# Ethambutol Stereochemistry: Step-by-Step Solution ## 1. **Understanding the Problem** - **Ethambutol** is a drug used to treat tuberculosis. - It has **two stereogenic (chiral) centers**. - The question asks for the **structure with both chiral centers in the S configuration**. - You need to redraw the molecule showing the correct (S,S) configuration. --- ## 2. **Structure of Ethambutol** Ethambutol’s IUPAC name is: **(2S,2'S)-2,2'-[ethane-1,2-diylbis(azanediyl)]di-butan-1-ol** **Simplified skeletal structure:** ``` HO–CH₂–CH(NH₂)–CH₂–CH₂–CH(NH₂)–CH₂–OH ``` The chiral centers are at the two carbon atoms bonded to NH₂ groups. --- ## 3. **Identifying the Chiral Centers** Let’s number the chain for clarity: 1. HO–CH₂–**C2**H(NH₂)–CH₂–CH₂–**C5**H(NH₂)–CH₂–OH - **C2** and **C5** are the chiral centers. --- ## 4. **Assigning Priorities (Cahn-Ingold-Prelog Rules)** At both chiral centers (**C2** and **C5**): - The four groups attached are: 1. –OH via the chain 2. –NH₂ 3. –CH₂– (rest of chain) 4. –H **Priority order** (highest to lowest): 1. –NH₂ (N has highest atomic number) 2. –CH₂OH (O in –OH is higher than C in –CH₂–) 3. –CH₂– (the rest of the chain) 4. –H (lowest) --- ## 5. **Drawing the (S,S) Configuration** To assign **S configuration** to a chiral center: - Arrange so that the lowest priority group (H) is at the back. - Trace the path from highest (1) to next (2) to next (3). - If the path is **counterclockwise**, it is S. ### **Example for C2:** - **Groups:** - –NH₂ - –CH₂OH - –CH₂–CH₂–CH(NH₂)–CH₂–OH - –H ### **Wedge-Dash Notation** ``` NH2 | HO–CH2–C*–CH2– | H ``` To show the S configuration: - Place H (priority 4) at the back (dashed line). - Arrange NH₂, CH₂OH, and the rest of the chain so the sequence from 1→2→3 is counterclockwise. #### **Both Chiral Centers (C2 and C5):** Draw both centers with the same logic. Here is a simplified depiction: ``` HO–CH2–C*(NH2)(H)–CH2–CH2–C*(NH2)(H)–CH2–OH Where at both C*: - NH2 is on a wedge (coming out) - H is on a dash (going back) - The backbone continues in the plane ``` **Skeletal structure with stereochemistry:** ```text HO NH2 | | H–O–CH2–C*–CH2–CH2–C*–CH2–OH | | NH2 HO ``` But to show the (S,S) configuration in wedge-dash notation: ### **Expanded Structure (for each chiral center):** - **At each C*:** - NH₂: solid wedge (coming out) - H: dashed line (going back) - Rest of chain in the plane #### **2D Representation at Each Chiral Center:** ``` NH2 | HO–CH2–C*–CH2–... | H (back) ``` For both chiral centers, arrange so NH₂ is on the wedge, H on the dash, the chain continues in the plane. --- ## 6. **Final Answer: Redrawn Ethambutol with (S,S) Configuration** ### **Wedge-Dash Structure (Simplified)** ``` HO–CH₂–[C*(NH₂, H, rest)]–CH₂–CH₂–[C*(NH₂, H, rest)]–CH₂–OH At each C*: NH2 | ...–C*–CH2–... | H (dashed back) ``` ### **Visual Example** ```markdown NH2 | HO–CH₂–C*–CH₂–CH₂–C*–CH₂–OH | | H NH2 (dash) (wedge) ``` - At **both C2 and C5**, NH₂ is on the wedge, H is on the dash (behind), other substituents in the plane. - This configuration gives (S,S) at both stereogenic centers. --- ## 7. **Summary** - Identify the two chiral centers in ethambutol. - Assign priorities using Cahn-Ingold-Prelog rules. - Draw the structure so that at both centers, the sequence from –NH₂ (1) → –CH₂OH (2) → rest of chain (3) is counterclockwise when H (4) is at the back. - The final drawing shows both chiral centers with the S configuration. --- ### **[No image provided, but you can use the description above to draw the structure with correct stereochemistry.]**