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Chemistry
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Total number of possible isomers (both structural as well as stereoisomers) of cyclic ethers of molecular formula $\text{C}_4\text{H}_8\text{O}$ is:
5-membered ring: tetrahydrofuran (1); 4-membered ring: 2-methyloxetane (2: R,S), 3-methyloxetane (1); 3-membered ring: 2-ethyloxirane (2: R,S), 2,3-dimethyloxirane (3: cis-meso, trans-pair), 2,2-dimethyloxirane (1). Total $= 1 + 2 + 1 + 2 + 3 + 1 = 10$.
For cyclic ethers with formula $\text{C}_4\text{H}_8\text{O}$ (Degree of unsaturation = 1): 1. Oxolane (5-membered): Tetrahydrofuran (THF) $\implies 1$ isomer. 2. Oxetanes (4-membered): (a) 2-methyloxetane has one chiral center at C-2 $\implies 2$ enantiomers (R and S); (b) 3-methyloxetane is achiral $\implies 1$ isomer. 3. Oxiranes (3-membered): (a) 2-ethyloxirane has one chiral center $\implies 2$ enantiomers (R and S); (b) 2,3-dimethyloxirane has two chiral centers, giving 1 meso-isomer (cis) and 1 pair of enantiomers (trans-R,R and trans-S,S) $\implies 3$ stereoisomers; (c) 2,2-dimethyloxirane is achiral $\implies 1$ isomer. Summing all structural and stereoisomers: $1 + 2 + 1 + 2 + 3 + 1 = 10$ isomers.