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CONCEPT:

Oxidation-Reduction Reactions and Changes in Color

• In an acidic medium, $\text{K}_2\text{Cr}_2\text{O}_7$ (potassium dichromate) undergoes a reduction reaction. The dichromate ion $\text{Cr}_2\text{O}_7^{2-}$ is reduced to $\text{Cr}^{3+}$, causing a color change from orange to green.
• This color change occurs when the dichromate ion reacts with a reducing agent, which donates electrons to reduce $\text{Cr}_2\text{O}_7^{2-}$.
• Common reducing agents include species that can themselves undergo oxidation, such as $\text{Fe}^{2+}, \text{Sn}^{2+}, \text{I}^-, \text{S}^{2-}$, etc.

EXPLANATION:

• To determine which species will cause the color change of $\text{K}_2\text{Cr}_2\text{O}_7$ in acidic medium, we need to identify species that act as reducing agents and undergo oxidation.
• For the species in the given options:
• $\text{Fe}^{2+}$: Oxidized to $\text{Fe}^{3+}$, donating electrons and reducing $\text{Cr}_2\text{O}_7^{2-}$. Thus, it causes the color change.
• $\text{Sn}^{2+}$: Oxidized to $\text{Sn}^{4+}$, donating electrons and reducing $\text{Cr}_2\text{O}_7^{2-}$. Thus, it causes the color change.
• $\text{I}^-$: Oxidized to $\text{I}_2$, donating electrons and reducing $\text{Cr}_2\text{O}_7^{2-}$. Thus, it causes the color change.
• $\text{S}^{2-}$: Oxidized to sulfur ($\text{S}$), donating electrons and reducing $\text{Cr}_2\text{O}_7^{2-}$. Thus, it causes the color change.
• Other species, such as $\text{Fe}^{3+}$, $\text{Sn}^{4+}$, or $\text{SO}_4^{2-}$, cannot act as reducing agents in this context, as they are already in their oxidized states.

Conclusion: The species $\text{Fe}^{2+}, \text{Sn}^{2+}, \text{I}^-, \text{S}^{2-}$ will change the orange color of $\text{K}_2\text{Cr}_2\text{O}_7$ to green in acidic medium, as they are all capable of acting as reducing agents.

CONCEPT:

Crystal Field Splitting Energy (CFSE) and Ligand Strength

• The crystal field splitting energy ($\Delta_0$) is directly proportional to the strength of the ligand in the spectrochemical series.
• Stronger ligands cause greater splitting of the d-orbitals, resulting in higher $\Delta_0$ values.
• The spectrochemical series for the given ligands is as follows:

CN^- > en (ethylene diamine) > H₂O > F^-

Thus, CN^- is the strongest ligand, and F^- is the weakest ligand.

• Additionally, chelation (e.g., by ethylene diamine) increases the ligand strength compared to monodentate ligands of similar type.

EXPLANATION:

• The given complexes are:
• A. $[\text{Cr(CN)}_6]^{3-}$
• B. $[\text{CrF}_6]^{3-}$
• C. $[\text{Cr(H}_2\text{O)}_6]^{3+}$
• D. $[\text{Cr(en)}_3]^{3+}$
• The crystal field splitting energy ($\Delta_0$) values are:
• I. 15,060 cm^-1
• II. 17,400 cm^-1
• III. 22,300 cm^-1
• IV. 26,600 cm^-1
• Based on the ligand strength:
• $[\text{Cr(CN)}_6]^{3-}$ (CN^-, strongest ligand) should have the highest splitting energy, i.e., $\Delta_0$ = 26,600 , \text{cm}^{-1} \) (IV). - $[\text{Cr(en)}_3]^{3+}$ (chelating ligand, moderate strength) should have the second-highest splitting energy, i.e., $\Delta_0$ = 22,300 , \text{cm}^{-1} \) (III). - $[\text{Cr(H}_2\text{O)}_6]^{3+}$ (H₂O, weaker ligand) should have the second-lowest splitting energy, i.e., $\Delta_0$ = 17,400 , \text{cm}^{-1} \) (II). - $[\text{CrF}_6]^{3-}$ (F^-, weakest ligand) should have the lowest splitting energy, i.e., $\Delta_0$ $= 15,060 , \text{cm}^{-1} \) (I).

MATCHING:

FINAL ANSWER:

A-IV, B-I, C-II, D-III

CONCEPT:

Distillation and Separation of Mixtures

• Distillation is a separation technique based on the difference in boiling points of substances.
• Simple distillation is effective when the boiling points of the components differ significantly, while fractional distillation is used for closer boiling points.
• An azeotropic mixture is a mixture of two or more liquids that boils at a constant temperature and retains its composition in the vapor phase, making it impossible to separate by conventional fractional distillation.

