Complete isomerism in organic chemistry

 

100 Q&A on Isomerism in Organic Chemistry (JEE/NEET Focus)

Part 1: Basic Concepts and Structural Isomerism

Q. No.	Question	Answer
1	Define Isomers.	Compounds having the same molecular formula but different physical or chemical properties.
2	What is Structural Isomerism?	Isomers having the same molecular formula but different connectivity (sequence of bonding).
3	What are the five main types of structural isomerism?	Chain, Position, Functional, Metamerism, and Tautomerism.
4	Define Chain Isomerism.	Isomers differing in the arrangement of the carbon skeleton (straight chain vs. branched chain).
5	Give one example of chain isomers (C5​H12​).	n-Pentane and Isopentane (2-Methylbutane).
6	Define Position Isomerism.	Isomers differing in the position of the functional group or substituent on the same carbon chain.
7	Give one example of position isomers (C3​H7​Cl).	1-Chloropropane and 2-Chloropropane.
8	Define Functional Isomerism.	Isomers having the same molecular formula but different functional groups.
9	Give the common functional isomer of Alcohol (C2​H6​O).	Ether (Dimethyl ether).
10	Give the common functional isomer of Aldehyde (C3​H6​O).	Ketone (Acetone).
11	Give the common functional isomer of Carboxylic Acid.	Ester (RCOOR′).
12	Give the common functional isomer of Alkene.	Cycloalkane.
13	Give the common functional isomer of Alkyne.	Diene (two double bonds) or Cycloalkene.
14	Define Metamerism.	Isomers differing in the nature of the alkyl groups attached to the same polyvalent functional atom/group (e.g., Ether, Ketone).
15	Give one example of metamers (C4​H10​O).	Diethyl ether and Methyl propyl ether.
16	Define Tautomerism.	Isomers existing in dynamic equilibrium due to the migration of a proton (usually between C and O or N).
17	What is the most common type of Tautomerism?	Keto-Enol Tautomerism.
18	What is the difference between an enol form and a keto form?	Enol has a hydroxyl (OH) group attached to a C=C bond; Keto has a carbonyl (C=O) group.
19	What property stabilizes the enol form of compounds like ethyl acetoacetate?	Intramolecular Hydrogen Bonding (chelation).
20	How many structural isomers are possible for C4​H10​?	Two (n-Butane and Isobutane).
21	How many structural isomers are possible for C5​H12​?	Three (n-Pentane, Isopentane, and Neopentane).

Part 2: Introduction to Stereoisomerism

Q. No.	Question	Answer
22	Define Stereoisomerism.	Isomers having the same molecular formula and connectivity but a different spatial arrangement of atoms/groups.
23	What are the two main types of stereoisomerism?	Conformational and Configurational (Geometrical and Optical).
24	Define Configurational Isomerism.	Isomers that cannot be interconverted by simple rotation around a single bond (requires bond breaking).
25	Define Geometrical Isomerism (Cis-Trans Isomerism).	Stereoisomerism arising due to restricted rotation around a double bond or in a ring structure.
26	What is the minimum structural requirement for a compound to show Geometrical Isomerism?	The two atoms/groups attached to each carbon of the double bond must be different (a=b and c=d in abC=cd).
27	What is a Cis Isomer?	The identical groups or atoms are on the same side of the double bond or ring.
28	What is a Trans Isomer?	The identical groups or atoms are on the opposite sides of the double bond or ring.
29	Which geometrical isomer is generally more stable and why?	Trans isomer, due to lower steric repulsion between the bulkier groups.
30	Does Propene (CH3​CH=CH2​) show geometrical isomerism?	No, the C−2 carbon is attached to two identical H atoms.
31	Does But-2-ene (CH3​CH=CHCH3​) show geometrical isomerism?	Yes (Cis and Trans forms are possible).
32	What is the alternative nomenclature system for geometrical isomers when groups are different (abC=cd)?	E/Z System (E = Entgegen, Z = Zusammen).
33	What rule is used to assign priority to groups in the E/Z system?	Cahn-Ingold-Prelog (CIP) Rules.
34	How are the Z and Cis isomers related in a simple case like But-2-ene?	Z is equivalent to Cis (highest priority groups on the same side).
35	How are the E and Trans isomers related in a simple case like But-2-ene?	E is equivalent to Trans (highest priority groups on opposite sides).
36	Does Cyclopropane show geometrical isomerism?	Yes, if there are two different substituents on any two carbon atoms.
37	Which geometrical isomer of But-2-enedioic Acid (HOOC-CH=CH-COOH) is more acidic?	Maleic Acid (Cis isomer), due to intramolecular H-bonding stabilizing the monoanion.
38	What is the name of the Trans isomer of But-2-enedioic Acid?	Fumaric Acid.
39	Does Oxime (R−CH=N−OH) show geometrical isomerism?	Yes (due to restricted rotation around the C=N bond, called Syn/Anti isomerism).

