Ranking Acidity: Study with Video Lessons, Practice Problems & Examples

Rank these compounds in order of increasing acidity.

A. CH₃CH₃

B. CH₃OH

C. CH₃NH₂

D. CH₃SH

Rank these compounds from the least acidic to the most acidic.

FCH₂COOH, BrCH₂COOH, ClCH₂COOH

Rank these compounds in order of increasing acidity.

A. CH₃CH₂CH₂CH₂OH

B. CH₃CH₂CH═CHCH₂OH

C. CH₃CH₂C≡CCH₂OH

Predict the most reactive protonation site in each of the following compounds.

Determine and rationalize which will have a higher pK_a value between the two hydroxyl groups in the following structure.

Identify the stronger acid in each of the following pairs.

Identify the anion that would more readily react with H⁺.

There are four compounds given in increasing order of acidity. The most acidic hydrogen in each of these structures is circled. Explain why each of these compounds is more acidic than the preceding one.

Explain the relative acidities of the following conjugate acids.

Arrange the indicated hydrogens in these molecules in decreasing order of acidity.

Consider the following acid-base reactions:
I) ClCH₂COOH + Cl₃CCOONa ⇌
II) CH₃CH₂OH + HNO₃ ⇌
III) CH₃OH + H⁻ ⇌
Indicate the conjugate acids and bases in your answers and determine the direction of equilibrium. Try to determine reaction direction without the use of the pKa values. You can find the pKa values on the internet if required.

Compare and arrange these compounds in increasing order of acidity. Provide a suitable reason for your answer.
CH₃CH₂OH, HCOOH, HCl, HBr, CH₄, and CH₃CH(Cl)COOH

Explain why FCH₂COOH is more acidic than ICH₂COOH if HF is less acidic than HI.

Which of the compounds shown below is more acidic?

Between H_a and H_b (boxed in the structure), which is expected to have a lower pK_a?

Consider the following acid-base reaction and answer these questions:

(a) Which is stronger, the acid or the conjugate acid?

(b) Would the K_eq be less than, equal to, or greater than one?

(c) Which side is favored, the reactants or the products?

Rank these compounds from the least acidic to the most acidic.

What are the product(s) (if a reaction takes place) for the following reactions?

a) Acetic acid + sodium methoxide

b) Phenol + sodium formate

Identify the stronger base in each of the pairs given below.

(i) Pick out the most stable conjugate base among the following.

(ii) Which structural feature(s) did you analyze?

Which of the indicated oxygen atoms is the most basic in the following structures?

(i) Identify the stronger acid among the following pair.

(ii) Which structural feature(s) did you analyze?

(Hint: draw the conjugate base first)

Which of the following compounds is more acidic? Why?

(Note: The most acidic proton for each compound is encircled.)

Arrange the given compounds in increasing order of acidity.

Rank the following compounds in increasing order of acidity. Provide reasoning for your answer.

Rank these compounds from the least acidic to the most acidic.

Identify the substituents in each of the following compounds as inductively electron-withdrawing, electron-donating by hyperconjugation, electron-withdrawing by resonance, or electron-donating by resonance.

a. —Cl

b. —CH₃

c. —CHO

Explain the large difference in the p K_a values of acetic acid (pK_a = 4.8) and trifluoroacetic acid (pK_a = 0.2).

Why aniline is a stronger acid compared to cyclohexylamine?

(i) Identify the most stable conjugate base in the following pair.

(ii) Which structural feature(s) did you analyze?

Rank the following compounds in order of decreasing acidity.

Imidazole has two nitrogen atoms. Determine which nitrogen is more basic and which one is less basic in its structure.

Identify the stronger base in each pair.

Explain why the hydroxy proton of kojic acid at carbon-5 is more readily lost than the hydroxy proton at carbon-7.

Identify the more basic member among each pair of the compounds given below.

Observe the increasing order of acidity of the following compounds.

a. What effect does bromine, an electronegative atom, have on the acidity of a carboxylic acid?

b. How does the substituent position influence the acidity of a carboxylic acid?

(i) Pick out the stronger acid among the following.

