Organic Chemistry Exam Study Guidance: Radicals, Resonance, Aromaticity, and Reactions

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Q1.1 Draw the major mono-substitution product when the following molecule is heated with Br2. Be sure to include stereochemistry in your answer if applicable.

Background

Topic: Free Radical Halogenation

This question tests your understanding of radical bromination, regioselectivity, and stereochemistry in organic molecules.

Key Terms and Formulas:

Free radical halogenation: A reaction where a halogen (like Br2) reacts with an alkane to substitute a hydrogen atom with a halogen atom via a radical mechanism.
Regioselectivity: Bromine tends to substitute at the most stable radical position (often tertiary > secondary > primary).
Stereochemistry: If the substituted carbon is chiral, consider possible stereoisomers.

Step-by-Step Guidance

Identify the most reactive hydrogen atom in the molecule (usually the one that forms the most stable radical upon abstraction).
Draw the structure of the molecule after replacing that hydrogen with a bromine atom.
If the carbon where substitution occurs is chiral, indicate the stereochemistry (use wedges/dashes if required).
Check if more than one stereoisomer is possible, and note them if applicable.

Try solving on your own before revealing the answer!

Q1.2 For the following transformation:

Background

Topic: Radical Halogenation and Stereochemistry

This question tests your ability to predict products of radical halogenation and to determine the number of stereoisomers formed.

Key Terms and Formulas:

Mono-substitution: Only one hydrogen is replaced by a halogen.
Stereoisomers: Molecules with the same connectivity but different spatial arrangements.

Step-by-Step Guidance

Identify the site of halogenation (where the radical is most stable).
Draw the product with the halogen substituted at that position, ignoring stereochemistry for part (a).
For part (b), determine if the substituted carbon is chiral or if new chiral centers are created.
Count the possible stereoisomers based on the number of chiral centers.

Try solving on your own before revealing the answer!

Q1.2 The reaction shown produces two brominated products. What is the other one?

Background

Topic: Radical Halogenation – Regioselectivity

This question tests your ability to identify all possible mono-brominated products from a radical halogenation reaction.

Key Terms and Formulas:

Regioselectivity: Bromine prefers the most stable radical, but less stable radicals can also form minor products.
Mono-bromination: Only one bromine atom is added per molecule.

Step-by-Step Guidance

Identify all possible sites where bromine can substitute a hydrogen atom.
Draw the structure for the alternative product (the one not already drawn).
Consider the relative stability of the radicals formed at each site.

Try solving on your own before revealing the answer!

Q1.3 What monomer is used to make the following polymer?

Background

Topic: Polymer Chemistry – Monomers and Polymers

This question tests your ability to deduce the monomer structure from a given polymer repeat unit.

Key Terms and Formulas:

Monomer: The small molecule that repeats to form a polymer.
Polymer: A large molecule made up of repeating monomer units.

Step-by-Step Guidance

Examine the polymer repeat unit and identify the structural features that could come from a monomer.
Reverse the polymerization process to deduce the monomer structure.
Draw the monomer, ensuring it can link to form the given polymer.

Try solving on your own before revealing the answer!

Q1.4 The free-radical chlorination of cyclohexane is shown below. Provide the first propagation step, i.e., the one in which Cl• reacts. DO NOT draw curly arrows.

Background

Topic: Radical Chain Mechanism

This question tests your understanding of the propagation steps in a radical halogenation reaction.

Key Terms and Formulas:

Propagation step: The stage in a radical chain reaction where a radical reacts with a stable molecule to produce a new radical and a product.
Cl•: Chlorine radical.

Step-by-Step Guidance

Write the equation for the reaction between Cl• and cyclohexane.
Show the products: a cyclohexyl radical and HCl.
Ensure the equation balances in terms of atoms and radicals.

Try solving on your own before revealing the answer!

Q2.1 For the following ion:

Background

Topic: Resonance and Hybridization

This question tests your ability to draw resonance forms, use curved arrows to show electron flow, and determine hybridization.

Key Terms and Formulas:

Resonance: Delocalization of electrons across multiple atoms.
Curved arrows: Indicate movement of electrons.
Hybridization: The mixing of atomic orbitals to form new hybrid orbitals (sp, sp2, sp3).

Step-by-Step Guidance

Draw the original ion structure.
Use curved arrows to show how electrons move to create a resonance form.
Draw the most significant resonance form (the one with the most stable charge distribution).
Explain why this resonance form is the most significant (consider charge placement and octet rule).
For hybridization, examine the oxygen atom's bonding and lone pairs to determine its hybridization.

Try solving on your own before revealing the answer!

Q2.2 The following diene is treated with one equivalent of HCl.

Background

Topic: Electrophilic Addition to Conjugated Dienes

This question tests your understanding of kinetic vs. thermodynamic control in reactions of conjugated dienes.

