Synthesis of pyrazole MCQs With Answer

Synthesis of pyrazole MCQs With Answer provides B. Pharm students a focused, practical guide to pyrazole synthesis, reaction mechanisms, and laboratory conditions. This collection emphasizes common routes—condensation of hydrazines with 1,3-diketones, cyclocondensation of chalcones, and 1,3-dipolar cycloaddition—plus oxidative aromatization, regioselectivity, and reagent choice. Questions cover reaction steps, catalysts, solvents, typical oxidants, characterization methods, and pharmaceutical relevance of pyrazole derivatives like celecoxib. Designed to reinforce mechanistic understanding and synthetic strategy, these MCQs help students prepare for exams and lab work by highlighting common pitfalls and analytical verification techniques. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which common reagent is used to synthesize pyrazoles by condensing with 1,3-diketones?

• Hydrazine hydrate
• Sodium borohydride
• Grignard reagent
• t-Butyllithium

Correct Answer: Hydrazine hydrate

Q2. The condensation of phenylhydrazine with acetylacetone primarily yields which type of product?

• Pyrazole
• Pyridine
• Imidazole
• Thiophene

Correct Answer: Pyrazole

Q3. Which step commonly follows cyclocondensation of hydrazines with chalcones to obtain aromatic pyrazoles?

• Oxidative aromatization
• Hydrogenation
• Grignard addition
• Nucleophilic substitution

Correct Answer: Oxidative aromatization

Q4. Which oxidizing agent is frequently used for converting pyrazolines to pyrazoles?

• DDQ (2,3-dichloro-5,6-dicyano-p-benzoquinone)
• LiAlH4
• NaBH4
• HCl

Correct Answer: DDQ (2,3-dichloro-5,6-dicyano-p-benzoquinone)

Q5. The 1,3-dipolar cycloaddition to synthesize pyrazoles typically involves which dipole?

• Diazo compound
• Azide
• Nitrile oxide
• Nitrene

Correct Answer: Diazo compound

Q6. Regioselectivity in pyrazole formation from unsymmetrical alkynes depends mainly on what?

• Electronic and steric effects of substituents
• Color of solvent
• Molecular weight of solvent
• Atmospheric pressure only

Correct Answer: Electronic and steric effects of substituents

Q7. Which solvent is commonly used for hydrazone formation and cyclization to pyrazoles?

• Ethanol
• Hexane
• Carbon tetrachloride
• Petroleum ether

Correct Answer: Ethanol

Q8. For acid-catalyzed cyclization in pyrazole synthesis, which acid is often employed?

• Glacial acetic acid
• Sodium hydroxide
• Tetramethylammonium hydroxide
• Sodium carbonate

Correct Answer: Glacial acetic acid

Q9. Which pharmaceutical agent contains a pyrazole ring?

• Celecoxib
• Ibuprofen
• Amoxicillin
• Metformin

Correct Answer: Celecoxib

Q10. In the mechanism of hydrazine + 1,3-diketone cyclization, the initial step is formation of what intermediate?

• Hydrazone
• Enolate
• Carbocation
• Radical

Correct Answer: Hydrazone

Q11. Which analytical method is most diagnostic for confirming N–H stretching in pyrazoles?

• Infrared (IR) spectroscopy
• UV-Visible spectroscopy
• Gas chromatography
• Polarimetry

Correct Answer: Infrared (IR) spectroscopy

Q12. The formation of 3,5-disubstituted pyrazoles from chalcones and hydrazine proceeds via which intermediate?

• Pyrazoline
• Oxime
• Imine
• Aziridine

Correct Answer: Pyrazoline

Q13. Which catalyst can promote oxidative aromatization using molecular oxygen for pyrazoline to pyrazole conversion?

• Copper salts (CuCl2) or Cu catalysts
• Potassium chloride
• Sodium sulfate
• Magnesium oxide

Correct Answer: Copper salts (CuCl2) or Cu catalysts

Q14. In pyrazole nomenclature, the nitrogen atoms are numbered such that which position is N1?