EXPLANATION:

• Statement (I): 1,2,3-Trihydroxypropane (commonly known as glycerol) cannot be separated from water by simple distillation because glycerol has a very high boiling point (approximately 290°C) compared to water (100°C). Moreover, glycerol decomposes at high temperatures, so it is typically separated by distillation under reduced pressure to avoid decomposition.
• Statement (II): An azeotropic mixture cannot be separated by fractional distillation because the composition of the liquid and vapor phases remains constant during boiling, preventing further separation. This statement is true.
• Based on the above analysis:
• Statement (I) is false because glycerol cannot be separated from water by simple distillation.
• Statement (II) is true because azeotropic mixtures cannot be separated by fractional distillation.

So, Statement I is false but Statement II is true.

CONCEPT:

Rate of S_N1 Reaction

• The rate of an S_N1 reaction is directly proportional to the stability of the intermediate carbocation.
• Carbocation stability is influenced by factors like resonance, hyperconjugation, and inductive effects.
• Order of carbocation stability:

Ph-⁺C_H2 > CH₃-⁺C_H2

• Resonance stabilization is more effective than hyperconjugation in stabilizing carbocations.

EXPLANATION:

• Statement (I): Benzyl chloride reacts faster in an S_N1 mechanism than ethyl chloride.
• This is true because the benzyl carbocation (Ph-⁺C_H2) formed during the reaction is stabilized by resonance with the aromatic ring.
• In contrast, the ethyl carbocation (CH₃-⁺C_H2) is stabilized only by hyperconjugation, which is less effective compared to resonance.
• Statement (II): Ethyl carbocation intermediate is less stabilized by hyperconjugation than benzyl carbocation by resonance.
• This is also true because the benzyl carbocation has multiple resonance structures, allowing delocalization of the positive charge over the aromatic ring, which provides significant stabilization.
• On the other hand, the ethyl carbocation relies only on hyperconjugation, which provides limited stabilization compared to resonance.
• Since both statements are true and align with the principles of carbocation stability, the correct answer is that both Statement (I) and Statement (II) are true.

Therefore, the stability of the benzyl carbocation due to resonance explains the faster reaction rate of benzyl chloride in the S_N1 mechanism compared to ethyl chloride.

CONCEPT:

IUPAC Nomenclature Priority of Functional Groups

• In IUPAC nomenclature, the priority of functional groups is based on the specific rules established by IUPAC. The priority order is used to determine the principal functional group, which dictates the suffix of the compound name.
• The priority order is determined based on the oxidation state of the carbon atom to which the functional group is attached. Functional groups with higher oxidation states have higher priority.
• In general, the priority order from highest to lowest is:
• Carboxylic acids (-COOH)
• Esters (-COOR)
• Amides (-CONH₂)
• Aldehydes (-CHO)
• Ketones (>C=O)
• Nitriles (-CN)
• Alcohols (-OH)
• Amines (-NH₂)
• Alkenes (C=C)
• Alkynes (C≡C)

EXPLANATION:

• Memonics: "Crazy Elephants Always Ask Kind Neighbors About Amazing Airplanes Always."
• Functional groups provided in the question:
• -CONH₂ (Amide)
• -CHO (Aldehyde)

• C=O (Ketone)

• -NH₂ (Amine)
• -C≡C- (Alkyne)
• Based on the priority order of functional groups:
• Amides (-CONH₂) have the highest priority among the groups provided.
• Aldehydes (-CHO) have the next highest priority.
• Ketones (>C=O) follow aldehydes in priority.
• Amines (-NH₂) come after ketones.
• Alkynes (-C≡C-) have the lowest priority among the given groups.
• Thus, the correct decreasing order of priority is:

-CONH₂, -CHO, >C=O, -NH₂, -C≡C-

Therefore, the correct answer is Option 3.

CONCEPT:

Electrophilic Attack on Aromatic Compounds

• Electrophilic substitution reactions occur when an electrophile replaces a substituent (usually a hydrogen atom) in an aromatic ring.
• The position at which the electrophile attacks depends on the electron density of the aromatic ring and the nature of the substituents already present.
• Electron-donating groups (+M or +I effect) increase electron density at certain positions, making them more favorable for electrophilic attack.
• Electron-withdrawing groups (-M or -I effect) decrease electron density at specific positions, making them less favorable for electrophilic attack.

EXPLANATION:

• In the given molecule, the nitrogen atom is attached to the aromatic ring and exhibits a +M (resonance donating) effect.
• The +M effect of nitrogen increases the electron density at the ortho and para positions of the aromatic ring.
• Among the labeled positions ("r", "u", "p", and "s"), position "u" is predominantly favored for electrophilic attack because it is para to the nitrogen atom, and the +M effect enhances the electron density at this position.
• Position "r" also experiences an increase in electron density due to the +M effect, but it is less favorable than "u" because steric hindrance near the nitrogen may play a minor role in reducing reactivity at the ortho position.

Therefore, the preferred site for the attack of the electrophile is predominantly at "u" because of the +M nature of nitrogen.

CONCEPT:

The question involves distinguishing between different chemical compounds in mixtures using specific reagents. The concept relies on the unique chemical reactions of these compounds with the reagents provided.