Part 3: Relationship between Properties and Isomerism

Q. No.	Question	Answer
40	Why does the Cis isomer often have a higher dipole moment than the Trans isomer?	Dipoles often add up in the Cis form, but cancel out in the Trans form (due to symmetry).
41	Which isomer of 1,2-dichloroethene (ClCH=CHCl) has a net dipole moment of zero?	Trans isomer (dipoles cancel).
42	Which isomer of 1,2-dichloroethene has the higher boiling point?	Cis isomer (higher BP due to higher polarity/dipole moment).
43	Which isomer of 1,2-dichloroethene has the higher melting point?	Trans isomer (higher MP due to better packing in the crystal lattice).
44	What is the term for the stability gained by the enol form in tautomerism?	Intramolecular Hydrogen Bonding (chelation).
45	How does tautomerism affect the reactivity of a compound?	It leads to a mixture of isomers, and the reaction proceeds through the more reactive tautomer.
46	Can a compound exhibit both Functional and Position isomerism?	No, they are distinct categories based on structural differences.
47	How is the DBE (Double Bond Equivalent) of a compound related to its structural possibilities?	DBE gives the number of rings and/or π-bonds required for the formula.
48	What is the relationship between the number of isomers and the size of the carbon chain?	The number of possible isomers increases rapidly as the carbon chain length increases.
49	What type of isomerism is shown by But-1-yne and But-2-yne?	Position Isomerism (position of the triple bond changes).

Part 4: JEE/NEET Application and Advanced Concepts

Q. No.	Question	Answer
50	What is the geometrical configuration of the two double bonds in Hepta-2,4-diene?	Cis/Trans possibilities at both C−2 and C−4 (resulting in four stereoisomers).
51	What are Syn and Anti Isomers?	Geometrical isomers specifically used for oximes (C=N bond) instead of Cis/Trans.
52	In Syn/Anti Isomerism, what determines the nomenclature?	Syn when H on C and OH on N are on the same side; Anti when on the opposite side.
53	Does Allene (CH2​=C=CH2​) show geometrical isomerism?	No, the two π-bonds are perpendicular, preventing planar arrangements.
54	How many stereoisomers does 1-bromo-1-chloro-propene (CH3​CH=C(Br)Cl) have?	Two (Geometrical E/Z isomers only).
55	What is the maximum number of geometrical isomers possible for a compound with n identical double bonds?	Generally 2n (if n is the number of double bonds that can exhibit GI).
56	What are Conformers?	Stereoisomers that can be interconverted by rotation around a single bond (e.g., staggered and eclipsed ethane).
57	What is the maximum angle of rotation required to interconvert geometrical isomers?	180∘ rotation around the double bond (which requires bond breaking/high energy).
58	Which isomer (Cis or Trans) reacts faster with KMnO4​ (hydroxylation) and why?	Cis isomer is often slower, as its bulky groups can hinder the approach of the reagent.
59	How many pairs of Cis/Trans isomers are possible in Octa-2,4,6-triene?	Eight (23=8 total stereoisomers, 4 pairs of diastereomers, including Cis/Trans).

Part 5: More Structural Isomerism Examples

Q. No.	Question	Answer
60	What type of isomerism is exhibited by Propanal and Propanone?	Functional Isomerism (Aldehyde vs Ketone).
61	What type of isomerism is exhibited by 1-Butyne and Buta-1,3-diene?	Functional Isomerism (Alkyne vs Diene).
62	What type of isomerism is exhibited by Propan-1-ol and Propan-2-ol?	Position Isomerism.
63	What type of isomerism is exhibited by Butanoic Acid and Ethyl Ethanoate?	Functional Isomerism (Acid vs Ester).
64	What type of isomerism is exhibited by Propylamine (CH3​CH2​CH2​NH2​) and Ethylmethylamine (CH3​CH2​NHCH3​)?	Functional Isomerism (1∘ vs 2∘ amine).
65	What type of isomerism is exhibited by Propanal and Cyclopropanol (C3​H6​O)?	Ring-Chain Isomerism (also functional isomerism if viewed broadly).
66	What type of isomerism is exhibited by Pent-1-ene and Cyclopentane (C5​H10​)?	Ring-Chain Isomerism (also functional).
67	What type of isomerism is exhibited by Butanone and Diethyl Ketone?	Metamerism (Ketone group position).
68	Give one example of a compound showing Keto-Enol Tautomerism.	Acetone (CH3​COCH3​) or Acetaldehyde (CH3​CHO).
69	What is the formula to calculate the number of chain isomers for alkanes?	No simple formula, usually solved by drawing structures.

