. The pka values for ethylamine (CH3CH2NH2), ethyl alcohol (CH3CH2OH), water, and hydrogen are 33, 19, 14, and 35 respectively. List the following bases in order of increasing basicity and thoroughly explain your reasoning (15 pts): LIOH LIOCH2CH3 LINHCH2CH3 LIH
The Correct Answer and Explanation is
Sure! Let’s analyze the bases LiOH, LiOCH2CH3, LiNHCH2CH3, and LiH and order them by increasing basicity based on their pKa values and chemical reasoning.
Given pKa values (acid form):
- Ethylamine (CH3CH2NH2) ≈ 33
- Ethyl alcohol (CH3CH2OH) ≈ 19
- Water (H2O) ≈ 14
- Hydrogen (H2) ≈ 35
Step 1: Understand the relationship between pKa and basicity
- The pKa given corresponds to the acid form of each conjugate base.
- The lower the pKa of the acid, the stronger the acid.
- The conjugate base of a strong acid is a weak base.
- Conversely, the higher the pKa of the acid, the weaker the acid and the stronger the conjugate base (base is more basic).
So, basicity order is the reverse of acid strength (or pKa order):
Higher pKa acid → Stronger conjugate base → More basic
Step 2: Identify the conjugate acid of each base
- LiOH → conjugate acid is H2O (pKa 14)
- LiOCH2CH3 → conjugate acid is CH3CH2OH (ethyl alcohol, pKa 19)
- LiNHCH2CH3 → conjugate acid is CH3CH2NH2 (ethylamine, pKa 33)
- LiH → conjugate acid is H2 (hydrogen gas, pKa 35)
Step 3: Order of conjugate acid pKa (and base strength)
| Base | Conjugate Acid | pKa (acid) | Basicity (qualitative) |
|---|---|---|---|
| LiOH | H2O | 14 | Weakest base |
| LiOCH2CH3 | CH3CH2OH | 19 | More basic than LiOH |
| LiNHCH2CH3 | CH3CH2NH2 | 33 | More basic than LiOCH2CH3 |
| LiH | H2 | 35 | Strongest base |
Final order of increasing basicity:
LiOH < LiOCH2CH3 < LiNHCH2CH3 < LiH
Basicity is determined by the ability of a species to accept protons (H+). The strength of a base can be evaluated by looking at the pKa values of its conjugate acid: the higher the pKa of the acid, the weaker the acid, and consequently the stronger the conjugate base.
Here, the bases are the lithium salts of hydroxide (LiOH), ethoxide (LiOCH2CH3), ethylamine (LiNHCH2CH3), and hydride (LiH). Their conjugate acids are water, ethanol, ethylamine, and hydrogen gas respectively.
Water has a relatively low pKa (~14), making it a moderately strong acid, so its conjugate base, hydroxide (OH⁻), is relatively weak compared to others on the list. Ethanol is less acidic (pKa ~19), so the ethoxide ion is more basic than hydroxide. Ethylamine is a much weaker acid (pKa ~33), thus its conjugate base, the ethylamide ion, is significantly more basic. Finally, molecular hydrogen has a very high pKa (~35), which means it is an extremely weak acid. Its conjugate base, hydride (H⁻), is therefore the strongest base in the list.
This trend reflects the stability of the conjugate bases: the more stable the conjugate base, the less basic it is. Hydroxide and ethoxide ions are stabilized by electronegative oxygen atoms and resonance. Amide ions are less stabilized due to the presence of nitrogen, which is less electronegative than oxygen. Hydride ion is the least stabilized (very high charge density on hydrogen) and thus the most reactive and basic.
In summary, the increasing order of basicity matches the increasing pKa of the conjugate acid, confirming that LiOH < LiOCH2CH3 < LiNHCH2CH3 < LiH.
