Organic Chemistry Study Notes: Mechanisms, Alkanes, Stereochemistry, and Alkene Addition Reactions

Reaction Mechanisms and Carbocation Stability

Electron Flow in Ionic Reactions

Organic reaction mechanisms use curved arrows to depict the movement of electrons during chemical transformations. Understanding these conventions is essential for interpreting and predicting reaction outcomes.

• Curved Arrow Notation: The tail of the arrow shows the origin of electrons, and the head shows their destination.

• Four Characteristic Patterns of Electron Flow:

  1. Nucleophilic attack

  2. Loss of a leaving group

  3. Proton transfer

  4. Rearrangement

Carbocations and Rearrangements

Carbocations are ions in which a carbon atom bears a positive charge, typically formed as intermediates in many organic reactions.

• Carbocation rearrangements occur to generate a more stable carbocation, usually via:

  • Hydride shift

  • Methyl shift

• Stability increases with the number of alkyl substituents attached to the positively charged carbon.

• Order of stability: tertiary > secondary > primary > methyl carbocation.

• Key factors for carbocation stability:

  • Hyperconjugation

  • Inductive effect

Alkanes and Conformational Analysis

Types of Alkanes

Alkanes are saturated hydrocarbons and can be classified as acyclic or cyclic.

• Acyclic alkanes: General formula $C_nH_{2n+2}$

• Cycloalkanes: General formula $C_nH_{2n}$

Conformational Analysis

Rotation about C–C single bonds allows alkanes to adopt various conformations, which differ in energy due to steric interactions and torsional strain.

• Newman projections are used to visualize different conformations.

• Conformations are classified as:

  • Staggered

  • Eclipsed

  • Anti

  • Gauche

• Staggered conformations are lower in energy than eclipsed due to minimized torsional strain.

• Anti conformation is lower in energy than gauche due to reduced steric hindrance.

• In ethane, all staggered conformations are degenerate (equal energy), as are all eclipsed conformations.

• In butane, the anti conformation is the lowest in energy, while one eclipsed conformation is highest due to increased steric interactions.

Isomerism and Stereochemistry

Structural and Stereoisomers

Isomers are compounds with the same molecular formula but different arrangements of atoms.

• Structural (Constitutional) Isomers: Same formula, different connectivity.

• Stereoisomers: Same formula and connectivity, different spatial arrangement.

Cis-Trans and E/Z Isomerism

Disubstituted alkenes can exist as cis (same side) or trans (opposite side) isomers. The Cahn-Ingold-Prelog system assigns E/Z configuration based on atomic number priorities.

• Z (zusammen): Higher priority groups on the same side.

• E (entgegen): Higher priority groups on opposite sides.

Chirality and Stereogenic Centers

A molecule is chiral if it is not superimposable on its mirror image. The most common source of chirality is a carbon atom bonded to four different groups (stereogenic center).

• Enantiomers: Non-superimposable mirror images.

• Achiral: Molecules superimposable on their mirror image.

• Plane of symmetry: If present, molecule is achiral.

• Molecules with only one chiral center are always chiral.

Assigning R/S Configuration

The Cahn-Ingold-Prelog system is used to assign absolute configuration to chiral centers.

• Assign priorities to four groups based on atomic number.

• Orient molecule so the lowest priority group is away.

• R (rectus): Clockwise sequence of priorities.

• S (sinister): Counterclockwise sequence.

Multiple Stereocenters and Diastereomers

Compounds with multiple stereocenters can have several stereoisomers.

• Maximum number of stereoisomers: $2^n$, where $n$ is the number of chiral centers.

• Each stereoisomer has one enantiomer; others are diastereomers.

• Enantiomers: Mirror images.

• Diastereomers: Not mirror images; differ in physical and chemical properties.

• Meso compounds: Molecules with multiple chiral centers but achiral due to a plane of symmetry.

• Allenes: Can be chiral without asymmetric atoms if different groups are present at each end.

