Chemistry - STRUCTURAL ISOMERISM Question with Solution | TestHub

Look at each enol. (A) acetaldehyde $\to$ CH₂=CH-OH : one alkene carbon carries two H, no E/Z. (B) propanal $\to$ CH₃-CH=CH-OH : each double-bond carbon carries two different groups (CH₃/H and H/OH), so E and Z enols exist. (C) isobutyraldehyde $\to$ (CH₃)₂C=CH-OH : two methyls on one carbon, no E/Z. (D) cyclohexanone $\to$ cyclohex-1-en-1-ol : the double bond is inside a small ring, no E/Z. Hence (B).