Alkynes — preparation, acidity of terminal alkynes, addition reactions, hydration to ketones

4.2.7 · Chemistry › Hydrocarbons

1. Alkyne kya hai? (WHAT)

Triple-bond carbon == $s p$ hybridised== hota hai, bond angle $18 0^{\circ}$ , geometry linear hoti hai. $C \equiv C$ bond length ( $120$ pm) $C = C$ ( $134$ pm) aur $C - C$ ( $154$ pm) se chhoti hoti hai.

2. Preparation (HOW)

2.1 Calcium carbide se (industrial ethyne)

$C a C_{2} + 2 H_{2} O ⟶ C_{2} H_{2} ↑ + C a (O H)_{2}$ Kyun? Carbide ion $C_{2}^{2 -}$ ethyne ka conjugate base hai; paani (ek stronger acid) use do baar protonate karta hai.

2.2 Vicinal / geminal dihalides ki dehydrohalogenation se

Ek vic-dihalide do HX lose karta hai (do eliminations chahiye → strong base chahiye): $vic-dihalide C H_{3} - C H B r - C H_{2} B r KOH (alc.) Δ C H_{3} - C B r = C H_{2} N a N H_{2} Δ C H_{3} - C \equiv C H$

2.3 Terminal alkyne ki alkylation se (bade alkynes banate hain)

$H C \equiv C H N a N H_{2} H C \equiv C^{-} N a^{+} R - X H C \equiv C - R$ Yeh kyun kaam karta hai: Acetylide ion ek strong nucleophile hai jo primary alkyl halide par $S_{N} 2$ karta hai. (Secondary/tertiary R–X → elimination dominate karta hai.)

3. Terminal alkynes ki acidity (chapter ka WHY star)

4. Electrophilic addition reactions (HOW)

Do $π$ bonds → reagent ki ek ya do equivalents add ho sakti hain.

4.1 HX ka addition (Markovnikov)

$C H_{3} - C \equiv C H H B r C H_{3} - C B r = C H_{2} H B r C H_{3} - C B r_{2} - C H_{3}$ Markovnikov kyun? H⁺ add hota hai zyada stable (secondary) vinyl cation dene ke liye; phir X⁻ cationic carbon par add hota hai. Isliye Br zyada substituted carbon par aa jaata hai.

4.2 $H_{2} O$ ka addition → hydration dekho (Section 5).

4.3 Catalytic hydrogenation (product control karo)

$R - C \equiv C - R H_{2} / P d - C a C O_{3} (Lindlar) c i s R - C H = C H - R$ $R - C \equiv C - R N a / l i q . N H_{3} t r an s R - C H = C H - R$ Kyun? Lindlar (poisoned Pd) dono H same face se deliver karta hai → cis. Dissolving-metal reduction ek trans-radical anion se guzarta hai → trans alkene.

Figure — Alkynes — preparation, acidity of terminal alkynes, addition reactions, hydration to ketones

5. Hydration se ketones (headline reaction)

5.1 Markovnikov hydration ( $H_{2} O / H_{2} S O_{4} / H g S O_{4}$ )

$R - C \equiv C H H_{2} O H_{2} S O_{4}, H g S O_{4} [R - O H C = C H_{2}] tautomerise R - C ∥ O - C H_{3}$

Ketone kyun (aldehyde nahi)? OH Markovnikov ke hisaab se add hota hai → zyada substituted carbon par → enol carbon jo R uthaye hue hai → tautomerisation par methyl ketone banta hai.

5.2 Keto–enol tautomerism (rearrangement)

$enol C = C - O H ⇌ keto C ∥ O - C - H$ Keto kyun favoured hota hai: $C = O$ $π$ bond (≈ $360$ kJ/mol) $C = C$ (≈ $270$ ) se bahut strong hota hai + $O - H$ vs $C - H$ swap; net energy girta hai → keto dominate karta hai.

6. Quick recall

Recall Active recall — right side cover karo

Alkyne ka general formula ::: $C_{n} H_{2 n - 2}$
Hybridisation & geometry ::: $s p$ , linear, $18 0^{\circ}$
Terminal $\equiv C - H$ acidic kyun hota hai ::: 50% s-character → anion nucleus ke paas held hota hai → stable
Acidity order ::: $s p > s p^{2} > s p^{3}$ (ethyne > ethene > ethane)
Acetylide banane ke liye base ::: $N a N H_{2}$ (NaOH nahi)
Markovnikov hydration ka product ::: methyl ketone (ethyne → acetaldehyde)
Lindlar deta hai ::: cis alkene; Na/NH₃ deta hai ::: trans alkene

Recall Feynman: 12 saal ke bachche ko samjhao

Socho teen rubber bands do atoms ko jod rahe hain — yahi triple bond hai, extra electrons se bhara hua. Woh electrons sticky candy ki tarah hain; jo bhi "positive" hota hai woh unhe pakadna chahta hai, isliye alkynes eagerly react karte hain. Aur, bilkul end ka H dheelay se pakda hota hai — jaise ek topi jo tez hawa mein ud jaati hai. Ek bahut tez hawa ( $N a N H_{2}$ ) topi uda leti hai; ek halki hawa ( $N a O H$ ) nahi uda sakti. Jab tum triple bond par mercury helper ke saath paani dalte ho, toh pehle ek hilta-dolta "enol" banta hai jo turant stable ketone mein flip ho jaata hai, kyunki nayi oxygen double bond super strong aur comfortable hoti hai.

Connections

Alkenes — electrophilic addition (alkynes do baar add karte hain, same logic)
Hybridisation and s-character (acidity ki jad)
Keto–enol tautomerism (hydration product drive karta hai)
Aldehydes and Ketones — carbonyl chemistry (products)
SN2 reactions (acetylide alkylation)
Markovnikov rule and carbocation stability

General formula of an open-chain alkyne

C_{n} H_{2 n - 2}

Hybridisation and bond angle at triple-bond carbon

s p

18 0^{\circ}

, linear

Why is a terminal alkyne C–H acidic?

Conjugate-base lone pair ek

s p

orbital mein hai (50% s-character), nucleus ke paas held hota hai → stable carbanion

Order of acidity: ethyne, ethene, ethane

ethyne (

s p

) > ethene (

s p^{2}

) > ethane (

s p^{3}

)

Why does NaOH fail to deprotonate ethyne but NaNH₂ works?

p K_{a}

(water)=15.7 <

p K_{a}

(ethyne)=25 <

p K_{a}

(NH₃)=38; sirf woh base deprotonate karta hai jiska conjugate acid weaker ho (NH₃)

Reagents for Markovnikov hydration of an alkyne

H_{2} O

, dil.

H_{2} S O_{4}

H g S O_{4}

catalyst

Product of Markovnikov hydration of propyne

propan-2-one (ek methyl ketone)

Why does ethyne give an aldehyde, not a ketone, on hydration

Dono carbon equivalent hain; enol sirf acetaldehyde mein tautomerise ho sakta hai

Product of anti-Markovnikov (hydroboration–oxidation) of a terminal alkyne

ek aldehyde (

R C H_{2} C H O

)

Lindlar's catalyst converts an alkyne to

ek cis-alkene

Na in liquid NH₃ converts an internal alkyne to

ek trans-alkene

Test reagent for terminal alkynes

Ammoniacal

A g N O_{3}

(white ppt) ya ammoniacal

C u C l

(red ppt)

Two HX needed: which base for the 2nd elimination to form the triple bond

N a N H_{2}

(sodium amide)

What intermediate forms first in alkyne hydration before the carbonyl

ek enol (

C = C - O H

)