- What is the pKa value of benzoic acid?
A) 3.19
B) 4.19
C) 5.19
D) 2.19
Answer: B
Explanation: Benzoic acid has a pKa of 4.19, indicating it is a weak acid. - Why are aromatic carboxylic acids generally more acidic than aliphatic carboxylic acids?
A) Due to +I effect of the alkyl chain
B) Resonance stabilization of the carboxylate anion by the benzene ring
C) Hydrogen bonding in aliphatic chains
D) Lower molecular weight
Answer: B
Explanation: The benzene ring delocalizes the negative charge in the conjugate base. - Which of the following is a weak acid?
A) HCl
B) NaOH
C) Benzoic acid
D) CH3COONa
Answer: C
Explanation: Benzoic acid is a weak acid with partial ionization. - The acidity of benzoic acid is due to:
A) Donation of a proton from the carboxyl group
B) Resonance in the benzene ring only
C) Inductive effect of the phenyl group
D) All of the above
Answer: D
Explanation: Combination of proton donation, resonance, and inductive effects. - Compared to acetic acid (pKa 4.76), benzoic acid (pKa 4.19) is:
A) Less acidic
B) More acidic
C) Equally acidic
D) Not comparable
Answer: B
Explanation: Lower pKa indicates higher acidity due to resonance. - In aqueous solution, the degree of ionization of benzoic acid is:
A) 100%
B) Less than 1%
C) 50%
D) 10%
Answer: B
Explanation: As a weak acid, it ionizes minimally. - The conjugate base of benzoic acid is stabilized by:
A) Hyperconjugation
B) Delocalization over the aromatic ring
C) Steric hindrance
D) Solvation only
Answer: B
Explanation: Resonance delocalizes the negative charge. - Which factor does NOT contribute to the acidity of aromatic acids?
A) Resonance
B) Inductive effect
C) Electron donation from the ring
D) Proton donation from -COOH
Answer: C
Explanation: Electron donation would decrease acidity. - Benzoic acid reacts with NaOH to form:
A) Sodium benzoate
B) Benzene
C) Benzoquinone
D) Benzaldehyde
Answer: A
Explanation: Salt formation indicates acidity. - The pH of a 0.1 M benzoic acid solution is approximately:
A) 1
B) 3
C) 7
D) 11
Answer: B
Explanation: Weak acid, pH = ½pKa – ½log C ≈ 3. - Aromatic acids are weaker than mineral acids because:
A) They are strong bases
B) Partial ionization
C) Complete dissociation
D) High solubility
Answer: B
Explanation: Weak acids ionize partially. - The Hammett acidity function relates to:
A) Strong acids
B) Substituted benzoic acids
C) Aliphatic acids only
D) Gases
Answer: B
Explanation: Used for measuring acidity in substituted aromatic systems. - Which is more acidic: benzoic acid or phenol?
A) Phenol (pKa 10)
B) Benzoic acid (pKa 4.19)
C) Equal
D) Depends on solvent
Answer: B
Explanation: Carboxylic acids are more acidic than phenols. - The acidity order for carboxylic acids is influenced by:
A) Chain length only
B) Resonance in aromatic systems
C) Both A and B
D) Neither
Answer: B
Explanation: Resonance key in aromatics. - Benzoic acid’s Ka is:
A) 10-5
B) 6.46 × 10-5
C) 10-3
D) 10-7
Answer: B
Explanation: Ka = 10-pKa ≈ 6.46 × 10-5. - In gas phase, aromatic acids show:
A) Decreased acidity
B) Increased acidity due to no solvation
C) No change
D) Become bases
Answer: B
Explanation: Intrinsic acidity higher without solvent stabilization. - The ortho-effect in benzoic acid increases acidity due to:
A) Steric hindrance destabilizing the acid
B) Resonance
C) Inductive only
D) Hyperconjugation
Answer: A
Explanation: Steric strain favors ionization. - Which statement is false about aromatic acid acidity?
