Ch. 3 - Acids and BasesWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins

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

When to Use Factors Affecting Acidity

Concept #1: Why we need factors affecting acidity and when to use them.

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.

Concept #2: Understanding the Element Effect.

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?

 

Example #1: Without using pKa values, which of the following pairs is more acidic?

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

Example #2: Without using pKa values, which of the following pairs is more acidic?

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

Example #3: Without using pKa values, which of the following pairs is more acidic?

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

Example #4: Without using pKa values, which of the following pairs is more acidic?

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. 

Concept #3: Understanding the Inductive Effect.

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. 

Example #5: Using electron clouds to understand the inductive effect.

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

Concept #4: The 3 factors that determine the strength of inductive effects.

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

Example #6: Without using pKa values, which of the following pairs is more acidic?

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?

Concept #5: Understanding resonance effects. Which of the following –OH groups would be more acidic and why?

Hybridization Effects

Concept #6: Understanding hybridization effects. 

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? 

Example #7: Which of the following hydrocarbons is the most acidic? 

Steric Effects

This rule really only applies with alcohols for now.

Concept #7: Understanding steric effects. 

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?

Example #8: Which of the oxides is the most basic? 

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

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

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

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

Additional Problems
Circle the weakest acid:  
For each pair of compounds below, indicate which is more acidic.
Consider the molecule given below: a) What is its name? b) Give its approximate pKa. c) Draw a structure that is more acidic. d) Draw a structure that is more basic.
The following compound is one of the strongest acids known. Explain using detailed pictures and a minimum of words why it is such a strong acid.  Propose a change to the structure below that would significantly increase the acidity. The molecule you draw should have a different molecular formula.
Circle the most acidic hydrogens in the following molecules. If there are two or more equivalent hydrogens (hydrogens having the same chemical environment) circle ALL of them.
Match the following list of pKa values to the compounds shown below: 16, 19, 25. Your answer should include pictures and a minimum of words to support your choices.
Place the following compounds in order of their basicity with #1 being the most basic and #4 being the least basic.
What is the order of acidity of the indicated hydrogen atoms in this molecule? Rank from most acidic to least acidic? a) b > a > c b) c > b > a c) c > a > b d) b > c > a
Which of the indicated hydrogen atoms is most acidic?  a) a b) b c) c d) d
Rank the following in order of basicity. (1 – least basic, 3 – most basic)
Which is the correct order of decreasing acid strength for these species? (most acidic → least acidic) a) I > II > IV > III b) II > I > III > IV c) III > IV > II > I d) IV > III > I > II  
Which is the order from the strongest acid to the weakest acid for these species? A) II > IV > I > III B) III > I > IV > II C) III > IV > I > II D) II > I > IV > III
Which is an incorrect statement?  
Aluminum, silicon, phosphorus and sulfur are in the same period and are arranged in order of increasing atomic numbers. Which anion is the strongest base?  
Which choice ranks these compounds from strongest to weakest acid? a) I > II > III b) II > III > I c) III > I > II d) III > II > I
What is the strongest base?
Consider molecule A shown below. Remember that for a Lewis structure, you must show all atoms, all bonding valence electrons, all nonbonding valence electrons, and all nonzero formal charges. Using structural considerations, identify which of the two types of Hs (CH or NH) in molecule A is the more acidic one. Provide a clear, concise explanation why one is more acidic than the other.     
Circle the most acidic compound from the following list and explain why.  
For each pair, decide which structure has the most acidic H and circle you answer. Then, give the reason for your choice.   
There are only four constitutional isomers with molecular formula C4H9NO2 that contain a nitro group (–NO2). Three of these isomers have similar pKa values, while the fourth isomer has a much higher pKa value. Draw all four isomers and identify which one has the higher pKa. Explain your choice. 
Rank order all of the following with respect to relative acidity. The acidic H atom in question is indicated with the arrow for each molecule. On the answer sheet on page 1 write the letter corresponding to the correct order of acidity, ranked from most to least acidic for the molecules labeled as (a) - (d).
Rank order the following in terms of anion stability. On the answer sheet on page 1 write the letter corresponding to the correct order of anion stability, ranked from most to least stable for the molecules labeled as (a) - (e).
Arrange the following compounds in order of decreasing acidity: (b) CH3CH2CH2OH, CH3CH2CO2H, CH3CHClCO2H
Using inductive effect and resonance considerations, determine which molecule is most acidic:  
In Section 3.4, we learned four factors (ARIO) for comparing the relative acidity of compounds. When two of these factors are in competition, the order of priority is the order in which these factors were covered (“atom” being the most important factor and “orbital” being the least important). However, we also mentioned that there are exceptions to this order of priority. One such exception was covered at the end of Section 3.4. Compare the two compounds below and determine if they constitute another exception. Justify your choice:
The free-energy change, ΔG°, for the ionization of acid HA is 21 kJ mol-1 ; for acid HB it is -21 kJ mol-1 . Which is the stronger acid?  
Rank order the following in terms of anion stability. Write the letter corresponding to the correct order of anion stability, ranked from most to least stable for the molecules labeled as (a) - (d).
The pKa of the most acidic CH 2 group in each of the following compounds was measured in DMSO as solvent (J. Org. Chem. 1981, 46, 4327–4331).
In each of the four structures, circle the more acidic hydrogen and determine the effect responsible for such observation
Draw as many resonance structures as you can for the following compounds (Do not draw the same resonance structure twice). Circle the weaker base, and briefly justify your answer.  
For each of the pairs, circle the  more acidic hydrogen and determine the effect responsible for such observation:
The pKa of ethane is approximately 50; while, the pK a of acetone (CH3COCH3) is approximately 20. Using complete sentences, explain why acetone is 1030 times more acidic. (A correct answer MUST include drawn structures.)
Select the correct ranking of the compounds shown according to their acidity."A > B" means A is more acidic than B.a) 1 > 2 > 3 > 4b) 1 > 3 > 4 > 2c) 1 > 3 > 2 > 4d) 2 > 4 > 3 > 1e) none of the above  
Rank the following molecules in order of acidity from most acid to least acidic.
List the bases you gave as answers to Exercise 3.18 in order of decreasing basicity.
List the acids you gave as answers to Exercise 3.20 in order of decreasing acidity.
In each of the four structures, circle the  MORE ACIDIC  hydrogen and determine the effect responsible for such observation:
For each of the pairs, cicle the MORE ACIDIC hydrogen and determine the effect responsible for such observation:
Which of the following is the weakest acid and which is the strongest acid? a. 1 is the weakest, 2 is the strongest b. 2 is the weakest, 1 is the strongest c. 3 is the weakest, 2 is the strongest d. 1 is the weakest, 3 is the strongest e. 2 is the weakest, 3 is the strongest 
Part ARank the following compounds from most acidic to least.Rank from most acidic to least acidic. To rank items as equivalent, overlap them. 
Which is a stronger base? 
Which is a stronger base?
Which is a stronger base?
Which of the following alcohols is least acidic?