Ethers — Williamson synthesis, cleavage by HI

4.3.6 · Chemistry › Halides and Oxygenated Derivatives

1. Ether kya hota hai? (KYA)

C–O–C bond angle lagbhag $11 0^{\circ}$ hota hai (oxygen $s p^{3}$ hai, jaise paani mein dono H ko carbon se replace kar do). O par do lone pairs hone ki wajah se ethers thodi weakly basic hoti hain — cleavage ke liye yeh important hai.

2. Williamson Ether Synthesis (HOW ethers banate hain)

Mechanism ko first principles se derive karna

Humein $R O^{-}$ ke O aur $R^{'} - X$ ke C ke beech ek bond chahiye.

Alkoxide kyun? Ek neutral alcohol oxygen sirf weakly nucleophilic hoti hai. Iska H ek base se remove karo (jaise Na metal, ya NaH) to $R O^{-}$ milta hai — ek negatively charged O jisme electron density bahut zyaada hoti hai → strong nucleophile. $R - O H + N a ⟶ R - O^{-} N a^{+} + \frac{1}{2} H_{2}$
Alkyl halide kyun? C–X bond polarized hoti hai $C^{δ +} - X^{δ -}$ . Carbon electrophilic hota hai; X, $X^{-}$ (stable anion) ki tarah chala ja sakta hai. Yeh perfect $S_{N} 2$ partner hai.
Attack ( $S_{N} 2$ ): alkoxide lone pair, $C^{δ +}$ ke backside par attack karta hai, X ek concerted step mein chala jaata hai:

$R - O^{-} + backside C δ + - X ⟶ [R - O \dots C \dots X]^{‡} ⟶ R - O - C + X^{-}$

3. HI se Ethers ki Cleavage (HOW ethers kaatte hain)

Ethers unreactive hoti hain (koi achha leaving group nahin: $R O^{-}$ ek bura leaving group hai). Inhe todne ke liye humein strong acid + achha nucleophile chahiye → HI (ya HBr) dono kaam karta hai.

Mechanism derive karna

Step 1 — Protonation (bure LG ko achhe LG mein badlo): $R - O - R^{'} + H^{+} \to R - O + (H) - R^{'}$ Kyun? Neutral ether O ek bura leaving group hai. Protonation ke baad, leaving group ek neutral alcohol $R - O H$ ban jaata hai — kaafi better.

Step 2 — Iodide attack karta hai ( $S_{N} 2$ ya $S_{N} 1$ ): $I^{-} + R^{'} - O + (H) - R \to R^{'} - I + R - O H$

Figure — Ethers — Williamson synthesis, cleavage by HI

4. 80/20 — jo actually marks dilata hai

Williamson = $S_{N} 2$ : bulky group → alkoxide, chhota group → halide.
Aryl/3° hamesha alkoxide hona chahiye (kabhi halide nahin).
HI cleavage: O ko protonate karo, phir $I^{-}$ chhote C par attack karta hai ( $S_{N} 2$ ) ya stable-cation C par ( $S_{N} 1$ ).
Alkyl–aryl ether → phenol + alkyl iodide (aryl kabhi iodide nahin banta).
Reactivity $H I > H B r > H C l$ .

Flashcards

Williamson synthesis kaun sa mechanism hai?

S_{N} 2

(alkoxide + alkyl halide).

Williamson mein alkyl halide kaun sa partner hona chahiye?

Kam hindered (methyl/1°) group; bulkier group alkoxide hoti hai.

3° alkyl halide Williamson mein kyun use nahin ho sakta?

Strong alkoxide base E2 elimination (alkene) karti hai

S_{N} 2

ether formation ki jagah.

Williamson se anisole kaise banate hain?

C_{6} H_{5} O^{-} N a^{+} + C H_{3} I \to C_{6} H_{5} O C H_{3}

(phenoxide nucleophile hai; aryl halide nahin ban sakta).

HI se ether cleavage ka pehla step kya hai?

Ether oxygen ka protonation, jisse

R O^{-}

(bura LG) neutral alcohol (achha LG) ban jaata hai.

Anisole + HI se kya milta hai?

Phenol + methyl iodide (

C_{6} H_{5} O H + C H_{3} I

), iodobenzene NAHIN.

C H_{3} - O - C (C H_{3})_{3}

+ HI ke products kya hain?

C H_{3} O H + (C H_{3})_{3} C I

(S_N1: stable 3° carbocation

I^{-}

pakad leta hai).

Ether cleavage ke liye HI > HBr > HCl kyun?

I^{-}

sabse zyaada polarizable/best nucleophile hai aur HI sabse strong acid hai (O ka sabse achha protonator).

Diethyl ether + excess HI se kya milta hai?

2 C_{2} H_{5} I + H_{2} O

S_{N} 1

cleavage mein kaun sa carbon iodide leta hai?

Woh jo zyaada stable carbocation banaata hai (3°/benzylic/allylic).

Recall Feynman: 12-saal ke bachche ko samjhao

Ek ether do LEGO bricks hain jo ek oxygen connector se judi hain. Ise banane ke liye: ek brick lo jiska connector "charged up aur grabby" ho (alkoxide) aur doosri brick lo jisme ek "loose, easy-to-pop-off cap" ho (halide). Grabby wali carbon par snap karti hai aur cap pop off ho jaata hai. Tip: jis brick par attack ho raha hai use chhota aur khula rakho, warna grabby piece tak pahunch nahin sakta. Ise todne ke liye: strong acid (HI) daalo. Acid oxygen par ek chhota H chipka deta hai taaki connector "slippery aur jaane ko taiyaar" ho jaaye, phir iodine carbon ko dhakka deke apni jagah le leta hai. Agar ek side ek flat ring (benzene) hai, toh iodine usme push nahin kar sakta, isliye woh side oxygen rakh leti hai aur phenol ban jaati hai.