CSIR NET Chemical Science: Aromaticity PYQs with Detailed Solutions
Overview: Mastering Aromaticity for CSIR-UGC NET
Aromaticity and Non-Benzenoid Systems form one of the most reliable scoring sections in the CSIR-UGC NET Chemical Sciences examination. Appearing consistently in both Part B (2 marks) and Part C (4 marks), aromaticity questions evaluate fundamental orbital theory, cyclic conjugation, and spectroscopic properties ($^1\text{H}$-NMR chemical shifts in diatropic versus paratropic rings).
This dedicated study guide compiles high-frequency Previous Year Questions (PYQs) from CSIR NET and GATE Chemistry. It categorizes recurring problem types, explains key diagnostic rules, and provides step-by-step solutions to help you identify aromatic, antiaromatic, non-aromatic, and homoaromatic systems quickly.
CSIR NET Aromaticity: Topic Weightage & Trend Analysis
| Exam Component | Typical Question Count | Marks Weightage | Common Question Formats |
| Part B | 1 to 2 Questions | 2 to 4 Marks | Direct classification (Aromatic vs Antiaromatic vs Non-aromatic), simple heterocycles, charged ions. |
| Part C | 1 to 2 Questions | 4 to 8 Marks | Annulene conformations, $^1\text{H}$-NMR diatropic/paratropic shifts, Craig’s rule, Baird aromaticity, quasi-aromatic dipoles. |
| Total Impact | 2 to 3 Questions | 6 to 12 Marks | Consistently tested in June and December cycles. |
Essential Diagnostic Rules for Solving PYQs
Before approaching the questions, review these high-yield diagnostic criteria:
- Huckel’s Rule ($4n+2$ $\pi$-electrons):
- Applies strictly to planar, monocyclic, fully conjugated systems.
- Aromatic: $(4n+2)\ \pi$-electrons (where $n = 0, 1, 2, \dots$).
- Antiaromatic: $4n\ \pi$-electrons (where $n = 1, 2, \dots$) in a planar ring.
- Non-Aromatic: Any system lacking planarity, broken conjugation ($\text{sp}^3$ carbons), or open chain structures.
- Annulene Planarity Rules:
- [8]-Annulene (Cyclooctatetraene): Adopts a non-planar tub conformation to relieve angle strain; non-aromatic.
- [10]-Annulene: Non-planar due to steric clash of internal hydrogens ($1,6$-positions); non-aromatic unless bridged (e.g., 1,6-methano[10]annulene is planar and aromatic).
- [14]-Annulene and [18]-Annulene: [14] is weakly aromatic due to steric hindrance, whereas [18]-annulene is planar and strongly aromatic with distinct internal ($\delta \approx -3.0\ \text{ppm}$) and external ($\delta \approx +9.3\ \text{ppm}$) $^1\text{H}$-NMR signals.
- Specialized Aromatic Classifications:
- Homoaromaticity: Conjugation interrupted by one or more $\text{sp}^3$ hybridized carbons, bypassing through space (e.g., homotropylium cation).
- Quasi-Aromatic Systems: Compounds whose resonance contributors have a major aromatic dipolar structure (e.g., Azulene, Tropone, Cyclopentadienone derivatives).
- Craig’s Rule for Polycyclic Systems: Used for non-benzenoid conjugated systems with a plane of symmetry ($C_{2v}$ symmetry). If $(N – f)$ is an integer multiple of 2, the system is non-aromatic/antiaromatic; if odd, it is aromatic.
- Baird’s Rule (Excited State): In the lowest triplet excited state ($T_1$), the rule inverts: $4n\ \pi$-systems are aromatic, while $(4n+2)\ \pi$-systems are antiaromatic.
Categorized PYQs with Step-by-Step Solutions
Type 1: Annulenes and $^1\text{H}$-NMR Spectroscopy (Part C)
Question 1:
In the $^1\text{H}$-NMR spectrum of [18]-annulene recorded at $-60^\circ\text{C}$, the ratio of peak areas and their respective chemical shift values are:
- (A) 12H at $\delta\ 9.28\ \text{ppm}$ (outer) and 6H at $\delta\ -2.99\ \text{ppm}$ (inner)
- (B) 6H at $\delta\ 9.28\ \text{ppm}$ (outer) and 12H at $\delta\ -2.99\ \text{ppm}$ (inner)
- (C) 18H single peak at $\delta\ 5.40\ \text{ppm}$
- (D) 12H at $\delta\ -2.99\ \text{ppm}$ (outer) and 6H at $\delta\ 9.28\ \text{ppm}$ (inner)
Correct Answer:(A)
Explanation:
- [18]-Annulene contains $18\ \pi$-electrons ($4n+2$ where $n=4$), making it an aromatic diatropic system.
- An induced diamagnetic ring current reinforces the external magnetic field outside the ring (deshielding outer protons, $\delta \approx 9.28\ \text{ppm}$) and opposes the field inside the ring (shielding inner protons, $\delta \approx -2.99\ \text{ppm}$).
- The structure contains 12 outer protons and 6 inner protons, giving an intensity ratio of $2:1$ ($12\text{H} : 6\text{H}$).
