17. Organic Reactions Problems

Organic Reactions Problems

1. Arrange the following in order from the worst to the best nucleophile.

  1. HCOO¯, HO¯, (CH3)2N¯
  2. CH3COO¯, H2N¯, HO¯
  3. (CH3)2N¯, HO¯ , CH3COO¯

2. You are given two compounds, acetone and ammonia and asked to predict the products.  Which of the following reaction schemes using curved arrows do you think gives the correct products? [C]

 

3. You are given the following compound and are asked to predict the product, based on the electron motion indicated by the curly arrows.

Which of the following represents the correct product? [A]
a.         b.        c.       d. 

4. A reaction where one of the reactants is a chiral compound produces a single product that is the optical inverse of the original reactant.  What kind of reaction is it? [b]

  1. SN1
  2. SN2
  3. Addition
  4. Dehydrogenation
  5. Lewis acid base reaction

5. A certain reaction produces two products that together form a racemic mixture.  What kind of reaction is it? [a]

  1. SN1
  2. SN2
  3. Addition
  4. Dehydrogenation
  5. Lewis acid base reaction

6. What is the compound that will result when the following reactants are allowed to react?  What kind of reaction is this?
(CH3)2C=CH2 + HCl → ?

  1. (CH3)2CH-CH2Cl
  2. (CH3)2CCl-CH3
  3. (CH2Cl)2C-CH3
  4. (CH3)2CCl-CH2Cl
  5. (CH3)3C-CH3

[b, Markovnikov addition]

6. Which of the following products is correct for the following reaction HClC=C(CH3)2 + HCl → ? What kind of reaction is this?

  1. CH3-C(CH3)2Cl
  2. CH3-C(CClH2)2
  3. CClH2-C(CH3)3
  4. CCl3-C(CH3)3
  5. CClH2-CH(CH3)2

[e, anti-Markovnikov addition]

7. Name the reaction type for the following reaction. HClC=C(CH3)2 + H2 → H2ClC=CH(CH3)2 .

  1. addition
  2. dehydrogenation
  3. hydration
  4. substitution
  5. hydrogenation

[a,e]

8.  Which of the following reactions is an SN2 reaction?

  1. A reaction between a nucleophile and a chiral molecule produces a racemic mixture.
  2. A reaction between a nucleophile and a chiral molecule produces a single product with the symmetry inverted.
  3. A reaction between a substrate molecule and a base involves a carbocation intermediate and a single product.
  4. The reaction between a substrate molecule and a base occurs in a single concerted step involving both reactants.
  5. A nucleophile bonds to a carbocation to produce products.

9.  Rank the following leaving group from strongest to weakest,  a. Cl¯, b H3C¯, c H2N¯

10. Which of the following would make the best electrophile?                       [d]

  1. H2C=CH2
  2. (CH3)2S
  3. (CH3)3CH
  4. CH3CH2Br

11.  Only one of the following reaction schemes is correct.  Which is it?    [a]

12.  A hydrocarbon with a single double bond is heated in a cracking reaction.  Draw all the products that might result.  Note that there will be multiple products in some cases.

Answers:

Question 1 a.

The worst to best nucleophile ranking in polar aprotic or general basicity order is \text{HCOO}^{-} (formate) < \text{OH}^{-} (hydroxide) < (\text{CH}_3)_2\text{N}^- (dimethylamide). Nucleophilicity generally tracks with basicity when comparing atoms of the same element or similar trends, where a stronger negative charge and lower electronegativity make (\text{CH}_3)_2\text{N}^- the strongest electron donor.
Nucleophile Ranking
  • \text{HCOO}^{-} (Formate ion): Worst nucleophile. The negative charge is spread out (delocalized) over two oxygen atoms through resonance. This makes the atom less reactive and a weak electron donor.
  • \text{OH}^{-} (Hydroxide ion): Intermediate nucleophile. The negative charge sits fully on a single oxygen atom. It is more reactive than the formate ion because it holds its charge locally.
  • (\text{CH}_3)_2\text{N}^- (Dimethylamide ion): Best nucleophile. Nitrogen is less electronegative than oxygen. It shares its lone pair and negative charge much more easily than oxygen does.
Question 1 b. From worst to best nucleophile in polar protic solvents, the order is \text{CH}_3\text{COO}^- < \text{HO}^- < \text{H}_2\text{N}^-.
Nucleophile Strength Factors
Nucleophile strength depends on charge, electronegativity, and solvent effects. A stronger negative charge and lower electronegativity make a species share its electrons faster.
Ranking Details

\text{CH}_3\text{COO}^- (Acetate ion)

    • Worst nucleophile in this group.
    • The negative charge is spread out over two oxygen atoms. This resonance stabilization makes the electrons less available to attack an electrophile.

\text{HO}^{-} (Hydroxide ion)

    • Intermediate nucleophile.
    • Oxygen is more electronegative than nitrogen. It holds its electrons tighter than an amide ion, making it less reactive.
\text{H}_2\text{N}^- (Amide ion)
    • Best nucleophile in this group.
    • Nitrogen is less electronegative than oxygen. It gives up its lone pair easily. It also carries a full negative charge without resonance sharing.

Question 1 c.  From worst to best nucleophile in a polar protic solvent, the order is: \text{CH}_3\text{COO}^- (acetate) < \text{HO}^{-} (hydroxide) < (\text{CH}_3)_2\text{N}^- (dimethylamide).

Nucleophile Ranking

\text{CH}_3\text{COO}^- (Worst)

    • Resonance stabilization shares the negative charge over two oxygen atoms.
    • This makes the atom less likely to share its electrons with a carbon atom.

\text{HO}^{-} (Middle)

    • The negative charge is on a single oxygen atom with no resonance help.
    • Oxygen is more electronegative than nitrogen, so it holds its electrons tightly.

(\text{CH}_3)_2\text{N}^- (Best)

    • The negative charge is on a nitrogen atom.
    • Nitrogen is less electronegative than oxygen.
    • It gives up its electrons easily to form a new bond.

Question 9 answer

The strongest leaving group is chloride (\text{Cl}^{-}), followed by the amide ion (\text{H}_2\text{N}^-), and the weakest is the methanide ion (\text{H}_3\text{C}^-).
Leaving Group Strength Ranking
  • 1. Strongest: \text{Cl}^{-} (Chloride)
      • Chloride is the conjugate base of a strong acid (\text{HCl}, \text{p}K_a \approx -7).
      • It is very stable with a negative charge.
      • It leaves easily in substitution reactions.

     

  • 2. Moderate: \text{H}_2\text{N}^- (Amide ion)
      • Amide is the conjugate base of ammonia (\text{NH}_{3}, \text{p}K_a \approx 38).
      • It is a strong base and is not very stable on its own.
      • It is a poor leaving group under normal conditions.

     

  • 3. Weakest: \text{H}_3\text{C}^- (Methanide ion)
      • Methanide is the conjugate base of methane (\text{CH}_{4}, \text{p}K_a \approx 50).
      • Carbon holds a negative charge very poorly here.
      • It is an extremely strong base and almost never acts as a leaving group. [1]

     

    Key Rule

  • Base strength: Stronger bases are worse leaving groups.
  • Stability: More stable anions make better leaving groups. [1]

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