Organic Chemistry: Stereochemistry, Conformational Analysis, and Free Radical Reactions

Exam Structure and Allowed Materials

This exam is a closed book, closed notes assessment. Only molecular models in a clear plastic bag and a non-programmable calculator are permitted. A periodic table is provided. No electronic devices or scratch paper are allowed. The exam consists of multiple-choice and short-answer questions, with a focus on stereochemistry, conformational analysis, and free radical reactions.

Conformational Analysis of Cyclohexane and Substituted Cyclohexanes

Chair Conformations

Cyclohexane adopts a chair conformation to minimize torsional and angle strain. Substituents on cyclohexane rings can occupy either axial or equatorial positions, which affects the molecule's stability.

• Axial positions: Perpendicular to the ring plane, alternate up and down around the ring.

• Equatorial positions: Extend outward from the ring, roughly in the plane of the ring.

• Substituent preference: Bulky groups prefer the equatorial position to minimize 1,3-diaxial interactions.

Example: In cis-1,3-dimethylcyclohexane, the lowest energy chair conformation places both methyl groups in equatorial positions if possible.

Counting Axial and Equatorial Hydrogens

Each carbon in cyclohexane has one axial and one equatorial hydrogen. Substituents replace hydrogens, so the number of axial hydrogens depends on the number and position of substituents.

Conformational Energy

• Chair conformations with bulky groups in axial positions are higher in energy due to steric strain.

• Chair conformations with bulky groups in equatorial positions are lower in energy.

Stereochemistry: Isomerism and Configuration

Types of Isomers

• Constitutional isomers: Compounds with the same molecular formula but different connectivity.

• Stereoisomers: Compounds with the same connectivity but different spatial arrangement of atoms.

• Enantiomers: Non-superimposable mirror images.

• Diastereomers: Stereoisomers that are not mirror images.

• Conformers: Different spatial arrangements due to rotation about single bonds.

Example: (2R,3R,4S)-2,3,4-trichloroheptane and (2R,3S,4R)-2,3,4-trichloroheptane are diastereomers.

Assigning Stereochemistry (R/S)

• Assign priorities to substituents using the Cahn-Ingold-Prelog rules.

• Orient the molecule so the lowest priority group is away from you.

• Trace a path from highest (1) to lowest (3) priority: clockwise = R, counterclockwise = S.

Example: D-Threonine (2R,3S) – correct stereochemistry must be assigned to each chiral center.

Optical Activity

• Optical rotation () is a measure of a compound's ability to rotate plane-polarized light.

• Concentration can be calculated using the formula: where is the specific rotation, is the observed rotation, is the path length in decimeters, and is the concentration in g/mL.

Free Radical Reactions

Mechanism of Free Radical Halogenation

Free radical halogenation involves three main steps: initiation, propagation, and termination.

• Initiation: Formation of radicals, usually by homolytic cleavage of a halogen molecule (e.g., ).

• Propagation: Radicals react with substrate to form new radicals and products (e.g., ).

• Termination: Two radicals combine to form a stable molecule, ending the chain reaction.

Example: Chlorination of isobutane can yield multiple dichlorination products depending on which hydrogens are substituted.

Stability of Free Radicals

• Radical stability increases in the order: methyl < primary < secondary < tertiary.

• Allylic and benzylic radicals are especially stable due to resonance.

Example: The order of stability for the radicals (CH3)3C·, CH2=CHCH2·, CH3CH2·, CH3·, (CH3)2CH· is: CH3· < CH3CH2· < (CH3)2CH· < (CH3)3C· < CH2=CHCH2· (allylic, most stable).

Bond Dissociation Energies

Bond dissociation energy (BDE) is the energy required to break a bond homolytically. Lower BDE indicates a weaker bond.

Application: The carbon-bromine bond is weakest (lowest BDE) when bromine is bound to a tertiary carbon.

Reactivity and Mechanisms of Alkyl Halides

SN2 Reactivity

• SN2 reactions proceed via a backside attack, leading to inversion of configuration.

• Reactivity order: methyl > primary > secondary > tertiary (steric hindrance slows the reaction).

• Good leaving groups and strong nucleophiles favor SN2 reactions.

Example: 1-iodobutane reacts faster than 1-fluorobutane in SN2 reactions due to the better leaving ability of iodide.

Nucleophilicity

• Nucleophilicity is the ability of a species to donate a pair of electrons to an electrophile.

• Factors affecting nucleophilicity: charge, electronegativity, solvent, and steric hindrance.

• In general, negatively charged species are more nucleophilic than their neutral counterparts.

Example: Among , , , and , is the least nucleophilic because it is electron-deficient and acts as a Lewis acid.

Carbocation Stability and Substituent Effects

Stabilization of Carbocations

• Carbocations are stabilized by electron-donating groups through inductive and hyperconjugation effects.

• Alkyl groups stabilize carbocations by donating electron density via sigma bonds (hyperconjugation).

• Resonance stabilization occurs when the positive charge is delocalized over multiple atoms.

Example: Allyl substituents stabilize carbocations through both inductive and resonance effects.

Halides: Classification and Reactivity

Classification of Alkyl Halides

• Primary halide: Halogen attached to a carbon bonded to one other carbon.

• Secondary halide: Halogen attached to a carbon bonded to two other carbons.

• Tertiary halide: Halogen attached to a carbon bonded to three other carbons.

Example: CH3CH2CHBrCH3 is a secondary halide.

Practice Problems and Applications

• Draw all monochlorination products for a given alkane and identify the major product based on the stability of the intermediate radical.

• Provide the major organic product for nucleophilic substitution reactions (e.g., alkyl halide with NaCN).

• Identify the relationship between pairs of compounds (enantiomers, diastereomers, or identical).

• Draw structures of complex stereoisomers given their IUPAC names and stereochemistry.

• Explicitly draw all axial and equatorial hydrogens on cyclohexane derivatives.

Summary Table: Types of Isomers

Additional info: This study guide is based on a midterm exam covering key concepts in organic chemistry, including conformational analysis, stereochemistry, free radical reactions, and alkyl halide reactivity. The guide includes definitions, examples, and tables to aid in exam preparation.