Part 6: More Geometrical Isomerism Scenarios

Q. No.	Question	Answer
70	Does 1,1-Dichloroethene (Cl2​C=CH2​) show geometrical isomerism?	No, the C−1 carbon has two identical Cl atoms.
71	Does Cyclohexane show geometrical isomerism?	No, but its disubstituted derivatives (e.g., 1,2-Dimethylcyclohexane) do.
72	What is the relationship between Cis-1,2-Dimethylcyclopropane and Trans-1,2-Dimethylcyclopropane?	They are Diastereomers (and geometrical isomers).
73	Assign the configuration (E or Z) to 1-bromo-1-chloro-propene where Br and H are Cis.	Z (Br on C−1 and CH3​ on C−2 are highest priority; if they are Cis →Z).
74	What is the name given to isomers that are neither Cis nor Trans but are E/Z isomers?	Diastereomers.
75	How many E/Z isomers are possible for 3-Hexene?	Two (E and Z).
76	Can a compound exhibit both Geometrical and Optical isomerism?	Yes, if it has both restricted rotation and a chiral center.
77	In the E/Z system, what does the letter 'Z' mean?	Zusammen (Together/Same side).
78	In the E/Z system, what does the letter 'E' mean?	Entgegen (Opposite side).
79	What is the most common reason for the Trans isomer being generally more stable?	Reduced Steric Strain.
80	Give an example of a cyclic compound showing geometrical isomerism.	1,2-Dimethylcyclobutane (Cis/Trans).

Part 7: Tautomerism and Isomerism in Functional Groups

Q. No.	Question	Answer
81	What is the term for the migration of the proton in tautomerism?	Prototropy.
82	What type of isomerism is shown by Methylacetylene (CH3​C≡CH) and Allene (CH2​=C=CH2​)?	Functional Isomerism (Alkyne vs Allene).
83	What type of isomerism is exhibited by 1∘,2∘, and 3∘ Amines (C3​H9​N)?	Functional Isomerism.
84	What type of isomerism is exhibited by Di-n-propylamine and Ethyl-n-butylamine (C6​H15​N)?	Metamerism (2∘ amines with different alkyl groups).
85	Why is the Keto form generally more stable than the Enol form?	The C=O bond is much stronger than the C=C bond.
86	Which Carbonyl compound exists almost entirely in the enol form and why?	Phenol (due to aromatic stabilization of the enol form).
87	Does Benzaldehyde show Keto-Enol tautomerism?	No, it lacks an α-hydrogen.
88	Does Tautomerism affect the chemical properties of a compound?	Yes, as the minor tautomer might be the one that reacts (reaction proceeds via the most reactive species).
89	What type of isomerism is shown by Cyclohexane and Hex-1-ene?	Ring-Chain and Functional Isomerism.
90	What are the general structural formulas for Metamers in the Ketone family?	R−C=O−R′ and R′′−C=O−R′′′ where R+R′=R′′+R′′′.

Part 8: Final Applications and Revision

Q. No.	Question	Answer
91	What is the DBE (Degree of Unsaturation) for C6​H6​?	4 (3 double bonds and 1 ring).
92	How many structural isomers are possible for Dichlorobenzene (C6​H4​Cl2​)?	Three (o−,m−,p−).
93	How many Cis/Trans isomers does 1,2-dibromocyclobutane have?	Three (Cis and two optically active Trans forms).
94	What is the stability order of conformations in cyclohexane?	Chair > Twist-boat > Boat > Half-chair.
95	Does 1,1-Dimethylcyclohexane show geometrical isomerism?	No, as the C−1 carbon has two identical CH3​ groups.
96	Name the type of isomerism responsible for the high boiling point of Cis alkenes.	Geometrical Isomerism (leading to higher polarity).
97	What is the relationship between the Cis/Trans pair of But-2-ene?	They are Diastereomers.
98	In Oximes, if the H on C and OH on N are Anti, what is the Cis/Trans equivalent?	Trans equivalent.
99	How many total stereoisomers are possible for 3-chloro-4-fluoro-2-pentene?	Four (One C=C for GI, two chiral centers for OI).
100	What is the total number of Cis/Trans isomers for 2,4-Hexadiene?	Three (Cis-Cis, Trans-Trans, and Cis-Trans ≡ Trans-Cis).
101	What is the structural requirement for an alicyclic compound to exhibit geometrical isomerism?	Presence of at least two different substituents on two different ring carbon atoms.
102	What is the relationship between the two Chair conformations of cyclohexane?	They are Conformational Isomers (easily interconvertible).
103	Which type of isomerism is responsible for the unique properties of Fructose (a Ketose) and Glucose (an Aldose)?	Functional Isomerism (C6​H12​O6​).
104	What is the condition for a Metamer to be possible in the Ether series?	The ether must have at least four carbon atoms in total (e.g., C4​H10​O).
105	What kind of isomerism is observed between 2-pentene (C=C at C−2) and 1-pentene (C=C at C−1)?	Position Isomerism.

