Chemistry Test 237

CONCEPT:

Electrochemical Theory of Aqueous Corrosion

• In aqueous corrosion, there are two main reactions: the anodic reaction (oxidation) where the metal loses electrons, and the cathodic reaction (reduction) where electrons are consumed.
• The cathodic reaction typically involves the reduction of oxygen in acidic or neutral solutions, which happens by gaining electrons from the oxidation reaction occurring at the anode.
• Corrosion is an electrochemical process where both anodic and cathodic reactions must occur simultaneously for corrosion to proceed.

EXPLANATION:

• However, this reaction is more common in basic environments and less common in oxygen-driven corrosion.

CONCLUSION:

The correct answer is: Option A 

CONCEPT:

Nitriding Process in Steel Treatment

• Nitriding is a surface-hardening process used to improve the hardness and wear resistance of steel by introducing nitrogen into its surface.
• The process involves heating the steel in an atmosphere of ammonia gas (NH₃) at temperatures between 500°C to 550°C. The ammonia decomposes into nitrogen, which diffuses into the steel’s surface and reacts with alloying elements to form nitrides.
• This process results in the formation of a hard, wear-resistant nitride layer on the surface of the steel without the need for quenching, unlike other heat treatments such as carburizing.

EXPLANATION:

• In the nitriding process, the steel is exposed to ammonia gas in a furnace. At high temperatures, the ammonia decomposes into nitrogen and hydrogen:

• The nitrogen atoms diffuse into the surface of the steel and react with elements like chromium, molybdenum, and aluminum to form nitrides. This results in a hard surface that improves the wear resistance and fatigue strength of the steel component.
• The main advantage of nitriding is that it does not require quenching, which means there is minimal distortion and less risk of cracking, making it an ideal treatment for precision components.
• Nitriding enhances the surface properties of the steel without altering its core properties, making it especially useful in industries where components face high wear and tear, such as automotive gears, crankshafts, and aerospace components.

FINAL ANSWER:

• The correct answer is: Option 1: Heating steel in an atmosphere of ammonia

CONCEPT:

Passivation of Iron in Concentrated Sulfuric Acid

• Passivation is the process by which a material, typically a metal, becomes inert or less reactive due to the formation of a protective oxide layer on its surface.
• Iron is usually more reactive than copper and can displace it from copper sulfate solution in a typical redox reaction.
• However, when iron is exposed to concentrated sulfuric acid (H₂SO₄), it forms a passive, protective layer, stopping further reactions.

EXPLANATION:

• In concentrated H₂SO₄, the sulfuric acid acts as a strong oxidizing agent and reacts with iron, leading to the formation of a protective layer on its surface, usually composed of iron sulfate or iron oxide.
• This layer is inert, meaning it does not allow further reactions between the iron and the surrounding environment. Therefore, iron does not react with copper sulfate solution as expected, even though iron is more reactive than copper.
• The layer effectively blocks the contact between the iron metal and the copper ions in the copper sulfate solution, preventing the typical displacement reaction (Fe + CuSO₄ → FeSO₄ + Cu).
• This phenomenon is called "passivation," where the iron loses its reactivity due to the formation of this layer, which makes it behave as though it were less reactive.

CONCLUSION:

The correct answer is: Option 3 - An inert layer of iron oxide is deposited on it

CONCEPT:

Blue Colouration in Copper-Based Reactions

• Copper(II) ions (Cu²⁺) typically form blue solutions when they dissolve in water because of the formation of hydrated copper complexes, such as [Cu(H₂O)₆]²⁺.
• When certain reagents like ammonium hydroxide (NH₄OH) are added to copper sulfate, they create a deep blue tetraammine copper(II) complex, [Cu(NH₃)₄]²⁺, leading to a blue color.
• Other reactions involving copper ions may not produce blue coloration but instead form different precipitates or compounds with varying colors, depending on the chemical species involved.

CONCLUSION:

The correct answer is: Option 2 - Copper sulfate solution reacts with K₄Fe(CN)₆

CONCEPT:

Gatterman–Koch Reaction

• The Gatterman–Koch reaction is used to introduce a formyl group (-CHO) into an aromatic ring, such as benzene, using carbon monoxide (CO) and hydrochloric acid (HCl) in the presence of a Lewis acid catalyst like AlCl₃ or CuCl.
• This reaction results in the formation of benzaldehyde (C₆H₅CHO) from benzene (C₆H₆).
• It is a special type of formylation reaction and serves as an alternative to Friedel-Crafts acylation, specifically for introducing the -CHO group.

REACTION:

The overall reaction for the Gatterman–Koch reaction is:

• Step 1: CO reacts with HCl in the presence of AlCl₃ or CuCl to form a highly reactive intermediate, formyl chloride (HCOCl).
• Step 2: The formyl chloride then reacts with benzene in the presence of AlCl₃ to form a benzene complex.
• Step 3: This complex loses a proton (H⁺), yielding benzaldehyde (C₆H₅CHO) as the final product.

