Q.Arrange the following compounds in the increasing order of their boiling points: Formaldehyde, ethane, methyl alcohol.

Step 1. Recall the chapter's own force-strength ranking (section 12.6.4's 'Remember' box). Hydrogen bond > dipole-dipole attraction > van der Waals force -- and matching this, boiling points run carboxylic acids > alcohols > ketones > aldehydes > ethers > alkanes.

Step 2. Place each of the three named compounds on that scale. Ethane (CH3-CH3) is a plain alkane, with only weak van der Waals forces between molecules -- the lowest boiling point of the three. Formaldehyde (H-CHO) is the simplest aldehyde, with a polar C=O giving dipole-dipole attraction -- higher than ethane, but still below an alcohol (Table 12.4 lists methanal's boiling point as 252 K). Methyl alcohol (CH3-OH) has an -OH group capable of full hydrogen bon …

Applying section 12.6.4's intermolecular-force-strength ranking (van der Waals < dipole-dipole < hydrogen bond) to place an alkane, an …

• Ranking purely by molecular mass instead of by the type of intermolecular force available, which would wrongly predict methyl alcohol (mass 32) below formaldehyde (mass 30) despite methanol's much stronger hydrogen bonding. …