• The nitrogen bearing the hydrogen in the 1H-pyrazole tautomer
• The nitrogen farthest from substituents
• The carbon adjacent to nitrogen
• Arbitrary position 2

Correct Answer: The nitrogen bearing the hydrogen in the 1H-pyrazole tautomer

Q15. What is a common laboratory oxidant alternative to DDQ for pyrazoline aromatization?

• Manganese dioxide (MnO2)
• Sodium borohydride (NaBH4)
• Hydrazine
• Tertiary amine

Correct Answer: Manganese dioxide (MnO2)

Q16. Which starting material gives pyrazoles via Knorr-type condensation?

• 1,3-diketone and hydrazine
• Aldehyde and amine
• Epoxide and amine
• Alkene and peracid

Correct Answer: 1,3-diketone and hydrazine

Q17. The 1,3-dipolar cycloaddition to alkynes yields which pyrazole substitution pattern when using terminal alkynes and stabilized diazo compounds?

• 1,3,5-trisubstituted or 1,4-disubstituted depending on dipole
• Only monosubstituted pyrazoles
• Pyrrole derivatives
• No reaction occurs

Correct Answer: 1,3,5-trisubstituted or 1,4-disubstituted depending on dipole

Q18. Which reagent is commonly used to generate diazo compounds for [3+2] cycloaddition in pyrazole synthesis?

• p-Toluenesulfonyl azide or diazo transfer reagents
• Hydrochloric acid only
• Sodium azide alone
• Acetic anhydride

Correct Answer: p-Toluenesulfonyl azide or diazo transfer reagents

Q19. Which property of substituents on a 1,3-diketone affects the acidity of alpha hydrogens and thus cyclization ease?

• Electron-withdrawing groups increase acidity
• Electron-donating groups increase acidity
• Nonpolar groups increase acidity
• Bulky groups always increase acidity

Correct Answer: Electron-withdrawing groups increase acidity

Q20. When synthesizing pyrazole derivatives, which analytical technique best distinguishes regioisomers?

• 1H and 13C NMR spectroscopy
• Thin-layer chromatography only
• Refractive index
• Elemental analysis only

Correct Answer: 1H and 13C NMR spectroscopy

Q21. In hydrazone formation prior to cyclization, which functional group on the carbonyl compound reacts with hydrazine?

• Carbonyl (C=O)
• Alkene double bond
• Nitrile
• Ether linkage

Correct Answer: Carbonyl (C=O)

Q22. Which condition favors intramolecular cyclization to form pyrazoles from hydrazones?

• Acidic conditions and heating
• Strongly basic and cold
• Photolysis only in absence of solvent
• Anhydrous neutral conditions only

Correct Answer: Acidic conditions and heating

Q23. Which substituent pattern is typical for pyrazoles obtained from asymmetric 1,3-diketone + hydrazine reactions?

• 3,5-disubstituted pyrazoles with substitution reflecting diketone positions
• Only 2-substituted pyrazoles
• Polysubstituted pyrroles
• Monosubstituted imidazoles

Correct Answer: 3,5-disubstituted pyrazoles with substitution reflecting diketone positions

Q24. Which reagent pair is useful to prepare chalcones as precursors for pyrazole synthesis?

• Aldehyde + acetophenone under Claisen-Schmidt condensation
• Alkane + peroxide
• Carboxylic acid + amine
• Epoxide + Grignard reagent

Correct Answer: Aldehyde + acetophenone under Claisen-Schmidt condensation

Q25. What is a practical green oxidant frequently used for pyrazoline to pyrazole conversion?

• Air or molecular oxygen with catalyst
• Chromium trioxide
• Mercury(II) oxide
• Sodium cyanide

Correct Answer: Air or molecular oxygen with catalyst

Q26. Which safety consideration is important when handling diazo transfer reagents for pyrazole synthesis?

• Avoid shock and heat; they can be explosive
• They are completely non-reactive and safe
• They should be stored in bright sunlight
• Mix with strong acids to stabilize

Correct Answer: Avoid shock and heat; they can be explosive

Q27. In synthesizing biologically active pyrazoles, which modification is commonly explored to tune activity?