Over 100 Q&A for Optical Isomerism (JEE/NEET Focus)

Part 1: Fundamentals and Chiral Molecules

Q. No.	Question	Answer
1	Define Optical Isomerism.	Isomerism where compounds have the same molecular formula but differ in their effect on plane-polarized light (PPL).
2	What is Plane-Polarized Light (PPL)?	Light waves oscillating in only one single plane.
3	What is a Polarimeter?	The instrument used to measure the angle of rotation of PPL caused by an optically active substance.
4	Define an Optically Active substance.	A substance that rotates the plane of PPL when passed through its solution or pure liquid form.
5	Define an Optically Inactive substance.	A substance that does not rotate the plane of PPL.
6	What does the prefix Dextrorotatory (d or +) signify?	Rotation of PPL to the Right (clockwise).
7	What does the prefix Laevorotatory (l or −) signify?	Rotation of PPL to the Left (anticlockwise).
8	Define Specific Rotation (α)	The angle of rotation produced by a solution of concentration 1 g/mL in a tube of 1 dm length.
9	Define a Chiral Molecule.	A molecule that is non-superimposable on its mirror image.
10	Define an Achiral Molecule.	A molecule that is superimposable on its mirror image.
11	What is the necessary and sufficient condition for a molecule to be chiral?	Absence of a Plane of Symmetry (σ) or Center of Symmetry (i).
12	What is a Chiral Centre?	An atom (usually Carbon) bonded to four different atoms or groups.
13	Why is Glycine (H2​NCH2​COOH) optically inactive?	Its α-carbon is attached to two H atoms (it lacks a chiral centre).
14	What is the term for a carbon atom bonded to four different groups?	Asymmetric Carbon or Chiral Carbon.
15	What is the total number of stereoisomers for a molecule with n different chiral centres?	2n (van't Hoff rule, max possible stereoisomers).

Part 2: Enantiomers, Diastereomers, and Meso Compounds

Q. No.	Question	Answer
16	Define Enantiomers.	Stereoisomers that are non-superimposable mirror images of each other.
17	How do enantiomers differ in their physical properties?	They have identical physical properties (melting point, boiling point, solubility, refractive index, etc.).
18	How do enantiomers differ in their optical activity?	They rotate PPL by the same magnitude but in opposite directions (+ and −).
19	Define a Racemic Mixture (dl or ±).	An equimolar mixture of two enantiomers.
20	Is a racemic mixture optically active or inactive?	Optically inactive (external compensation/cancellation of rotations).
21	Define Racemisation.	The process of converting a pure enantiomer into a racemic mixture.
22	Define Diastereomers.	Stereoisomers that are not mirror images of each other.
23	How do diastereomers differ in their physical properties?	They have different physical properties (can be separated by fractional distillation or crystallization).
24	Define a Meso Compound.	An optically inactive compound despite having chiral centres.
25	What is the structural condition for a meso compound?	It must possess a Plane of Symmetry (σ) or a Center of Symmetry (i).
26	What type of compensation leads to the optical inactivity of a meso compound?	Internal Compensation (one half of the molecule rotates light equally and oppositely to the other half).
27	How many stereoisomers are possible for Tartaric Acid (2,3-dihydroxybutanedioic acid)?	Three (one d, one l, and one meso form).
28	What is the relationship between the meso form and its enantiomers (the d and l forms)?	The meso form is a diastereomer of the d and l forms.
29	What is the formula for calculating the number of meso forms possible for a symmetric molecule?	2(n/2)−1 (where n is the number of chiral centres, if n is even).

Part 3: R/S Nomenclature (CIP Rules)

Q. No.	Question	Answer
30	What is the R/S System used for?	To give an absolute configuration to each chiral centre.
31	What does the letter R signify in R/S nomenclature?	Rectus (Latin for Right).
32	What does the letter S signify in R/S nomenclature?	Sinister (Latin for Left).
33	What is the fundamental principle of the Cahn-Ingold-Prelog (CIP) Rules?	Assigning priority to the four groups attached to the chiral centre based on Atomic Number.
34	How are isotopes prioritized in the CIP rules?	The higher mass number isotope gets higher priority (e.g., D>H).
35	How are double/triple bonds treated in the CIP rules?	The multiply-bonded atom is counted as being bonded to an equivalent number of single-bonded atoms (fictitious atoms).
36	How is the configuration determined if the lowest priority group (4) is pointing away (dashed bond)?	The configuration is R for clockwise rotation of 1→2→3, and S for anticlockwise.
37	How is the configuration determined if the lowest priority group (4) is pointing towards the observer (wedge bond)?	The determined configuration (from 1→2→3) is Inverted (R→S and vice versa).
38	What is the relationship between R and S forms?	They are Enantiomers.

Part 4: Chirality in Reactions and Resolution

Q. No.	Question	Answer
39	What is the stereochemical result of a nucleophilic substitution reaction (SN​1) on a chiral substrate?	Racemisation (or partial racemisation).
40	What is the stereochemical result of a nucleophilic substitution reaction (SN​2) on a chiral substrate?	Inversion of Configuration (Walden Inversion).
41	When does Walden Inversion occur?	When the nucleophile attacks from the backside of the leaving group (SN​2 mechanism).
42	Define Resolution.	The process of separating a racemic mixture into its constituent pure enantiomers.
43	Why can't enantiomers be separated by fractional distillation?	They have identical boiling points.
44	What is the standard method for resolving a racemic mixture?	Conversion to Diastereomeric Salts, separation by crystallization, and then regeneration of the enantiomers.
45	Why can diastereomeric salts be separated easily?	They have different physical properties (solubility, melting point).
46	What is the term for a molecule that rotates PPL but has no chiral centre (e.g., substituted allenes)?	Axial Chirality.
47	Give an example of a common resolving agent.	An optically active acid (+ Tartaric acid) or base (+ Strychnine).

