Which of the following has the lowest boiling point?
- $CH_3CH_2OH$
- $CH_3-CH_2-NH_2$
- $CH_3-O-CH_3$
- $HCOOH$
The Correct Option is C
Approach Solution - 1
The boiling point of a substance depends on its molecular interactions. The stronger the intermolecular forces (such as hydrogen bonding), the higher the boiling point. Let's analyze each option:
1. HCOOH (Formic acid): Formic acid has strong hydrogen bonding due to the presence of both a hydroxyl group (-OH) and a carbonyl group (C=O), leading to a high boiling point.
2. CH\(_3\)CH\(_2\)OH (Ethanol): Ethanol also has strong hydrogen bonding due to the hydroxyl group (-OH), resulting in a relatively high boiling point.
3. CH\(_3\)CH\(_2\)NH\(_2\) (Ethanamine): Ethanamine contains an amine group (-NH\(_2\)) which can form hydrogen bonds, but the hydrogen bonding in amines is generally weaker than in alcohols and acids. Thus, it has a lower boiling point compared to the previous two compounds.
4. CH\(_3\)OCH\(_3\) (Dimethyl ether): Dimethyl ether has weaker London dispersion forces and no hydrogen bonding compared to the alcohols and acids. Therefore, it has the lowest boiling point.
Thus, CH\(_3\)CH\(_2\)NH\(_2\) has the lowest boiling point among the options.
Therefore, the correct answer is (C) CH\(_3\)CH\(_2\)NH\(_2\).
Approach Solution -2
To determine which compound has the lowest boiling point, we need to consider the types of intermolecular forces present in each compound:
1. HCOOH (Formic Acid): Formic acid has strong hydrogen bonding between molecules due to the presence of both an -OH group and a -C=O group. This results in a relatively high boiling point.
2. CH$_3$CH$_2$OH (Ethanol): Ethanol also exhibits hydrogen bonding due to the presence of the -OH group. The hydrogen bonds make ethanol’s boiling point relatively high, but lower than formic acid’s.
3. CH$_3$CH$_2$NH$_2$ (Ethanamine): Ethanamine has hydrogen bonding but not as strong as alcohols or carboxylic acids because the nitrogen atom is less electronegative than oxygen, and the hydrogen bonding is not as effective. This results in a relatively lower boiling point.
4. CH$_3$OCH$_3$ (Dimethyl Ether): Dimethyl ether has weak dipole-dipole interactions and London dispersion forces but lacks hydrogen bonding. This results in a much lower boiling point compared to the other compounds.
Thus, CH$_3$CH$_2$NH$_2$ (ethanamine) has the lowest boiling point because it has weaker intermolecular forces (hydrogen bonding is weaker than in alcohols and acids), making it the correct answer.
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Concepts Used:
Ethers
A class of organic compounds that mostly contain an ether group in which the oxygen atom is bonded to two alkyl or aryl groups are referred to as Ethers. For the same, C-O-C is the functional group.
Chemical Properties of Ether:
Ethers are less polar as well as less reactive, and they do not react with active metals (Na, K), cold dilute acid, oxidizing and reducing agents or other chemicals.
The reason is that they do not have an active functional group.
- Because of the presence of a lone pair on the oxygen atom, ether has a basic nature and behaves like a Lewis base. Formation of oxonium ion occurs when ethers react with cold conc. acid. Ethers, such as BF3, AICI3, and RMgX, form a dative bond with Lewis acid.
- There is a significant difference between the boiling points of ethanol and dimethyl ether. Ethanol has very much higher boiling point of 78 degrees Celsius, whereas dimethyl ether has a negative 25 degrees boiling point. And this explains the molecules' state of matter.
- Ethanol is, of course, a liquid at room temperature and pressure because its boiling point is higher than room temperature. On the other hand, Dimethyl ether already has turned into a gas due to its much lower boiling point.