1. Arrange these three compounds in order of decreasing ability to form intermolecular hydrogen bonds: CH3OH, CH3SH, and 1CH3 2 2NH.
2. Arrange these compounds in order of increasing boiling point: CH3CH2CH2CH3, CH3CH2CH2OH, and CH3CH2CH2NH2. Boiling point values from lowest to highest are 20.5°C, 7.2°C, and 77.8°C.
The Correct Answer and Explanation is :
1. Order of Decreasing Ability to Form Intermolecular Hydrogen Bonds
The three compounds in question are:
- CH3OH (Methanol)
- CH3SH (Methyl mercaptan)
- CH3NH2 (Methylamine)
Explanation:
Hydrogen bonding occurs when a hydrogen atom is attached to an electronegative atom such as oxygen (O), nitrogen (N), or fluorine (F), which has lone pairs of electrons. Among the three compounds, the ability to form hydrogen bonds depends on the presence of a highly electronegative atom bonded to hydrogen.
- CH3OH (Methanol): This compound has an -OH (hydroxyl) group, where oxygen is highly electronegative and can form hydrogen bonds with hydrogen atoms from other molecules. Methanol can both donate and accept hydrogen bonds due to the lone pairs of electrons on oxygen and the hydrogen attached to it.
- CH3NH2 (Methylamine): This compound has an -NH2 (amine) group. Nitrogen, like oxygen, is electronegative and can form hydrogen bonds. However, nitrogen is less electronegative than oxygen, meaning the hydrogen bonds formed by methylamine will be weaker than those formed by methanol.
- CH3SH (Methyl mercaptan): This compound contains a -SH (thiol) group. While sulfur is electronegative, it is significantly less so than oxygen and nitrogen, meaning sulfur’s hydrogen bonding ability is weaker compared to oxygen and nitrogen.
Order of Decreasing Ability to Form Hydrogen Bonds:
- CH3OH > CH3NH2 > CH3SH
Methanol has the strongest hydrogen bonding capability due to the highly electronegative oxygen atom, followed by methylamine with nitrogen, and methyl mercaptan having the weakest ability due to sulfur.
2. Order of Increasing Boiling Point
The compounds under consideration are:
- CH3CH2CH2CH3 (Butane)
- CH3CH2CH2OH (Propanol)
- CH3CH2CH2NH2 (Propylamine)
Boiling Points:
- Butane: 7.2°C
- Propanol: 77.8°C
- Propylamine: 20.5°C
Explanation:
Boiling points are influenced by the strength of intermolecular forces. The three major types of intermolecular forces are:
- Van der Waals Forces (Dispersion Forces): Present in all molecules, these forces are the weakest.
- Dipole-Dipole Interactions: Occur in polar molecules and are stronger than Van der Waals forces.
- Hydrogen Bonds: Occur when hydrogen is bonded to a highly electronegative atom like O or N, and these are the strongest type of intermolecular force.
- Butane (CH3CH2CH2CH3): Butane is a nonpolar molecule, so it only experiences Van der Waals forces, which are weak. Hence, it has the lowest boiling point.
- Propylamine (CH3CH2CH2NH2): Propylamine is polar and contains an -NH2 group, allowing it to form hydrogen bonds. Hydrogen bonding is stronger than Van der Waals forces, so propylamine has a higher boiling point than butane.
- Propanol (CH3CH2CH2OH): Propanol is also polar and contains an -OH group, which can form hydrogen bonds, similar to propylamine. However, oxygen is more electronegative than nitrogen, so the hydrogen bonds in propanol are stronger than those in propylamine. Therefore, propanol has the highest boiling point.
Order of Increasing Boiling Point:
- Butane < Propylamine < Propanol
The order of boiling points corresponds to the strength of intermolecular forces, with hydrogen bonds being the most significant factor in determining boiling points.