• Phenol vs Ethanol: Phenol reacts with neutral $FeCl_3$ to give a violet color, whereas ethanol does not react with neutral $FeCl_3$.
• Diethyl amine vs Ethyl amine: Ethyl amine (a primary amine) reacts with $CHCl_3/KOH$ to give a carbylamine (foul-smelling test), whereas diethyl amine (a secondary amine) does not give this test.
• Acetaldehyde vs Acetone: Acetaldehyde (an aldehyde) gives a positive test with ammonical silver nitrate (Tollens' test), forming a silver mirror, whereas acetone (a ketone) does not.
• Benzoic acid vs Cinnamic acid: Cinnamic acid reacts with bromine water due to the presence of a double bond in its structure, whereas benzoic acid does not react with bromine water.

EXPLANATION:

1. A. Diethyl amine + Ethyl amine: Ethyl amine (primary amine) reacts with $CHCl_3/KOH$, producing a foul smell (carbylamine test). Diethyl amine (secondary amine) does not react. Therefore, the reagent used is II. $CHCl_3 + KOH, \Delta$.
2. B. Acetaldehyde + Acetone: Acetaldehyde gives a positive test with ammonical silver nitrate (Tollens' test), forming a silver mirror, whereas acetone does not. Therefore, the reagent used is IV. Ammonical silver nitrate.
3. C. Ethanol + Phenol: Phenol reacts with neutral $FeCl_3$ to produce a violet color, whereas ethanol does not. Therefore, the reagent used is III. Neutral $FeCl_3$.
4. D. Benzoic acid + Cinnamic acid: Cinnamic acid reacts with bromine water due to the presence of a double bond, whereas benzoic acid does not. Therefore, the reagent used is I. Bromine water.

So the correct matching is A-II, B-IV, C-III, D-I

CONCEPT:

Electrophilic Substitution Reaction (ESR) Reactivity

• The reactivity of an aromatic compound towards electrophilic substitution reactions (ESR) depends on the electron density of the benzene ring.
• Electron-donating groups increase the nucleophilicity of the benzene ring, enhancing its reactivity with electrophiles.
• Electron-withdrawing groups decrease the nucleophilicity of the benzene ring, reducing its reactivity with electrophiles.

EXPLANATION:

• Rate of ESR $\propto$ nucleophilicity of benzene ring.
• The nucleophilicity of the benzene rings follows the order is P > R > Q
• Thus, the reactivity of these compounds towards the diazonium salt in a slightly acidic medium is P > R > Q

Therefore, the correct order regarding the reactivity of these compounds with $Ph-N \equiv N \ Cl^{(-)}$ is P > R > Q.

CONCEPT:

Vitamins and Their Chemical Names

• Vitamins are organic compounds essential for various metabolic processes in the human body.
• Each vitamin has a specific chemical name that represents its structure and function.
• Matching vitamins with their chemical names helps in understanding their roles in human nutrition.

EXPLANATION:

• The matching of vitamins with their chemical names is as follows:
• Vitamin B₁ (A): Its chemical name is Thiamine. Thiamine plays a crucial role in energy metabolism and nerve function.
• Vitamin B₂ (B): Its chemical name is Riboflavin. Riboflavin is involved in energy production and cellular function.
• Vitamin B₆ (C): Its chemical name is Pyridoxine. Pyridoxine is important for amino acid metabolism and neurotransmitter synthesis.
• Vitamin C (D): Its chemical name is Ascorbic acid. Ascorbic acid is an antioxidant and is essential for collagen synthesis and immune function.

Therefore, the correct matching is: A - III, B - IV, C - I, D - II.

CONCEPT:

Flame Test and Identification of Ions

• The flame test is used to identify certain metal ions based on the characteristic color they produce when heated in a flame.

• Flame Colors:

• Ba+2Ba+2

  produces an apple green color.

• Ca+2Ca+2

  produces a brick red color.

• Sr+2Sr+2

  produces a crimson red color.

• Reactions for Identification:

• Ba+2+CrO2−4→BaCrO4Ba+2+CrO42−→BaCrO4

: Yellow precipitate indicates the presence of Ba+2Ba+2

.

• The sodium carbonate extract test with ammonium molybdate and concentrated nitric acid produces a canary yellow precipitate if PO3−4PO43−

  is present.

EXPLANATION:

• From the flame test:
• The apple green color in the flame test confirms the presence of Ba+2Ba+2

.

• From the reaction with K2CrO4K2CrO4

:

• The formation of a yellow precipitate confirms the presence of Ba+2Ba+2

  as BaCrO4BaCrO4

.

• From the sodium carbonate extract test:

• Heating the sodium carbonate extract with concentrated HNO3HNO3

  and ammonium molybdate produces a canary yellow precipitate, confirming the presence of PO3−4PO43−

.

• Based on the observations:
• The cation present in the salt is Ba+2Ba+2

.

• The anion present in the salt is PO3−4PO43−

.

Therefore, the cation and anion present in the salt are Ba2+Ba2+

and PO3−4PO43−

respectively.