Chemistry Test 238

CONCEPT:

Acidity of Phenols and Substituted Phenols

• The acidity of phenols is influenced by the presence and nature of substituents on the benzene ring.
• Electron-withdrawing groups (like -NO₂) increase the acidity of phenols by stabilizing the negative charge on the oxygen atom after deprotonation.
• Electron-donating groups (like -CH₃) decrease the acidity by destabilizing the phenoxide ion.
• The position of these substituents relative to the hydroxyl group also affects the extent of their influence on acidity.

ORDER OF ACIDITY:

• p-Nitrophenol (Compound 4): Most acidic due to the strong electron-withdrawing effect of the nitro group at the para position.
• m-Nitrophenol (Compound 3): Second most acidic due to the electron-withdrawing effect of the nitro group at the meta position.
• Phenol (Compound 1): Less acidic than the nitrophenols as it lacks an electron-withdrawing group.
• o-Cresol (Compound 2): Least acidic because the methyl group is electron-donating, which decreases the acidity.

CONCLUSION:

• The correct order of acidity is 4 > 3 > 1 > 2.

CONCEPT:

Conversion of Alcohol to Alkyl Chloride and Back

• Alcohols react with PCl₅ to form alkyl chlorides.
• Alkyl chlorides undergo elimination when treated with alcoholic KOH, forming alkenes.
• Alkenes can be hydrated using conc. H₂SO₄ and boiling water to form alcohols, typically yielding the Markovnikov product.

EXPLANATION:

• Step 1: "Z" is an alcohol, specifically isopropyl alcohol (CH₃-CH(OH)-CH₃).
• Step 2: Reacting with PCl₅ converts it to isopropyl chloride (CH₃-CHCl-CH₃).
• Step 3: Treating with alcoholic KOH causes elimination to form propene (CH₂=CH-CH₃).
• Step 4: Hydration of propene yields isopropyl alcohol (CH₃-CH(OH)-CH₃), which matches "Z".

CONCEPT:

Reactivity of Alcohols with Concentrated HCl

• Alcohols react with concentrated HCl through a nucleophilic substitution reaction.
• The rate of this reaction is influenced by the stability of the carbocation intermediate formed during the process.
• Tertiary carbocations are the most stable, followed by secondary and then primary carbocations.

EXPLANATION:

• Option 1: This is a tertiary alcohol (2-methyl-2-propanol). It forms a tertiary carbocation, which is highly stable and reacts most readily with HCl.

• Option 2: This is a secondary alcohol (butan-2-ol). It forms a secondary carbocation, which is less stable than a tertiary carbocation.
• Option 3: This is a primary alcohol (butan-1-ol), which forms a primary carbocation, the least stable.
• Option 4: This is a primary alcohol (isobutanol), which also forms a primary carbocation.

CONCLUSION:

The correct answer is Option 1: (2-methyl-2-propanol).

CONCEPT:

Electrolysis of Aqueous Sodium Chloride Solution Containing Ethyl Alcohol

• Electrolysis involves passing an electric current through a solution to cause a chemical reaction that would not normally occur.
• In the case of an aqueous solution of sodium chloride containing ethyl alcohol, the primary reaction involves the formation of chlorine gas at the anode and hydrogen gas at the cathode.
• Chlorine gas, when produced in the presence of ethyl alcohol, can react to form different chlorinated organic compounds depending on the conditions.

EXPLANATION:

During the electrolysis of the given solution, the following primary reactions occur:

• At the anode:

  2Cl⁻ → Cl2 + 2e⁻

• At the cathode:

  2H2O + 2e⁻ → H2 + 2OH⁻

• Chlorine gas (Cl₂) produced at the anode can react with ethyl alcohol (C₂H₅OH):

  C2H₅OH + Cl₂ → CH₃CHO + 2HCl

• This acetaldehyde can then react further:

  CH₃CHO + 3Cl₂ → CCl₃CHO + 3HCl

• Finally, chloral (CCl₃CHO) can react with water:

  CCl₃CHO + H₂O → CHCl₃ + HCOOH

• This leads to the formation of chloroform (CHCl₃) and formic acid (HCOOH).

CONCLUSION:

• The primary organic product of this series of reactions is chloroform (CHCl₃).

CONCEPT:

Reactions Involving Ethyl Bromide (C₂H₅Br)

• Organic halides such as ethyl bromide can undergo various reactions depending on the reagents used.
• Different reactions include elimination, substitution, and coupling reactions, each producing distinct products.
• Key reactions include:
  • Reaction with alcoholic KOH (dehydrohalogenation or elimination)
  • Wurtz reaction with metallic sodium
  • Williamson ether synthesis with sodium ethoxide
  • Reaction with silver cyanide

EXPLANATION:

• Statement 1: C₂H₅Br reacts with alcoholic KOH to form C₂H₅OH
  • Ethyl bromide reacts with alcoholic KOH to undergo dehydrohalogenation, forming ethene (C₂H₄), not ethanol.
• Statement 2: C₂H₅Br when treated with metallic sodium gives ethane
  • When ethyl bromide is treated with metallic sodium, it actually undergoes the Wurtz reaction to produce butane (C₄H₁₀), not ethane.

