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. KOH KOCH2CH3 NaNHCH2CH3 LiH
The Correct Answer and Explanation is:
A. LiH < NaNHCH2CH3 < KOCH2CH3 < KOH
B. NaNHCH2CH3 < KOCH2CH3 < LiH < KOH
C. KOH < KOCH2CH3 < NaNHCH2CH3 < LiH
D. KOCH2CH3 < KOH < Li
Given:
- pKa values:
- Ethylamine (CH3CH2NH2): 33
- Ethyl alcohol (CH3CH2OH): 19
- Water (H2O): 14
- Hydrogen (H2): 35
- Bases to be ranked by increasing basicity:
- KOH (potassium hydroxide)
- KOCH2CH3 (potassium ethoxide)
- NaNHCH2CH3 (sodium ethylamide)
- LiH (lithium hydride)
Step 1: Understand what pKa means here
The pKa values given correspond to the conjugate acids of the bases mentioned:
- For KOH, the conjugate acid is H2O (pKa 14).
- For KOCH2CH3 (potassium ethoxide), the conjugate acid is ethyl alcohol (pKa 19).
- For NaNHCH2CH3 (sodium ethylamide), the conjugate acid is ethylamine (pKa 33).
- For LiH, the conjugate acid is hydrogen gas H2 (pKa 35).
Step 2: Relationship between conjugate acid pKa and base strength
- The stronger the base, the weaker its conjugate acid (higher pKa).
- So, higher pKa of the conjugate acid → stronger base.
Using this, the order of base strength from weakest to strongest is:
- KOH (conjugate acid H2O, pKa 14)
- KOCH2CH3 (conjugate acid ethyl alcohol, pKa 19)
- NaNHCH2CH3 (conjugate acid ethylamine, pKa 33)
- LiH (conjugate acid hydrogen gas, pKa 35)
Step 3: Final order
Increasing basicity:
KOH < KOCH2CH3 < NaNHCH2CH3 < LiH
Step 4: Match with the options
Option C: KOH < KOCH2CH3 < NaNHCH2CH3 < LiH — This is correct.
The basicity of a base is related to the acidity of its conjugate acid. Specifically, the weaker the conjugate acid (indicated by a higher pKa), the stronger the base. This is because a base’s ability to accept protons is inversely proportional to the tendency of its conjugate acid to lose protons.
Here, we are given pKa values of the conjugate acids corresponding to four bases:
- For KOH, the conjugate acid is water (H2O) with a pKa of 14.
- For potassium ethoxide (KOCH2CH3), the conjugate acid is ethanol with a pKa of 19.
- For sodium ethylamide (NaNHCH2CH3), the conjugate acid is ethylamine with a pKa of 33.
- For lithium hydride (LiH), the conjugate acid is hydrogen gas (H2) with a pKa of 35.
The higher the pKa of the conjugate acid, the less acidic it is, and therefore, its conjugate base is stronger. Applying this principle:
- Water (pKa 14) is more acidic than ethanol (pKa 19), so KOH is a weaker base than KOCH2CH3.
- Ethanol (pKa 19) is more acidic than ethylamine (pKa 33), so KOCH2CH3 is weaker than NaNHCH2CH3.
- Ethylamine (pKa 33) is more acidic than hydrogen gas (pKa 35), so NaNHCH2CH3 is weaker than LiH.
Thus, the bases rank in increasing order of basicity as:
KOH < KOCH2CH3 < NaNHCH2CH3 < LiH
This ordering also aligns well with chemical intuition:
- Hydroxide ion (OH⁻) is a strong base but not the strongest here.
- Ethoxide ion (CH3CH2O⁻) is stronger than hydroxide due to the electron-donating effect of the alkyl group.
- Ethylamide ion (CH3CH2NH⁻) is stronger still, as nitrogen is less electronegative than oxygen, better stabilizing the negative charge.
- Hydride ion (H⁻) is the strongest base here, reflecting the very high pKa of H2, and its great tendency to accept protons.
Answer:
C. KOH < KOCH2CH3 < NaNHCH2CH3 < LiH
