Organic Chemistry Stereochemistry and Conformational Analysis Syllabus

For cis- vs trans-1,2-dimethylcyclohexane, which is more stable and why?

Trans-1,2-dimethylcyclohexane is more stable. Its diequatorial (e,e) chair places both methyls equatorial, whereas the cis isomer must always be one axial/one equatorial (a,e), giving one methyl unavoidable 1,3-diaxial strain.

For 1,4-disubstituted cyclohexanes, which stereoisomer can be diequatorial?

The trans-1,4 isomer can adopt a diequatorial (e,e) chair (most stable), while the cis-1,4 isomer is forced into one axial/one equatorial (a,e) arrangement regardless of which chair it adopts.

Give the general rule for cis/trans equatorial arrangements at odd vs even carbon separations.

For substituents an even number of carbons apart (1,2 or 1,4 with even difference conventions): trans can be diequatorial. For 1,3: cis can be diequatorial. In general, whether cis or trans allows (e,e) depends on whether the positions are both 'up-type' or mixed—determined by the ring geometry (1,2 trans e,e; 1,3 cis e,e; 1,4 trans e,e).

What is a chirality center (stereocenter)?

A chirality center is typically an $sp^{3}$ carbon bonded to four different groups. Interchanging any two groups produces a different stereoisomer. More broadly, a stereocenter is any atom at which swapping two attached groups yields a distinct stereoisomer.

What is chirality?

Chirality is the property of a molecule being non-superimposable on its mirror image. A chiral molecule lacks an internal plane of symmetry (and lacks other improper symmetry elements) and can exist as two enantiomers.

What symmetry element, if present, makes a molecule achiral?

If a molecule possesses an internal mirror plane of symmetry ($\sigma$), a center of inversion, or more generally an improper rotation axis ($S_{n}$), it is superimposable on its mirror image and is therefore achiral.

Define enantiomers.

Enantiomers are stereoisomers that are non-superimposable mirror images of each other. They have identical physical properties (melting point, boiling point, density) except that they rotate plane-polarized light in equal but opposite directions and interact differently with other chiral molecules.

What is optical activity and which molecules exhibit it?

Optical activity is the ability of a chiral substance to rotate the plane of plane-polarized light. Only chiral molecules (or non-racemic mixtures of enantiomers) are optically active; achiral molecules and racemic mixtures are not.

What is specific rotation and how is it calculated?

Specific rotation is a standardized measure of optical activity: $$[\alpha]_{D}^{T} = \frac{\alpha}{c \cdot l}$$ where $\alpha$ is the observed rotation in degrees, $c$ is concentration in $\text{g/mL}$, and $l$ is path length in decimeters ($\text{dm}$).

What are dextrorotatory and levorotatory, and how are they labeled?

Dextrorotatory ($+$ or $d$) compounds rotate plane-polarized light clockwise; levorotatory ($-$ or $l$) compounds rotate it counterclockwise. The sign of rotation is determined experimentally and has no fixed relationship to R/S configuration.

State the four steps of the Cahn–Ingold–Prelog rules for assigning R/S.

1) Assign priorities to the four groups by decreasing atomic number of the first point of difference. 2) Point the lowest-priority group away from you. 3) Trace $1 \to 2 \to 3$. 4) Clockwise = $R$ (rectus), counterclockwise = $S$ (sinister).

In CIP rules, how do you break a priority tie at the first atom?

When two atoms attached to the stereocenter are identical, move outward to the next set of atoms and compare them at the first point of difference, using the highest atomic numbers. Duplicate a doubly/triply bonded atom as phantom atoms (e.g., $\ce{C=O}$ counts as C bonded to two O's).

When the lowest-priority group points toward the viewer, how do you assign R/S?

Determine the direction of $1 \to 2 \to 3$ as drawn, then reverse the assignment. For example, if the observed rotation appears clockwise (which would be R) but the lowest-priority group faces you, the actual configuration is $S$.

How many stereoisomers are possible for a molecule with n stereocenters?

A molecule with $n$ stereocenters has a maximum of $2^{n}$ possible stereoisomers. This number is reduced when meso compounds (internal symmetry) are present.

Define diastereomers.

Diastereomers are stereoisomers that are NOT mirror images of each other. They arise in molecules with two or more stereocenters and differ in configuration at some (but not all) stereocenters. Unlike enantiomers, they have different physical properties.

How do the properties of enantiomers and diastereomers differ?

Enantiomers have identical physical properties (except opposite optical rotation) and can only be distinguished in a chiral environment. Diastereomers have different melting points, boiling points, solubilities, and other physical properties, so they can be separated by ordinary methods like distillation or chromatography.

What is a meso compound?

A meso compound contains two or more stereocenters but is achiral overall because it possesses an internal plane of symmetry (or other improper symmetry element). It is superimposable on its mirror image and is optically inactive despite having stereocenters.