A) pKa < 5
B) Stronger than alcohols
C) Weaker than HCl
D) More acidic than aliphatic due to +R effect
Answer: D
Explanation: -R effect from ring stabilizes anion. - Benzoic acid solubility in water is low due to:
A) Acidity
B) Hydrophobicity of ring
C) Both
D) Neither
Answer: C
Explanation: Affects ionization and solubility. - The pKa of benzoic acid in DMSO is:
A) Higher than in water
B) Lower than in water
C) Same
D) Unmeasurable
Answer: A
Explanation: Less solvated anion in aprotic solvent. - Aromatic acids form salts with:
A) Strong bases only
B) Weak bases
C) Acids
D) Neither
Answer: A
Explanation: Neutralization reaction. - The acidity of benzoic acid compared to formic acid:
A) Less acidic
B) More acidic
C) Equal
D) Depends on temperature
Answer: A
Explanation: Alkyl/aryl substitution decreases acidity slightly. - Resonance in benzoate ion involves:
A) 2 structures
B) 5 structures
C) 1 structure
D) None
Answer: B
Explanation: Delocalization over ring and oxygens. - Which is the strongest aromatic acid among isomers?
A) o-Nitrobenzoic acid
B) m-Nitrobenzoic acid
C) p-Nitrobenzoic acid
D) Benzoic acid
Answer: A
Explanation: Ortho-effect enhances. - Benzoic acid is used in:
A) Buffer solutions
B) Due to its acidity
C) Both
D) Neither
Answer: C
Explanation: Weak acid for buffers. - The pH at equivalence in benzoic acid-NaOH titration is:
A) 7
B) >7
C) <7
D) 0
Answer: B
Explanation: Hydrolysis of benzoate. - Acidity decreases in order:
A) Benzoic > acetic > formic
B) Formic > acetic > benzoic
C) Acetic > benzoic > formic
D) Benzoic > formic > acetic
Answer: B
Explanation: Inductive effects and resonance. - The conjugate acid of benzoate is:
A) Benzene
B) Benzoic acid
C) Phenol
D) Benzoquinone
Answer: B
Explanation: Reverse of deprotonation. - In non-polar solvents, benzoic acid acidity:
A) Increases
B) Decreases
C) Unchanged
D) Becomes basic
Answer: B
Explanation: Less stabilization of ions. - Which test confirms acidity of benzoic acid?
A) Litmus test
B) Reaction with NaHCO3
C) Both
D) Neither
Answer: C
Explanation: Effervescence with bicarbonate.
- An electron-withdrawing group (EWG) on benzoic acid:
A) Decreases acidity
B) Increases acidity
C) No effect
D) Makes it basic
Answer: B
Explanation: Stabilizes conjugate base. - Which substituent increases acidity most at para position?
A) -CH3
B) -NO2
C) -OH
D) -NH2
Answer: B
Explanation: Strong -I and -R effects. - The Hammett constant σ for -NO2 (para) is:
A) 0.78 (positive, EWG)
B) -0.78
C) 0
D) 1.0
Answer: A
Explanation: Positive σ indicates acidity increase. - Ortho-substituted benzoic acids are stronger due to:
A) Ortho-effect (steric + electronic)
B) Meta-directing only
C) Para-resonance
D) Inductive only
Answer: A
Explanation: Steric hindrance aids ionization. - p-Anisic acid (p-methoxybenzoic acid) pKa is:
A) 4.49
B) 3.89
C) 4.79
D) 5.19
Answer: A
Explanation: -OCH3 is electron-donating, increases pKa. - Rank: p-nitro > m-nitro > o-nitro benzoic acid acidity?
