Nomenclature of geometrical isomers – Syn/Anti MCQs With Answer

Nomenclature of geometrical isomers – Syn/Anti MCQs With Answer

The nomenclature of geometrical isomers is essential for B.Pharm students to understand stereochemistry, drug-receptor interactions, and structure–activity relationships. This introduction clarifies how terms like syn/anti, cis/trans, and E/Z are applied in alkenes, cyclic systems, and conformational analysis, and how IUPAC rules prioritize substituents for unambiguous naming. Key concepts include dihedral angles in Newman projections, relative orientation on ring systems, and converting older syn/anti descriptors to modern E/Z notation where appropriate. Mastery of these terms aids in predicting physical properties and pharmacological behavior. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. In 2-butene, when both methyl groups are on the same side of the double bond, the geometric descriptor is:

• anti
• syn
• E
• Z

Correct Answer: syn

Q2. Which nomenclature system is preferred by IUPAC for disubstituted alkenes to avoid ambiguity?

• syn/anti
• cis/trans
• E/Z
• alpha/beta

Correct Answer: E/Z

Q3. For a substituted alkene where priority groups are on the same side of the double bond, the correct E/Z designation is:

• E
• Z
• syn
• anti

Correct Answer: Z

Q4. In Newman projection terminology, two substituents with a dihedral angle of 180° are described as:

• gauche
• syn
• anti
• eclipsed

Correct Answer: anti

Q5. The historical syn/anti descriptors are still used in specific contexts. Which of the following is a correct modern replacement for syn/anti in many cases?

• cis/trans always replaces syn/anti
• E/Z replaces syn/anti where priority rules apply
• alpha/beta replaces syn/anti
• R/S replaces syn/anti

Correct Answer: E/Z replaces syn/anti where priority rules apply

Q6. In a substituted cyclohexane, two substituents that are on the same face of the ring (both axial or both equatorial facing up) are referred to as:

• anti
• syn
• E
• trans-diaxial

Correct Answer: syn

Q7. Which of the following statements best describes the difference between cis/trans and E/Z notation?

• cis/trans is based on CIP priority while E/Z is not
• E/Z is based on CIP priority rules and applies universally, while cis/trans is limited to simpler cases
• cis/trans is used for alkynes while E/Z is used for alkanes
• E/Z refers to optical rotation, cis/trans does not

Correct Answer: E/Z is based on CIP priority rules and applies universally, while cis/trans is limited to simpler cases

Q8. For the alkene with substituents of higher priority on opposite sides according to CIP rules, the descriptor is:

• syn
• Z
• E
• cis

Correct Answer: E

Q9. In peptide chemistry, syn and anti rotamers may describe orientation around which bond?

• The peptide C-N amide bond (ω) exclusively
• The side-chain χ torsion angles and backbone φ/ψ torsions
• The peptide bond cannot have syn/anti rotamers
• Only disulfide bonds show syn/anti rotamers

Correct Answer: The side-chain χ torsion angles and backbone φ/ψ torsions

Q10. Which criterion is used by CIP rules to assign priority when determining E/Z?

• Atomic number of directly attached atoms
• Molecular weight of entire substituent
• Boiling point of substituent groups
• Number of hydrogen atoms only

Correct Answer: Atomic number of directly attached atoms

Q11. In a 1,2-disubstituted alkene where substituents are opposite in space but substituent priorities are equal complexity, which system resolves ambiguity best?

• cis/trans
• syn/anti
• E/Z using full CIP comparison of subsequent atoms
• alpha/beta

Correct Answer: E/Z using full CIP comparison of subsequent atoms

Q12. For 1,3-disubstituted cyclohexane, when substituents are on adjacent carbons and point to the same face, the relationship is commonly called:

• axial-equatorial
• syn-1,3
• anti-1,3
• geminal

Correct Answer: syn-1,3

Q13. Which description correctly matches “anti” in Newman projection terms for vicinal substituents?

