benzene?
What is the role of concentrated sulfuric acid in the nitration of benzene?
What type of reaction is Friedel-Crafts acylation?
What type of reaction is Friedel-Crafts acylation?
What is the key difference between side chain and ring halogenation of methylbenzene?
What is the key difference between side chain and ring halogenation of methylbenzene?
Which of these properties is typical of arenes?
Which of these properties is typical of arenes?
During Friedel-Crafts alkylation, what potential issue arises due to the carbocation intermediate?
During Friedel-Crafts alkylation, what potential issue arises due to the carbocation intermediate?
When benzene undergoes combustion with insufficient oxygen, what is mainly produced?
When benzene undergoes combustion with insufficient oxygen, what is mainly produced?
What does the term 'delocalized pi electrons' refer to in the context of benzene's structure?
What does the term 'delocalized pi electrons' refer to in the context of benzene's structure?
In the electrophilic substitution mechanism, which step is typically the rate-determining step?
In the electrophilic substitution mechanism, which step is typically the rate-determining step?
Which of the following is true about deactivating groups in electrophilic substitution?
Which of the following is true about deactivating groups in electrophilic substitution?
What structural feature of benzene is responsible for its planarity?
What structural feature of benzene is responsible for its planarity?
Which of the following is true regarding the rate of methylbenzene compared to benzene?
Which of the following is true regarding the rate of methylbenzene compared to benzene?
Why does benzene require a stronger electrophile than alkenes for electrophilic reactions?
Why does benzene require a stronger electrophile than alkenes for electrophilic reactions?
Which best describes the Friedel-crafts acylation reaction?
Which best describes the Friedel-crafts acylation reaction?
What is a condition required for benzene to form nitrobenzene?
What is a condition required for benzene to form nitrobenzene?
Flashcards
Aromatic Compounds
Aromatic Compounds
Hydrocarbons containing sigma bonds and delocalized pi electrons between carbon atoms in a ring.
Electrophilic Substitution
Electrophilic Substitution
A chemical reaction where a functional group on a compound is replaced by an electrophile.
Friedel-Crafts Reaction
Friedel-Crafts Reaction
An organic coupling reaction where substituents attach to aromatic rings through electrophilic aromatic substitution.
Nitration
Nitration
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Benzene structure
Benzene structure
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Combustion of Arenes
Combustion of Arenes
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Reactivity of Methyl Benzene
Reactivity of Methyl Benzene
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Free Radical Substitution
Free Radical Substitution
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Oxidation of Alkylbenzenes
Oxidation of Alkylbenzenes
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Halogenation of Arenes
Halogenation of Arenes
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Nitration of Methylbenzene
Nitration of Methylbenzene
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Friedel-Crafts Acylation
Friedel-Crafts Acylation
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Reactions of Substituted Benzene Rings
Reactions of Substituted Benzene Rings
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2,4-Directing Groups
2,4-Directing Groups
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3-Directing Groups
3-Directing Groups
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Study Notes
Arenes
- Arenes chemistry is exemplified by reactions of benzene and methyl benzene.
- Benzene structure can be described, and different benzene compounds named.
- The mechanism of electrophilic substitution in arenes can be described.
- Halogenation in arenes can be predicted to occur either in the side-chain or in the aromatic ring, depending on reaction conditions.
- In the electrophilic substitution of arenes, different substituents direct to different ring positions.
Definitions
- Aromatic hydrocarbons contain sigma bonds and delocalized pi electrons between carbon atoms in a ring.
- Electrophilic substitution is a chemical reaction where a functional group attached to a compound is replaced by an electrophile.
- The Friedel-Crafts reaction is an organic coupling reaction where substituents attach to aromatic rings through electrophilic aromatic substitution.
- Alkylation and acylation reactions are the two primary types of Friedel-Crafts reactions.
- Nitration is electrophilic substitution where a hydrogen atom is replaced by a nitro group (-NO2).
Benzene Structure
- Benzene structure consists of six carbon atoms arranged in a regular hexagon, joined to a hydrogen atom and to neighboring carbons by σ bonds.
- Carbons in benzene form 3 bonds instead of 4.
- P orbitals form sp2 hybrid bonds.