Key Terms and Formulas:

Kinetic product: The product formed fastest, often at the less substituted carbon.
Thermodynamic product: The most stable product, often at the more substituted carbon.
Conjugated diene: A molecule with alternating double bonds.

Step-by-Step Guidance

Draw the possible carbocation intermediates after HCl adds to the diene.
Draw the products formed from each intermediate (ignore stereochemistry for now).
Identify which product is formed under kinetic control and which under thermodynamic control.

Try solving on your own before revealing the answer!

Q2.3 Below is an MO energy level diagram for the allyl system, H2C=CH2–CH2.

Background

Topic: Molecular Orbital Theory – Allyl System

This question tests your ability to assign electrons to molecular orbitals, label orbitals, and identify HOMO/LUMO.

Key Terms and Formulas:

HOMO: Highest Occupied Molecular Orbital.
LUMO: Lowest Unoccupied Molecular Orbital.
π, π*, n: Bonding, anti-bonding, and non-bonding orbitals.

Step-by-Step Guidance

Determine the number of π electrons in the allyl cation.
Assign electrons to the MO diagram (use up and down arrows).
Label each orbital as bonding (π), anti-bonding (π*), or non-bonding (n).
Identify which orbital is the HOMO and which is the LUMO.

Try solving on your own before revealing the answer!

Q2.4 Which molecule absorbs UV–visible light at the longest wavelength?

Background

Topic: UV–Visible Spectroscopy and Conjugation

This question tests your understanding of how conjugation affects absorption wavelength in UV–visible spectroscopy.

Key Terms and Formulas:

Conjugation: Extended π systems lower the energy gap, increasing absorption wavelength.
λmax: Wavelength of maximum absorption.

Step-by-Step Guidance

Compare the degree of conjugation in each molecule.
Identify which molecule has the most extended π system.
Recall that more conjugation means longer λmax.

Try solving on your own before revealing the answer!

Q2.5 Choose the major product of the following Diels–Alder reaction, indicating stereochemistry.

Background

Topic: Diels–Alder Reaction – Stereochemistry

This question tests your ability to predict the major product and its stereochemistry in a Diels–Alder reaction.

Key Terms and Formulas:

Diels–Alder reaction: [4+2] cycloaddition between a diene and a dienophile.
Stereochemistry: Endo vs. exo products, use of wedges/dashes.

Step-by-Step Guidance

Draw the diene and dienophile structures.
Show the cycloaddition, forming the new ring.
Indicate the stereochemistry of substituents (endo rule).

Try solving on your own before revealing the answer!

Q2.6 Rank the following dienophiles for reactivity in the Diels–Alder reaction (1 = most reactive).

Background

Topic: Diels–Alder Reactivity

This question tests your understanding of how substituents affect dienophile reactivity.

Key Terms and Formulas:

Dienophile: The alkene or alkyne that reacts with a diene.
Electron-withdrawing groups: Increase dienophile reactivity.

Step-by-Step Guidance

Identify the substituents on each dienophile.
Rank them based on the number and strength of electron-withdrawing groups.
Assign numbers (1 = most reactive) to each.

Try solving on your own before revealing the answer!

Q3.1 Choose the correct IUPAC name for the following compound.

Background

Topic: Aromatic Nomenclature

This question tests your ability to apply IUPAC rules to name substituted benzene derivatives.

Key Terms and Formulas:

IUPAC nomenclature: Systematic naming of organic compounds.
Substituent positions: ortho (2), meta (3), para (4).

Step-by-Step Guidance

Identify the functional groups attached to the benzene ring.
Assign numbers to the ring positions according to IUPAC rules.
Choose the correct name from the options given.

Try solving on your own before revealing the answer!

Q3.2 Which nitrogen atom in histamine is LEAST basic? Explain briefly.

Background

Topic: Basicity in Aromatic Heterocycles

This question tests your understanding of how resonance and hybridization affect basicity in nitrogen atoms.

Key Terms and Formulas:

Basicity: Ability of a molecule to accept a proton.
Resonance: Delocalization of lone pairs can decrease basicity.

Step-by-Step Guidance

Identify the nitrogen atoms in histamine.
Analyze their electronic environments (are lone pairs involved in resonance?).
Determine which nitrogen is least able to accept a proton.

Try solving on your own before revealing the answer!

Q3.3 Classify the molecules below as aromatic, non-aromatic, or anti-aromatic. Assume the rings are planar.

Background

Topic: Aromaticity Criteria

This question tests your ability to apply Huckel's rule and other criteria to classify ring systems.

Key Terms and Formulas:

Aromatic: Planar, cyclic, fully conjugated, and 4n+2 π electrons.
Anti-aromatic: Planar, cyclic, fully conjugated, and 4n π electrons.
Non-aromatic: Does not meet criteria for aromaticity or anti-aromaticity.

