Q.Predict the order of reactivity of the following compounds in $S_N1$ and $S_N2$ reactions:

Concept and Intuition

Nitrite, $NO_2^-$, is an ambident nucleophile — it can bond to an electrophile through either its nitrogen or one of its oxygens, giving two different constitutional products from the same formal reagent. The identity of the counter-cation changes which end reacts: with the more ionic alkali-metal nitrites (Na/KNO$_2$), the reaction tends toward O-attack, giving alkyl nitrites (esters, R–O–N=O); with the more covalent silver salt $AgNO_2$, the reaction instead proceeds through nitrogen, giving the nitroalkane R–NO$_2$ as the major product — mirroring the well-known $KCN$ (C-attack, nitrile) vs $AgCN$ (N-attack, isocyanide) contrast for cyanide.

Step-by-Step Solution

1. The question states the major product's formula as $C_4H_9NO_2$, i.e. a nitroalkane (R–NO$_2$), not an alkyl nitrite ester — this identifies the reagent needed as $AgNO_2$, which favours N-attack.
2. The arrow is explicitly labelled $S_N2$: a clean, single-step backside-attack substitution with inversion at the reacting carbon and, crucially, no carbocation intermediate — so no skeletal rearrangement is possible.
3. Taking $C_4H_9Br$ in its default (straight-chain) reading as $n$-butyl bromide, $S_N2$ attack by $AgNO_2$ gives 1-nitrobutane, $CH_3CH_2CH_2CH_2NO_2$, with the carbon skeleton unchanged. …

Concept and Intuition

Isonitriles ($\mathrm{R-NC}$) and nitriles ($\mathrm{R-CN}$) are structural isomers formed from the ambident cyanide ion $\mathrm{CN^-}$, which can attack an electrophile through either its carbon end or its nitrogen end. With KCN (essentially ionic), the more nucleophilic and less electronegative carbon end attacks preferentially, giving the nitrile as major product. With AgCN, the compound is largely covalent — silver is bonded to the carbon of $\mathrm{CN^-}$ (soft–soft Ag–C interaction), which ties up the carbon and leaves the lone pair on nitrogen free to act as the nucleophile. So AgCN + R–X gives the isocyanide as the major product. Separately, the carbylamine reaction — a 1° amine treated with chloroform and alcoholic KOH — is a completely different, classical route to isocyanides: KOH generates the electrophilic carbene $:\mathrm{CCl_2}$ from $\mathrm{CHCl_3}$, which is attacked by the amine nitrogen's lone pair, and after loss of HCl twice, gives $\mathrm{R-NC}$ directly. This reaction is in fact used as a qualitative test for primary amines (the isocyanide has a distinctive foul smell).

Step-by-Step Solution

1. Statement I: $\mathrm{CH_3CH_2Cl}$ + $\mathrm{AgCN}$. Because AgCN is covalent (Ag–C≡N), the nitrogen lone pair performs the $S_N2$ attack on the alkyl halide's carbon, displacing $\mathrm{Cl^-}$, giving $\mathrm{CH_3CH_2-N \equiv C}$ (ethyl isocyanide) as the major product. Correctly matched. …

Concept and Intuition

Cyanide salts react with alkyl halides at either the carbon end (giving a nitrile, R-CN) or the nitrogen end (giving an isocyanide/carbylamine, R-NC), and which end attacks depends on the counter-cation. $KCN$ is largely ionic, so the more nucleophilic carbon end of $CN^-$ attacks, giving the nitrile. $AgCN$ is more covalent (Ag–C bond character ties up the carbon), forcing the alkyl halide to be attacked through nitrogen, giving the isocyanide. Isocyanides, $R-N\equiv C$, are then reduced by $H_2$/catalyst by sequential addition across both the $\pi$ bonds of the N$\equiv$C triple bond, ending in a secondary amine $R-NH-CH_3$ (the terminal carbon becomes a methyl group bonded to N).

Step-by-Step Solution

1. $C_2H_4 + HBr \rightarrow CH_3CH_2Br$ (simple electrophilic addition of HBr to ethylene; both carbons are equivalent so there's no regiochemistry issue).
2. $CH_3CH_2Br + AgCN \rightarrow CH_3CH_2-NC$ (ethyl isocyanide) — this is X. AgCN's covalent Ag–C bond makes the nitrogen end of cyanide the nucleophile, giving the isocyanide rather than the nitrile that KCN would give. …

Concept and Intuition

Cyanide is an ambident nucleophile: with alkyl halides in $KCN$ it attacks through carbon to give a nitrile ($R-CN$), while the carbylamine (isocyanide) test attacks through nitrogen to give an isocyanide ($R-NC$) — these are structural isomers, not the same compound. Nitriles can also be made by dehydrating a primary amide (losing water from $-CONH_2$) or by dehydrating an aldoxime (itself made from an aldehyde + hydroxylamine), both of which retain the original carbon count of the starting compound.