Part 5: Symmetry Elements and Optical Activity

Q. No.	Question	Answer
48	Define a Plane of Symmetry (σ).	An imaginary plane that divides the molecule into two identical halves that are mirror images of each other.
49	Define a Centre of Symmetry (i).	A point within the molecule from which lines drawn to identical atoms are equal in length but opposite in direction.
50	Does a molecule with a centre of symmetry exhibit optical activity?	No, it is achiral.
51	Does a molecule with a chiral centre always exhibit optical activity?	No (the exception is the meso compound).
52	What is a Stereogenic Centre?	Any point in a molecule where the interchange of two groups leads to a stereoisomer (includes chiral centre and C=C double bond).
53	Which common form of substituted cyclohexane is typically achiral due to a plane of symmetry?	Trans-1,4-disubstituted cyclohexane.
54	What is the relationship between the stability of the eclipsed conformation of Butane and its optical activity?	Conformations are interconvertible, so the overall molecule is optically inactive.

Part 6: Quickfire Revision and Numerical Application

Q. No.	Question	Answer
55	What is the configuration of C−2 in L-Glyceraldehyde?	S (S-configuration).
56	What is the configuration of C−2 in D-Glyceraldehyde?	R (R-configuration).
57	How many stereoisomers does 2-chlorobutane have?	Two (a single pair of enantiomers, 21).
58	What is the maximum number of stereoisomers for 2,3-dichlorobutane?	Four (22), but only three are unique (due to one meso form).
59	How many chiral centres are in Glucose?	Four chiral centres.
60	What is the term for non-mirror-image stereoisomers that are rapidly interconverting?	Conformers.
61	A mixture of 75% R and 25% S enantiomer has what % enantiomeric excess (ee)?	50% (ee=75−25).
62	If ee is 80%, what is the percentage of the major enantiomer?	90% (Major=50+ee/2).
63	The addition of H2​ to Cis-But-2-ene using a catalyst gives a product. Is the product optically active?	No, the product (Butane) is achiral.
64	The reaction of Butan-2-ol with HBr proceeds via SN​1. What is the resulting mixture called?	Racemic Mixture.
65	The separation of a racemic mixture is called...	Resolution.
66	What is the chemical that causes the rotation of PPL called?	The Optically Active Substance.
67	What is the simplest chiral alkane?	3-Methylhexane (C7​H16​).
68	What is the simplest chiral alcohol?	Butan-2-ol (C4​H10​O).
69	What is the structural relationship between D-Glucose and L-Glucose?	They are Enantiomers.
70	What is the structural relationship between D-Glucose and D-Mannose?	They are Diastereomers (specifically, C−2 epimers).
71	What are Epimers?	Diastereomers that differ in configuration at only one chiral centre.
72	What is the common name for the D/L system based on Glyceraldehyde?	Fischer Projection (used to denote relative configuration).
73	Is the rotation of PPL by a molecule related to its R/S configuration?	No, R does not imply + and S does not imply −.
74	What type of isomers are the α and β forms of Glucose?	Anomers (a specific type of diastereomer).
75	How do you check for a centre of symmetry in a molecule?	Check if every atom can be reflected through the center point to an identical atom.

Part 7: Optical Isomerism in Rings and Other Systems

Q. No.	Question	Answer
76	Does Cis-1,2-Dimethylcyclopropane have a plane of symmetry?	Yes (a plane perpendicular to the ring).
77	Is Cis-1,2-Dimethylcyclopropane optically active?	No (it is a meso compound / achiral).
78	Does Trans-1,2-Dimethylcyclopropane have a plane of symmetry?	No.
79	Is Trans-1,2-Dimethylcyclopropane optically active?	Yes (it exists as a pair of enantiomers).
80	What is a Pseudochiral Centre?	A carbon atom bonded to four different groups, two of which are enantiomers of each other.
81	What is the configuration of the C−3 carbon in Meso-Tartaric Acid?	The configuration is r or s (lower case, depends on the other chiral centre).
82	What happens to the optical activity of an enantiomer when it is dissolved in a non-chiral solvent?	No change (remains optically active).
83	What happens to the optical activity of an enantiomer when it is mixed with its mirror image?	It becomes optically inactive (racemic mixture).
84	What is the term for the stability of a racemic mixture?	External compensation (thermodynamic stability).
85	Can a compound with double bonds show optical isomerism?	Yes, if it exhibits axial chirality (like substituted allenes) or has chiral centres elsewhere.

