Organic Chemistry : Help with Acid-Base Reactions

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #4 : Mechanisms And Intermediates

For which of the following acid-base reactions will the equilibrium lie on the left side?

Possible Answers:

 

Correct answer:

Explanation:

The pKa value indicates how strong an acid is, and acid strength increases as pKa decreases. The side of a reaction with a lower pKa is going to dissociate more, pushing the equilibrium over to the other side. The equilibrium will thus lie on the side with the HIGHER pKa.

Since the pKa of acetic acid (4.76) is higher than the pKa of trifluoroacetic acid (0), the reaction will shift to the left to reach equilibrium.

Example Question #1 : Help With Acid Base Reactions

A carboxylic acid has a pKa of 5. At a pH of 8, what is the ratio of salt to acid?

Possible Answers:

Correct answer:

Explanation:

Use the Henderson Hasselbalch equation:

Example Question #2 : Help With Acid Base Reactions

Screen shot 2015 07 19 at 10.49.13 am

List the given compounds in order of decreasing basicity.

Possible Answers:

II, I, III, IV

IV, I, III, II

II, III, IV, I

III, II, IV, I

II, III, I, IV

Correct answer:

II, I, III, IV

Explanation:

An easy way to consider relative base strengths is to consider the strength of the compounds' conjugate acids.The stronger the conjugate acid, the weaker the base. Water (compound IV) is the least basic of the compounds because its conjugate acid,  is the strongest of the given compounds' conjugate acids . Carboxylate ions (compound III) are highly stabilized by resonance and predominate at neutral pH. The conjugate carboxylic acids readily donate protons (acetic acid: ). Ammonia (compound I) has considerable basicity; binding a fourth hydrogen produces ammonium ion , which predominates at neutral pH  Sodium propoxide (compound II) is a strong base, bearing a full negative charge on its oxygen. Its conjugate acid, 1-propanol, is a rather weak acid . Since its conjugate acid is the weakest (highest ), sodium propoxide is the strongest base. Based on the previous observations the correct ordering of the compounds is: II, I, III, IV.

 

Example Question #3 : Help With Acid Base Reactions

Screen shot 2015 07 19 at 11.40.05 am

Rank these weak acids by decreasing  (of the expressed hydrogens).

 

Possible Answers:

III, IV, I, II

IV, III, I, II

II, III, I, IV

III, IV, II, I

IV, I, II, III

Correct answer:

III, IV, II, I

Explanation:

The governing principle regarding the prediction of  values (relative to other compounds) is to assess the stability of the product formed by the release of a proton. The release of the alkyne hydrogen in compound III results in a carbanion, a highly unstable species, so it is expected that this compound is the least acidic and has the highest . Intuition serves well in this instance and we see that hydrogens bound to a triple bond have a  value of around 25. The relative stabilities of the remaining compounds may be assessed in the same manner. Compound IV is the second weakest acid  because the three methyl groups donate electron density such that if the oxygen is deprotonated, the resulting negative charge is destabilized. Methanol and water have a unique, non-intuitive relationship regarding their relative acidities. One would assume that water should be a stronger acid than other acids bound to alkyl groups (by the reasoning expressed for compound IV). This is the case for all alcohols except methanol, in which the delocalization of charge allowed by the increased molecular size outweighs the destabilization caused by electron donation. Thus methanol is a slightly stronger acid than water. This is evidenced in their  values: 15.7 for water and 15.5 for methanol. The correct ordering of the given compounds is: III, IV, II, I.

Example Question #4 : Help With Acid Base Reactions

A.Question 2

B.Question 2 acetone

Which of the above molecules is expected to have a more acidic alpha-carbon, and why?

Possible Answers:

Molecule A, because its conjugate base has more stability.

Molecule B, because its conjugate base has more stability.

Molecule B, because its conjugate base has less stability.

Molecule A, because its conjugate base has less stability.

Correct answer:

Molecule B, because its conjugate base has more stability.

Explanation:

Molecule B will have a more acidic alpha-carbon because once the alpha proton becomes dissociated, the conjugate base will have relatively more stability than the conjugate base of molecule A.

When the alpha-carbon on molecule A loses it's proton, the conjugate base is not as stable. The reason for this is because the oxygen that is involved in the ester bond can contribute its electrons towards a resonance structure. Therefore, after the alpha-proton is lost, the alpha-carbon will have a negative charge that will be destabilized by the delocalized negative charge of the resonance structures.

Example Question #5 : Help With Acid Base Reactions

Which of the following is the strongest acid?

Possible Answers:

Correct answer:

Explanation:

From the start, we know we can eliminate answer choice  because it is the only answer choice that is not a strong acid. Now we have three strong acids, but we have to determine which is strongest. To do so, we take the conjugate base of each strong acid to see which conjugate base is the weakest acid. Remember: weaker conjugate base means a stronger acid.  is the largest ion of the bunch. Its large size allows it to better stabilize the negative charge and so it is the weakest (most stable) conjugate base. Because the weakest conjugate base leads to the strongest acid,  is our correct answer.

Example Question #6 : Help With Acid Base Reactions


 

Which of the following sets of bases are listed from most basic to least basic?

Possible Answers:

Correct answer:

Explanation:

The correct ranking from most basic to least basic is:

The best bases are negatively charged, and the worst bases are positively charged (acidic). The stronger the base, the weaker (more stable) it's conjugate acid. An alkane is a very stable conjugate acid, which tells us that is the most basic of the set.

We know based on charge alone, that is more basic than but less basic than or .

We know that is more basic because the electronegativity of  is less than that of . This means the lone pair of electrons on are held less tightly and more likely to pick up a proton.

Example Question #7 : Help With Acid Base Reactions

Which of the following sets of acids are correctly listed from most to least acidic?

Possible Answers:

Correct answer:

Explanation:

Remember that the strongest acids have the weakest conjugate bases.  is more acidic than  because iodine has a larger atomic radius than bromine. , , and  are strong acids and should be at the beginning of the list. Alkanes are not acidic. Acetic acid is a weak acid (pKa =4-5).

Example Question #8 : Help With Acid Base Reactions

Screen shot 2015 09 21 at 12.14.52 pm

Rank the given molecules in order of increasing pKa.

Possible Answers:

III, I, IV, V, II

III, II, V, I, IV

IV, II, V, I, III

I, II, V, IV, III

II, V, IV, I, III

Correct answer:

II, V, IV, I, III

Explanation:

Recall that the stronger an acid, the lower the pKa.

II (two fluorine atoms really close to  has largest inductive effect, so bond is most weakened, and pKa is lowest)

V (one fluorine really close to  has strong inductive effect)

I (one fluorine a little further away from  has weaker inductive effect)

IV (no inductive effect)

III (alcohols are much less acidic than carboxylic acids, and it has the highest pKa of all)

Example Question #9 : Help With Acid Base Reactions

Which side (left or right) of the following reaction is favored and why?

Screen shot 2015 11 09 at 3.36.33 pm

Possible Answers:

Right side, because the  of  is lower than the  of 2-butanol.

Left side, because the  of  is lower than the  of 2-butanol.

Left side, because the  of  is higher than the  of 2-butanol.

Right side, because the  of  is higher than the  of 2-butanol.

Correct answer:

Right side, because the  of  is lower than the  of 2-butanol.

Explanation:

The side of the reaction that is favored will have the acid with the higher , because the reaction goes (strong acid + strong base  weak acid + weak base).

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