• Substituents separated by 60° dihedral angle
• Substituents overlapped (0°) in eclipse
• Substituents separated by 180° dihedral angle
• Substituents in perpendicular orientation (90°)

Correct Answer: Substituents separated by 180° dihedral angle

Q14. When converting an older syn/anti description for an alkene to modern nomenclature, what must be considered?

• Only the bond lengths
• Use CIP priority to determine E or Z
• Replace syn with R and anti with S
• Always change syn to cis and anti to trans directly

Correct Answer: Use CIP priority to determine E or Z

Q15. In bicyclic compounds, syn/anti descriptors refer to relative orientation of substituents with respect to:

• The same ring junction face
• Only the bridgehead atoms’ oxidation state
• Optical rotation sign
• Presence of double bonds exclusively

Correct Answer: The same ring junction face

Q16. In pharmacology, why is correct geometric isomer nomenclature important?

• It determines the color of the drug only
• Geometric isomers can have different pharmacological activities and toxicities
• Nomenclature has no impact on formulation
• Only synthesis yield depends on isomer name

Correct Answer: Geometric isomers can have different pharmacological activities and toxicities

Q17. Which pair of descriptors specifically indicates opposite faces of a ring or double bond?

• syn and gauche
• anti and syn
• syn and cis
• cis and gauche

Correct Answer: anti and syn

Q18. A molecule described as E-2-chloro-2-butene has the higher priority groups:

• On the same side of the double bond
• On opposite sides of the double bond
• Always syn to each other
• Not determinable without R/S

Correct Answer: On opposite sides of the double bond

Q19. For a molecule with two different substituents on each carbon of a C=C, which method is essential to assign E or Z?

• Counting hydrogen atoms only
• Assigning CIP priorities to substituents on each carbon
• Measuring bond angles experimentally only
• Using syn/anti interchangeably

Correct Answer: Assigning CIP priorities to substituents on each carbon

Q20. In monosubstituted cycloalkanes, the term syn is meaningful when comparing:

• Only identical substituents on same carbon
• Relative orientation of two substituents on non-adjacent carbons with respect to the ring face
• Substituent acidity
• Mass spectrometric fragmentation patterns

Correct Answer: Relative orientation of two substituents on non-adjacent carbons with respect to the ring face

Q21. Which of these structural descriptions is most consistent with the term “antiperiplanar” commonly equivalent to anti?

• Dihedral angle ~0°
• Dihedral angle ~120°
• Dihedral angle ~180°
• Dihedral angle ~60°

Correct Answer: Dihedral angle ~180°

Q22. When naming a disubstituted alkene with identical substituents on both carbons, cis/trans can be used. Which case would force using E/Z instead?

• When each carbon has two identical substituents
• When each carbon has two different substituents
• When one substituent is hydrogen on both carbons
• When molecule is achiral

Correct Answer: When each carbon has two different substituents

Q23. In carbohydrate nomenclature, syn/anti descriptors are sometimes used to describe the orientation of which groups?

• Glycosidic oxygen and ring oxygen relationship
• Orientation of exocyclic substituents or oxocarbenium approach during reactions
• Molecular weight differences
• Only the anomeric carbon’s R/S configuration

Correct Answer: Orientation of exocyclic substituents or oxocarbenium approach during reactions

Q24. In an alkene where the higher priority groups are on the same side, the compound often shows which of the following properties compared to its E-isomer?

• Lower dipole moment always
• Higher steric strain and often higher dipole moment
• Identical physical properties to E-isomer
• Always more stable than E-isomer

Correct Answer: Higher steric strain and often higher dipole moment

Q25. Which technique is most commonly used to experimentally distinguish E and Z isomers in the lab?

• Infrared spectroscopy alone
• NMR spectroscopy and coupling constants, often combined with NOE experiments
• Melting point only
• Mass spectrometry alone

Correct Answer: NMR spectroscopy and coupling constants, often combined with NOE experiments

Q26. For a disubstituted alkene with identical substituents, cis is equivalent to which descriptor when substituents are on the same carbon-side orientation?

