Problem 101 - Chemistry The Central Science

Question

Triethylamine, \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{3} \mathrm{~N},\) has a \(\mathrm{p} K_{b}\) value of \(2.99 .\) Is triethylamine a stronger base than ammonia, \(\mathrm{NH}_{3} ?\)

Step-by-Step Solution

Answer

Yes, triethylamine is a stronger base than ammonia, as its \(\mathrm{p}K_b\) value (2.99) is lower than ammonia's \(\mathrm{p}K_b\) value (4.75).

Step 1: Compare \(\mathrm{p}K_b\) Values

Compare the \(\mathrm{p}K_b\) values of triethylamine and ammonia: Triethylamine: \(\mathrm{p}K_b = 2.99\) Ammonia: \(\mathrm{p}K_b = 4.75\)

Step 2: Determine Which Base is Stronger

Since the \(\mathrm{p}K_b\) of triethylamine (2.99) is lower than the \(\mathrm{p}K_b\) of ammonia (4.75), triethylamine is a stronger base than ammonia.

Key Concepts

Understanding pKb Values

When it comes to analyzing the strength of bases in chemistry, one essential tool is understanding pK_b values. The pK_b value is a numeric scale used to specify the strength of a base. It is closely related to the pK_a value, which is used for acids. The lower the pK_b value, the stronger the base.

But why is this the case? The pK_b value is logarithmically linked to the base dissociation constant (K_b), which provides a measure of how readily a base forms its conjugate acid by accepting a proton in water. Mathematically, it is expressed as pK_b = -log(K_b). The negative logarithm means that as K_b increases (indicating a stronger base), the resulting pK_b decreases. Therefore, bases with lower pK_b values are considered stronger because they are more effective at accepting protons.

Base Strength Determination

To determine the relative strength of a base, comparing pK_b values is a convenient method. As seen in the exercise, triethylamine has a pK_b of 2.99, while ammonia has a pK_b of 4.75. Since triethylamine's value is lower, it indicates a stronger base. However, understanding the concept in depth requires recognizing that the base dissociation constant (K_b) is the true descriptor of base strength and the pK_b value is simply a more approachable representation of this constant.

In the aqueous solutions where base strength is typically measured, a stronger base will be more likely to accept a proton, thus shifting the equilibrium towards the conjugate acid. This results in a higher K_b and consequently, a lower pK_b. It’s crucial to remember that this measurement is relative and depends on the solvent and the specific base being compared.

Amine Chemistry

Amines are a key class of compounds in organic chemistry, characterized by the presence of a nitrogen atom with a lone pair that can accept a proton, making them bases. Generally speaking, the basicity of an amine is influenced by its alkyl substituents – the groups attached to the nitrogen.

Triethylamine, for instance, has three ethyl groups connected to its nitrogen, while ammonia lacks these alkyl substituents. The presence of alkyl groups in amines such as triethylamine tends to increase the electron density available to the nitrogen, enabling it to more readily accept a proton. This effect is known as alkyl group inductive effect, where the electron-donating nature of the alkyl groups enhances the base strength of the amine. Consequently, triethylamine, with its increased electron density due to the ethyl groups, is a stronger base compared to ammonia, which solely relies on its lone pair of electrons for basicity.