- Remaining p-orbitals are non-hybridized and have an unpaired electron, forming part of the central delocalized electron cloud.
- Six spare p orbitals, one on each carbon atom, are parallel and perpendicular to the ring's plane.
- Each carbon has a 120° bond angle and a trigonal planar shape.
- Carbon-carbon bonds are equal in length and strength.
- Six delocalized electrons at the center create a planar, regular hexagonal shape and prevent normal addition reactions.
- The π bond also makes benzene more stable.
Aromatic compounds
- Aromatic compounds have rings of delocalized electrons.
- Arenes consist of aromatic compounds like: Benzene, methylbenzene, and naphthalene.
- Benzene is the simplest arene.
Naming Arenes
- When naming arenes, the ring is numbered so you can indicate the positions of substituents on the benzene ring.
- The lowest order of priority ensures that the lowest number is achieved.
- Priority of functional groups: CO2H > OH > NH2 > Alkyl > Halogen > Aldehyde > NO2.
- If the benzene ring is a substituent, it is named phenyl.
Physical Properties
- Non-polar
- Liquid at room temperature due to strong intermolecular forces
- Colourless liquid with a characteristic odor
- Insoluble in polar solvents but soluble in polar solvents
- Boiling point is similar to equivalent cycloalkanes
- Poor conductor as non-polar
Reactivity
- Benzene's π bond has high electron density, which attracts electrophiles.
- Benzene requires very powerful electrophiles because of its stability, due to delocalized electrons.
- Bromine water and aqueous acid have no effect on benzene.
- Alkenes react by electrophilic addition, while arenes react by electrophilic substitution.
- In benzene, the carbocation intermediate loses a proton to reform the ring of π electrons after an electrophile attacks.
Reactions
- Combustion
- Addition
- Electrophilic substitution
Combustion
- Benzene and methylbenzene are components of unleaded petrol.
- They completely burn to carbon dioxide and steam in sufficient oxygen.
- C6H6 + 7.5O2 → 6CO2 + 3H2O
- Liquid arenes produce very smoky flames when lit, due to insufficient oxygen for complete combustion.
- A smoky flame indicates a compound with a high carbon:hydrogen ratio.
- C6H6 + 1.5O2 → 6C(s) + 3H2O
- The enthalpy of combustion of benzene is less exothermic than expected due to the stability of the six delocalized pi electrons.
Electrophilic Substitution
- A powerful electrophile is attracted to benzene's π bond, breaks the ring of electrons, and forms a σ bond to one of the carbon atoms.
- Two of the six π electrons are used to form the dative bond to the electrophile.
- The other four π electrons spread over the remaining five carbon atoms of the ring in a five-center delocalized orbital.
- The intermediate carbocation loses a proton, which reforms the sextet of π electrons.
Halogenation-bromination
- Benzene reacts with non-aqueous bromine on warming in the presence of anhydrous iron (III) bromide to form bromobenzene.
- The bromine electrophile is formed by reacting anhydrous iron (III) bromide with Br2.
- FeBr3 reacts with bromine by accepting a lone pair of electrons on bromine, causing strong polarization and weakening the bond, which leads to heterolytic breaking.
- Br - Br + FeBr3 → Br⁺ + [FeBr4]⁻
- The final stage regenerates the catalyst, by the reaction between the proton H⁺ formed with the [FeBr4]⁻: H⁺ + (FeBr4)⁻ → HBr + FeBr3
Halogenation-chlorination
- A similar electrophilic substitution reaction occurs when chlorine gas is bubbled through benzene at room temperature.
- A catalyst such as iron (III) chloride or aluminum chloride may be used.
- These catalysts are known as halogen carriers, e.g., FeBr3, AlCl3, and FeCl3.
Friedel-Crafts Alkylation
- When benzene is heated under reflux with chloroalkane in the presence of aluminum chloride, the alkyl group attaches to the benzene ring.
Friedel-Crafts Acylation
- A similar reaction occurs when benzene is refluxed with acyl chloride, ethanoyl chloride, and aluminum chloride as a catalyst.
Nitration
- When benzene reacts with a mix of concentrated nitric and sulphuric acid heated under reflux to around 50 °C, nitrobenzene forms.