• Changing substituents at C3 and C5 positions
• Varying the solvent only
• Altering the molecular weight without changing substituents
• Removing the heterocycle entirely

Correct Answer: Changing substituents at C3 and C5 positions

Q28. Which reagent is commonly used to form phenylhydrazones from aldehydes for subsequent pyrazole synthesis?

• Phenylhydrazine
• Hydrochloric acid alone
• Sodium nitrite
• Thionyl chloride

Correct Answer: Phenylhydrazine

Q29. Which mechanism element is involved in 1,3-dipolar cycloaddition leading to pyrazoles?

• Concerted [3+2] cycloaddition
• Free radical chain propagation exclusively
• SN1 nucleophilic substitution
• Pericyclic [4+2] Diels-Alder only

Correct Answer: Concerted [3+2] cycloaddition

Q30. Which chromatographic technique is best for checking purity of synthesized pyrazoles in lab?

• TLC followed by melting point and NMR confirmation
• Paper chromatography only
• Reversed-phase HPLC without standards only
• Colorimetric spot test only

Correct Answer: TLC followed by melting point and NMR confirmation

Q31. Which electronic effect on an aryl substituent directs regioselectivity during cycloaddition to give a particular pyrazole isomer?

• Electron-withdrawing groups stabilize positive charge development and influence regiochemistry
• Electron-donating groups never affect regioselectivity
• Only steric effects matter; electronic never does
• Isomer distribution is completely random

Correct Answer: Electron-withdrawing groups stabilize positive charge development and influence regiochemistry

Q32. For synthesis of N-substituted pyrazoles, which starting material modification is effective?

• Use N-substituted hydrazines (e.g., methylhydrazine)
• Use protected alcohols only
• Use tertiary amines instead of hydrazine
• Remove carbonyl groups from diketone

Correct Answer: Use N-substituted hydrazines (e.g., methylhydrazine)

Q33. Which temperature condition typically accelerates cyclization in pyrazole synthesis?

• Gentle heating (reflux in ethanol)
• Deep freeze (-78°C)
• Room temperature always prevents reaction
• Excessive heating to decomposition

Correct Answer: Gentle heating (reflux in ethanol)

Q34. What is the role of an acid catalyst in hydrazone cyclization to pyrazole?

• Activate carbonyl for nucleophilic attack and promote dehydration
• Reduce the substrate to alcohol
• Oxidize pyrazoline directly
• Scavenge radicals

Correct Answer: Activate carbonyl for nucleophilic attack and promote dehydration

Q35. Which reagent is used to prepare 1,3-diketones (acetoacylation) as pyrazole precursors?

• Claisen condensation or acetoacylation methods
• Hydrogenation over Pd/C
• Buchwald-Hartwig amination
• Wittig reaction only

Correct Answer: Claisen condensation or acetoacylation methods

Q36. Which spectroscopic change indicates aromatization of pyrazoline to pyrazole?

• Appearance of downfield aromatic proton signals in 1H NMR
• Disappearance of all carbon signals in 13C NMR
• Loss of molecular ion in MS
• Colorless solution turning dark without spectral change

Correct Answer: Appearance of downfield aromatic proton signals in 1H NMR

Q37. What is the common outcome when chalcones react with hydrazine hydrate under acidic reflux?

• Formation of pyrazoles after oxidation of intermediate pyrazolines
• Formation of pyrroles directly
• Complete polymerization
• No reaction occurs

Correct Answer: Formation of pyrazoles after oxidation of intermediate pyrazolines

Q38. Which base-catalyzed side reaction can reduce yield during hydrazine condensation?

• Aldol condensation of diketone components
• Electrophilic aromatic substitution
• Radical polymerization of solvent
• Isomerization to pyridine

Correct Answer: Aldol condensation of diketone components

Q39. In medicinal chemistry, extending conjugation on a pyrazole often affects which property?

