hce_nchu
114年
化學
第 6 題
Phenyl Grignard reagent reacts with 2-methylpropanal to form a secondary alcohol. The $^1H$ NMR spectrum of 2-methylpropanal displays the two methyl groups as equivalent (a single doublet at $\delta$ 1.1). However, in the product alcohol (a racemic mixture, the $^1H$ NMR now shows two distinct 3H doublets, one at $\delta$ 0.75 and one around $\delta$ 1.0. Why do the two methyl groups in the product have different NMR chemical shifts? What is the term applied to such protons?
- A The presence of the hydroxyl group creates an asymmetric center, making the methyl groups homotopic.
- B The formation of a chiral center in the product makes the two methyl groups diastereotopic, meaning they experience different chemical environments.
- C The hydroxyl group participates in hydrogen bonding, causing different splitting patterns for the methyl groups, which are now homotopic.
- D The two methyl groups in the product remain chemically equivalent, and the observed difference in NMR chemical shifts is due to rotational averaging.
- E The steric hindrance of the bulky phenyl group shields one methyl group, making them enantiotopic rather than diastereotopic.
思路引導 VIP
請試著畫出反應後產物的立體結構，並仔細觀察原本醛基的碳原子在接上格氏試劑後，其周圍的四個基團是否相同？接著思考，如果我們分別將左邊與右邊的甲基暫時標記起來，從這個新形成的碳原子中心看過去，這兩個甲基所處的空間環境還會是對稱的嗎？
🤖
AI 詳解
AI 專屬家教
恭喜你準確捕捉到了分子對稱性的微細變化！這題的關鍵在於產物結構中**不對稱中心（Chiral center）**的生成。在反應物 2-甲基丙醛（2-methylpropanal）中，兩個甲基處於鏡像對稱的環境，因此在 $^1H$ NMR 中表現為化學等價。然而，當格氏試劑（Grignard reagent）攻擊羰基後，原本的羰基碳轉變成了手性碳，生成了 $1$-苯基-$2$-甲基-$1$-丙醇 ($C_6H_5CH(OH)CH(CH_3)_2$)。
化學環境的空間差異
由於手性中心的存在，這兩個甲基在空間中相對於羥基（$-OH$）或苯基（$-C_6H_5$）的位置變得不再對稱。這種在結構上雖然看似對稱，但在手性環境下變為「不等價」的質子或基團，我們稱之為非對映異位（Diastereotopic）。這正是為什麼你在光譜中會觀察到兩組不同的化學位移（$\delta$ 0.75 與 1.0），因為它們在磁場中所感受到的電子屏蔽效應已經截然不同。
▼ 還有更多解析內容