- 2H2SO4 + HNO3 → 2HSO4⁻ + H3O⁺ + NO2⁺
Addition
- Benzene reacts with hydrogen gas and nickel catalyst to form cyclohexane at 200°C.
Reactions of Substituted Benzene Ring
- Substituted benzene rings undergo electrophilic substitution.
- Position and reaction rate are determined by the substituent's nature relative to unsubstituted benzene.
- Ortho and para substitutions occur at positions 2, 4, and 6.
- Meta substitutions occur at positions 3 and 5.
- Orientations depend on current substituent, not the electrophile.
2,4-Directing
- Groups that are either capable of donating electrons to the ring by the inductive effect, or they have a lone pair of electrons on the atom joined to the ring.
- This lone pair can incorporate into the π system by sideways overlap of p orbitals.
3-Directing
- Substituents favoring 3-substitution have a + atom joined directly to the ring.
Rate of Electrophilic Substitution
- Normally in 2,4-directing groups cause substitution faster than benzene, and are called activating groups (chlorine is an exception).
- 3- directing groups cause slower substitution than benzene, and are called deactivating groups.
- In electrophilic substitution, the rate-determining step is the attack of the electrophile on the ring.
- An activating group donates electrons into the ring, making it more negative and attracting the electrophile more strongly causing a faster reaction.
- A deactivating group withdraws electrons, the electrophile is less strongly attracted and the reaction occurs slowly.
Methyl Benzene (Toluene)
- Methyl benzene also reacts via electrophilic substitution, and is an activating group.
- Reaction conditions are milder than those of benzene's reactions.
- The methyl group donates electron density into the benzene ring, positive inductive effect.
- Increases electron density which increases attraction to electrophiles for a faster reaction.
- The methyl group is a 2,4- directing group.
Reactions
- Side chain reactions
- Free radical substitution
- Oxidation with aq KMnO4
- Ring reactions-electrophilic substitution
- Halogenation
- Alkylation
- Acylation
- Nitration
Side Chain Reactions
- Free Radical Substitution
- When methyl benzene reacts with chlorine in UV light, side chain substitution occurs.
- A hydrogen atom in the methyl group is substituted by a Cl atom.
- The mechanism involved is similar to free radical substitution in alkanes.
Oxidation
- When alkylbenzenes react with hot acidified potassium manganate (IV), then oxidation of the whole side chain occurs.
- The carbon closest to the ring is left as a carboxylic acid group.
Ring Reactions
- Halogenation
-- Methyl benzene reacts with chlorine and a halogen carrier catalyst AlCl3 (anhydrous) at room temperature.
- The products are 2-chloromethylbenzene and 4-chloromethylbenzene.
Nitration
- Methylbenzene reacts faster than benzene in nitration: The reaction is about 25 times faster.
- Lower temperatures can prevent substitution of more than one nitro group (30°C rather than 50°C).
- Using the same nitrating mixture of concentrated sulphuric and nitric acids is a must.
- The main products are: 2-nitromethylbenzene and 4-nitromethylbenzene.
- Higher temperatures (100°C) cause trinitration producing 2,4,6-trinitramethyl benzene
- Nitro groups are directed toward 2nd, 4th and 6th positions.
Friedel-Crafts Acylation
- The Friedel-Crafts Acylation reaction is similar with methylbenzene reaction except the acyl group attaches to the ring relative to the methyl group.
- Normally, the methyl group in methylbenzene directs new groups into the 2- and 4- positions (assuming the methyl group is in the 1- position).
- In acylation virtually all the substitution takes place in the 4- position.
Points to Note
- Arenes are hydrocarbons that contain one or more benzene rings.
- The benzene ring contains a delocalized group of six p electrons, which confers great stability.
- Despite their unsaturation, arenes do not undergo the same addition reactions as alkenes
- The preferred reaction type is electrophilic substitution.
- Aromatic compounds can be halogenated either in the ring or in the side chain, depending on the conditions.
- Aromatic compounds with alkyl side chains can be oxidized by potassium manganate (VII) to benzenecarboxylic acids.
- Under strong conditions with hydrogen, the benzene ring can undergo addition rather than substitution.
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