Type 2: Charged Species and Quasi-Aromatic Systems (Part B)
Question 2:
Among the following compounds, the one which displays a high dipole moment and behaves as an aromatic salt is:
- (A) Cyclopentadiene
- (B) Cycloheptatrienone (Tropone)
- (C) Cyclooctatetraene
- (D) 1,3-Cyclohexadiene
Correct Answer:(B)
Explanation:
- Tropone (cycloheptatrienone) possesses a polarized carbonyl group: $\text{C}=\text{O} \longleftrightarrow \text{C}^+ – \text{O}^-$.
- Resonance pushes electron density to oxygen, generating a seven-membered cyclic carbocation known as the tropylium cation.
- The tropylium ring has 6 $\pi$-electrons ($4n+2$, $n=1$), fully delocalized across seven carbons, imparting strong quasi-aromatic stability and an unusually high dipole moment ($\approx 4.3\ \text{D}$).
Type 3: Antiaromatic vs Non-Aromatic Classifications (Part B/C)
Question 3:
Consider the following species:
- Pentalene
- Cyclopropenyl cation
- Cyclopentadienyl cation
- Azulene
The pair of antiaromatic species among the above is:
- (A) 1 and 2
- (B) 1 and 3
- (C) 2 and 4
- (D) 3 and 4
Correct Answer:(B)
Explanation:
- Pentalene (1): A planar, bicyclic $8\ \pi$-electron system ($4n$$\pi$-electrons, $n=2$); highly reactive and antiaromatic.
- Cyclopropenyl cation (2):$2\ \pi$-electrons ($4n+2$, $n=0$); exceptionally stable and aromatic.
- Cyclopentadienyl cation (3):$4\ \pi$-electrons ($4n$, $n=1$); planar and antiaromatic with open-shell triplet ground state character.
- Azulene (4):$10\ \pi$-electrons ($4n+2$, $n=2$); non-benzenoid aromatic compound with a dipole moment of $1.08\ \text{D}$.
- Therefore, species 1 and 3 are antiaromatic.
Quick-Reference Identification Matrix
| Species / Ring System | π-Electrons | Geometry / Conformation | Classification | Characteristic Diagnostic Feature |
| Cyclopropenyl Cation | 2 | Planar | Aromatic | High $pK_R^+$ stability |
| Cyclobutadiene | 4 | Rectangular distortion | Antiaromatic | Jahn-Teller distortion, dimerizes rapidly |
| Cyclopentadienyl Anion | 6 | Planar regular pentagon | Aromatic | Common ligand in ferrocene |
| Tropylium Ion | 6 | Planar regular heptagon | Aromatic | Forms stable bromide and fluoroborate salts |
| Cyclooctatetraene (COT) | 8 | Tub-shaped ($D_{2d}$) | Non-Aromatic | Conjugated polyene behavior, addition reactions |
| COT Dianion ($\text{C}_8\text{H}_8^{2-}$) | 10 | Planar regular octagon | Aromatic | Formed by reduction with 2 equivalents of K metal |
| Homotropylium Cation | 6 | Non-planar ($sp^3$ bridge) | Homoaromatic | $\text{CH}_2$ bridge excluded from $\pi$-overlap |
| 1,6-Methano[10]annulene | 10 | Bridged perimeter | Aromatic | Peripheral delocalization, diatropic NMR |
Solved PYQ Practice Questions & PDF Download
Review these additional examination problems for comprehensive revision:
- Practice Problem 1: Determine the aromaticity of Pyrrole, Furan, and Thiophene, and arrange them in descending order of resonance stabilization energy. (Hint: Thiophene > Pyrrole > Furan)
- Practice Problem 2: Explain why heptalene is non-aromatic using Craig’s rule of symmetry.
- Practice Problem 3: Contrast the ground state ($S_0$) and excited triplet state ($T_1$) aromaticity of cyclobutadiene and benzene according to Baird’s rule.
[Download Complete Solved Aromaticity PYQ Booklet PDF]
Frequently Asked Questions (FAQs)
What is the primary difference between antiaromatic and non-aromatic compounds?
Antiaromatic compounds are planar, cyclic, and fully conjugated with $4n$$\pi$-electrons, which destabilizes them relative to open-chain analogs. Non-aromatic compounds fail one or more requirements of aromaticity (such as planarity or continuous conjugation), giving them stability comparable to open-chain polyenes.
Why is [10]-annulene non-aromatic despite having 10 pi electrons?
Although it satisfies the $4n+2$ count ($n=2$), the steric repulsion between inward-pointing hydrogen atoms at C-1 and C-6 prevents the ring from achieving planarity. Without planar $p$-orbital overlap, it behaves as a non-aromatic conjugated polyene.
How does Baird’s rule apply to photochemical reactions in CSIR NET?
Baird’s rule states that in the lowest triplet excited state ($T_1$), $(4n)\ \pi$-electron systems become aromatic and stabilized, while $(4n+2)\ \pi$-electron systems become antiaromatic and destabilized. This principle is commonly tested in photochemical ring-opening and pericyclic reaction pathways.