Part 8: Numerical and Conceptual Mix

Q. No.	Question	Answer
86	How many stereoisomers are possible for 2,3,4-trichlorohexane?	Eight (23=8) as all three chiral centres are non-identical.
87	If a single pure enantiomer has specific rotation +50∘, what is the rotation of a 1:3 mixture of the d and l forms?	−25∘ (ee=25% l, so 0.5×50∘).
88	What kind of strain is minimized when using the anti conformation in Fisher projections?	Torsional strain.
89	The reduction of Acetophenone (C6​H5​COCH3​) by NaBH4​ yields a product. Is the product chiral?	Yes (1-phenylethanol is chiral).
90	What is the main reason for the optical inactivity of a solution containing D- and L-tartaric acid?	Equal and opposite rotation by the two forms (external compensation).
91	What is the R/S configuration of C−2 in L-Alanine?	S (Alanine is generally S configuration).
92	What is the relationship between d/l and D/L notation?	d/l refers to optical rotation; D/L refers to relative configuration (based on glyceraldehyde).
93	How do you check for a plane of symmetry in a Fischer Projection?	Check for a plane that bisects the molecule horizontally or vertically.
94	What does Syn addition of Br2​ to an alkene mean stereochemically?	Both Br atoms add to the same face of the double bond.
95	What does Anti addition of Br2​ to an alkene mean stereochemically?	Br atoms add to opposite faces of the double bond.
96	Does Trans-1,3-dimethylcyclohexane show optical activity?	Yes (it is chiral).
97	What is the term for the process of converting a single enantiomer into a racemic mixture?	Racemisation.
98	Why are meso compounds possible only when the number of chiral centres is even?	To allow for the existence of an internal plane of symmetry.
99	What is the simplest molecule that exhibits optical isomerism?	Glyceraldehyde (C3​H6​O3​).
100	Are Cis/Trans isomers also optical isomers?	No, they are diastereomers (a type of stereoisomer), but they are geometrical isomers.
101	What does the term chiroptical property refer to?	Any molecular property related to chiral asymmetry (like optical rotation, CD).
102	What is the R/S configuration of D-lactic acid?	R.
103	What is the relationship between the two chair conformations of Trans-1-tert-butyl-4-methylcyclohexane?	Conformers (easily interconvertible) but not enantiomers.
104	What is the name of the reaction that produces a single enantiomer product from a non-chiral starting material?	Asymmetric Synthesis.

Over 100 Q&A for Geometrical and Conformational Isomerism (JEE/NEET Focus)

Part 1: Geometrical Isomerism (GI) - Fundamentals & Alkenes

Q. No.	Question	Answer
1	What is the fundamental requirement for a molecule to exhibit Geometrical Isomerism (GI)?	Restricted rotation around a bond (like a double bond or in a ring).
2	What is the structural requirement for GI around a C=C bond?	Each carbon atom of the double bond must be attached to two different groups.
3	What is a Cis-isomer?	Identical groups are on the same side of the plane of restricted rotation.
4	What is a Trans-isomer?	Identical groups are on the opposite sides of the plane of restricted rotation.
5	Why does Propene (CH3​CH=CH2​) not show GI?	The C−1 carbon is attached to two identical H atoms.
6	What is the relationship between Cis and Trans isomers?	They are Diastereomers (a type of stereoisomer).
7	Which geometrical isomer is generally more stable and why?	Trans isomer, due to lower steric repulsion.
8	How does GI affect the dipole moment of simple symmetrical alkenes (e.g., 1,2-dichloroethene)?	Cis has a net dipole moment (μ=0); Trans has μ=0 (dipoles cancel).
9	Which isomer (Cis or Trans) of 1,2-dichloroethene has the higher boiling point?	Cis isomer (due to higher polarity/dipole moment).
10	Which isomer has the higher melting point?	Trans isomer (due to better packing in the crystal lattice).
11	What is the GI of Cycloalkenes typically restricted to?	Larger rings (≥8 carbons) can exist as stable Trans forms; smaller rings only Cis.
12	Does 1,1-Dichloroethene (Cl2​C=CH2​) show GI?	No, one carbon has two identical Cl groups.
13	What GI is shown by But-2-ene?	Cis (Z) and Trans (E) isomers.
14	What is the product of Anti-addition of Br2​ to Cis-but-2-ene?	meso-2,3-dibromobutane.
15	What is the product of Anti-addition of Br2​ to Trans-but-2-ene?	Racemic mixture of d,l-2,3-dibromobutane.

