To determine the minimum number of carbon atoms needed for an optically active ether, we need to consider the definition of chirality and the structure of ethers.
### Step-by-Step Solution:
1. **Understanding Chirality**:
- An optically active compound must have at least one chiral center. A chiral center is typically a carbon atom that is bonded to four different groups.
2. **Identifying the Ether Structure**:
- Ethers have the general structure R-O-R', where R and R' are hydrocarbon groups. For an ether to be optically active, at least one of these carbon atoms must be chiral.
3. **Constructing a Chiral Ether**:
- Let's consider a simple ether structure. If we have a carbon atom bonded to an -OCH3 group, a hydrogen atom, a -CH3 group, and a -C2H5 group, we can visualize this as:
- Chiral center: C (chiral carbon)
- Groups attached: OCH3, H, CH3, C2H5.
- This configuration gives us a total of 5 carbon atoms (1 from OCH3, 1 from CH3, 2 from C2H5, and 1 from the chiral carbon).
4. **Exploring Other Configurations**:
- We can also consider a cyclic ether. For example, a three-membered cyclic ether (like oxirane) can have a chiral carbon with different groups attached.
- If we have a carbon with a hydrogen and a methyl group, and if we add a -CH2- group (which can be part of the ether), we can still achieve chirality with only 3 carbon atoms.
5. **Conclusion**:
- After analyzing different structures, we find that it is indeed possible to create an optically active ether with a minimum of 2 carbon atoms. For example, a carbon bonded to OCH3, H, and two different isotopes of hydrogen (like deuterium and tritium) can create a chiral center with only 2 carbon atoms.
### Final Answer:
The minimum number of carbon atoms needed for an optically active ether is **2**.