Step-by-Step Solution

1. $C_2H_5Cl + KCN \to C_2H_5-CN$ (X = propanenitrile) via C-attack (SN2).
2. Route I: $C_2H_5NH_2 \xrightarrow{CHCl_3/KOH,\Delta} C_2H_5NC$ — this is the carbylamine reaction, producing the isocyanide $R-NC$, NOT the nitrile $R-CN$. So I does not give X.
3. Route II: $C_2H_5CONH_2$ on dehydration loses $H_2O$ to directly give $C_2H_5CN$ — same compound as X. …

Concept and Intuition

Cyanide ion, $CN^-$, is an ambident nucleophile — it can attack through either its carbon or its nitrogen atom, giving two different products from the same alkyl halide:

• $R-X + CN^- \to R-CN$ (alkyl cyanide/nitrile) — attack through carbon.
• $R-X + CN^- \to R-NC$ (alkyl isocyanide/carbylamine) — attack through nitrogen.

Which product dominates depends on the counter-ion/solvent:

• KCN (or NaCN) in a polar solvent like ethanol is largely ionic, so the more nucleophilic carbon end of $CN^-$ attacks preferentially — nitrile is the major product.
• AgCN is much more covalent (Ag–C bond character), which leaves the nitrogen lone pair more available/nucleophilic — isocyanide is the major product with AgCN.

The two products differ sharply in what their hydrolysis gives:

• Nitrile hydrolysis: $R-CN + 2H_2O \xrightarrow{H^+/OH^-} RCOOH + NH_3$ — releases ammonia gas, a colourless pungent gas that turns red litmus blue (basic gas) — matching the clue in the question.
• Isocyanide hydrolysis: $R-NC + 2H_2O \to RNH_2 + HCOOH$ — releases an amine and formic acid, not free ammonia gas in the same simple sense.

Step-by-Step Solution

1. $C_3H_7Cl$ (n-propyl chloride) + $CN^-$ replaces Cl with CN, giving a 4-carbon, 1-nitrogen product $C_4H_7N$ — consistent with either $CH_3CH_2CH_2-CN$ (nitrile) or $CH_3CH_2CH_2-NC$ (isocyanide); both share the molecular formula $C_4H_7N$. …

Concept and Intuition

Cyanide is an ambident nucleophile — it can attack an electrophile through either its carbon or its nitrogen lone pair. With ionic $KCN$/$NaCN$, the free $CN^-$ ion attacks predominantly through carbon (since a C-C bond is more stable than a C-N bond), giving alkyl cyanides (nitriles) as the major product. With $AgCN$, however, the Ag-C bond is largely covalent, tying up the carbon and leaving the nitrogen lone pair free to act as the nucleophile, giving isocyanides (isonitriles) as the major product.

Step-by-Step Solution

1. Evaluate the Assertion: 1-bromopentane reacting with $AgCN$ indeed gives pentyl isocyanide ($C_5H_{11}-NC$) as the major product — this matches known behaviour of $AgCN$ in nucleophilic substitutions, so A is TRUE. …

Concept and Intuition

$CN^-$ is an ambident nucleophile — it can attack through carbon (giving a nitrile, $R-CN$) or through nitrogen (giving an isocyanide/isonitrile, $R-NC$). With the more ionic, aqueous/ethanolic KCN, C-attack (nitrile) dominates as the major product, but some N-attack (isocyanide) also occurs as a minor product. Isocyanides are hydrolysed (acid-catalyzed) to a primary amine plus formic acid — this reaction is in fact used as a diagnostic (carbylamine test) and as a method to convert an alkyl halide into a primary amine with one carbon degradation avoided (unlike nitrile hydrolysis/reduction, which keeps the extra carbon).

Step-by-Step Solution

1. 1-Bromopropane + ethanolic KCN → major product: $CH_3CH_2CH_2-CN$ (butanenitrile precursor, C-attack); minor product: $CH_3CH_2CH_2-NC$ (propyl isocyanide, N-attack).
2. Hydrolysis of the isocyanide (minor product): $CH_3CH_2CH_2-NC + 2H_2O \xrightarrow{H^+} CH_3CH_2CH_2-NH_2 + HCOOH$. …