Organic Chemistry Exam #2 Review

Focuses on stereoisomerism and chemical reactivity & mechanisms

Crestor® (rosuvastatin)

A medication used to reduce cholesterol that has 8 possible stereoisomers based on its molecular structure.

Singulair® (montelukast)

A medication used to manage asthma that contains a chiral center with an R absolute configuration.

Plavix® (clopidogrel)

A medication used to manage blood clots which features a chirality center assigned the S absolute configuration.

Plane of symmetry

A tool used to determine chirality; a molecule contains one if it can be divided into two halves that would look identical if reflected in a mirror.

Cahn-Ingold-Prelog system

A system used to rank substituents by priority based on atomic number, where priorities in a specific case were ranked as -SH > -F > -OH > -H.

Intermediate

A species on a reaction pathway that represents a local minimum on an energy diagram.

Heterolytic cleavage

A type of bond cleavage where both electrons from the bond are taken by one of the atoms, typically occurring in ionic reactions.

Specific rotation

A physical property of chiral molecules; examples include $-61$ for $(R)$-carvone and $-116$ for $(S)$-limonene.

Enantiomeric excess (% ee)

A measurement of the purity of a chiral sample; for a carvone sample with an observed rotation of $-40$ and a pure rotation of $-61$, the % ee is $66\%$.

Meso isomer

An achiral compound that contains chiral centers and a plane of symmetry, often illustrated in structures containing substituted tartaric acid derivatives.

Transition state

A high-energy point on a reaction coordinate diagram representing a maximum energy state as bonds are being formed and broken.

Hydride

A hydrogen atom with a negative charge and two electrons, symbolized as $H^-$.

Concerted endothermic reaction

A chemical reaction that occurs in a single step (one transition state) where the energy of the products is higher than the energy of the reactants.

Enantiomers

Stereoisomers that are non-superimposable mirror images of each other.

Diastereomers

Stereoisomers that are not mirror images of each other.

Constitutional isomers

Compounds that have the same molecular formula but different connectivity of atoms.

Nucleophile

A reactant that provides an electron pair to form a new bond; frequently identified as electron-rich sites such as lone pairs or pi bonds.

Electrophile

An electron-deficient atom or molecule that accepts an electron pair from a nucleophile to form a new bond.

$\Delta S$ (Entropy)

A thermodynamic property representing disorder; it is predicted to be positive when a reaction results in an increase in the number of molecules.

Gibbs Free Energy Favorability

A reaction with $\Delta H = 20\,kJ/mol$ and $\Delta S = 10\,J/mol \cdot K$ at $298\,K$ will favor reactants.

Fischer projection

A 2D representation of a 3D molecule where horizontal lines represent bonds coming out of the page (wedges) and vertical lines represent bonds going into the page (dashes).

Enantiomers

Chiral molecules that are non-superimposable mirror images of each other, sharing identical physical properties except for the direction of rotation of plane-polarized light.

Diastereomers

Stereoisomers that are not mirror images of one another, which can occur in molecules with multiple chiral centers such as $1,3,5$-trimethylcyclohexane.

Constitutional Isomers

Compounds that share the same molecular formula but differ in the connectivity of their atoms.

Specific Rotation

A physical property of a chiral compound, such as pure $(S)$-carvone or $(R)$-carvone, which is a measure of its ability to rotate plane-polarized light.

Enantiomeric Excess ($ee$)

A measurement of the purity of a chiral substance; for example, a sample of $(R)$-carvone with an $85\%$ $ee$ would show a specific rotation relative to the pure enantiomer's value of $-61$.

Proton Transfer

A common pattern of arrow pushing in reaction mechanisms involving the movement of an $H^+$ ion between species, often seen with reagents like $H_2O$ or $MeOH$.

Nucleophilic Attack

A mechanism step where an electron-rich nucleophile, such as a bromide anion ($Br^-$), initiates a bond with an electrophilic carbon atom.

Loss of Leaving Group

A fundamental reaction step where a bond breaks and the leaving group takes both electrons from the bond, such as the departure of a halide ion.

Rearrangement

A mechanism step involving the migration of atoms or groups within a molecule to form a more stable isomer, such as generating a more stable cation.

Plane of Symmetry

An internal mirror plane that divides a molecule into two identical halves; its presence renders a molecule achiral even if it contains chiral centers.

Meso Isomer

An achiral compound that possesses multiple chiral centers but also contains an internal plane of symmetry.

Absolute Configuration

The precise spatial arrangement of atoms at a chiral center, designated as either $R$ or $S$ based on the Cahn-Ingold-Prelog priority rules.

Fischer Projection

A standard two-dimensional representation of a three-dimensional organic molecule, where horizontal lines represent bonds coming toward the viewer and vertical lines represent bonds going away.

Chiral Chromatography

An effective laboratory strategy used to separate enantiomers by utilizing a stationary phase that is itself chiral.

Distillation

A separation technique that is generally ineffective for separating enantiomers because they possess identical boiling points.

Enthalpy Change ($\Delta H$)

The heat energy change of a reaction, such as a value of $20\,kJ\,mol^{-1}$, which helps determine if a reaction is exothermic or endothermic.

Entropy Change ($\Delta S$)

The change in molecular disorder during a reaction, such as a value of $10\,J\,mol^{-1}\,K^{-1}$, used alongside temperature ($298\,K$) to predict if a reaction favors reactants or products.

Reaction Intermediate

A transient chemical species formed during a multi-step reaction; on an energy diagram, it corresponds to a local minimum between two transition states.

Cation Stability

The relative stability of positively charged carbon species (carbocations), which determines the likelihood of certain reaction pathways.

Chiral Center

An atom, typically a carbon, that is bonded to four distinct groups, creating the potential for optical activity in a molecule like Singulair® (montelukast).