(ii) Which structural feature(s) you analyzed?

(Hint: draw the conjugate base first)

Ammonia (NH₃), methanol (CH₃OH), and propanoic acid (CH₃CH₂COOH) can act as either acids or bases, depending on the conditions.

Arrange the three compounds from the least basic to the most basic. Additionally, write a reaction for each compound reacting with a generic acid (HA).

Identify which molecule loses the indicated proton more readily.

Rank the following anions in order of increasing basicity.

A. CH₃O⁻ 

B. Cl⁻

C. CH₃NH⁻

D. CH₃CH₂⁻

Identify the stronger acid in each pair.

a. (i) CH₃CH₂OH or (ii) CH₃CH₂NH₂

b. (i) CH₃CH₂OH or (ii) CH₃CH₂SH

Why is the conjugate acid of piperazine more acidic than the conjugate acid of piperidine?

Consider the following acid-base reaction and answer these questions:

(a) Which is stronger, the base or the conjugate base?

(b) Which side is favored, the reactants or the products?

(c) Would the K_eq be less than, equal to, or greater than one?

Which one of the given molecules is expected to have a greater K_a? Which one is a stronger acid?

Why is the pK_a of the α-hydrogen of N,N-dimethylacetoacetamide higher than the pK_a of the α-hydrogen of ethyl acetoacetate?

Consider the following nitrogenous compounds. Rank these compounds in order of increasing basic strength. Use the concept of hybridization. Additionally, rank their conjugate acids in order of decreasing acidic strength.

The following molecule has two nitrogen atoms. Determine which of these nitrogens would be protonated preferentially.

Identify the stronger acid in each pair.

a. (i) CH₃CH₂CH₂OCH₂CH₂OH or (ii) CH₃CH₂OCH₂CH₂CH₂OH 

b. 

Explain why 2,4-dinitrophenol (pK_a = 4.0) is a stronger acid than 3,5-dinitrophenol (pK_a = 6.7).

Identify the stronger acid in each of the following pairs.

Rank the following acids from the strongest to the weakest acid. Provide a brief explanation for your answer.

Consider the reaction of the given compound with a strong base: 

Which labeled proton is expected to be removed first?

Identify the following substituents as inductively electron-withdrawing, electron-donating by resonance, electron-withdrawing by resonance, or electron-donating by hyperconjugation.

a. —CH(CH₃)₂

b. —CH₂COOCH₃

c. —OH

Rank the following anions in order of increasing basicity.

A. CH₃CH₂CH₂⁻

B. CH₃C≡C⁻

C. CH₃CH=CH⁻

Identify the acid that would more readily donate a proton to a base.

Ammonia (NH₃), methanol (CH₃OH), and propanoic acid (CH₃CH₂COOH) can act as either acids or bases, depending on the conditions.

Arrange the three compounds from the least acidic to the most acidic. Additionally, write a reaction for each compound reacting with a generic base (B⁻).

A negative charge on an adjacent carbon atom can be stabilized by a nitrile group (-CN) through resonance. Of the three hydroxybenzonitrile derivatives shown below, one is only slightly more acidic than phenol, while the other two are much more acidic than phenol. Use resonance structures of the appropriate phenoxide ions to explain why two of the ions are unusually stable.

The following diagram shows two pairs of organic bases. For each pair, determine which compound is a stronger base and draw their conjugate acids. Additionally, determine which conjugate acid in a pair is stronger.

Rank the following compounds in the order from strongest to weakest acid.

CH₃OH
CCl₃CH₂COOH
CCl₃COOH
CH₃CH₂COOH

Predict the most reactive protonation site in each of the following compounds.

Identify the stronger base in each pair.

Identify the stronger base in each pair.

Below are the pK_a values of 2-iodobenzoic acid, 3-iodobenzoic acid, and 4-iodobenzoic acid.

Explain why the ortho-substituted benzoic acid is the most acidic, while the para-substituted is the least acidic.

An ascorbic acid derivative is slightly more acidic than propanoic acid, even though the derivative lacks the carboxylic acid group. Compare the conjugate base of propanoic acid with the most stable conjugate base of the ascorbic acid derivative and suggest why these two compounds have similar acidities.

