# Homework Help: Acidic strength

1. Mar 25, 2015

### gracy

1. The problem statement, all variables and given/known data

tetrahydrothiophene is stronger acid than tetrahydrofuran
2. Relevant equations
Acidic strength depends on stability of conjugate base.And stability of conjugate base in turn depends on
following factors
resonance,inductive effect,hybridization,atomic size i.e electron density and electronegativity.

3. The attempt at a solution
As electronegativity of oxygen is greater than sulphur ,tetrahydrofuran should give more stable conjugate base after removal of acidic hydrogen.,But my teacher says electron density plays bigger role in determining stability of conjugate base than electronegativity .So,sulphur in tetrahydrothiophene is bigger than oxygen in size that's why it will have less electron density that's why will give more stable conjugate base .Hence tetrahydrothiophene more stable than tetrahydrofuran.Right?

Last edited: Mar 25, 2015
2. Mar 25, 2015

### Ygggdrasil

A few issues here:
1) Do you expect these compounds to be acidic? Which hydrogen atom is lost when you deprotonate thf and tht?

2) When the atoms you are comparing reside on different rows of the periodic table, atomic size is indeed more important than electronegativity when determining the stability of a conjugate base. As you correctly state, electron and charge density is the main factor.

Sulfur is larger than oxygen. Given that they have the same number of valence electrons, which will have the greater electron density?

3. Mar 25, 2015

### gracy

I just corrected myself.

4. Mar 25, 2015

### Ygggdrasil

Ok, now that section is correct. But I'm not still clear about what you're measuring. Are you trying to look at the ability of thf and tht to donate protons (their acidity) or their ability to accept protons (their basicity). Which do you think would be more likely, the deprotonation of these molecules (what would the conjugate base look like?) or the protonation of these molecules (what would the conjugate acid look like?)?

5. Mar 25, 2015

### gracy

I think
. The problem statement, all variables and given/known data

tetrahydrothiophene is stronger acid than tetrahydrofuran
2. Relevant equations
Acidic strength depends on stability of conjugate base.And stability of conjugate base in turn depends on
following factors
resonance,inductive effect,hybridization,atomic size i.e electron density and electronegativity.

3. The attempt at a solution

As electronegativity of oxygen is greater than sulphur ,tetrahydrofuran should give more stable conjugate base ..,But my teacher says electron density plays bigger role in determining stability of conjugate base than electronegativity .So,sulphur in tetrahydrothiophene is bigger than oxygen in size that's why it will have less electron density So it can accept electron(which is tendency of an acid)without any electron repulsion better than tetrahydrofuran which will have less tendency to accept electron because of electronic repulsion due to greater electronic density. .Hence tetrahydrothiophene is better acid than tetrahydrofuran.Right?

6. Mar 25, 2015

### gracy

Please answer.Is my post 5 correct?

7. Mar 25, 2015

### Ygggdrasil

If tetrahydrofuran is acting as an acid, the oxygen is not the atom accepting a pair of electrons/donating a proton. It may be helpful to draw out the conjugate base you expect from the deprotonation of tetrahydrofuran.

Now, tetrahydrofuran is acting as a base, the oxygen atom is the atom donating the pair of electrons/accepting the proton.

Can you double check the question to see whether it is asking about the acidity of THF and THT or their basicity? Both are extremely poor acids, so it would make more sense to talk about their ability to act as bases.

8. Mar 25, 2015

### gracy

Here is the video

but the audio is not clear and his accent is hard to understand.But you just look at only one instant at time 4:34

9. Mar 25, 2015

### Ygggdrasil

So if tetrahydrofuran is acting as an acid, where does the proton come from, and what does the conjugate base look like?

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