Q.Match the items of Column I and Column II.
Column I:

The key idea is that each reaction or compound class in Column I has a defining characteristic in Column II.

Reasoning:

1. $\mathrm{S_N1}$ reactions proceed via a planar carbocation intermediate, leading to a racemic mixture of products. Hence, (i) matches with (c).
2. Fire extinguishers often contain halons, which are chlorobromocarbons. Hence, (ii) matches with (e).
3. Bromination of an alkene adds bromine across the double bond to give a vicinal dibromide (bromines on adjacent carbons). Hence, (iii) matches with (a). …

This is a matching question that tests your grasp of key terms and reactions in organic chemistry. Each item in Column I describes a reaction, a class of compounds, or a chemical application. Column II contains the outcomes, rules, or structural features that are directly linked to them. The trick is to recall the precise definition or the characteristic result of each process.

Let’s go through each match one by one.

1. (i) $\mathrm{S_N1}$ reaction → (c) Racemisation

   An $\mathrm{S_N1}$ reaction proceeds through a planar carbocation intermediate. The nucleophile can attack from either face of this plane with equal probability. This leads to a mixture of two enantiomers in equal amounts — a racemic mixture. The process is called racemisation. No other item in Column II describes this stereochemical outcome.

2. (ii) Chemicals in fire extinguisher → (e) Chlorobromocarbons

   Fire extinguishers, especially older or specialised ones, often contain halocarbons like carbon tetrachloride ($\mathrm{CCl_4}$) or halons (bromochlorodifluoromethane, etc.). These compounds work by interrupting the combustion chain reaction. The general term for such compounds is chlorobromocarbons (or halons). Note that “vic-dibromides” and “gem-dihalides” are specific structural types, not general fire-extinguishing agents.

3. (iii) Bromination of alkenes → (a) vic-dibromides

   When bromine ($\mathrm{Br_2}$) adds across a carbon-carbon double bond, one bromine atom attaches to each of the two formerly double-bonded carbons. The product has two bromine atoms on adjacent (vicinal) carbons. Hence, the product is a vicinal dibromide, or simply a vic-dibromide.

4. (iv) Alkylidene halides → (b) gem-dihalides

   An alkylidene halide has the general structure $\mathrm{R_2C=X_2}$, where both halogen atoms are attached to the same carbon. The prefix “gem-” (from Latin geminus, meaning twin) indicates that two identical substituents are on the same carbon. So alkylidene halides are geminal dihalides, or gem-dihalides. …

Concept: Reaction Mechanisms & Nomenclature in Haloalkanes/Haloarenes

Method: Conceptual Matching by Reaction Type & Product

This method works by identifying the defining feature of each item in Column I and linking it to the unique term in Column II.

Steps

Step 1: Analyse each entry in Column I for its key characteristic

• (i) $\mathrm{S_N1}$ reaction → proceeds via a carbocation intermediate → nucleophile can attack from either face → racemic mixture is formed.
• (ii) Chemicals in fire extinguisher → common agents are halons (e.g., bromochlorodifluoromethane) → these are chlorobromocarbons.
• (iii) Bromination of alkenes → addition of $\mathrm{Br_2}$ across double bond → bromine atoms attach to adjacent carbons → product is a vic-dibromide.
• (iv) Alkylidene halides → both halogen atoms on the same carbon of a double-bonded carbon → these are gem-dihalides.
• (v) Elimination of HX from alkyl halide → follows Saytzeff rule (more substituted alkene is major product).

Step 2: Match each with Column II

| Column I | Key Feature | Column II Match | …

Here are the common mistakes students make when matching these organic chemistry concepts, along with the correct reasoning to avoid them.

Mistake 1: Confusing vic-dibromides with gem-dihalides

• The Mistake: Students often mix up the terms "vicinal" and "geminal." They might match Bromination of alkenes (iii) with gem-dihalides (b), or Alkylidene halides (iv) with vic-dibromides (a).
• Why it happens: Both terms describe dihalides (two halogen atoms), and the prefixes sound similar. Students memorize the words without visualizing the carbon skeleton.
• How to avoid it:
  • Visualize the carbon atoms:
    • vic (vicinal): Halogens are on adjacent (neighbor) carbon atoms. Think "V" for "Vicinal" and "V" for "Voisin" (French for neighbor). Bromination of an alkene adds Br₂ across the double bond, placing Br atoms on two neighboring carbons → vic-dibromides.
    • gem (geminal): Both halogens are on the same carbon atom. Think "Gemini" (twins) — they are together on one carbon. Alkylidene halides (like $\mathrm{R_2CCl_2}$) have two halogens on the same terminal carbon → gem-dihalides.
  • Correct Match:
    • (iii) Bromination of alkenes → (a) vic-dibromides
    • (iv) Alkylidene halides → (b) gem-dihalides

Mistake 2: Associating $\mathrm{S_N1}$ with Inversion Instead of Racemisation

• The Mistake: Students match $\mathrm{S_N1}$ reaction (i) with a single stereochemical outcome like "inversion" (which is characteristic of $\mathrm{S_N2}$).
• Why it happens: Many students memorize "$\mathrm{S_N2}$ = inversion" and "$\mathrm{S_N1}$ = racemisation" but forget the reason. Under exam pressure, they might default to "inversion" for any substitution.
• How to avoid it:
  • Understand the mechanism:
    • $\mathrm{S_N1}$ involves a planar carbocation intermediate. The nucleophile can attack from either the top or the bottom with equal probability. This produces a 50:50 mixture of both enantiomers → racemisation (loss of optical activity).
    • $\mathrm{S_N2}$ involves a backside attack on a tetrahedral carbon, which inverts the configuration (like an umbrella turning inside out).
  • Correct Match:
    • (i) $\mathrm{S_N1}$ reaction → (c) Racemisation

Mistake 3: Confusing the Application of Saytzeff’s Rule

• The Mistake: Students match Elimination of HX from alkyl halide (v) with a rule about substitution (like Markovnikov's rule) or forget that Saytzeff's rule specifically governs alkene stability in elimination.
• Why it happens: Students often confuse Saytzeff's rule (for elimination) with Markovnikov's rule (for addition). Both deal with "more substituted" products, but in different contexts.
• How to avoid it:
  • Remember the reaction type:
    • Saytzeff rule: In elimination reactions (dehydrohalogenation), the major product is the more substituted alkene (the one with more alkyl groups on the double bond). This is because more substituted alkenes are more stable (hyperconjugation).
    • Markovnikov rule: In addition reactions (like HX adding to an alkene), the hydrogen goes to the carbon with more hydrogens.
  • Correct Match:
    • (v) Elimination of HX from alkyl halide → (d) Saytzeff rule …