A) True
B) False (o > p > m)
C) Equal
D) p > o > m
Answer: B
Explanation: Ortho-effect dominates. - Electron-donating groups like -CH3 cause:
A) Bathochromic shift in UV
B) Hypsochromic in acidity (weaker acid)
C) No change
D) Hyperchromic
Answer: B
Explanation: Destabilizes anion. - The inductive effect (-I) of -Cl increases acidity by:
A) Withdrawing electrons
B) Donating electrons
C) Resonance only
D) Hyperconjugation
Answer: A
Explanation: Pulls electrons from -COOH. - For -F substituent, acidity order:
A) o > m > p
B) p > m > o
C) m > o > p
D) Equal
Answer: B
Explanation: Resonance stronger at para. - Which group has strongest +R effect, decreasing acidity?
A) -NO2
B) -NH2
C) -CN
D) -CF3
Answer: B
Explanation: Donates electrons via resonance. - pKa of p-chlorobenzoic acid vs benzoic acid:
A) Higher (less acidic)
B) Lower (more acidic)
C) Same
D) Unpredictable
Answer: B
Explanation: -Cl is EWG. - Meta substituents affect acidity via:
A) Inductive effect only
B) Resonance only
C) Both
D) Neither
Answer: A
Explanation: No resonance conjugation at meta. - The substituent -CF3 at ortho:
A) Increases acidity most
B) Decreases
C) No effect
D) Makes basic
Answer: A
Explanation: Strong -I, ortho proximity. - Hammett plot for benzoic acids is:
A) Linear for meta/para
B) Curved for ortho
C) Both
D) None
Answer: A
Explanation: Ortho deviates due to steric. - Which is least acidic: p-methylbenzoic acid?
A) Yes
B) No, o-methyl is least
C) p-aminobenzoic
D) m-nitro
Answer: C
Explanation: Strong +R from -NH2. - Resonance effect of -NO2 stabilizes:
A) Carboxyl group
B) Carboxylate anion
C) Benzene ring only
D) Proton
Answer: B
Explanation: Delocalizes negative charge. - For halogens, acidity: F > Cl > Br > I due to:
A) Electronegativity
B) Size
C) Both
D) Neither
Answer: A
Explanation: Higher electronegativity, stronger -I. - pKa increases with:
A) EWG
B) EDG
C) Neutral group
D) No substituent
Answer: B
Explanation: EDG destabilizes anion. - Ortho-hydroxybenzoic acid (salicylic acid) acidity:
A) Decreases
B) Increases
C) No effect
D) Variable
Answer: B
Explanation: Intramolecular H-bond stabilizes anion. - Sigma meta for -CH3 is:
A) -0.07 (EDG)
B) +0.07
C) 0
D) 1
Answer: A
Explanation: Weak EDG, increases pKa. - Which substituent has σ_p = 0.78?
A) -NO2
B) -OH
C) -Cl
D) -CH3
Answer: A
Explanation: Strong EWG. - The effect is more pronounced at para than meta for:
A) Inductive
B) Resonance
C) Both
D) Steric
Answer: B
Explanation: Resonance requires conjugation. - p-Trifluoromethylbenzoic acid pKa:
A) 3.6 (more acidic)
B) 4.8
C) 4.19
D) 5.0
Answer: A
Explanation: Strong -I from -CF3. - Electron-releasing substituent decreases acidity in order:
A) Ortho > meta > para
B) Para > meta > ortho
C) Meta > para > ortho
D) Equal
Answer: A
Explanation: Closer proximity stronger effect. - For -NH2 at para, pKa:
A) 4.19
B) 4.90
C) 3.50
D) 5.50
Answer: B
Explanation: Strong +R, weaker acid. - The gas-phase acidity trend for EWG:
A) Ortho < meta < para
B) Para < meta < ortho
C) Equal
D) Opposite
Answer: A
Explanation: No steric in gas phase. - Which group has dual effect (I and R)?
