Question

Which compound in each of the following pairs is more basic? (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CONH}_{2}\) (b) \(\mathrm{NaOH}\) or \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{NHCH}_{3}\) or pyridine

Short answer

(a) Ethylamine is more basic; (b) NaOH is more basic; (c) Dimethylamine is more basic.

Step-by-step solution

Understanding Basicity

Basicity is a measure of how readily a molecule can donate an electron pair to a proton (H+). A stronger base will have a higher tendency to accept a proton.

Analyzing Pair (a)

We compare \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{NH}_2\) and \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CONH}_2\). The \(-\mathrm{NH}_2\) group in ethylamine is free to donate its electron pair, making it more basic. In acetamide, the lone pair is involved in resonance between N and the carbonyl group, reducing its ability to act as a base.

Analyzing Pair (b)

We compare \(\mathrm{NaOH}\) and \(\mathrm{CH}_3\mathrm{NH}_2\). \( ext{NaOH}\) is a strong base because it dissociates completely in water, releasing OH- ions, while methylamine is a weak base. Therefore, \( ext{NaOH}\) is more basic.

Analyzing Pair (c)

We compare \( ext{dimethylamine, } \mathrm{CH}_3\mathrm{NHCH}_3\) and pyridine. Dimethylamine has a nitrogen with an unshared pair of electrons not involved in resonance with aromatic systems, making it more available to bond with H+. Pyridine's lone pair participates in the aromatic system, making it a weaker base.

Key concepts

Electron Donation

Electron donation is crucial in determining the basicity of organic compounds. When a molecule donates a pair of electrons to a proton (\(\text{H}^+\)), it acts as a base. The more available these electrons are for donation, the stronger the base. Consider ethylamine (\(\text{CH}_3\text{CH}_2\text{NH}_2\)). The nitrogen atom has lone pair electrons readily available for donation, making it a strong base. This ability to donate electrons strengthens the molecule’s capacity to accept protons.

• Free electrons increase basicity.
• Accessibility of electrons plays a key role.

In contrast, when electrons are tied up, such as in resonance, the molecule becomes less basic. Understanding electron donation helps explain why certain compounds act more aggressively as bases.

Resonance Effects

Resonance effects significantly impact the basicity of a molecule. Resonance occurs when electrons are delocalized across multiple atoms, spreading their effect and stabilizing the molecule. However, this can also reduce the availability of electrons for donation.
In the example of acetamide (\(\text{CH}_3\text{CH}_2\text{CONH}_2\)), the lone pair on nitrogen participates in resonance with the carbonyl group.

• Delocalization stabilizes the molecule.
• Electron sharing reduces electron donation ability.

This interaction makes the nitrogen less able to donate electrons compared to ethylamine, reducing the basicity of acetamide. Recognizing resonance helps in assessing why some compounds are less basic despite having electron pairs.

Amines

Amines are a key class of compounds in organic chemistry defined by having one or more nitrogen atoms bonded to carbon atoms. The lone pair of electrons on nitrogen makes amines typically basic, as they can easily bond with protons.
Types of amines, such as methylamine, dimethylamine, and other alkylamines, differ in their basic strengths based on the availability of these electrons.

• Primary amines have one alkyl group attached.
• Secondary amines have two alkyl groups.
• Tertiary amines have three alkyl groups.

In comparing a secondary amine like dimethylamine with a compound like pyridine, the amine’s nitrogen’s lone pair is more readily available for donation, making it a stronger base. Amines’ basicity makes them essential in reactions involving proton transfer.

Proton Acceptance

Proton acceptance is a foundational concept in understanding basicity. A base's strength relates to its tendency to accept protons. This is influenced by how readily the molecule can donate its lone pair of electrons to the proton.
In the case of sodium hydroxide (\(\text{NaOH}\)) and methylamine (\(\text{CH}_3\text{NH}_2\)), NaOH is a strong base because it completely dissociates in water to produce hydroxide ions (\(\text{OH}^-\)), which are excellent proton acceptors.

• Strong bases dissociate completely in solution.
• Weaker bases partially dissociate.

On the other hand, methylamine is a weaker base as it doesn’t dissociate fully in solution. Understanding proton acceptance helps in predicting how different bases will behave in chemical reactions.