Part 2: E/Z System and GI in Other Systems

Q. No.	Question	Answer
16	What is the E/Z system used for?	Assigning absolute configuration to geometrical isomers with multiple different substituents.
17	What does the configuration Z stand for?	Zusammen (Together), equivalent to Cis in simple cases.
18	What does the configuration E stand for?	Entgegen (Opposite), equivalent to Trans in simple cases.
19	What rule is used to determine priority in the E/Z system?	Cahn-Ingold-Prelog (CIP) Rules.
20	How is priority assigned in the E/Z system?	Based on the higher atomic number of the atoms directly attached to the double bond carbons.
21	If the two highest priority groups are on the same side, what is the configuration?	Z.
22	If the two highest priority groups are on opposite sides, what is the configuration?	E.
23	Does Propanone Oxime (CH3​C(=NOH)CH3​) show GI?	No, the C atom has two identical CH3​ groups.
24	What are the names for GI in Oximes (C=N bond)?	Syn (equivalent groups on same side) and Anti (opposite sides).
25	Does Cyclohexane show GI?	No, but its disubstituted derivatives do.
26	What are the GI isomers of 1,2-Dimethylcyclopropane called?	Cis and Trans isomers.
27	What is the relationship between Cis-1,2-dimethylcyclohexane and Trans-1,2-dimethylcyclohexane?	They are Diastereomers.
28	How many geometrical isomers are possible for Hepta-2,4-diene?	Four (22) (GI at C−2 and C−4).
29	How many geometrical isomers are possible for But-2-yne?	Zero (linear geometry of the triple bond prevents restricted rotation).
30	Does Allene (CH2​=C=CH2​) show GI?	No (π-orbitals are perpendicular).

Part 3: Conformational Isomerism - Alkanes

Q. No.	Question	Answer
31	Define Conformational Isomerism.	Isomers that are interconvertible by rotation around a single (σ) bond.
32	What is another name for Conformational Isomers?	Conformers or Rotamers.
33	What is the preferred representation for viewing conformations?	Newman Projections.
34	Define Torsional Strain.	The repulsive strain due to the close proximity of electron clouds of the bonds/atoms in the adjacent carbons.
35	Which conformation of ethane has maximum stability?	Staggered conformation.
36	Which conformation of ethane has minimum stability (maximum energy)?	Eclipsed conformation.
37	What is the energy difference between the eclipsed and staggered forms of ethane called?	Torsional energy (≈12.5 kJ/mol).
38	What is the Dihedral Angle (θ) in the staggered conformation of ethane?	60∘.
39	What is the Dihedral Angle (θ) in the eclipsed conformation of ethane?	0∘.
40	Why are conformers not isolated at room temperature?	The energy barrier for interconversion is very low.
41	What is the most stable conformation of Butane?	Anti-staggered (Dihedral angle 180∘).
42	What is the least stable conformation of Butane?	Fully Eclipsed (Dihedral angle 0∘).
43	What is the term for the strain caused by the interaction of groups across space (non-bonded atoms)?	Steric Strain.
44	What is the conformation of butane that is slightly higher in energy than anti but lower than eclipsed?	Gauche (Dihedral angle 60∘).
45	What is the energy difference between the anti and gauche conformations of butane mainly due to?	Steric hindrance (CH3​/CH3​ interaction).

Part 4: Conformational Isomerism - Cycloalkanes

Q. No.	Question	Answer
46	Which cycloalkane is perfectly planar and why?	Cyclopropane (rigid, bond angle 60∘).
47	What is the main cause of instability in Cyclopropane and Cyclobutane?	Angle Strain (deviation from 109.5∘) and Torsional Strain.
48	What is the most stable non-planar conformation of Cyclobutane?	Puckered or Folded conformation.
49	What is the most stable conformation of Cyclopentane?	Envelope conformation (non-planar).
50	What is the most stable conformation of Cyclohexane?	Chair form.
51	What are the two types of C−H bonds in the Chair form?	Axial (perpendicular to ring) and Equatorial (parallel to ring).
52	What is the process of interconverting two Chair forms called?	Ring flipping (or Chair-Chair interconversion).
53	What happens to the axial/equatorial bonds during ring flipping?	Axial bonds become equatorial, and equatorial bonds become axial.
54	Which conformation of Cyclohexane has the highest energy?	Half-chair form.
55	What is the next most stable conformation after the Chair form?	Twist-boat (or Skew boat).
56	What are the H atoms at C−1 and C−4 in the Boat form called?	Flagpole Hydrogens (cause severe steric strain).
57	In monosubstituted cyclohexanes (e.g., Methylcyclohexane), which position is more stable?	The substituent in the Equatorial position (avoids 1,3-diaxial interaction).
58	What is 1,3-Diaxial Interaction?	Steric repulsion between an axial substituent at C−1 and the axial H atoms at C−3 and C−5.
59	How many 1,3-diaxial interactions does an axial methyl group cause?	Two (with H at C−3 and C−5).
60	What is the relationship between the two Chair forms of an achiral substituted cyclohexane (e.g., methylcyclohexane)?	They are Diastereomers (since they have different energies).

Part 5: Mixed Concepts and Calculations (JEE/NEET)