Give an example of a meso compound and explain why it is achiral.

Meso-tartaric acid, $(2R,3S)$-tartaric acid, has two stereocenters of opposite configuration with an internal mirror plane between C2 and C3. The two halves cancel each other's optical rotation, making the molecule achiral and optically inactive.

What is a Fischer projection and what do its lines represent?

A Fischer projection depicts a stereocenter as a cross: horizontal lines represent bonds coming toward the viewer (wedges), and vertical lines represent bonds going away from the viewer (dashes). The carbon chain is drawn vertically with the most oxidized carbon at the top.

What two operations are allowed vs forbidden when manipulating a Fischer projection?

Allowed: rotating the entire projection by $180^{\circ}$ in the plane (retains configuration), or holding one group fixed and rotating the other three. Forbidden: rotating by $90^{\circ}$ or lifting/flipping out of the plane—these invert the configuration to the enantiomer.

How do you assign R/S directly from a Fischer projection?

Assign CIP priorities. If the lowest-priority group is on a vertical bond (pointing away), read $1\to2\to3$ directly: clockwise = $R$. If the lowest-priority group is on a horizontal bond (pointing toward you), determine the rotation and then reverse it.

What distinguishes a stereospecific from a stereoselective reaction?

A stereospecific reaction is one in which a particular stereoisomer of the reactant yields a particular stereoisomer of the product (different stereoisomeric reactants give different stereoisomeric products), determined by mechanism. A stereoselective reaction is one that produces predominantly one stereoisomer out of several possible, even from a single reactant.

Is every stereospecific reaction also stereoselective? Explain.

Yes—every stereospecific reaction is necessarily stereoselective, because favoring a specific product stereochemistry from a given reactant means one stereoisomer predominates. However, the converse is not true: a stereoselective reaction need not be stereospecific.

Give an example of a stereospecific reaction and its stereochemical outcome.

The $\ce{S_N2}$ reaction is stereospecific: the nucleophile attacks opposite the leaving group, producing inversion of configuration (Walden inversion). Similarly, anti addition of $\ce{Br2}$ to cis- vs trans-2-butene gives different (meso vs d,l) dibromide diastereomers.

What is racemization and how does it typically occur?

Racemization is the conversion of a single enantiomer (or an optically active sample) into a racemic mixture (50:50 of both enantiomers), which is optically inactive. It commonly occurs through a planar achiral intermediate such as a carbocation ($\ce{S_N1}$) or an enol/enolate, which can be attacked from either face.

Why does an $\ce{S_N1}$ reaction at a stereocenter typically give racemization (or partial racemization)?

The $\ce{S_N1}$ mechanism forms a planar $sp^{2}$ carbocation intermediate. The nucleophile can attack either face with roughly equal probability, generating both R and S products. In practice slight excess of inversion is often seen due to the leaving group shielding one face.

What is inversion of configuration and which mechanism causes it?

Inversion of configuration (Walden inversion) is the flipping of a stereocenter's spatial arrangement, like an umbrella turning inside out. It is the hallmark of the concerted $\ce{S_N2}$ mechanism, where the nucleophile attacks $180^{\circ}$ from the leaving group (backside attack).

What is a racemic mixture (racemate) and what is its optical rotation?

A racemic mixture is an equimolar $50:50$ mixture of two enantiomers. Because the rotations of the two enantiomers cancel exactly, the net optical rotation is $0^{\circ}$—the racemate is optically inactive, often denoted $(\pm)$ or $d,l$.

What is enantiomeric excess (ee) and how is it calculated?

Enantiomeric excess measures the purity of one enantiomer over the other: $$\%\,ee = \frac{|\,\%R - \%S\,|}{\%R + \%S} \times 100 = \frac{[\alpha]_{\text{obs}}}{[\alpha]_{\text{pure}}} \times 100.$$ For example, a sample that is $75\%$ R and $25\%$ S has $50\%\ ee$.

What is the resolution of enantiomers?

Resolution is the process of separating a racemic mixture into its two pure enantiomers. Because enantiomers have identical physical properties, they cannot be separated directly; a chiral resolving agent is required to convert them into separable diastereomers.

Describe the classic method of resolving a racemic acid into pure enantiomers.

React the racemic acid with a single enantiomer of a chiral base (resolving agent) to form two diastereomeric salts. Because diastereomers have different solubilities/physical properties, they are separated (e.g., by crystallization), then each salt is treated with strong acid to regenerate and recover the individual pure enantiomeric acids.

Besides diastereomeric salt formation, name two other methods used to resolve enantiomers.

Chiral chromatography (a chiral stationary phase retains the two enantiomers differently) and enzymatic/kinetic resolution (an enzyme selectively reacts with one enantiomer, leaving the other unreacted for separation).