You have JavaScript disabled!

MendelSet works best with JavaScript enabled. Please enable JavaScript.

If you are using NoScript or another JavaScript blocker, please add MendelSet.com to your whitelist.

MS 4526 - Chem 420 Unit 3 Homework

MS 4526 - Chem 420 Unit 3 Homework

Submitted by brucezenner on January 29, 2015.

Keywords: acid-base, alpha carbons, relative acidity, resonance

Description: Please Read Chapter 3 of "2nd Language" and do the associated problems.
In addition, complete the following problem set.

Total Problems: 10

Printer-friendly version

Problem # 308

Rank the group of molecules below in in order of decreasing basicity. (1 = most basic)

Explain your reasoning.
Solution
The periodic trend for acidity is increasing acid stength as you move from left to right or from up to down on the periodic table, so the trend for basicity will be opposite; amines (which contain nitrogen) are the most basic neutral compounds, and oxygen is most basic than sulfur.

Problem Details
MendelSet practice problem # 308 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): base strength, periodic trends
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 # 717

Rank each of the eight compounds A through H below in order of decreasing acidity (1 = most acidic).

Don't get intimidated! What are the differences between these compounds?

Consider electron withdrawing groups (EWG), resonance, hybridization, and the functional group of the acidic proton.
Solution
Carboxylic acids are much more acidic than alcohols (because of resonance in their conjugate bases), so A and D are the least acidic of the group. The conjugate base of D has resonance, so D is more acidic than A:

(everything) > D > A

Electronegative atoms increase acid strength (by stabilizing the conjugate base), and sp² carbons are more electronegative than sp³ carbons (due to higher s-character). So B (all sp³ carbons) will be the least acidic of the carboxylic acids, and H will be more acidic than B (two sp² carbons):

(everything) > H > B > D > A

Of the remaining 4 benzoic acids, C has the most EWG (two nitros) and so is the most acidic. F has no EWG and so is the least acidic. Both E and G have only one EWG and so will be similar in acidity, with E slightly more acidic than G because the EWG affect is strongest in the ortho position (closer to the action). So the overal order is:

(strongest acid) C > E > G > F > H > B > D > A (weakest acid)

Problem Details
MendelSet practice problem # 717 submitted by Matt on July 23, 2011.
Subject: Organic Chemistry
Subject Topic(s): Carboxylic Acids (Inductive Effects, Esterification)
Question Type: Concept (explain why this is so..)
Keyword(s): acid strength, carboxylic acids
Problem # 721

Rank each of the four compounds below in order of decreasing acidity (1 = most acidic).
Solution
The general rule is that the pK₁ or a dicarboxylic acid is lower (more acidic) than the pK_a of a regular carboxylic acid, and the pK₂ is a little higher (less acidic).

Why is this? Because carboxylic acids are electron withdrawing groups (EWG). They stabilize the conjugate base of an acid, making the acid more acidic. So Both D and A should be more acidic than C. Because A has its carboxylic acid groups closer together, the EWG effect is stronger, and it will be the most acidic.

B will be the least acidic of the group. Why? Because a carboxylate group has a negative charge, which is electron donating and destablizing to the conjugate base (is a molecule is already -1, it doesn't help to become -2.)

So the overall order is:

(strongest acid) A > D > C > B (weakest acid)

Problem Details
MendelSet practice problem # 721 submitted by Matt on July 24, 2011.
Subject: Organic Chemistry
Subject Topic(s): Carboxylic Acids (Inductive Effects, Esterification)
Question Type: Concept (explain why this is so..)
Keyword(s): acid strength, carboxylic acids
Problem # 753

Rank the amines W through Z below in order of decreasing basicity (1 = most basic). Explain your reasoning.
Solution
Electron withdrawing groups (EWG) reduce electron density and reduce basicity. Resonance delocalizes electron density and so also decreases basicity (the electrons are less available to pick up a proton).

Electron donating groups (EDG) add electron density and increase basicity.

Alkyl groups are weakly EDG (induction), and hydrogen is neither EDG nor EWG. So cyclohexylamine (compound X) is a stronger base than ammonia (compound W).