Stereochemical Definitions Table

The following table summarizes key stereochemical terms and their definitions:

Term	Definition
Structural Isomers (Constitutional Isomers)	Different compounds with the same molecular formula but different orders of attachment.
Stereoisomers	Different compounds with the same structure and connectivity, differing only in the arrangement of the atoms in space.
Chiral	Any object that cannot be superposed on its mirror image.
Achiral	Any object that can be superposed on its mirror image.
Enantiomers	A pair of stereoisomers that are non-superimposable mirror images. Pairs of enantiomers have identical physical and chemical properties except for interactions with other chiral molecules and with polarized light.
Diastereomers	Any pair of stereoisomers that are not enantiomers. Diastereomers are chemically and physically different. They have different melting points and different substituents and often undergo chemical reactions in a different fashion.
Meso form	A stereoisomer that contains chiral carbons but can be superposed on its mirror image due to a plane of symmetry.

Examples of Structural and Stereoisomers

The following table describes examples of structural isomers, enantiomers, and diastereomers:

Type	Example	Description
Structural Isomers	CH3OCH3 vs. CH3CH2OH	Same formula, different connectivity.
Enantiomers	Two molecules with opposite configurations at all chiral centers	Non-superimposable mirror images.
Diastereomers	Two molecules with different configurations at one or more (but not all) chiral centers	Not mirror images; different physical properties.
Meso Compound	Molecule with two chiral centers and a plane of symmetry	Achiral despite having chiral centers.

Alkene Addition Reactions

General Features

Alkenes typically undergo addition reactions in which atoms or groups are added to the carbons of the double bond. These reactions are classified based on the groups being added.

• Addition of H and X (hydrohalogenation)

• Addition of H and OH (hydration)

• Addition of H and H (hydrogenation)

• Addition of X and X (halogenation)

• Addition of X and OH (halohydrin formation)

• Addition of OH and OH (dihydroxylation)

Carbocation Intermediates and Stability

Many alkene addition reactions proceed via carbocation intermediates, whose stability is crucial for reaction outcome.

• Stabilized by hyperconjugation, resonance, and inductive effects.

• Stability order: tertiary > secondary > primary > methyl.

Hydrohalogenation (Section 10.2)

Hydrohalogenation is the addition of HX (where X is a halogen) to an alkene.

• Alkene acts as nucleophile; carbocation intermediate forms.

• Reaction follows Markovnikov's rule: hydrogen adds to the less substituted carbon, halogen to the more substituted carbon.

• Carbocation rearrangements may occur for increased stability.

• Intermediate is $sp^2$ hybridized and planar; X can add from either side, yielding a racemic mixture.

Hydration (Section 10.3)

Hydration adds H and OH across a double bond, typically via acid-catalyzed mechanism to form an alcohol.

• Follows Markovnikov's rule; racemic mixture forms.

• Mechanism similar to hydrohalogenation.

Hydroboration-Oxidation (Section 10.4)

Hydroboration-oxidation adds H and OH across a double bond, but with anti-Markovnikov regioselectivity.

• Reagents: 1) $BH_3$, THF; 2) $H_2O_2$, NaOH.

• No carbocation intermediate; no rearrangements.

• Stereochemistry is syn: H and OH add to the same side.

Oxymercuration-Demercuration (Section 10.5)

Oxymercuration-demercuration produces a Markovnikov alcohol without carbocation rearrangements.

• Reagents: 1) $Hg(OAc)_2$, H_2O; 2) $NaBH_4$.

• No carbocation intermediate; no rearrangements.

• Other nucleophiles (alcohols, amines) can be used to give ethers or amines.

Summary Table: Alkene Addition Reactions

Reaction	Regioselectivity	Intermediate	Stereochemistry
Hydrohalogenation	Markovnikov	Carbocation	Racemic
Hydrohalogenation (anti-Markovnikov)	Anti-Markovnikov	None	Syn
Acid-catalyzed Hydration	Markovnikov	Carbocation	Racemic
Oxymercuration-Demercuration	Markovnikov	None	Syn
Hydroboration-Oxidation	Anti-Markovnikov	None	Syn

Additional info:

• All reactions above are fundamental to introductory organic chemistry and are commonly tested in college-level courses.

• Understanding the mechanism and stereochemistry is crucial for predicting products and their properties.