Step-by-Step Guidance

Count the number of π electrons in each ring.
Check if the ring is planar and fully conjugated.
Apply Huckel's rule to determine aromaticity.

Try solving on your own before revealing the answer!

Q3.4 Use the inscribed polygon (Frost’s circle) method to show the relative energy levels and electron configuration of the cyclopropenyl radical. Is the ion aromatic, anti-aromatic or not aromatic? Explain with reference to Huckel’s rule.

Background

Topic: Frost’s Circle and Aromaticity

This question tests your ability to use Frost’s circle to determine MO energy levels and apply Huckel’s rule.

Key Terms and Formulas:

Frost’s circle: A method to visualize MO energy levels in cyclic systems.
Huckel’s rule: Aromatic if 4n+2 π electrons, anti-aromatic if 4n π electrons.

Step-by-Step Guidance

Draw the Frost’s circle for cyclopropenyl radical.
Place the appropriate number of electrons in the energy levels.
Determine if the electron configuration matches aromatic or anti-aromatic criteria.
Explain your reasoning using Huckel’s rule.

Try solving on your own before revealing the answer!

Q3.5 The structure of the aromatic nucleobase uracil is shown below.

Background

Topic: Resonance in Aromatic Heterocycles

This question tests your ability to draw resonance structures and show electron delocalization in aromatic rings.

Key Terms and Formulas:

Resonance: Delocalization of π electrons.
Curved arrows: Show electron movement.

Step-by-Step Guidance

Draw the original structure of uracil.
Use curved arrows to show electron movement creating a resonance structure.
Draw the resonance structure with delocalized π electrons.

Try solving on your own before revealing the answer!

Q4.1 Provide a detailed mechanism (provide electron arrows and intermediates) for the acetylation of benzene using only the species shown.

Background

Topic: Electrophilic Aromatic Substitution – Acetylation

This question tests your ability to draw mechanisms for Friedel–Crafts acetylation, including intermediates and electron flow.

Key Terms and Formulas:

Electrophilic aromatic substitution: Benzene reacts with an electrophile to substitute a hydrogen.
Acetylation: Introduction of an acetyl group (–COCH3).
Mechanism: Show intermediates and electron movement with arrows.

Step-by-Step Guidance

Draw the generation of the acetyl cation (electrophile) from the reagents provided.
Show benzene attacking the acetyl cation, forming a sigma complex (arenium ion).
Draw the deprotonation step to restore aromaticity.
Include all intermediates and electron arrows.

Try solving on your own before revealing the answer!

Q4.2 Which is the least reactive toward electrophilic aromatic substitution?

Background

Topic: Reactivity of Aromatic Compounds

This question tests your understanding of how substituents affect the reactivity of benzene rings toward electrophilic substitution.

Key Terms and Formulas:

Electron-withdrawing groups: Decrease reactivity.
Electron-donating groups: Increase reactivity.

Step-by-Step Guidance

Identify the substituents on each aromatic compound.
Determine whether each substituent is electron-donating or electron-withdrawing.
Rank the compounds by reactivity toward electrophilic aromatic substitution.

Try solving on your own before revealing the answer!

Q4.3 Provide monosubstitution products for the following electrophilic aromatic substitution reactions. If more than one product will form, show both. If no substitution will occur, write “NR”.

Background

Topic: Electrophilic Aromatic Substitution – Regioselectivity

This question tests your ability to predict the products and regioselectivity of aromatic substitution reactions.

Key Terms and Formulas:

Monosubstitution: Only one substituent is added.
Regioselectivity: Ortho, meta, para positions.

Step-by-Step Guidance

Identify the directing effects of existing substituents.
Predict the positions where substitution will occur.
Draw the possible products for each reaction.
If no reaction occurs, write “NR”.

Try solving on your own before revealing the answer!

Q4.4 Provide the main organic product for each of the following reactions.

Background

Topic: Aromatic Compound Reactions

This question tests your ability to predict the main product of various reactions involving aromatic compounds.

Key Terms and Formulas:

Electrophilic aromatic substitution: Main mechanism for aromatic reactions.
Product prediction: Based on reagents and conditions.

Step-by-Step Guidance

Identify the type of reaction and reagents used.
Predict the main organic product based on the mechanism.
Draw the product structure.

Try solving on your own before revealing the answer!

Q4.5 How would you synthesize the following compound from benzene and any reagents you need?

Background

Topic: Organic Synthesis – Aromatic Compounds

This question tests your ability to plan a synthetic route from benzene to a target compound using appropriate reagents.

Key Terms and Formulas:

Synthesis: Sequence of reactions to build a target molecule.
Reagents: Chemicals used to transform benzene into the desired product.

Step-by-Step Guidance

Analyze the target compound and identify the functional groups.
Determine the sequence of reactions needed to introduce each group.
Choose appropriate reagents for each step.
Outline the synthetic route from benzene to the product.