Q.How will you bring about the following conversions?

Concept: Each conversion uses a specific organic reaction — elimination, substitution, addition, or coupling. The key is to identify the functional group change and choose the correct reagent.

1. Ethanol → but-1-yne Step 1: Dehydrate ethanol to ethene ($\text{conc. H}_2\text{SO}_4$, 170°C). Step 2: Brominate ethene to 1,2-dibromoethane ($\text{Br}_2$). Step 3: Double dehydrohalogenation with $\text{NaNH}_2$ in liquid $\text{NH}_3$ gives ethyne. Step 4: Alkylate ethyne with $\text{CH}_3\text{CH}_2\text{Br}$ using $\text{NaNH}_2$ to get but-1-yne.
2. Ethane → bromoethene Step 1: Free-radical bromination of ethane gives bromoethane ($\text{Br}_2$, hv). Step 2: Dehydrohalogenation with alcoholic KOH yields ethene. Step 3: Brominate ethene to 1,2-dibromoethane, then dehydrobrominate with alcoholic KOH to bromoethene.
3. Propene → 1-nitropropane Step 1: Add HBr to propene (peroxide effect, anti-Markovnikov) to get 1-bromopropane. Step 2: Treat with aqueous $\text{AgNO}_2$ (nitrite) — $\text{S}_\text{N}2$ gives 1-nitropropane.
4. Toluene → benzyl alcohol Step 1: Free-radical chlorination of toluene ($\text{Cl}_2$, hv) gives benzyl chloride. Step 2: Hydrolysis with aqueous NaOH yields benzyl alcohol.
5. Propene → propyne Step 1: Brominate propene to 1,2-dibromopropane ($\text{Br}_2$). Step 2: Double dehydrohalogenation with $\text{NaNH}_2$ in liquid $\text{NH}_3$ gives propyne.
6. Ethanol → ethyl fluoride Step 1: Convert ethanol to ethyl chloride using $\text{PCl}_5$ or $\text{SOCI}_2$. Step 2: Swarts reaction — treat ethyl chloride with $\text{AgF}$ (or Hg₂F₂, CoF₂, SbF₃) to get ethyl fluoride. (Simple NaF does not drive this exchange the way NaI does in a true Finkelstein reaction — fluorination needs one of the Swarts-reaction metal fluorides.)
7. Bromomethane → propanone Step 1: Convert $\text{CH}_3\text{Br}$ to $\text{CH}_3\text{MgBr}$ (Mg, dry ether — Grignard reagent). Step 2: React $\text{CH}_3\text{MgBr}$ with $\text{CH}_3\text{CN}$ (acetonitrile), then hydrolyse the resulting imine-magnesium complex with dilute acid to get propanone. (A nitrile is used rather than an acid chloride because the initial addition product is stable to further Grignard attack until hydrolysis — an acid chloride would over-react with excess Grignard to give a tertiary alcohol instead.)
8. But-1-ene → but-2-ene Step 1: Add HBr to but-1-ene (no peroxide, Markovnikov addition) to get 2-bromobutane. …

Let's work through each conversion one by one, focusing on the why behind each step.

(i) Ethanol to but-1-yne

Concept: We need to increase the carbon chain from 2 to 4 carbons and introduce a terminal triple bond. The strategy: convert ethanol to ethene (dehydration), then to 1,2-dibromoethane (bromination), then to ethyne (double dehydrohalogenation), and finally alkylate the terminal alkyne.

1. Ethanol → Ethene: Dehydrate ethanol using concentrated $H_2SO_4$ at 170°C.

   $CH_3CH_2OH \xrightarrow{H_2SO_4,\,170^\circ C} CH_2=CH_2 + H_2O$

2. Ethene → 1,2-Dibromoethane: Add bromine across the double bond.

   $CH_2=CH_2 + Br_2 \rightarrow CH_2Br-CH_2Br$

3. 1,2-Dibromoethane → Ethyne: Double dehydrohalogenation with alcoholic KOH (or NaNH$_2$).

   $CH_2Br-CH_2Br \xrightarrow{alc.\,KOH,\,heat} HC\equiv CH$

4. Ethyne → But-1-yne: Alkylate the terminal alkyne. First, treat ethyne with NaNH$_2$ in liquid NH$_3$ to form sodium acetylide. Then react with ethyl bromide ($CH_3CH_2Br$).

   $HC\equiv CH \xrightarrow{NaNH_2} HC\equiv C^-Na^+ \xrightarrow{CH_3CH_2Br} HC\equiv C-CH_2CH_3$ …

Let's tackle these conversions one by one. The key is to think backwards from the product to the reactant, identifying the functional group transformations needed.

Here is one clear, named method for each, with step-by-step reasoning.

(i) Ethanol to but-1-yne

Method: Build ethyne first, then alkylate its acetylide with an ethyl halide

Why this works: But-1-yne is a terminal alkyne — the cleanest NCERT route is to make ethyne from ethanol, deprotonate it to sodium acetylide, and add the remaining two carbons as bromoethane.