Rank the following compounds in order of decreasing acidity.

Determine the more acidic molecule in each of the given pairs of isomers. Provide a suitable reason for your answer.

(a)      
                                                         

    (b)

Protonation can take place on any of the three nitrogen atoms of the following compound. However, one of the three nitrogen atoms is much more basic. Identify the most basic nitrogen atom in the compound.

Identify the stronger acid in the given pair.

Why is m-methoxyphenol (pK_a = 9.65) more acidic than p-methoxyphenol (pK_a = 10.2)?

Which one is the most basic compound in the following pair? Explain why.

Discuss the relative acidities of the following alcohols.

(Most acidic) a. CH₃C≡CCH₂OH > b. CH₃CH═CHCH₂OH > c. CH₃CH₂CH₂CH₂OH (Least acidic)

Which of the two compounds, HI and HBr, is more acidic?

Which of the following compounds is expected to have higher acidity?

True or False. The expected order of the pK_a values of the carboxyl groups of aspartic acid, valine, and glycine corresponds to the following. 

Arrange the following amines in increasing order of their basicity.

Guanidine (HNC(NH₂)₂) is an organic base that is used to produce hair relaxers. Draw a conjugate acid for guanidine. Do you anticipate guanidine to be a weak base or a strong base?

What prevents the protonation of the encircled nitrogen at physiological pH?

Which of the encircled atoms is more basic?

Arrange the following alcohols in decreasing order based on their pK_a values. Explain.

For the given acid-base reaction:

(i) Is the reactant base or conjugate base stronger?

(ii) Identify the side of the reaction that is favored.

(iii) Is the K_eq less than or greater than 1?

Choose the more stable base between the given pair of ions.

Identify the stronger base in the pair below. Provide a brief explanation without the pK_a values.

The molecule shown below is a heteroatomic base. Which of the two nitrogens is more basic?

Consider the following pair:

Without referring to the pK_a values, choose the least reactive base. Explain your answer.

Consider the following acid-base reaction:

(i) Is the reactant acid stronger than the conjugate acid?

(ii) Determine which direction of the reaction (forward or reverse) is more favored.

(iii) Will the equilibrium constant (K_eq) be less than, equal to, or greater than 1?

Rank the following alkenes in order of decreasing rate of protonation.

Note: Only focus on basicity at the double bonds ignoring any other sites that may be more basic.

Which in the given pair of ions would react with H⁺ most quickly?

Determine the acid you would expect to donate a proton more readily to a basic compound.

Identify the more stable conjugate base in the pair of structures shown below. Which structural feature did you analyze to answer the question?

Which is a more stable conjugate base in the following pair of structures? Provide the basis for your answer.

(i) Which of the two molecules below has a more acidic proton?

(ii) Which structural feature did you analyze for your answer?

[Hint: Draw the structure of the conjugate bases.]

(i) Which has a more acidic proton in the pair of molecules shown below?

(ii) What structural feature did you analyze to answer the question?

[Hint: Start by drawing the conjugate bases.]

Choose the more stable base between the given compounds.

H₂O and H₂S

Shown below are the structures of drugs and the diseases they treat. Predict whether the activity of these drugs will increase, decrease, or not be affected when taken with medicine for acid reflux diseases.

Provide an explanation for the following: A base can deprotonate the α-carbon of methyl propionate but not the α-carbons of propionic acid and propionamide.

Arrange compounds X, Y, and Z in order of decreasing ease of proton removal from the methyl substituent.

Malic acid is a compound with two —COOH groups. Why is its first pK_a lower than the pK_a of acetic acid? (The first pK_a of malic acid is for the —COOH group adjacent to the —OH group.)

One of the given four amino acid side chains can deprotonate the ɑ-carbon of an aldehyde. Which one is it?

Explain why the pK_a of 2,2,2-trifluoroethanol (12.4) is lower than the pK_a of ethanol (16.0).

The structure shown below has four possible acidic groups. (Note that phosphorus is a period 3 element, hence it can expand its octet and have more than four bonds.)