A) -NO2 (-I, -R)
B) -CH3 (+I only)
C) -H
D) -OH (+R, -I)
Answer: D
Explanation: Halogens similar. - Acidity of o-fluorobenzoic acid vs p-fluoro:
A) o > p
B) p > o
C) Equal
D) o < p
Answer: B
Explanation: Resonance dominates at para for F. - Sigma for -OH (para):
A) -0.37
B) +0.37
C) 0
D) 1
Answer: A
Explanation: EDG via +R. - Substituent stabilizing anion most:
A) -SO2CF3 (para)
B) -CH3
C) -NH2
D) -H
Answer: A
Explanation: Strongest EWG. - In Hammett equation, ρ for benzoic acids is:
A) 1
B) 2.5
C) 0
D) -1
Answer: A
Explanation: Standard reference. - Ortho-methylbenzoic acid acidity:
A) Less than benzoic due to steric
B) More due to ortho-effect
C) Equal
D) Variable
Answer: B
Explanation: Ortho-effect increases. - For -CN group:
A) Increases acidity via -R
B) Decreases
C) No effect
D) Only inductive
Answer: A
Explanation: Both -I and -R. - pKa of m-chlorobenzoic:
A) 3.98
B) 4.19
C) 4.50
D) 3.50
Answer: A
Explanation: Inductive effect lowers pKa. - EDG effect on sigma:
A) Negative
B) Positive
C) Zero
D) Infinite
Answer: A
Explanation: Increases pKa. - Strongest acid among: o/p/m-toluic acid?
A) o-Toluic
B) p-Toluic
C) m-Toluic
D) All equal
Answer: A
Explanation: Ortho steric. - Resonance contribution highest for:
A) Para-NO2
B) Meta-NO2
C) Ortho-NO2
D) No substituent
Answer: A
Explanation: Full conjugation. - Halogen at ortho increases acidity more than para due to:
A) Inductive proximity
B) Resonance
C) Steric
D) All
Answer: A
Explanation: Inductive dominates halogens. - pKa trend for -NHCOCH3:
A) Slightly higher than benzoic
B) Lower
C) Same
D) Much higher
Answer: A
Explanation: Weak EDG. - Fukui function shows for EWG:
A) Increased acidity para > meta > ortho
B) Opposite
C) Equal
D) No trend
Answer: A
Explanation: Electrophilic stabilization.
- Benzoic acid reacts with NaOH to give:
A) Sodium benzoate
B) Benzoyl chloride
C) Benzamide
D) Benzyl alcohol
Answer: A
Explanation: Acid-base salt formation. - Esterification of benzoic acid with ethanol gives:
A) Ethyl benzoate
B) Methyl benzoate
C) Benzyl acetate
D) No reaction
Answer: A
Explanation: Fischer esterification. - Decarboxylation of benzoic acid yields:
A) Benzene
B) Toluene
C) Phenol
D) Benzal chloride
Answer: A
Explanation: Soda lime heating. - Reaction with SOCl2 gives:
A) Benzoyl chloride
B) Benzoic anhydride
C) Sodium benzoate
D) Benzamide
Answer: A
Explanation: Acid chloride formation. - Hell-Volhard-Zelinsky reaction on benzoic acid:
A) Not applicable (aromatic)
B) Gives alpha-bromo
C) Decarboxylates
D) Esterifies
Answer: A
Explanation: HVZ for aliphatic with alpha-H. - Nitration of benzoic acid occurs at:
A) Meta position
B) Ortho/para
C) All positions
D) No reaction
Answer: A
Explanation: -COOH meta-directing. - Reduction of benzoic acid with LiAlH4 gives:
A) Benzyl alcohol
B) Benzaldehyde
C) Benzene
D) Benzoic anhydride
Answer: A
Explanation: Alcohol from acid. - Reaction with PCl5:
A) Benzoyl chloride
B) Benzamide
C) Ester
D) Salt
Answer: A
Explanation: Similar to SOCl2. - Bromination of benzoic acid:
A) Meta-bromobenzoic acid
B) Ortho/para
C) No reaction
D) Side chain
Answer: A