Steps:

1. Ethanol → Ethene: Acid-catalysed dehydration.

2. Ethene → 1,2-Dibromoethane: Electrophilic addition of bromine.

3. 1,2-Dibromoethane → Ethyne: Double dehydrohalogenation.

4. Ethyne → Sodium acetylide: Deprotonation of the terminal C–H.

5. Acetylide + Bromoethane → But-1-yne: SN2 alkylation adds the two-carbon chain (bromoethane itself is made from ethanol + $PBr_3$).

Final Product: But-1-yne

(ii) Ethane to bromoethene

Method: Four Steps: Halogenation → Elimination → Addition → Selective Elimination

Why this works: Dehydrohalogenation of bromoethane would remove its ONLY bromine and give plain ethene — a two-step route can never give bromoethene. The vinyl bromide must come from a 1,2-dibromide that loses just ONE HBr.

Steps:

1. Free Radical Halogenation: React ethane with $Br_2$ in the presence of UV light (hv). This gives bromoethane.

2. Dehydrohalogenation: Treat bromoethane with alcoholic $KOH$. This eliminates HBr to form ethene.

3. Addition of Bromine: Add $Br_2$ (in $CCl_4$) across the double bond to get 1,2-dibromoethane.

4. Selective Dehydrohalogenation: Treat with ONE equivalent of alcoholic $KOH$ to eliminate a single HBr.

Final Product: Bromoethene (Vinyl bromide)

(iii) Propene to 1-nitropropane

Method: Anti-Markovnikov Addition (via Free Radical Mechanism)

Why this works: Direct nitration of an alkene with $HNO_3$ is messy. We need to add the nitro group ($-NO_2$) to the terminal carbon (C-1), which is the anti-Markovnikov position.

Steps:

1. Anti-Markovnikov Addition of HBr: React propene with $HBr$ in the presence of a peroxide (e.g., benzoyl peroxide). This gives 1-bromopropane.

2. Nucleophilic Substitution: React 1-bromopropane with alcoholic $AgNO_2$ (silver nitrite). The nitrite ion ($NO_2^-$) acts as an ambident nucleophile, but with $AgNO_2$, the major product is the nitroalkane.

Final Product: 1-Nitropropane

(iv) Toluene to benzyl alcohol

Method: Free Radical Halogenation followed by Nucleophilic Substitution

Why this works: We need to selectively oxidize the methyl group to a $-CH_2OH$ group without touching the ring. Free radical bromination targets the benzylic position.

Steps:

1. Benzylic Bromination: React toluene with $N$-bromosuccinimide (NBS) in the presence of light (hv) and a radical initiator (like benzoyl peroxide). This selectively brominates the methyl group.

2. Hydrolysis: Treat benzyl bromide with aqueous $NaOH$ or $Na_2CO_3$. This substitutes the bromine with a hydroxyl group.

Final Product: Benzyl alcohol

(v) Propene to propyne

Method: Two-Step: Halogenation then Double Dehydrohalogenation

Why this works: We need to introduce a triple bond. This is done by adding two halogen atoms to the double bond, then removing two molecules of HX.

Steps:

1. Electrophilic Addition of Bromine: React propene with $Br_2$ in $CCl_4$. This gives 1,2-dibromopropane.

2. Double Dehydrohalogenation: Treat the dibromide with a strong base like alcoholic $KOH$ (first elimination) followed by sodamide ($NaNH_2$) in liquid $NH_3$ (second elimination, which is more difficult).

Final Product: Propyne

(vi) Ethanol to ethyl fluoride

Method: Nucleophilic Substitution (via a good leaving group)

Why this works: Direct substitution of $-OH$ by $F^-$ is poor because $OH^-$ is a bad leaving group. We must first convert the alcohol into a better leaving group.

Steps:

1. Convert Alcohol to Alkyl Halide (Chloride): React ethanol with $PCl_5$ or $SOCl_2$ to form chloroethane.

2. Halogen Exchange (Swarts Reaction): React chloroethane with $AgF$ (or $Hg_2F_2$, $CoF_2$, $SbF_3$). The driving force is the precipitation of $AgCl$. (The name Finkelstein reaction is reserved for the Cl/Br → iodide exchange with NaI in acetone.)

Final Product: Ethyl fluoride

(vii) Bromomethane to propanone

Method: Grignard Reaction + Oxidation …

Common Mistakes in Markovnikov Addition & Organic Conversions

Students often lose marks in these conversions due to conceptual confusion and procedural errors. Let's break down the key mistakes and how to avoid them.

(i) Ethanol → But-1-yne

✗ Common Mistakes

• Trying to add 2 carbons directly to ethanol without first converting to a better leaving group.
• Using NaNH₂ directly on ethanol (alcohols are not acidic enough for this).
• Forgetting that but-1-yne has a terminal alkyne (triple bond at C1).

✓ Correct Approach

1. Ethanol → Ethene (dehydration with conc. H₂SO₄, 170°C)
2. Ethene → 1,2-dibromoethane (Br₂ addition)
3. 1,2-dibromoethane → But-1-yne (2 moles NaNH₂ in liq. NH₃ to form sodium acetylide, then alkylate with CH₃CH₂Br — ethyl bromide, not methyl bromide, since ethyne (2 C) + ethyl (2 C) = the 4 carbons but-1-yne needs; using CH₃Br would only reach propyne)

Why this works: The double dehydrohalogenation creates the triple bond, and the alkylation adds the extra carbon.