Aniline (compound Y) has resonance, so it is less basic than compound X, which does not have resonance.

N-acyl aniline (compound Z) has more resonance forms than aniline, so it will be the least basic.

So the overall over is:

(most basic) X > W > Y > Z (least basic)

Problem Details
MendelSet practice problem # 753 submitted by Matt on July 27, 2011.
Subject: Organic Chemistry
Subject Topic(s): Amines (Basicity, Hoffman Elimination, Gabriel Amine Synthesis)
Question Type: Concept (explain why this is so..)
Keyword(s): base strength, amine bases, EDG and EWG
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
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 # 720

Base your answers to the three problems below on your knowledge of electron donating groups and electron withdrawing groups (EDG and EWG).

a) Based on the pK_a's listed below, Is formic acid more or less acidic than acetic acid? Propose an explanation why.

b) If acetic acid were added to a pH = 4.7 buffer solution, what percentage of it would be in its acetate (conjugate base) form?

c) Methoxy (-OCH₃) is usually considered an EDG. But based on the pK_a of methoxy acetic acid, do you think this is always the case? Explain.
Solution
a) The pK_a of formic acid is less than that of acetic acid, so formic acid is the stronger acid.

The pK_a difference is about ~1, and pK_a is logorithmic, so formic acid is ~10x as acidic as acetic acid.

Why? Because formate (formic acid's conjugate base) is more stable than acetate (acetic acid's conjugte base).

Why? Probably because of EDG. Alkyl groups such as methyl as weakly electron donating. Carboxylate is already negatively charged, and adding an EDG (the methyl group in acetate) makes it less stable, so acetate is a strong base, and acetic acid is the weaker acid. Formate has an hydrogen, which is neither EDG nor EWG, so doesn't have this problem.

b) The Henderson–Hasselbalch equal is pH = pK_a + log (A^-/HA). In this case, A^- is ^-OAc (acetate) and HA is HOAc (acetic acid). So plugging in the numbers:

pH = pK_a + log (^-OAc/HOAc)

4.7 = 4.7 + log (^-OAc/HOAc)

0 = log (^-OAc/HOAc),

so (^-OAc/HOAc) =1, and ^-OAc = HOAc.

So the ratio of acid to conjugate base (acetic acid to acetate) would be about 1:1.

The math above illustrates a rule you might have learned in general chemistry- when pH = pK_a, the concentration of acid equals the concentration of conjugate base.

c) The pKa of methoxy acetic acid (~3.6) is less than that of formic acid (~3.8), which means methoxy acetic acid is the strong acid, so methoxy acetate must be more stable than formate. This implies that methoxy must be somewhat electron withdrawing (an EWG).

This might be suprising because during the aromatic reactions chapter(s) (EAS) methoxy is considered an EDG. Oxygen is electron donating in terms of resonance. But remember that oxygen is also very electronegative. So in this case, it seems that the inductive EWG effect (electronegativity) outweighs the EDG effect.

Problem Details
MendelSet practice problem # 720 submitted by Matt on July 24, 2011.
Subject: Organic Chemistry
Subject Topic(s): Carboxylic Acids (Inductive Effects, Esterification)
Question Type: Concept (explain why this is so..)
Keyword(s): acid strength, carboxylic acids
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 # 745

The molecule below has five different types of hydrogens (A through E). Rank each in order of decreasing acidity.

(1 = most acidic). Explain your reasoning.
Solution
Hydrogens D and E are alpha to two carbonyls and so will be the more acidic than C, which is only alpha to one carbonyl. This is because the enolates that arise from deprotonate at D and E have more resonance forms than the enolate that arrises at C.

Because ketones are more electron withdrawing than esters, D will be more acidic than E. So far we have:

D > E > C > (other)

The carbanions that would arrise from deprotonation of carbons A and B both do not have any resonance forms, so we don't expect either to be acidic. But because A is next to a fluorine atom, which is an electron withdrawing group, A will be more acidic than B. So the overall order is:

(most acidic) D > E > C > A > B (least acidic)

Problem Details
MendelSet practice problem # 745 submitted by Matt on July 27, 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): acid strength

Follow MendelSet on Twitter, become a fan on Facebook.

Printer-friendly version