Question

1. in which of the following pure substances will hydrogen bonding be an important intermolecular force? 1) dichloromethane, ch₂cl₂, 2) ch₃ch₂oh, 3) methylamine, (ch₃nh₂), 4) trimethylamine, n(ch₃)₃

a) 2 and 3
b) all
c) none
d) 3 and 4
e) 1 and 2

Explanation:

Step1: Recall hydrogen - bonding criteria

Hydrogen bonding occurs when a hydrogen atom is bonded to a highly electronegative atom (N, O, F) and there is another electronegative atom (N, O, F) in the vicinity to act as a hydrogen - bond acceptor.

Step2: Analyze dichloromethane ($CH_2Cl_2$)

In $CH_2Cl_2$, hydrogen is bonded to carbon. Carbon is not a highly electronegative atom like N, O, or F. So, no hydrogen bonding.

Step3: Analyze ethanol ($CH_3CH_2OH$)

In $CH_3CH_2OH$, the hydrogen atom is bonded to oxygen. Oxygen is highly electronegative. There can be hydrogen - bonding between ethanol molecules where the hydrogen of one ethanol molecule is attracted to the oxygen of another ethanol molecule.

Step4: Analyze methylamine ($CH_3NH_2$)

In $CH_3NH_2$, hydrogen is bonded to nitrogen. Nitrogen is highly electronegative. Hydrogen - bonding can occur between methylamine molecules where the hydrogen of one methylamine molecule is attracted to the nitrogen of another methylamine molecule.

Step5: Analyze trimethylamine ($N(CH_3)_3$)

In $N(CH_3)_3$, all the hydrogens are bonded to carbon. There is no H - N, H - O, or H - F bond. So, no hydrogen bonding.

Answer:

a) 2 and 3