(i) Which group is the strongest acid? Justify your answer.

(ii) Which group is the weakest acid?

Consider the following compounds: 

Explain the relative acidities.

Are the pK_a values of alanylalanine expected to be higher or lower than the pK_a values of alanine (pK_a1 = 2.34 and pK_a2 = 9.69)?

Which of the following statements is true?

(i) The 4-methylbenzenaminium ion has a greater K_a than the 4-methylcyclohexan-1-aminium ion. 

(ii) 4-methylcyclohexan-1-amine is a stronger base than p-toluidine.

True or False: When 1,3-dimethylguanidine is protonated, its double-bonded nitrogen is the nitrogen that accepts the proton because the lone pair electrons on the other nitrogens are delocalized.

Determine which among the given molecules is the strongest base.

The pK_a values of ortho-, meta-, and para-methoxybenzoic acids are shown below:

The relative pK_a values depend on the substituent. The ortho isomer is the most acidic, and the para isomer is the least acidic. Explain these relative acidities.

Examine the compounds below with their pK_a values. Show the conjugate base of each compound and rationalize why the acidity increases with increasing nitro substituents.

(i) CH₃CH₃, pK_a ≈ 50

(ii) CH₃CH₂NO₂, pK_a = 8.98

(iii) CH₃CH(NO₂)₂, pK_a = 5.50

Identify the more acidic isomer and illustrate why it is more acidic by removing a proton at the indicated atom and showing resonance forms (if applicable)

Identify the more acidic member of each pair and illustrate why it is more acidic by removing a proton at the indicated atom and showing resonance forms (if applicable).

Consider the molecule shown below:

(i) Draw the four different conjugate bases that can be formed from deprotonation of each -OH group.

(ii) Identify the most acidic -OH group in the molecule.

Between each pair, which is more basic? Explain.

Choose the more basic of the two structures below. Why?

Rank the conjugate acids of the following bases from the most stable to the least stable.

(i) NH₃, (ii) CH₃CH₂CH₂OH, (iii) KOH, (iv) CH₃CH₂CONH₂

Consider the listed properties of ethanol and 2,2,2-trifluoroethanol below and answer the following questions.

(i) Compound 2 has a molecular weight that is more than twice that of compound 1, but compound 2 has a slightly lower boiling point. Why?

(ii) Despite three electronegative fluorine atoms, the dipole moment of compound 2 is lower than that of compound 1. Explain why.

(iii) Compound 2 is a stronger acid than compound 1. Why is this the case?

A pair of compounds is shown below. When treated with a base, compound (b) reacts more quickly than compound (a). Explain the enhanced acidity of compound (b) compared to compound (a).

Explain, using the resonance forms of the conjugate bases, why benzenesulfonic acid (C₆H₅SO₃H) exhibits a higher acidity than benzoic acid (C₆H₅COOH).

Predict the structure of the anion formed by the abstraction of a proton from the following compound using NaOH. Also, draw the important resonance forms involved in the stabilization of this anion.

Determine the stronger base between (i) and (ii). Rationalize your choice using the concept of element effect.

Choose the stronger base between 1 and 2. Justify your choice without the use of p  K_a values.

Choose the stronger acid between cyclohexylmethanol and cyclomethanethiol. Explain your answer in terms of element effects. 

Without knowing the pK_a values, identify the least basic structure among the given pair. Justify your choice.

Without knowing the pK_a values of the following structures, identify the least reactive base among them. Justify your choice.

Which of these anions is more basic?

F⁻ or I⁻

Without knowing the pK_a values, identify the more acidic member among the following pair. Justify your choice.

Without knowing the pK_a values of the following structures, identify the more reactive base among them. Justify your choice.

4-(3-(cyclopentylamino)-2-hydroxypropoxy)benzamide is an analog of a beta-blocker that is used to treat high blood pressure. Which nitrogen atom in 4-(3-(cyclopentylamino)-2-hydroxypropoxy) is the most basic?

Acetic acid, with a pK_a value of 4.74, is approximately 15 times more acidic compared with acetone, with a pK_a value of 20. Why?