# Organic Chemistry: Stereochemistry, Chirality, Isomerism & Conformational Analysis > 32 organic chemistry review cards on chiral centers, Cahn–Ingold–Prelog (R/S and E/Z) configuration, enantiomers, diastereomers, meso compounds, optical activity, Fischer projections, and cyclohexane chair conformations. - **Canonical URL:** https://flashcards.world/flashcards/sets/97967932-8ac0-4507-a6a0-266017b82dc1/ - **Markdown URL:** https://flashcards.world/flashcards/sets/97967932-8ac0-4507-a6a0-266017b82dc1.md - **Level:** basic - **Languages:** en → en - **Total Cards:** 32 ## Flashcards (32 Cards) | # | Front (Term / Prompt) | Back (Definition / Answer) | Example | Multiple-Choice Options | | :--- | :--- | :--- | :--- | :--- | | 1 | What is atropisomerism, and how does BINAP illustrate it? | Atropisomers arise when restricted rotation about a single bond makes distinct conformations isolable on the relevant timescale. BINAP has a hindered biaryl axis and isolable axially chiral enantiomers; a tetrahedral carbon stereocenter is not required. | | | | 2 | How is enantiomeric excess calculated, and when can optical rotation estimate it? | For one enantiomer pair, % ee = 100 × \|n₁ − n₂\|/(n₁ + n₂). It also equals 100 × \|[α]_mixture/[α]_pure\| only when rotation is additive, conditions match, and other optically active components do not interfere. Identify the major enantiomer separately. | | | | 3 | What are the 1,3-diaxial interactions in a monosubstituted cyclohexane chair? | An axial substituent has unfavorable close contacts with the axial hydrogens at the two ring carbons two positions away, on the same face. These interactions help explain why bulky alkyl substituents usually prefer an equatorial position. | | | | 4 | How do anti, gauche and methyl–methyl eclipsed n-butane rank in energy? | Anti, with the methyl groups 180° apart, is lowest. Gauche at ±60° is about 3.8 kJ/mol higher, and methyl–methyl eclipsed at 0° is about 19 kJ/mol higher in a common introductory model. These are approximate relative energies, with anti chosen as zero. | | | | 5 | Why is the cyclohexane chair a low-strain conformation? | Its C–C–C angles are close to tetrahedral values and neighboring bonds are largely staggered. This minimizes angle and torsional strain; it does not require every angle to equal exactly 109.5°. | | | | 6 | Why does a racemate have zero net optical rotation in an achiral medium? | It contains equal amounts of the two enantiomers. Their equal and opposite contributions cancel under the same measurement conditions. The individual molecules remain chiral. | | | | 7 | How are epimers and anomers related in carbohydrate stereochemistry? | Epimers are diastereomers differing at one stereocenter. Anomers differ specifically at the anomeric center, derived from the sugar's carbonyl carbon. The term also applies to suitable derivatives such as glycosides, not only to free cyclic hemiacetals or hemiketals. | | | | 8 | How do you recognize a tetrahedral carbon stereocenter, and does it guarantee a chiral molecule? | It is a tetrahedral carbon with four distinguishable substituent paths. Assess the whole molecule: meso compounds contain stereocenters but are achiral, and some chiral molecules have no tetrahedral carbon stereocenter. | | | | 9 | How do staggered and eclipsed ethane appear in a Newman projection? | Looking along the C–C bond, staggered C–H bonds are offset by 60° from the nearest bond on the other carbon; eclipsed bonds line up at 0°. Staggered ethane is lower in energy. | | | | 10 | What are enantiomers, and how do their properties compare? | They are stereoisomers that are nonsuperimposable mirror images. In matched achiral conditions they have the same ordinary physical properties, such as melting point and solubility, and opposite optical rotations. Their interactions with chiral environments, including enzymes, can differ. | | | | 11 | Why is cyclopentane puckered rather than planar? | Puckering reduces eclipsing and torsional strain. Cyclopentane moves among closely related envelope and twist conformations; the envelope picture has four carbons approximately coplanar and one displaced. It is not locked into one envelope. | | | | 12 | When can a C=C bond give two geometrical stereoisomers? | Each double-bond carbon must carry two distinguishable substituents, and rotation about the double bond is restricted. E/Z gives a general designation; cis/trans is suitable only when the reference groups are unambiguous. | | | | 13 | How do you assign R or S once the four CIP priorities are known? | Point priority 4 away from you. The sequence 1 → 2 → 3 gives R if clockwise and S if counterclockwise. R/S describes configuration, not the sign of optical rotation. | | | | 14 | How is a solution's specific rotation calculated using concentration in g/mL? | [α] = α_obs/(l c), where α_obs is the observed rotation in degrees, l is path length in decimeters and c is concentration in g/mL. Report wavelength, temperature, solvent and concentration; use compatible units when comparing measurements. | | | | 15 | Why is cyclopropane strongly angle-strained? | Its three-carbon ring has C–C–C angles of 60°, far from the preferred tetrahedral geometry of an unstrained alkane. Its bonding is distorted, often described using bent bonds. Eclipsed neighboring C–H bonds add torsional strain. | | | | 16 | What are diastereomers, and must every measured property distinguish them? | They are stereoisomers that are not mirror images. Ordinary physical properties