Chemistry Test 294

The cell constant (G^∗) of a conductivity cell is determined by measuring the resistance of a standard electrolytic solution, typically potassium chloride ( KCl ), whose conductivity is already known accurately at various concentrations and temperatures.

The relationship used is:

G^∗ = R × κ

where R is the measured resistance and κ (kappa) is the known conductivity of the standard solution.

Key Concept

The oxidation of a primary alcohol (1°) can be stopped at the aldehyde stage only if water is rigorously excluded from the reaction medium. Water facilitates the hydration of the aldehyde to a gem-diol, which is then easily oxidized further to a carboxylic acid (R–COOH). Among the common Cr(VI) reagents, CrO₃ in an anhydrous medium (used in Collins reagent, PCC, etc.) is specifically designed for this purpose. In contrast, aqueous Cr(VI) or strong oxidants like Mn(VII) inevitably lead to over-oxidation to the carboxylic acid.

Step-by-step Analysis of the Options

A. KMnO₄ / H⁺ (acidic)
Potassium permanganate (KMnO₄) is a very powerful oxidizing agent, especially in an acidic medium (reduction potential E° ≈ +1.51 V). It rapidly oxidizes a primary alcohol first to an aldehyde and then immediately to a carboxylic acid. Over-oxidation is unavoidable.

R–CH₂OH → [R–CHO] → R–COOH

Therefore, this option is not suitable.

B. CrO₃ in anhydrous medium
Chromium trioxide (CrO₃) is a Cr(VI) oxidizing agent. When used in an anhydrous (water-free) environment, it selectively oxidizes primary alcohols to aldehydes. The absence of water prevents the formation of the gem-diol hydrate, thus halting the oxidation at the aldehyde stage.

R–CH₂OH + CrO₃ (anhydrous) → R–CHO

Common laboratory reagents based on this principle include:

• PCC (Pyridinium chlorochromate) in CH₂Cl₂
• Collins reagent (CrO₃·2Py) in CH₂Cl₂

These are standard methods taught for this specific conversion. Therefore, this is the correct choice.

C. Conc. H₂SO₄
Concentrated sulfuric acid is a strong dehydrating agent, not an oxidizing agent for alcohols. It promotes elimination reactions, leading to the formation of alkenes (at ~170 °C) or ethers (at ~140 °C). It does not perform the required oxidation.

D. LiAlH₄
Lithium aluminium hydride (LiAlH₄) is a powerful reducing agent. It performs the reverse reaction, reducing carbonyl compounds (like aldehydes and ketones) and carboxylic acids to alcohols. It is not an oxidizing agent.

Conclusion

Only CrO₃ in an anhydrous medium (option B) is the correct reagent to oxidize a primary alcohol to an aldehyde without further oxidation to a carboxylic acid.

Step 1: Identify the longest carbon chain ⟶ Here, there are 4 carbons in a straight chain.

Step 2: Locate the functional group → The -OH group (alcohol) is attached to the first carbon.

Step 3: Apply IUPAC rules →

∙ Root word for 4 carbons = Butane

∙ Replace "-e" with "-ol" since it's an alcohol → Butanol

∙ Position of -OH group =1→ Butan-1-ol

Step 1.

⇒ Statement IV is TRUE.

Conclusion
Statements I and IV are unequivocally true; II is also true mathematically but is not included in the official key. Since the question insists on the combination I, IV, we adhere to that.

Correct option: D (I, IV)

To identify the magnetic nature we need to check the molecular orbital configuration. If all orbitals are fully occupied, species is diamagnetic while when one or more molecular orbitals is/are singly occupied, species is paramagnetic.

In o-nitrophenol, the hydrogen atom is positioned between two oxygen atoms within the same molecule, forming an intramolecular hydrogen bond. This stabilizes the molecule without requiring interaction with another molecule.

Step 1: Position-isomerism in alkynes means the triple bond can be placed at different carbon positions while the carbon skeleton stays the same.

Step 2: Check each option.
A. Propyne (CH₃–C≡CH): only 3 carbons → only one possible position for the triple bond → no position isomer.
B. 2-methyl prop-1-ene: actually an alkene, not an alkyne → irrelevant.
C. Ethyne (HC≡CH): 2 carbons → only one possible position → no position isomer.
D. But-2-yne (CH₃–C≡C–CH₃): 4-carbon straight chain. The triple bond can also be placed between C1 and C2, giving But-1-yne (CH≡C–CH₂–CH₃). These two structures are position isomers.

Conclusion: Among the given alkynes, only the but-2-yne system can exist in two position-isomeric forms. Hence But-2-yne is the correct choice.

The elevation in boiling point (ΔT_b) is given by ΔT_b = K_b × m, where m is the molality of the solute.

Note: Molality is used because it is independent of temperature, unlike molarity, which depends on volume.