Q. No.	Question	Answer
61	How many stereoisomers does 1,3-Dichlorocyclopentane have?	Three (Cis [achiral] and a pair of Trans [enantiomers]).
62	What is the GI requirement for a compound with two double bonds (A-CH=CH-CH=CH-B)?	GI is possible at both double bonds (C−2 and C−4).
63	What is the maximum number of stereoisomers possible for Hepta-2,4-diene?	Four (22).
64	What is the relationship between Cis and Trans isomers in terms of rotation?	Cis and Trans are Configurational Isomers (not easily rotatable).
65	What is the relationship between the Gauche and Anti forms of butane?	They are Conformational Isomers (Rotamers).
66	Does Allene (CH2​=C=CH2​) show GI?	No (GI requires co-planarity, which Allene lacks).
67	Which isomer is formed exclusively in the E2 elimination of alkyl halides (if possible)?	Trans isomer (due to anti-periplanar transition state/Saytzeff's rule).
68	What is the approximate energy required for ring-flipping in cyclohexane?	≈42−45 kJ/mol (easily accessible at room temperature).
69	In 1,2-Dimethylcyclohexane, which isomer exists as a meso compound?	Cis isomer (Cis-1,2 is achiral/meso).
70	Which bond is shorter: the C=C bond in the Cis isomer or the Trans isomer?	They are essentially the same length.
71	What causes the flagpole interaction in the boat conformer?	Steric repulsion between the H atoms at C−1 and C−4.
72	What is the term for the stability gained by the gauche form of 2-chloroethanol?	Intramolecular H-bonding (instead of steric hindrance).
73	What is the maximum number of eclipsed interactions in the eclipsed conformation of ethane?	Three (3 pairs of C−H bonds).
74	What is the major strain present in the Chair form of cyclohexane?	None (ideally, both angle strain and torsional strain are minimal/zero).
75	Why is the Twist-boat more stable than the Boat form?	The Twist-boat has less torsional strain and avoids the flagpole interaction.

Part 6: Applications and Advanced GI (Quickfire)

Q. No.	Question	Answer
76	What is the effect of GI on the hydration of maleic acid (cis) versus fumaric acid (trans)?	Maleic acid (cis) hydrates faster than fumaric acid (trans).
77	How does increasing the size of the substituent affect the axial/equatorial preference?	The preference for the Equatorial position increases dramatically.
78	What is the strain in Cyclohexane primarily due to?	Torsional strain and Steric strain in the boat form; angle strain is negligible in the chair.
79	How many geometrical isomers does 1,3-Dimethylcyclohexane have?	Four (Cis and Trans, each existing as a pair of enantiomers).
80	What is the configuration of the C=N bond in Acetophenone Oxime (C6​H5​C(=NOH)CH3​)?	Syn (CH3​/OH same side) and Anti (CH3​/OH opposite side).
81	What is the relationship between the Cis and Trans isomers of 1,3-Dimethylcyclohexane?	Diastereomers.
82	Which isomer of But-2-enoic Acid (Cis or Trans) has a higher pKa​?	Trans isomer (Fumaric acid), as it lacks the H-bonding stabilization of the cis form's anion.
83	What is the effect of ring size on the Trans isomer of Cycloalkenes?	The Trans isomer becomes more stable as the ring size increases.
84	Is the boat conformation of cyclohexane optically active?	No, it contains a plane of symmetry.
85	What is the preferred conformation of Trans-1,4-dibromocyclohexane?	The Diequatorial (e,e) chair form.
86	Does Buta-1,3-diene show GI?	No (C−1 and C−4 carbons are attached to two H atoms).
87	What is the name for the rotation of axial H bonds towards the center of the ring in the chair form?	Axial-Axial interaction (in trans-1,2 forms, for example).
88	How many GI isomers are possible for 1-chloro-1-propene (CH3​CH=CHCl)?	Two (Cis and Trans).
89	What is the major product of the E2 elimination of menthyl chloride?	Anti-Saytzeff product (due to the required anti-periplanar geometry).
90	Is Cyclooctene (C8​H14​) capable of showing GI?	Yes, it is the smallest cycloalkene for which Trans form is stable.

Part 7: Final Revision Mix

Q. No.	Question	Answer
91	Is a molecule with GI always achiral?	No, Cis/Trans isomers can also be chiral (if they possess chiral centres).
92	What is the energy difference between the chair and boat conformations of cyclohexane?	≈29 kJ/mol.
93	What is the primary method to separate geometrical isomers?	Fractional distillation (due to difference in BP).
94	What is the GI nomenclature used for azo compounds (N=N bond)?	Syn/Anti (similar to oximes).
95	What is the most stable form of Buta-1,3-diene (conformationally)?	s-Trans (single bond trans to each other).
96	What is the dihedral angle between the two methyl groups in the anti conformation of butane?	180∘.
97	What is the dihedral angle between the two methyl groups in the gauche conformation of butane?	60∘.
98	Why are eclipsed conformations less stable than staggered?	Due to Torsional Strain.
99	What is the relationship between the Chair and Twist-boat forms of cyclohexane?	Conformational Isomers.
100	What is the GI name for the Cis isomer of But-2-enedioic Acid?	Maleic Acid.
101	What is the GI name for the Trans isomer of But-2-enedioic Acid?	Fumaric Acid.
102	What is the total number of stereoisomers for a compound with one double bond (GI) and one chiral center (OI)?	Four (2×2=4).
103	What is the general name for the method used to analyze conformations?	Conformational Analysis.
104	What is the smallest cycloalkene for which a Trans isomer is known to be isolated?	Trans-Cyclooctene (C8​).
105	What is the minimum number of carbons required for a simple alkene to show GI?	Four (But-2-ene).

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