and NMR spectra can differ even in an achiral environment, but a particular value or spectral signal may coincide. | | | | 17 | When does 2ⁿ give the maximum number of stereoisomers? | For a fixed connectivity with n independently assignable tetrahedral stereocenters and no other stereogenic elements, 2ⁿ is an upper bound. Symmetry can make assignments identical and reduce the number of distinct stereoisomers. | | | | 18 | How do you assign R/S in a Fischer projection when priority 4 is horizontal? | Priority 4 points toward you, so reverse the usual result from 1 → 2 → 3: an apparent clockwise sequence gives S, and an apparent counterclockwise sequence gives R. | | | | 19 | How can a chiral resolving agent help separate a racemate? | For a suitable compound, reaction with an enantiopure agent gives diastereomeric salts or derivatives. Their differing properties may permit separation, for example by crystallization, followed by recovery of the enantiomers without racemization. This is one resolution method, not a universal procedure. | | | | 20 | What does a large positive A-value mean for tert-butylcyclohexane? | A = G_axial − G_equatorial for the two chair conformations. The tert-butyl group's large positive value strongly favors its equatorial chair at room temperature. Calling it a conformational lock is a useful approximation, not a claim that ring inversion is impossible. | | | | 21 | What makes a compound meso despite having stereocenters? | It is an achiral stereoisomer: its whole structure is superimposable on its mirror image despite containing stereocenters. Internal symmetry, commonly a mirror plane in a suitable conformation, accounts for this; it is not a racemic mixture. | | | | 22 | What does torsional strain describe in ethane's conformations? | It describes the energy penalty associated with twisting from the staggered arrangement toward eclipsing around the C–C bond. The rotational barrier reflects electronic interactions; attributing it solely to classical electron–electron repulsion is an oversimplification. | | | | 23 | What do horizontal and vertical bonds mean at a Fischer-projection crossing? | The crossing represents a tetrahedral center. Horizontal bonds point toward the viewer; vertical bonds point away. The drawing is a projection, not a claim that all the bonds lie in one plane. | | | | 24 | What are the basic CIP rules for ranking substituents at a tetrahedral stereocenter? | Rank the directly attached atoms by atomic number, with higher numbers taking priority; for isotopes of one element, higher mass number ranks higher. Resolve ties by comparing attached-atom lists in descending priority at the first difference. Multiple bonds are represented with duplicate atoms for ranking. | | | | 25 | Which chair of trans-1,2-dimethylcyclohexane is favored, and why? | The diequatorial chair is favored over the diaxial chair because it avoids four methyl–hydrogen 1,3-diaxial interactions. It still has a gauche interaction between the methyl groups; a simple additive strain model estimates an advantage of about 11 kJ/mol. | | | | 26 | What do (+) and (−) mean in polarimetry? | Viewed toward the light source, clockwise rotation is dextrorotatory (+) and counterclockwise rotation is levorotatory (−). These measured signs do not determine R/S configuration. | | | | 27 | Why is trans-1,4-dimethylcyclohexane favored over the cis isomer in simple chair analysis? | The trans isomer has a diequatorial chair, whereas each cis chair has one axial and one equatorial methyl group. The trans diequatorial arrangement avoids the axial methyl group's 1,3-diaxial contacts. A chair flip does not interconvert cis and trans. | | | | 28 | How do replacement tests distinguish enantiotopic from diastereotopic protons? | Replace each proton in turn with the same label: enantiomeric products mean enantiotopic, and diastereomeric products mean diastereotopic. Enantiotopic protons have the same chemical shift in an achiral environment; diastereotopic protons are not symmetry-equivalent, but their signals need not be resolved or have different numerical shifts. | | | | 29 | What changes, and what stays fixed, during a cyclohexane chair flip? | Axial positions become equatorial and equatorial positions become axial. Each substituent keeps its up or down orientation, so a ring flip does not change a cis relationship into trans or vice versa. | | | | 30 | How are E and Z assigned to an alkene? | Rank the two substituents on each double-bond carbon separately using CIP rules. Higher-priority groups on the same side give Z; on opposite sides they give E. Each carbon must have two distinguishable substituents. | | | | 31 | How do axial and equatorial positions differ in a cyclohexane chair? | The six axial bonds are approximately parallel to the chair's axis and alternate up and down. Equatorial bonds point outward around the ring. At each carbon, one position is up and the other down; equatorial does not mean all bonds lie in a flat plane. | | | | 32 | What is steric strain in conformational analysis? | It is destabilization from unfavorable close contacts between nonbonded atoms or groups. Strong short-range repulsion develops when their electron clouds overlap; van der Waals radii provide approximate contact distances, not rigid boundaries. | | | --- Study this deck interactively with FSRS-6 spaced repetition, audio review, and multiple-choice mode at [Flashcards World](https://flashcards.world/flashcards/sets/97967932-8ac0-4507-a6a0-266017b82dc1/).