CHEM 222 Organic Chemistry: Chap 15 Electrophilic Addition

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Questions and Answers

What factor greatly influences the amount of 1,2- and 1,4-addition products formed in electrophilic addition reactions of conjugated dienes?

The concentration of reactants

The presence of catalysts

The reaction conditions (correct)

The type of conjugated diene used

Which product becomes the major product at equilibrium when a mixture containing predominantly the 1,2-product is heated?

1,3-butadiene

1-bromo-2-butene (correct)

Butadiene

3-bromo-1-butene

What is the simplest aromatic hydrocarbon?

Naphthalene

Toluene

Benzene (correct)

Cyclohexane

Which of the following statements is true about the structure of benzene?

It is a cyclic compound with a six-membered ring. (B) Signup and view all the answers

What do the dashed lines in the resonance hybrid of benzene represent?

The position of the bonds (B) Signup and view all the answers

Benzene has how many π electrons?

6 (D) Signup and view all the answers

What is the bond length of carbon–carbon in benzene?

1.39 Å (C) Signup and view all the answers

Which of the following is NOT a characteristic of benzene?

It has four degrees of unsaturation. (D) Signup and view all the answers

What characterizes a kinetic product in a chemical reaction?

It is formed faster at lower temperatures and is irreversible. (C) Signup and view all the answers

Which of the following statements about thermodynamic products is true?

They depend on the stability of the product and are formed at equilibrium. (C) Signup and view all the answers

In the case of a reaction yielding multiple products, what determines which product is the kinetic product?

The product that is formed faster at lower temperatures. (C) Signup and view all the answers

What is meant by 'thermodynamic control' in chemical reactions?

The reaction eventually produces the most stable product. (C) Signup and view all the answers

In an electrophilic addition of HBr to conjugated dienes, which products are formed?

A mixture of 1,2- and 1,4-addition products. (C) Signup and view all the answers

Which factor is crucial in distinguishing between kinetic and thermodynamic products?

The temperature at which the reaction occurs. (D) Signup and view all the answers

If a reaction produces product B first and eventually equilibrates to produce product C, what type of product was B?

Kinetic product. (D) Signup and view all the answers

How does the concept of 'kinetic control' affect product distribution?

It favors the rapid formation of products over stability. (D) Signup and view all the answers

What describes the arrangement of carbon atoms in a benzene ring?

Each carbon atom is trigonal planar with bond angles of 120 degrees. (D) Signup and view all the answers

How is a substituted benzene compound named?

By naming the substituent first and adding the word benzene. (D) Signup and view all the answers

What is used to indicate the relative position of two substituents on a benzene ring?

The prefixes ortho, meta, or para. (A) Signup and view all the answers

What is the correct abbreviation for a phenyl group when represented in structures?

Ph− (C) Signup and view all the answers

Why does benzene exhibit unusual reactivity compared to alkenes?

It has a lower heat of hydrogenation than expected. (C) Signup and view all the answers

What are the criteria for a compound to be classified as aromatic?

Must be cyclic, planar, and have overlapping p orbitals. (D) Signup and view all the answers

What distinguishes a benzyl group from a phenyl group?

Benzyl groups contain a -CH2- link to benzene. (D) Signup and view all the answers

What does the term 'aryl groups' refer to?

Substituents that contain a benzene ring. (B) Signup and view all the answers

Which characteristic is necessary for a compound to be classified as aromatic?

It must be completely conjugated with a p orbital on every atom. (A) Signup and view all the answers

What is Hückel's rule regarding aromatic compounds?

It requires that a compound contains 6 π electrons. (A) Signup and view all the answers

What is the significance of the lone pair in pyrrole in terms of its aromaticity?

It is part of the delocalized π system and satisfies aromatic criteria. (B) Signup and view all the answers

Which type of compound is formed when two or more six-membered rings with alternating bonds are fused?

Poly Aromatic Hydrocarbons (PAH). (A) Signup and view all the answers

Why is cyclooctatetraene not considered aromatic?

It is nonplanar and does not allow for electron delocalization. (C) Signup and view all the answers

What happens to the structure of annulene due to strain?

