pKa: Videos & Practice Problems

Understanding pKa values is crucial in organic chemistry, as they indicate the strength of acids. Alkanes have the highest pKa (~50), making them the weakest acids, while alkynes are stronger with a pKa of 25. Alpha hydrogens near carbonyls have a significantly lower pKa of 20 due to tautomerization. Water and alcohols have a pKa around 16, serving as a neutral reference. Strong acids, like carboxylic acids (pKa ~5) and HCl (pKa <0), are vital for various reactions. Memorizing these values aids in predicting reactivity and understanding acid-base behavior.

The 12 pKa values you want to memorize (because they are important!).

Understanding the concept of pKa is essential in organic chemistry, as it helps determine the acidity of various molecules. pKa values indicate how well a substance can donate protons (H⁺ ions), with lower pKa values signifying stronger acids. While memorization of specific pKa values is often necessary, grasping the underlying principles can enhance comprehension.

Starting with the least acidic compounds, alkanes, which are sp³ hybridized, have a pKa around 50. This high value reflects their lack of reactivity, as they do not possess dipoles, charges, or double bonds that would facilitate proton donation. Next, alkenes (sp² hybridized) exhibit slightly better acidity with a pKa of 44 due to the presence of double bonds, which increase reactivity.

Amines, characterized by an H atom bonded to nitrogen, have a pKa of 38, indicating they are still relatively weak acids. Diatomic hydrogen (H₂) has a pKa of 35, which is another value to memorize. Alkynes, with sp hybridization, show improved acidity with a pKa of 25, as the triple bond enhances their ability to donate protons.

Alpha hydrogens, which are attached to carbons adjacent to carbonyl groups, have a notably lower pKa of 20. This increased acidity is attributed to tautomerization, a concept that will be explored further in advanced studies. Alcohols and water both have a pKa of approximately 16, serving as a reference point for comparing other acids.

Moving to stronger acids, a positively charged nitrogen with at least one hydrogen has a pKa around 10, indicating significant acidity. Carboxylic acids, which include compounds like acetic acid, have a pKa of about 5. This value is derived from calculations involving their dissociation in solution.

As we approach stronger acids, an oxygen atom with a positive charge and at least one hydrogen has a pKa of around -2, marking it as a very strong acid. Finally, strong acids such as HCl, HBr, and HI are grouped together with pKa values that are negative, indicating their high acidity. While specific negative values may vary, the key takeaway is that these acids are significantly more potent than those with positive pKa values.

In summary, the pKa values of various organic compounds range widely, from alkanes at 50 to strong acids with negative values. Familiarity with these values and the factors influencing acidity will be crucial for success in organic chemistry.

Identifying pKa values

Understanding the acidity of different functional groups is crucial in organic chemistry, particularly when evaluating their pKa values. The pKa is a measure of the strength of an acid; lower pKa values indicate stronger acids. In this context, we will analyze various functional groups and their corresponding pKa values to rank them in order of increasing acidity.

Starting with the first functional group, an alcohol (R-OH) typically has a pKa around 16. In contrast, a carboxylic acid (R-COOH) is a stronger acid with a pKa of approximately 5. In the case of molecule A, the hydrogen attached to the oxygen is part of an alcohol, as it is two carbons away from a carbonyl group, confirming its pKa of about 16.

Next, we examine molecule B, which features a nitrogen atom attached to a hydrogen. If the nitrogen is neutral (an amine), it has a pKa of around 38. However, if the nitrogen carries a positive charge (an ammonium ion), the pKa drops significantly to about 10. Since molecule B is an ammonium derivative, its pKa is approximately 10.

Moving on to molecule C, we find a hydrogen attached to an alpha carbon, which is adjacent to a carbonyl group. This positioning makes the hydrogen unusually acidic, with a pKa of around 20. It is important to note that while both sides of the carbon may appear similar, only the hydrogen directly off the alpha carbon exhibits this unique acidity.

For molecule D, we analyze a carbon-hydrogen bond. The acidity of this bond depends on the hybridization of the carbon atom. An sp³ hybridized carbon (with four groups attached) has a very high pKa of about 50, indicating it is a weak acid. An sp² hybridized carbon has a pKa of around 44, while an sp hybridized carbon has a pKa of approximately 25. In this case, molecule D is sp³ hybridized, confirming its pKa of about 50.

Finally, we consider another carbon-hydrogen bond in molecule E. The hybridization here is sp, as it has only two groups attached, leading to a pKa of about 25.

Now, to rank these functional groups in order of increasing acidity based on their pKa values, we find the following: molecule D is the weakest acid with a pKa of 50, followed by molecule E with a pKa of 25. Next is molecule C with a pKa of 20, then molecule A with a pKa of 16. The strongest acid is molecule B, with a pKa of about 10. This ranking illustrates the relationship between structure and acidity, emphasizing that positive charges often correlate with increased acidity due to their ability to donate protons.

Rank the following organic compounds in the order of increasing pKa.

• A. a < e < d < c < b
• B. d < e < a < c < b
• C. d < a < e < c < b
• D. b < c < e < a < d

Answer: (D)

Rank the following compounds in the order of increasing acidity.

• A. c < d < a < b
• B. d < a < b < c
• C. b < a < d < c

Answer: (B)

Here’s what students ask on this topic:

What is the definition of pKa in organic chemistry?

pKa is a measure of the acidity of a molecule, specifically the negative logarithm of the acid dissociation constant (Ka). It indicates how easily a molecule donates a proton (H⁺). A lower pKa value means a stronger acid, as it more readily donates protons. Conversely, a higher pKa value indicates a weaker acid. Understanding pKa values helps predict the reactivity and stability of molecules in various chemical reactions.

Why is it important to memorize pKa values in organic chemistry?

Memorizing pKa values is crucial because they help predict the behavior of acids and bases in chemical reactions. Knowing these values allows you to determine which molecules will act as acids or bases, understand reaction mechanisms, and predict the outcome of reactions. This knowledge is essential for success in organic chemistry courses, exams like the MCAT, and practical applications in laboratory settings.

What are the pKa values of common functional groups like alkanes, alkenes, and alkynes?

Common functional groups have distinct pKa values: Alkanes have a pKa of around 50, making them very weak acids. Alkenes have a pKa of approximately 44, slightly more acidic than alkanes. Alkynes are more acidic with a pKa of about 25. These values are important to remember as they help in understanding the relative acidity and reactivity of these functional groups in organic reactions.

How does tautomerization affect the pKa of alpha hydrogens near carbonyl groups?

Tautomerization significantly lowers the pKa of alpha hydrogens near carbonyl groups. Normally, an sp³ hybridized C-H bond has a pKa around 50. However, the pKa drops to about 20 for alpha hydrogens due to the stabilization provided by tautomerization. This process involves the shifting of a hydrogen atom and a double bond, creating an enol form that stabilizes the conjugate base, making the alpha hydrogen more acidic.

What is the pKa of water and why is it considered a neutral reference point?

Water has a pKa of approximately 15.7, often rounded to 16 for simplicity. It is considered a neutral reference point because it lies in the middle of the pKa scale, with values lower than 16 indicating stronger acids and values higher indicating weaker acids. This reference helps in comparing the acidity of other molecules and understanding their behavior in aqueous solutions.