Organic Chemistry

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3. Acids and Bases

Ranking Acidity

Why is this section important? Because not all acid-base reactions are that easy.

When to Use Factors Affecting Acidity

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Why we need factors affecting acidity and when to use them.

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There are two situations in particular that making predicting equilibrium challenging:

  1. You don’t know the pKas of the acids (so how can you tell equilibrium?)
  2. The pKas for the acid and the conjugate acid are both the same (again, what do we do?)

No matter what, we know that the stronger acid will have the more stable conjugate base. Remember, reactivity and stability have that inverse relationship we talked about. 

The Element Effect

This effect describes the way different atoms donate protons. For example C-H vs. N-H. 

It consists of two trends:

  1. Electronegativity (EN) – the stronger the EN, the more stable the conjugate base will be with extra electrons.
  2. Size – The bigger (squishier) the atom, the less the conjugate base “feels” extra electrons.
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concept

Understanding the Element Effect.

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Content

NOTE: This effect can only be used when comparing the way different atoms are attached to hydrogen. If you are comparing O-H vs. another O-H, it won’t work!

Without using pKa values, which of the following pair is more acidic?

 

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example

Using factors affecting acidity to rank acids

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Without using pKa values, which of the following pair is more acidic?

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Using factors affecting acidity to rank acids

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Without using pKa values, which of the following pair is more acidic?

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Using factors affecting acidity to rank acids

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Without using pKa values, which of the following pair is more acidic?

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Using factors affecting acidity to rank acids

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The Inductive Effect

This effect describes the way that electronegative atoms that are NOT CONNECTED to the acidic proton make the conjugate base more stable. 

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concept

Understanding the Inductive Effect.

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Any EN force that helps to pull electrons away from the conjugate is called an inductive effect.  If you can spread out that negative charge over multiple atoms, that base will be more stable. 

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example

Using electron clouds to understand the inductive effect.

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Factors that increase inductive effects:

  1. Strength of EN forces:-F > -Cl > -Br > -I
  2. Number of EN forces:The more the better
  3. Closeness of EN forces:The closer the better
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The 3 factors that determine the strength of inductive effects.

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Without using pKa values, which of the following pairs is more acidic?

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example

Using Inductive Effect to determine acidity

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Resonance Effects

If a conjugate base is able to make a resonance structure, it will be more stable. 

Understanding resonance effects. Which of the following –OH groups would be more acidic and why?

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concept

Understanding Resonance Effects

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Hybridization Effects

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Understanding hybridization effects.

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The higher the %s-character of the conjugate base, the more stable it will be.

  • Recall, %s-character = (s-orbitals)/(total hybrid orbitals) x 100.
  • Aka, sp3  = ¼ = 25% s-character

Which of the following hydrocarbons is the most acidic? 

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Which of the following hydrocarbons is the most acidic?

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Steric Effects

This rule really only applies with alcohols for now.

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Understanding steric effects.

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Particularly with alcohols, the more easily solvated the conjugate base is, the more stable it will be.

  • The smaller the R group, the more acidic the alcohol
  • The bigger the R group, the more basic the alkoxide

Which of the oxides is the most basic?

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example

Which of the oxides is the most basic?

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16
Problem

Would the following reactions go to the right or the left? Draw the products and label ALL species. Provide the full mechanism.

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Problem

Would the following reactions go to the right or the left? Draw the products and label ALL species. Provide the full mechanism.

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Problem

Would the following reactions go to the right or the left? Draw the products and label ALL species. Provide the full mechanism.

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Problem

Would the following reactions go to the right or the left? Draw the products and label ALL species. Provide the full mechanism.