It adopts a nonplanar conformation to relieve the strain. (C) Signup and view all the answers

Which of the following is an example of an aromatic heterocycle?

Pyridine. (D) Signup and view all the answers

How does pyrene differ from simpler aromatic compounds like benzene?

It contains more fused rings and thus more resonance structures. (D) Signup and view all the answers

What distinguishes the electronic configuration of pyridine from that of pyrrole?

Pyridine's nonbonded electron pair is localized on the N atom, while pyrrole's is delocalized over the ring. (A) Signup and view all the answers

Which statement is true regarding the cyclopentadienyl anion?

It possesses six π electrons and meets the criteria for aromaticity. (C) Signup and view all the answers

What is the significance of aromatic molecules being flat?

Flat structures allow for better resonance and electron delocalization. (A) Signup and view all the answers

Which feature of the tropylium cation confirms its aromaticity?

It has three double bonds and six π electrons. (B) Signup and view all the answers

Which statement about cyclopentadiene is correct?

It becomes aromatic upon loss of a proton, leading to increased stability. (D) Signup and view all the answers

Which of the following best describes anti-aromatic molecules?

They are cyclic, conjugated, and have (4n) π electrons. (B) Signup and view all the answers

What must a species fulfill to qualify as aromatic?

It must have a cyclic, conjugated structure and (4n + 2) π electrons. (B) Signup and view all the answers

Which statement regarding ionic aromatic compounds is accurate?

Both negatively and positively charged ions can be aromatic if they meet the criteria. (D) Signup and view all the answers

Study Notes

Kinetic vs Thermodynamic Products

Two situations can arise when multiple products are formed
Kinetic product forms faster; thermodynamic product is more stable at equilibrium
Kinetic product: irreversible reaction, depends on reaction rate
Thermodynamic product: reversible reaction, depends on product stability
If a reaction forms faster at lower temperatures, it's the kinetic product
If a reaction predominates at equilibrium, it's the thermodynamic product

Example: Kinetic vs Thermodynamic

Electrophilic addition of HBr to isolated dienes (separate double bonds) follows Markovnikov's rule, yielding one product
Electrophilic addition to conjugated dienes (connected double bonds) yields a mixture of 1,2- and 1,4-addition products

Mechanism 14.2: Electrophilic Addition of HBr to 1,3-Dienes

Step 1: H⁺ from HBr adds to a terminal carbon of the diene, forming a resonance-stabilized allylic carbocation
Step 2: Nucleophilic attack of Br⁻ can occur at either site of the allylic carbocation, forming 1,2- and 1,4-addition products

Reaction Conditions and Addition Products

The ratio of 1,2- to 1,4-addition products depends on reaction conditions
At low temperatures (e.g., -80°C), 1,2-addition is the major product
At high temperatures (e.g., 40°C), 1,4-addition is the major product

Kinetic vs Thermodynamic Products (cont.)

The isolated kinetic product can become the thermodynamic product when heated
The 1,2-product is faster, so it predominants at lower temperatures
The 1,4-product is more stable, so it predominates at equilibrium

Why Does This Occur?

The 1,4-product is more stable than the 1,2-product because it has more alkyl groups bonded to the double bond
Disubstituted alkenes are more stable than monosubstituted alkenes

Benzene

Benzene is the simplest aromatic hydrocarbon (C₆H₆)
It has four degrees of unsaturation: cyclic, with three double bonds
It does not undergo addition reactions like other double bonds, as it's more stable in its current form

Structure of Benzene

Benzene's structure must account for:
- A six-membered ring
- Three additional degrees of unsaturation
- Equal bond lengths
The Kekule description shows benzene as an equilibrium of two structures
Benzene's structure is a hybrid of the resonance structures, meaning electrons are delocalized around the ring

Resonance Hybrid of Benzene

The true structure of benzene is a resonance hybrid of two equivalent Lewis structures
The dashed lines in the hybrid depict the position of bonds
Delocalization of electrons makes the description of benzene easier

Representations of Benzene

Benzene has 6 π electrons
Actual bond length is intermediate between a single and double carbon-carbon bond (1.39 Å)