• trans
• anti
• syn
• E

Correct Answer: syn

Q27. In stereochemistry problems, which projection is most appropriate to analyze syn/anti relationships of vicinal hydrogens?

• Fischer projection only
• Newman projection
• Cram projection
• Hückel diagram

Correct Answer: Newman projection

Q28. If two substituents on a double bond have the same priority on both carbons, what can be concluded about E/Z assignment?

• E/Z cannot be assigned; molecule is meso
• Use secondary atoms to break the tie following CIP rules
• Assign E arbitrarily
• Assign Z arbitrarily

Correct Answer: Use secondary atoms to break the tie following CIP rules

Q29. Which statement accurately links syn/anti with reaction stereochemistry in addition reactions?

• Syn addition places substituents on opposite faces of the double bond
• Anti addition places new substituents on the same face of the double bond
• Syn addition places new substituents on the same face of the double bond
• Addition stereochemistry is unrelated to syn/anti terminology

Correct Answer: Syn addition places new substituents on the same face of the double bond

Q30. In naming a substituted bicyclic system, syn and anti refer to substituents relative to:

• The orientation of substituents relative to the bridge connecting rings
• The CIP priority of substituents only
• The number of pi bonds in the system
• Optical rotation sign

Correct Answer: The orientation of substituents relative to the bridge connecting rings

Q31. When two substituents on non-adjacent carbons are described as anti, they are typically:

• On the same face of a ring
• On opposite faces of a ring
• Attached to the same carbon
• Always axial

Correct Answer: On opposite faces of a ring

Q32. Which of the following is true for E isomers in general compared to Z isomers?

• E isomers always have higher boiling points
• E isomers often have lower steric hindrance and can be more stable
• E isomers are always soluble in water
• E isomers always rotate plane-polarized light to the right

Correct Answer: E isomers often have lower steric hindrance and can be more stable

Q33. In molecules where syn/anti describe conformers, which energy relationship is typical?

• Syn conformer is always lower in energy than anti
• Anti conformer is often lower in energy due to reduced steric repulsion
• Both conformers have identical energy always
• Energy cannot be compared without chirality data

Correct Answer: Anti conformer is often lower in energy due to reduced steric repulsion

Q34. For a 1,2-disubstituted alkene, which physical method can give direct evidence of syn or anti arrangement of substituents across the double bond?

• 1H NMR coupling constant measurements (vicinal coupling) and NOE experiments
• Mass spectrometry fragmentation pattern alone
• Thin-layer chromatography retention factor only
• UV-Vis absorption maximum alone

Correct Answer: 1H NMR coupling constant measurements (vicinal coupling) and NOE experiments

Q35. In older literature, syn/anti might be used for 1,3-relationships. Which modern approach clarifies such descriptors?

• Ignore older descriptors completely
• Use three-dimensional models and specify relative stereochemistry (e.g., cis/trans, syn/anti with ring-face specification)
• Always convert to R/S notation
• Replace with alphabetical labels

Correct Answer: Use three-dimensional models and specify relative stereochemistry (e.g., cis/trans, syn/anti with ring-face specification)

Q36. In the context of azobenzene photoisomerization, syn and anti terminology is often analogous to:

• R/S enantiomer assignment
• cis and trans isomerism around the N=N bond
• Only conformational rotamers
• Optical isomers

Correct Answer: cis and trans isomerism around the N=N bond

Q37. Which of these is a correct step when applying CIP rules to assign E/Z?

• Ignore multiple bonds and consider atoms twice
• Consider higher atomic number at the first point of difference along substituent chains
• Prioritize by atomic weight of the substituent group
• Assign priority based on alphabetical order of substituent names

Correct Answer: Consider higher atomic number at the first point of difference along substituent chains

Q38. When substituents on an alkene are hydrogen and methyl on one carbon, and ethyl and propyl on the other, which concept is key to determining E/Z?

• Molecular formula only
• CIP priority applied to the atoms directly attached to the double bond
• Boiling point comparison
• Mass spectrometric base peak

Correct Answer: CIP priority applied to the atoms directly attached to the double bond

Q39. In stereochemical descriptions, which term most precisely describes substituents located on the same side of a planar double bond when using modern nomenclature?