Explanation: Meta-directing group. - With NH3, benzoic acid forms:
A) Benzamide (via acid chloride)
B) Direct no reaction
C) Aniline
D) Benzene
Answer: A
Explanation: Amidation requires activation. - Schotten-Baumann reaction involves benzoic acid derivative:
A) Benzoyl chloride with amine
B) Acid itself
C) Ester
D) Salt
Answer: A
Explanation: Acylation of phenols/amines. - Heating with CaO gives:
A) Benzene
B) Benzoquinone
C) Phthalic acid
D) Salicylic acid
Answer: A
Explanation: Decarboxylation variant. - Reaction with CH3OH/H+:
A) Methyl benzoate
B) Methanol no change
C) Benzyl methyl ether
D) No reaction
Answer: A
Explanation: Esterification. - Friedel-Crafts on benzoic acid:
A) Deactivated, no reaction
B) Occurs at meta
C) Ortho/para
D) Side chain
Answer: A
Explanation: -COOH deactivates ring. - Conversion to benzamide:
A) Via NH3 + heat
B) Direct
C) Via SOCl2 then NH3
D) Both A and C
Answer: C
Explanation: Standard amidation. - Oxidation of toluene gives:
A) Benzoic acid
B) Benzaldehyde
C) Benzyl chloride
D) Phenol
Answer: A
Explanation: KMnO4 oxidation. - With NaHCO3, benzoic acid:
A) Effervescence (CO2)
B) No reaction
C) Salt only
D) Ester
Answer: A
Explanation: Acidity test. - Cannizzaro reaction: Benzoic acid from:
A) Benzaldehyde (no alpha-H)
B) Acetophenone
C) Toluene
D) No
Answer: A
Explanation: Disproportionation. - Hydrogenation of benzoyl chloride:
A) Benzaldehyde (Rosenmund)
B) Benzoic acid
C) Benzyl alcohol
D) Benzene
Answer: A
Explanation: Partial reduction. - With diazomethane:
A) Methyl benzoate
B) Benzyl diazomethane
C) No reaction
D) Salt
Answer: A
Explanation: Methylation of acid. - Kolbe-Schmitt reaction product:
A) Salicylic acid from phenol, but for benzoic? Not direct
B) Benzoic from CO2
C) No
D) Phthalic
Answer: C
Explanation: For phenols. - Benzoic acid to benzophenone:
A) Via Friedel-Crafts with PhCOCl
B) Direct
C) Reduction
D) Oxidation
Answer: A
Explanation: Acylation, but ring deactivated—use derivative. - Saponification of ethyl benzoate gives:
A) Benzoic acid + ethanol
B) Benzene
C) Ester
D) Amide
Answer: A
Explanation: Hydrolysis. - With KMnO4, benzoic acid:
A) Stable
B) Oxidizes to CO2
C) No reaction
D) Decarboxylates
Answer: C
Explanation: Aromatic acids resistant. - Formation of anhydride:
A) With (COCl)2 or P2O5
B) Direct
C) With base
D) Reduction
Answer: A
Explanation: Dehydration. - Reaction with Grignard:
A) No, salts form; use ester for tertiary alcohol
B) Benzene
C) Direct addition
D) Ketone
Answer: A
Explanation: Acids react with RMgX to salt. - Commercial production of benzoic acid:
A) Oxidation of toluene
B) From benzaldehyde
C) Hydrolysis of benzamide
D) All
Answer: A
Explanation: Industrial process. - With LiAlH4 then H3O+:
A) Benzyl alcohol
B) Benzoic acid
C) Benzaldehyde
D) Benzene
Answer: A
Explanation: Full reduction. - Electrophilic substitution on ring:
A) Meta-directed by -COOH
B) Ortho/para
C) No
D) Ipso
Answer: A
Explanation: Deactivating, meta. - Important use of benzoic acid reaction:
A) Preservative via sodium benzoate
B) Dye
C) Fuel
D) Polymer
Answer: A
Explanation: Antimicrobial salt.
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