The products A and B in the following reactions, respectively are A A g - N O _ 2 C H _ 3 - C H _ 2 - C H _ 2 - B r A g C N B$$\mathrm {A} \xleftarrow {\mathrm {A g} - \mathrm {N O} _ {2}} \mathrm {C H} _ {3} - \mathrm {C H} _ {2} - \mathrm {C H} _ {2} - \mathrm {B r} \xrightarrow {\mathrm {A g C N}} \mathrm {B}$$

Related Formula

Markovnikov addition of HCl$HCl$ across an alkene:

Nucleophilic substitution with AgCN$AgCN$ favors coordinate carbon bonding over nitrogen, yielding covalent isocyanides (R-NC$R-NC$).

Core Logic

Step 1: The starting material contains a double bond. Treating it with HCl$HCl$ leads to addition. According to Markovnikov's rule, the chloride ion attaches to the secondary position, creating chloride intermediate [A]$[A]$.

Step 2: Compound [A]$[A]$ reacts with AgCN$AgCN$. Since AgCN$AgCN$ is covalent, the lone pair on nitrogen acts as the attacking nucleophile, leading to substitution with an isocyanide group (-NC$-NC$) rather than a cyanide group (-CN$-CN$).

Step 1: Structural Synthesis

The intermediate [A]$[A]$ possesses a chlorine atom at the secondary carbon position. Substituting this chlorine with -NC$-NC$ provides the product corresponding to option (4).

Pattern Recognition

Distinguish between ionic vs covalent cyanide sources:

• KCN / NaCN$KCN / NaCN \implies$ forms alkyl nitriles (R-CN$R-CN$)
• AgCN$AgCN \implies$ forms alkyl isocyanides (R-NC$R-NC$)

Chapter Mix

Class 12 Chemistry: Haloalkanes and Haloarenes

Related Formula

Nucleophilic Aromatic Substitution (SNAr$S_NAr$) occurs via a Meisenheimer complex intermediate, where strong electron-withdrawing groups (-NO₂$-\mathrm{NO}_2$) activate positions strictly ortho and para to themselves.

Core Logic

The reactant is 1,2-dibromo-4-nitrobenzene. Let us evaluate the two bromine positions relative to the nitro group:

• The bromine at C-1 is para to the strong activating -NO₂$-\mathrm{NO}_2$ group.
• The bromine at C-2 is meta to the -NO₂$-\mathrm{NO}_2$ group.

Since the para position facilitates effective negative charge delocalization onto the oxygen atoms of the nitro group during intermediate formation, the para-bromine undergoes substitution exclusively by the ethoxide ion (^-OC₂H₅$^-\mathrm{OC}_2\mathrm{H}_5$). This yields the final product shown below:

Pattern Recognition

In aromatic pathways activated by -NO₂$-\mathrm{NO}_2$, substitution happens exclusively at positions ortho or para relative to the nitro flag; meta positions remain unactivated.

Chapter Mix

Class 12 Chemistry: Haloalkanes and Haloarenes

Core Logic

KCN$KCN$ is predominantly ionic and provides cyanide ions (CN^-$CN^-$) in solution. Although both carbon and nitrogen are in a position to donate electron pairs, the attack takes place mainly through carbon because C-C bond is more stable than C-N bond, forming alkyl cyanides (nitriles) as major product.

However, AgCN$AgCN$ is mainly covalent in nature and nitrogen is free to donate an electron pair forming isocyanide as the main product.

Step 1: Final Conclusion

Assertion (A) is correct. Reason (R) states both are highly ionic, which is incorrect because AgCN$AgCN$ is largely covalent. Therefore, (A) is correct but (R) is not correct.

Pattern Recognition

Ambidentate nucleophile shortcut: Alkali metal cyanides (KCN$KCN$, NaCN$NaCN$) are ionic arrow$\rightarrow$ attack from Carbon arrow$\rightarrow$ Cyanide. Heavy metal cyanides (AgCN$AgCN$) are covalent arrow$\rightarrow$ attack from Nitrogen arrow$\rightarrow$ Isocyanide.

Chapter Mix

Class 12 Chemistry: Haloalkanes and Haloarenes

Core Logic

The structure of 2-chlorobutane is CH₃-CH(Cl)-CH₂-CH₃$CH_3-CH(Cl)-CH_2-CH_3$. There is exactly one chiral center in this molecule, which is the carbon atom at position 2 (bonded to H$H$, Cl$Cl$, CH₃$CH_3$, and CH₂CH₃$CH_2CH_3$).

For a molecule with n$n$ distinct chiral centers and no plane of symmetry, the number of optical isomers (stereoisomers) is 2ⁿ$2^n$.

Step 1: Calculate Isomers

Number of chiral centers n = 1$n = 1$. Total optical isomers = 2¹ = 2$2^1 = 2$ (one pair of enantiomers: the (R) and (S) configurations).

Pattern Recognition

1 chiral center always gives 2 optical isomers (a pair of enantiomers).

Chapter Mix

Class 12 Chemistry: Haloalkanes and Haloarenes Class 11 Chemistry: Organic Chemistry Some Basic Principles and Techniques

Related Formula

Core Logic

Cyanide (CN^-$CN^-$) is an ambident nucleophile that can attack through either carbon or nitrogen:

• KCN$\text{KCN}$ and NaCN$\text{NaCN}$ are predominantly ionic compounds, leaving the carbon atom free and highly nucleophilic to yield alkyl cyanides (R-CN$\text{R-CN}$) as the major product.
• Silver cyanide (AgCN$\text{AgCN}$) is predominantly covalent. Because the silver-carbon bond is strong, the lone pair on nitrogen acts as the primary nucleophilic site, leading to the formation of alkyl isocyanides (R-NC$\text{R-NC}$).

Step 1: Selection

Therefore, reacting an alkyl halide with AgCN$\text{AgCN}$ selectively produces an alkyl isocyanide.

Pattern Recognition

Ionic cyanides like KCN$\text{KCN}$ yield nitriles, whereas covalent cyanides like AgCN$\text{AgCN}$ yield isocyanides due to the availability of the nitrogen lone pair.

Chapter Mix

Class 12 Chemistry: Organic Compounds Containing Halogens