Comparing Basicity: -OCH3, -OH, -NH2, and Alkyle with Negative Charge

[catan]

Homework Statement

-OCH₃, -OH, -NH₂, and the fourth choice is an alkyle, ethyne with a neg charge. the picture shows a triple bond attacked to a hydrogen.

Homework Equations

The Attempt at a Solution

I know that OH is a relatively common base and I thought it was strong, but I have no idea how it compares with the others. I know that alkyles are are less basic than alkenes and alkanes, but the oxygens throw me off a bit. I think that by having O in them it would suggest that their conj acid would be more acidic making them more basic. Is this correct at all? Is there an easy way to determine basicity?

 

[burningbend]

check the pKa of each conjugate acid. whichever has the highest pKa is the strongest base

water = 15.7
alcohols are between 16-18
NH3 is ~35
HCCH is ~25

so NH2- is the strongest base

 

[Blueshift5]

There are a few things to consider when comparing basicity: the strength of the conjugate acid, the stability of the conjugate base, and the inductive effect of substituents. Let's break it down for each of the choices given.

-OCH3: This molecule has a lone pair of electrons on the oxygen, making it a potential base. However, the presence of the methoxy (-OCH3) group actually decreases the basicity of the molecule. This is because the methoxy group is an electron-withdrawing group, which stabilizes the conjugate acid and makes it less likely to donate its proton. Therefore, -OCH3 is a weaker base compared to -OH and -NH2.

-OH: As you mentioned, -OH is a relatively common base and is considered a strong base. This is because the conjugate acid, H2O, is a weak acid, meaning it will readily donate its proton. Additionally, the lone pair of electrons on the oxygen is highly reactive and readily accepts a proton. Overall, -OH is a stronger base compared to -OCH3 and -NH2.

-NH2: This molecule has a lone pair of electrons on the nitrogen, making it a potential base. The conjugate acid, NH3, is a weak acid, so -NH2 is a stronger base compared to -OCH3. However, it is weaker than -OH due to the electronegativity difference between nitrogen and oxygen. Nitrogen is less electronegative than oxygen, so it is less likely to attract the proton and form a stable conjugate acid.

Alkyle with negative charge: This choice is a bit ambiguous, as there are many different alkyl groups that could have a negative charge. However, in general, alkyl groups are less basic than alkenes and alkanes due to the inductive effect of the carbon-hydrogen bonds. The more alkyl groups attached to a negatively charged carbon, the less basic it will be.

In conclusion, the order of basicity for the given choices is -OH > -NH2 > -OCH3 > alkyle with negative charge. However, it is important to note that basicity can also be affected by other factors such as steric hindrance and resonance effects. it is important to consider all of these factors when comparing basicity.