Question

Structure of C8H10O2 with one 4H singlet at 7 ppm, one 4H singlet at 4.7 ppm, and a 2H singlet at 2.99

Answer 1

• The formula is C₈H₁₀O₂
• Hydrogen deficiency index, HDI = (2C+2-H)/2 = (2*8+2-10)/2 = 4
• This indicates possibility of a benzene ring.
• 4H at 7ppm confirms the presence of benzene ring.
• But benzene ring has 6H. Thus, only 4H will mean that there are two substituents.
• Moreover, all 4H have same chemical shift. This means that the substituents might be symmetrically located. Further the substituents might be the same. If they were different then we might have got different peaks around 7 ppm
• Thus till now , we have accounted for 6 carbon and 4 Hydrogen. Remaining 4 Carbon, 6 Hydrogen and 2 Oxygens are yet to be placed
• We have 4H singlet. This means that there can possibly be 2 -CH₂- groups present somewhere. But both these groups cannot be adjacent to each other because that will give multiplet (and not singlet). Further, the chemical shift of 4.7 ppm is reasonably high. This means that they might be located near to an electronegative atom i.e. Oxygen.
• There are two oxygens left to account for along with 2 hydrogens. This means there has to be 2 -OH groups in the molecule. -OH group Hydrogen can have a wide range of nmr shift from 0.5 ppm to 5 ppm. Thus, 2H singlet at 2.99 ppm should belong to 2-OH groups.
• Putting everything together, we have the following structure