• E
• Z
• anti
• axial

Correct Answer: Z

Q40. For a drug candidate, why would a medicinal chemist care about syn vs anti conformations?

• They affect the molecular flavor only
• They determine receptor binding orientation, affecting potency and selectivity
• Conformations do not influence ADME properties
• Syn/anti are irrelevant if molecule is achiral

Correct Answer: They determine receptor binding orientation, affecting potency and selectivity

Q41. If two substituents are geminal on the same carbon, which descriptors apply?

• syn/anti between them as vicinal substituents
• geminal is used; syn/anti is not appropriate
• E/Z must be assigned
• They are always trans

Correct Answer: geminal is used; syn/anti is not appropriate

Q42. Which of the following most accurately describes the relationship of syn/anti to conformational analysis?

• Syn/anti only describe permanent stereoisomers
• Syn/anti can describe both conformational relationships (rotamers) and configurational relationships across double bonds or rings
• Syn/anti are only used for ionic compounds
• Syn/anti are identical to R/S configuration

Correct Answer: Syn/anti can describe both conformational relationships (rotamers) and configurational relationships across double bonds or rings

Q43. In a substituted alkene, if the higher priority groups are on adjacent carbon atoms and lie on the same side relative to the double bond plane, the descriptor is:

• E
• Z
• anti
• trans

Correct Answer: Z

Q44. When two substituents are described as syn-periplanar, what is their approximate dihedral angle?

• ~180°
• ~0°
• ~120°
• ~90°

Correct Answer: ~0°

Q45. For cyclic alkenes, cis/trans descriptors are often used. When should E/Z be used instead?

• When rings are large and substituents have ambiguous relative positions requiring CIP priority
• When molecule is achiral always
• Only when rings contain heteroatoms
• Never; cis/trans is always preferred for rings

Correct Answer: When rings are large and substituents have ambiguous relative positions requiring CIP priority

Q46. Which of these is an accurate statement about syn/anti and NMR NOE experiments?

• NOE enhancements are unrelated to spatial proximity and cannot detect syn relationships
• NOE experiments can show through-space proximity consistent with syn arrangements
• NOE gives only coupling constants, not distance information
• NOE is replaced by mass spectrometry for syn/anti determination

Correct Answer: NOE experiments can show through-space proximity consistent with syn arrangements

Q47. In the case of an alkene bearing H and Cl on one carbon and Br and I on the other, which step is necessary to determine whether the molecule is E or Z?

• Compare atomic numbers of H and Cl only
• Apply CIP rules considering atomic numbers of atoms directly attached (Cl vs H and I vs Br)
• Measure the dipole moment only
• Assign based on alphabetical order of substituents

Correct Answer: Apply CIP rules considering atomic numbers of atoms directly attached (Cl vs H and I vs Br)

Q48. Which of the following correctly describes a trans-1,2-disubstituted cycloalkane in terms of syn/anti?

• Substituents are syn on the same face of the ring
• Substituents are anti, located on opposite faces of the ring
• Trans always means syn in cyclic systems
• Trans indicates identical substituents on same carbon

Correct Answer: Substituents are anti, located on opposite faces of the ring

Q49. When assigning syn/anti descriptors to a pair of substituents on nonadjacent carbons, which visualization aids are most helpful?

• Line-angle formula without 3D model
• Molecular models, Newman projection or perspective drawings to determine relative face orientation
• Only molecular formula and empirical formula
• Using melting point tables

Correct Answer: Molecular models, Newman projection or perspective drawings to determine relative face orientation

Q50. Which of the following best summarizes why modern nomenclature prefers E/Z over syn/anti for alkenes?

• E/Z uses relative boiling points to name isomers
• E/Z provides an unambiguous, priority-based system applicable to complex substituents
• Syn/anti is shorter and therefore less preferred
• E/Z only applies to cyclic compounds

Correct Answer: E/Z provides an unambiguous, priority-based system applicable to complex substituents

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