CONCLUSION:

• The Gatterman–Koch reaction provides a direct method for synthesizing benzaldehyde from benzene using CO and HCl, with AlCl₃ and CuCl as catalysts.
• The correct answer to the original question is option 3: Gatterman–Koch reaction.

CONCEPT:

Reaction of Nitriles (RCN) with Grignard Reagents (R’MgX)

• Nitriles (RCN) react with Grignard reagents (R'MgX) to form an intermediate imine complex.
• This intermediate, upon hydrolysis, is converted into a ketone (R-CO-R').
• The reaction proceeds through the addition of the Grignard reagent to the carbon of the nitrile group, followed by hydrolytic cleavage to yield the ketone.

CONCEPT:

Reactivity of Carbonyl Compounds Towards Nucleophilic Addition with Reaction Examples

• The reactivity of carbonyl compounds towards nucleophilic addition reactions is determined by the electrophilicity of the carbonyl carbon. Electrophilicity is affected by the groups attached to the carbonyl carbon.
• Aldehydes are generally more reactive than ketones due to the presence of only one electron-donating alkyl group (or hydrogen) compared to two in ketones. This makes the carbonyl carbon in aldehydes more positively charged and more susceptible to nucleophilic attack.
• In ketones, especially those attached to aromatic rings, the electron-donating effect of phenyl groups reduces the electrophilicity of the carbonyl carbon, lowering its reactivity towards nucleophiles.

EXPLANATION:

• C₆H₅CHO (Benzaldehyde):
• The carbonyl group is attached to an aromatic ring (C6H5) and a hydrogen atom. Hydrogen does not donate electrons to the carbonyl carbon, making this carbonyl more electrophilic and more reactive towards nucleophilic addition. Aldehydes, in general, are more reactive than ketones for this reason.

ORDER OF REACTIVITY:

• The correct answer is: Option 2) 1 > 2 > 3

CONCEPT:

Identification of Carbonyl Compounds

• The molecular formula C₃H₆O suggests the compound could be an aldehyde or a ketone.
• The compound forms a 2,4-dinitrophenylhydrazone, indicating the presence of a carbonyl group (C=O) as 2,4-DNP reacts specifically with aldehydes and ketones to form a hydrazone derivative.
• A negative Tollens' test indicates that the compound is not an aldehyde, as aldehydes typically give a positive Tollens' test by forming a silver mirror. Therefore, the compound must be a ketone.

EXPLANATION:

• The molecular formula C₃H₆O could represent a few possibilities, including propanal (an aldehyde) and acetone (a ketone).
• Given the negative Tollens' test, the compound cannot be an aldehyde, ruling out options involving aldehyde structures.
• The only ketone with the formula C₃H₆O is acetone (CH₃COCH₃), which forms a 2,4-dinitrophenylhydrazone but gives a negative Tollens' test.

CONCLUSION:

The correct compound is: Option 3: CH₃COCH₃ (Acetone)

CONCEPT:

Matching Organic Compounds and Their Descriptions

• Formalin is a solution of formaldehyde in water, typically containing 40% formaldehyde by volume.
• Trioxane is a trimer of formaldehyde, formed by the polymerization of three formaldehyde molecules.
• Ketene is a reactive compound with the general formula R₂C=C=O, featuring a cumulated double bond.
• Metaldehyde is a cyclic tetramer of acetaldehyde, used as a solid fuel or in slug pellets.

MATCHING:

• (a) Formalin: 40% aqueous solution of HCHO.
  Match: (4)
• (b) Trioxane: Trimer of HCHO.
  Match: (1)
• (c) Ketene: Compounds of the general formula R₂C=C=O.
  Match: (2)
• (d) Metaldehyde: A tetramer of acetaldehyde.
  Match: (3)

CONCLUSION:

The correct answer is: Option 2: (a) - (4), (b) - (1), (c) - (2), (d) - (3)

CONCEPT:

Acidity of Phenols and Substituted Phenols

• The acidity of phenols is influenced by the presence and position 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.
• The position of the substituent relative to the hydroxyl group also plays a crucial role:
  • Para (p-) position: The -NO₂ group strongly withdraws electron density through resonance, making p-nitrophenol the most acidic.
  • Ortho (o-) position: The -NO₂ group also has a strong electron-withdrawing effect, but intramolecular hydrogen bonding slightly reduces its acidity compared to the para position.
  • Meta (m-) position: The -NO₂ group exerts only an inductive effect, making it less effective in increasing acidity compared to the ortho and para positions.

EXPLANATION:

• p-Nitrophenol (4): Most acidic due to strong electron-withdrawing resonance effect.
• o-Nitrophenol (2): Second most acidic due to both inductive and resonance effects, but reduced slightly by intramolecular hydrogen bonding.
• m-Nitrophenol (3): Less acidic than the ortho and para isomers, as the nitro group only provides an inductive effect.
• Phenol (1): Least acidic, with no electron-withdrawing groups.

CONCLUSION:

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