(ii) Ethane → Bromoethene

✗ Common Mistakes

• Trying direct bromination of ethane (gives bromoethane, not bromoethene).
• Using Br₂/CCl₄ on ethane (no reaction — alkanes need light/heat for substitution).
• Confusing bromoethene (CH₂=CHBr) with bromoethane (CH₃CH₂Br).

✓ Correct Approach

1. Ethane → Ethene (cracking or catalytic dehydrogenation, Cr₂O₃/Al₂O₃, 600°C)
2. Ethene → 1,2-dibromoethane (Br₂/CCl₄ addition)
3. 1,2-dibromoethane → Bromoethene (alc. KOH, heat — dehydrohalogenation)

Key insight: You need a double bond first, then add Br₂, then eliminate one HBr.

(iii) Propene → 1-Nitropropane

✗ Common Mistakes

• Assuming direct nitration of propene (gives mixture of nitroalkenes, not 1-nitropropane).
• Forgetting that Markovnikov addition would give 2-nitropropane, not 1-nitropropane.
• Using HNO₃/H₂SO₄ (nitrating mixture) — this works for aromatics, not alkenes.

✓ Correct Approach (Anti-Markovnikov needed)

1. Propene → 1-Bromopropane (HBr with peroxide — anti-Markovnikov addition)
2. 1-Bromopropane → 1-Nitropropane (NaNO₂ in DMF or aqueous ethanol, SN2 reaction)

Critical point: The first step must be anti-Markovnikov to get the Br at C1. Without peroxide, HBr adds to C2 (Markovnikov product).

(iv) Toluene → Benzyl Alcohol

✗ Common Mistakes

• Trying direct oxidation of methyl group (gives benzoic acid, not benzyl alcohol).
• Using KMnO₄ or K₂Cr₂O₇ — these over-oxidize to benzoic acid.
• Forgetting that benzyl alcohol is C₆H₅CH₂OH (alcohol, not acid).

✓ Correct Approach

1. Toluene → Benzyl chloride (Cl₂/hν or Cl₂/UV light — side-chain chlorination)
2. Benzyl chloride → Benzyl alcohol (aq. NaOH or KOH, hydrolysis)

Why this works: Free radical chlorination attacks the benzylic position (CH₃ group), not the ring. Then SN2 hydrolysis gives the alcohol.

(v) Propene → Propyne

✗ Common Mistakes

• Trying one-step elimination (propene → propyne requires two eliminations).
• Using alc. KOH directly on propene (no reaction — no leaving group).
• Forgetting that propyne has a terminal triple bond.

✓ Correct Approach

1. Propene → 1,2-Dibromopropane (Br₂/CCl₄ addition)
2. 1,2-Dibromopropane → Propyne (2 moles NaNH₂ in liq. NH₃)

Mechanism: Two successive dehydrohalogenations — first gives bromopropene, second gives propyne.

(vi) Ethanol → Ethyl Fluoride

✗ Common Mistakes

• Trying direct reaction with HF (poor yield, dangerous, and HF is weak acid).
• Using F₂ gas (explosive, non-selective).
• Confusing with other halides (Cl, Br, I are easier).

✓ Correct Approach

1. Ethanol → Ethene (conc. H₂SO₄, 170°C)
2. Ethene → Ethyl fluoride (HF addition — Markovnikov gives ethyl fluoride)

Alternative: Ethanol → Ethyl chloride (SOCl₂) → Ethyl fluoride (metathesis with AgF or KF in polar aprotic solvent).

(vii) Bromomethane → Propanone

✗ Common Mistakes

• Trying direct alkylation of bromomethane (gives ethane, not propanone).
• Forgetting that propanone (acetone) has a carbonyl group (C=O).
• Using Grignard reagent incorrectly.

✓ Correct Approach

1. Bromomethane → Methyl magnesium bromide (Mg/dry ether — Grignard reagent)
2. CH₃MgBr + CH₃CN → Propanone (react with acetonitrile, then hydrolyse the resulting imine-magnesium complex with dilute acid)

Why a nitrile, not an acid chloride: an acid chloride (e.g. CH₃COCl) reacts with a Grignard reagent to first give a ketone, but that ketone is itself attacked by any remaining Grignard reagent, over-reacting to a tertiary alcohol. A nitrile's initial addition product is stable to further Grignard attack, so hydrolysis cleanly stops at the ketone.

(viii) But-1-ene → But-2-ene

✗ Common Mistakes

• Thinking this is a simple isomerization (it requires bond migration).
• Using H⁺ alone (gives carbocation rearrangement but also polymerization).
• Forgetting that double bond migration needs specific conditions.

✓ Correct Approach

1. But-1-ene → But-2-ene (H₂SO₄ (dilute), heat — or Al₂O₃, 300°C)

Mechanism: Protonation gives secondary carbocation (more stable), then deprotonation gives the more substituted alkene (but-2-ene). …