Each Carbon Atom in Benzene

Each carbon atom in the benzene ring has three atoms bonded to it and no lone pairs of electrons
Each carbon is sp² hybridized and trigonal planar with 120° bond angles
Each carbon has a p orbital with one electron that lies above and below the plane of the molecule
There's a high electron density above and below the ring plane

Naming Substituted Benzenes

To name a monosubstituted benzene, name the substituent and add the word "benzene"
Many monosubstituted benzenes have common names (e.g., toluene, phenol, aniline)

Nomenclature of Benzene Derivatives

For naming two groups on a benzene ring use prefixes: ortho (1,2), meta (1,3), para (1,4)
For clarity of the molecule, the numbers 1,2,3.4 are used.

Naming Benzene as a Substituent

A benzene substituent is a phenyl group (Ph-)
Formed by removing one hydrogen from benzene
Benzene can be represented as PhH; phenol would be PhOH

Benzyl and Aryl Groups

The benzyl group (C₆H₅CH₂-) is different from the phenyl group (C₆H₅-)
Other substituted aromatic rings are collectively called aryl groups

Unusual Reactivity of Benzene

Benzene exhibits lower than expected heat of hydrogenation compared to other highly unsaturated compounds
It does not undergo typical addition reactions
The reaction would involve a substitution without breaking or rearrangement of bonds within the ring.

What Accounts for Aromaticity

Four criteria must be met for a molecule to be aromatic:
- Cyclic
- Planar
- Completely conjugated
- Satisfy Hückel's rule (4n + 2 π electrons)

Larger Aromatic Rings

Rings larger than benzene that meet criteria for aromaticity are called annulenes
Number of atoms in the ring is indicated in brackets e.g, [14]-annulene

Example of Not Aromatic Ring

[10]-Annulene: has 10π electrons, but the ring is nonplanar to relieve strain, so it isn't aromatic

Fused Ring Aromatics

Two or more six-membered rings fused together with alternating double and single bonds can be aromatic
Examples include naphthalene, anthracene, phenanthrene

Resonance Structures of Fused Ring Aromatics

Resonance structures increase as more fused rings exist in the compound
For example, naphthalene is a hybrid of three resonance structures, similar to how benzene is a hybrid of two

Heterocycles

Heterocycles containing oxygen, nitrogen, or sulfur can also be aromatic
Lone pairs of electrons in the heterocycle can be localized on the heteroatom or part of the delocalized system
Examples include pyridine and pyrrole

Effect of Delocalization

In pyridine, the nonbonded electron pair on the nitrogen atom is localized in an sp² hybridized orbital
In pyrrole, the nonbonded electron pair on the nitrogen atom is in a p orbital and delocalized over the entire ring

Ionic Aromatic Compounds

Negatively and positively charged ions can be aromatic, provided the criteria for aromaticity are met, including satisfying Hückel's rule
Examples include cyclopentadienyl anion and tropylium cation

Cyclopentadienyl examples

Cyclopentadienyl structures: anion, cation, and radical
Aromaticity is determined by electron count and if the molecule is flat, cyclic, and conjugated

Cyclopentadiene and Acidity

Cyclopentadiene itself is not aromatic because it's not fully conjugated
Loss of a proton makes the product and stabilize the product
The conjugate base (cyclopentadienyl anion) is both fully conjugated and aromatic, hence, stable

Aromaticity in a cation

The tropylium cation (seven-membered ring with a positive charge, having 3 double bonds) is aromatic due to satisfying Huckel's rule (having 6 π electrons, cyclic, planar, and conjugated

Molecular Orbital Theory

Molecular Orbital Theory will be covered in a separate lecture

Remember

Kinetic vs thermodynamic reactions (control)
Aromaticity: definition
Aromatic criteria: methodology of determining aromaticity

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Description

Test your knowledge on electrophilic addition reactions of conjugated dienes and the properties of benzene. This quiz covers key concepts including the formation of 1,2- and 1,4-addition products, resonance structures, and the characteristics of kinetic and thermodynamic products. Perfect for students studying organic chemistry concepts related to hydrocarbon reactions.