Which of the following is the strongest base?
- $\ce{LiOH}$
- $\ce{CH3Li}$
- $\ce{LiNH2}$
- $\ce{LiF}$
According to me the answer should be $\ce{LiF}$ as it has max number of lone pairs but the answer says its 2). I am confused.
Answer
In order of decreasing basicity they are ranked $\ce{LiCH3}$ > $\ce{LiNH2}$ > $\ce{LiOH}$ > $\ce{LiF}$. We are dealing with the ions $\ce{F-}$, $\ce{OH-}$, $\ce{NH2-}$ and $\ce{CH3-}$ and so the factor which influences their basicity is the stability of the ion compared to its conjugate acid.
The stability of the ions is determined by the energy of the lone pairs. Carbon has the least effective nuclear charge ($Z_{\text{eff}}$) and so the lone pair on the $\ce{CH3-}$ ion has the highest energy, making it the least stable ion and so the strongest base. The $Z_{\text{eff}}$ increases as we move across the period until we get to fluorine which has the greatest $Z_{\text{eff}}$ and so its lone pairs have the lowest energy, making $\ce{F-}$ the least basic ion.
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