• Lipophilicity and electronic distribution, altering activity and ADME
• Only the compound’s odor
• Production cost only
• It prevents characterization by NMR

Correct Answer: Lipophilicity and electronic distribution, altering activity and ADME

Q40. What safety precaution is important when using MnO2 as oxidant in pyrazole synthesis?

• Avoid inhalation of dust; use in a fume hood and proper PPE
• MnO2 is nontoxic and requires no precautions
• Always mix with strong acids to deactivate it
• Store below 0°C only

Correct Answer: Avoid inhalation of dust; use in a fume hood and proper PPE

Q41. Which structural feature distinguishes pyrazole from imidazole?

• Pyrazole has two adjacent nitrogen atoms (positions 1 and 2)
• Pyrazole contains sulfur in the ring
• Imidazole has no nitrogens
• Pyrazole is a five-membered ring with oxygen

Correct Answer: Pyrazole has two adjacent nitrogen atoms (positions 1 and 2)

Q42. Which reagent can be used to selectively N-alkylate pyrazoles after synthesis?

• Alkyl halide with a base (e.g., K2CO3)
• Strong oxidant like CrO3
• Hydrochloric acid to protonate only
• Diazomethane without base

Correct Answer: Alkyl halide with a base (e.g., K2CO3)

Q43. What is the effect of electron-donating groups on aromatic aldehyde used in chalcone formation for pyrazole precursors?

• They accelerate aldol condensation and influence subsequent reactivity
• They always prevent reaction
• They cause immediate polymerization only
• They have no effect whatsoever

Correct Answer: They accelerate aldol condensation and influence subsequent reactivity

Q44. Which method allows regioselective synthesis of 1,3,5-trisubstituted pyrazoles?

• Controlled 1,3-dipolar cycloaddition using substituted diazo compounds and alkynes
• Random heating of hydrazine and any ketone
• Mixing pyrazole with sodium metal
• Exposing pyrazoline to sunlight only

Correct Answer: Controlled 1,3-dipolar cycloaddition using substituted diazo compounds and alkynes

Q45. During work-up of pyrazole syntheses, which extraction solvent is commonly used to isolate organic product?

• Ethyl acetate
• Water only
• Concentrated hydrochloric acid
• Liquid ammonia

Correct Answer: Ethyl acetate

Q46. What is the role of molecular sieves in certain pyrazole syntheses?

• Remove water to drive condensation and improve yields
• Act as a catalyst for cycloaddition
• Donate protons to hydrazine
• Provide nitrogen for ring formation

Correct Answer: Remove water to drive condensation and improve yields

Q47. Which isotopic labeling (e.g., 15N) study helps determine which nitrogen in pyrazole derived from hydrazine?

• 15N NMR to trace incorporation into specific ring nitrogens
• Deuterium labeling of solvent only
• Carbon-14 labeling of nonreactive groups
• Fluorine-19 NMR of unrelated fluorinated substrate

Correct Answer: 15N NMR to trace incorporation into specific ring nitrogens

Q48. Which computational approach helps predict regioselectivity in pyrazole-forming cycloadditions?

• DFT calculations to analyze frontier molecular orbitals
• Simple molecular weight estimation
• Random number generation
• Only qualitative intuition without calculation

Correct Answer: DFT calculations to analyze frontier molecular orbitals

Q49. When preparing pyrazoles for biological testing, which purification step is crucial to avoid assay interference?

• Removal of residual oxidants and heavy metals via thorough washing and chromatography
• Leaving oxidant residues to enhance activity
• Not characterizing purity at all
• Only measuring color

Correct Answer: Removal of residual oxidants and heavy metals via thorough washing and chromatography

Q50. Which concept is important when designing a pyrazole-based drug candidate in medicinal chemistry?

• Balance between lipophilicity, electronic properties, and metabolic stability
• Only maximizing molecular weight
• Making molecule as hydrophobic as possible regardless of solubility
• Avoiding any heteroatoms other than carbon

Correct Answer: Balance between lipophilicity, electronic properties, and metabolic stability

Download