    2C₂H₅Br + 2Na → C₄H₁₀ + 2NaBr

• Statement 3: C₂H₅Br when treated with sodium ethoxide forms diethyl ether
  • Correct. Ethyl bromide reacts with sodium ethoxide in the Williamson ether synthesis to form diethyl ether (C₂H₅OC₂H₅).

    C₂H₅Br + C₂H₅ONa → C₂H₅OC₂H₅ + NaBr

• Statement 4: C₂H₅Br with AgCN forms ethyl cyanide
  • Incorrect. Ethyl bromide reacts with silver cyanide to form ethyl isocyanide (C₂H₅NC), not ethyl cyanide (C₂H₅CN).

CONCLUSION:

The correct statement is: C₂H₅Br when treated with sodium ethoxide forms diethyl ether.

CONCEPT:

Reactivity of Hydrogen Halides (HX) in Alcohol Substitution Reactions

• The reactivity of hydrogen halides with alcohols is influenced by two main factors: the strength of the acid (HX) and the bond dissociation energy of the H-X bond.
• Stronger acids are more effective at protonating the alcohol, facilitating the formation of the carbocation intermediate or directly substituting the hydroxyl group.
• The bond dissociation energy of H-X decreases down the group in the periodic table, making the bond easier to break and thus increasing reactivity.

EXPLANATION:

• HI: Hydroiodic acid (HI) is the strongest acid and has the weakest H-I bond, making it the most reactive hydrogen halide in this reaction.
• HBr: Hydrobromic acid (HBr) is slightly less reactive than HI due to a stronger H-Br bond but is still highly reactive.
• HCl: Hydrochloric acid (HCl) has a stronger H-Cl bond and is less reactive than both HI and HBr.
• HF: Hydrofluoric acid (HF) is the least reactive because it has the strongest H-F bond and is a weaker acid compared to the others.

CONCLUSION:

The correct order of reactivity is: HI > HBr > HCl > HF

CONCEPT:

Dehydration of Ethylene Glycol with Anhydrous ZnCl₂

• Ethylene glycol, when heated with anhydrous zinc chloride (ZnCl₂), undergoes a dehydration reaction.
• This reaction removes water molecules from ethylene glycol, leading to the formation of a new product.
• Dehydration of diols (like ethylene glycol) typically results in the formation of carbonyl compounds.

EXPLANATION:

• Ethylene glycol, when dehydrated with ZnCl₂, forms acetaldehyde (CH₃CHO) as the major product.

CONCLUSION:

Ethylene glycol, when heated with anhydrous ZnCl₂, forms Acetaldehyde.

CONCEPT:

Passivity of Iron in Concentrated Nitric Acid

• Passivity is a phenomenon where a metal becomes resistant to corrosion or further reaction due to the formation of a protective layer on its surface.
• In the case of iron, when it is exposed to concentrated nitric acid, it becomes passive because of a thin, protective oxide layer that forms on the surface of the metal.
• This oxide layer acts as a barrier, preventing further chemical reaction between the iron and nitric acid. This state of passivity helps iron resist further oxidation or dissolution in the aggressive acidic environment.

EXPLANATION:

• When iron comes into contact with concentrated nitric acid, a protective oxide layer, mainly composed of iron oxide (Fe₂O₃), forms on its surface. This layer is thin but very effective in shielding the metal underneath from reacting with the surrounding nitric acid.
• This oxide layer creates a passive state, which prevents further chemical reactions from taking place. The passivity arises because the oxide layer is stable, insoluble, and adheres tightly to the surface of the iron, preventing nitric acid from penetrating and attacking the underlying metal.
• The concept of passivity is crucial in preventing corrosion and is often seen in other metals like chromium and aluminum, which also form protective oxide layers under certain conditions.
• Without this passive oxide layer, iron would continuously react with nitric acid, leading to its corrosion. But the formation of this layer essentially makes the metal inert in concentrated nitric acid.

CONCLUSION:

The correct answer is: Option (3) - A thin oxide layer coating on the metal

CONCEPT:

Extraction of Copper: Bessemer Process

• In the extraction of copper, the Bessemer process is used to convert the remaining copper(I) sulfide and copper(I) oxide to copper metal.
• This reaction occurs in the Bessemer converter, where copper(I) oxide reacts with copper(I) sulfide, reducing both compounds to copper metal.
• The process also produces sulfur dioxide gas as a byproduct, which is removed from the system.

CONCEPT:

Inductive Effect

• The inductive effect refers to the transmission of charge through a chain of atoms in a molecule by the electrostatic attraction or repulsion of electrons.
• It is caused by the electronegativity difference between atoms and is a permanent effect within the molecule.
• As the distance from the substituent group increases, the inductive effect diminishes because its influence weakens over longer atomic chains.
• The inductive effect can influence the acidity of carboxylic acids, as electron-withdrawing groups increase acidity by stabilizing the negative charge on the conjugate base.

EXPLANATION:

• It is a permanent effect — Correct, the inductive effect is permanent and does not change with time or chemical reaction.
• It decreases as the distance from the substituent increases — Correct, the inductive effect weakens as the distance from the electron-withdrawing or donating group increases.
• It affects the acidity of carboxylic acids — Correct, the inductive effect influences acidity by affecting the electron density around the carboxyl group.
• It is a temporary effect — Incorrect, the inductive effect is not temporary but rather a permanent feature of a molecule's electronic structure.

CONCLUSION:

• The statement "It is a temporary effect" is incorrect regarding the inductive effect.