MS 901 - Acid-Base Practice

Submitted by Matt on August 6, 2011.

Textbook and Chapter: Carey and Giuliano 8th Ed. (2010), Chapter 1

Keyword: acid strength

Description: This mendel set goes through rules that affect acid and base strength:

Negative charges are best on more electronegative atoms (left to right on periodic table).
Negative charges are best on larger atoms (up to down on periodic table).
Resonance stabilizes charges. The more resonance forms the better.
Neutral compounds are in general more stable than changed ones.

"What makes a compound stable?" problems are key to understand for the MCAT.

Total Problems: 6

Printer-friendly version

Problem # 286

Draw the conjugate base forms of each acid listed below, then rank the acids in order or decreasing acidity (1 = most acidic).

Explain your reasoning.
Solution
Electronegativity increases as you travel from left to right along the periodic table, so the fluorine anion is more stable than a negative oxygen, which is more stable than a negative nitrogen, etc.

Because F^- is the most stable, it is the weakest base, and its conjugate acid (HF) is the most acidic.

Problem Details
MendelSet practice problem # 286 submitted by Matt on June 6, 2011.
Subject: Organic Chemistry
Subject Topic(s): Acid Strength Trends and Conjugate Base Stability
Question Type: Concept (explain why this is so..)
Keyword(s): periodic trends, acid strength
Problem # 288

Draw the conjugate base form of each acid listed below, then rank the acids in order or decreasing acidity (1 = most acidic).

Explain your reasoning.
Solution
Size increases as you go down the periodic table, so iodine is the larger than bromine, which is larger than chlorine, etc.

Because I^- is the largest anion, it is best able to "handle" its negative charge (due to its small charge:size ratio), and so is the most stable conjugate base, and therefore the weakest conjugate base. So HI is the strongest acid.

Problem Details
MendelSet practice problem # 288 submitted by Matt on June 6, 2011.
Subject: Organic Chemistry
Subject Topic(s): Acid Strength Trends and Conjugate Base Stability
Question Type: Concept (explain why this is so..)
Keyword(s): resonance, acid strength
Problem # 303

Draw the conjugate base form of each acid listed below, then rank the acids in order or decreasing acidity (1 = most acidic).

Explain your reasoning.
Solution
Perchlorate (ClO₄^-) has the most resonance forms and therefore has the most electron delocalization, so it is the most stable and weakest base, which makes its conjugate acid (HClO₄) the strongest acid.

Problem Details
MendelSet practice problem # 303 submitted by Matt on June 7, 2011.
Subject: Organic Chemistry
Subject Topic(s): Acid Strength Trends and Conjugate Base Stability
Question Type: Concept (explain why this is so..)
Keyword(s): resonance, acid strength
Problem # 304

Draw the conjugate base form of each acid listed below, then rank the acids in order or decreasing acidity (1 = most acidic).

Explain your reasoning.
Solution
The ammonium species is the most acidic because its conjugate base is the most stable (because its neutral).

Neutral amines aren't acidic because their conjugate bases are very unstable- a nitrogen with a negative formal charge is a very strong base.

Problem Details
MendelSet practice problem # 304 submitted by Matt on June 7, 2011.
Subject: Organic Chemistry
Subject Topic(s): Acid Strength Trends and Conjugate Base Stability
Question Type: Concept (explain why this is so..)
Keyword(s): acid strength
Problem # 305

Rank each group of acids in order of decreasing acidity. (1 = most acidic)

Explain your reasoning. You will have to use more than one rule in your explanation (resonance, electronegativity, atomic radius, etc.).
Solution
Phenol is more acidic than cyclohexanol because the conjugate base of phenol (phenolate) has resonance while the conjugate base of cyclohexanol does not.

Thiophenol is more acidic than phenol because sulfur is larger than oxygen, and so RS^- is more stable than RO^-.

Problem Details
MendelSet practice problem # 305 submitted by Matt on June 7, 2011.
Subject: Organic Chemistry
Subject Topic(s): Acid Strength Trends and Conjugate Base Stability, Phenols (Claisen Rearrangement, Kolbe-Schmitt, Phenol EAS)
Question Type: Concept (explain why this is so..)
Keyword(s): resonance, acid strength
Problem # 306

Rank each group of acids in order of decreasing acidity. (1 = most acidic)

Explain your reasoning. You will have to use more than one rule in your explanation (resonance, electronegativity, atomic radius, etc.).
Solution
Compounds 1 and 2 are more acidic than compound 3 because their conjugate bases have more resonance forms than that of compound 3.

Compound 1 is more acidic than compound 2 because the resonance form of its conjugate base has two oxygen atoms and is more stable than that of compound 2, which has one oxygen and one nitrogen; a negative oxygen atom is more stable than a negative nitrogen atom.

Problem Details
MendelSet practice problem # 306 submitted by Matt on June 7, 2011.
Subject: Organic Chemistry
Subject Topic(s): Acid Strength Trends and Conjugate Base Stability, Carbonyl Alpha-Substitution Reactions (Enols and Enolates, Malonic Ester Syn, Stork)
Question Type: Concept (explain why this is so..)
Keyword(s): resonance, acid strength

Follow MendelSet on Twitter, become a fan on Facebook.

Printer-friendly version

MS 901 - Acid-Base Practice

MS 901 - Acid-Base Practice

Login