Question

1. An organic compound has a molecular ion at m/z = 90, an M+1 isotope peak at m/z = 91, and an M+2 isotope peak at m/z = 92. The M+1 signal is 5% of the molecular ion’s intensity. The M+2 signal is 4% of the molecular ion’s intensity. What is the molecular formula of the compound? What functional groups are compatible with this molecular formula? Show your reasoning.

2. If the dihedral angle between two nonequivalent vicinal protons is approximately 90 degrees, then the magnitude of the coupling constant between these two protons will be

a. very small or possibly zero

b. intermediate
c. large

2.1. Electronegative substituents typically cause a nearby proton to be

a. shielded

b. unaffected

c. deshielded

2.2. Pent-1-yne can be converted into cis-non-3-ene with which sequence of reagents?

a. a.) NaNH2, EtI; b.) excess Na, NH3(l)
b. a.) excess Na, NH3(l); b.) NaNH2, EtI
c. a.) NaNH2, EtI; b.) H2, cat. Pd/BaSO4, quinoline

2.3. Which analytical technique can be used on its own to determine molecular formula?

a. IR

b. HRMS

c. 1H NMR

Answer 1

1. An organic compound has a molecular ion at m/z = 90, an M+1 isotope peak at m/z = 91, and an M+2 isotope peak at m/z = 92. The M+1 signal is 5% of the molecular ion’s intensity. The M+2 signal is 4% of the molecular ion’s intensity.

The molecular formula of the compound = C4H9S

The functional groups compatible with this molecular formula = -SH (thiol)

Sulfur has three isotopes S-32, S-33 and S-34 in the given ratio as discussed in mass spectrum here and thus compound has -SH group in it.

2.0 If the dihedral angle between two nonequivalent vicinal protons is approximately 90 degrees, then the magnitude of the coupling constant between these two protons will be

a. very small or possibly zero

2.1 Electronegative substituents typically cause a nearby proton to be,

c. deshielded

2.2 Pent-1-yne can be converted to cis-non-3-ene with sequence of reagents,

c.

2.3 Analytical technique to be used on its own to determine molecular formula,

b. HRMS