Organic Pharmaceutical Chemistry PCH 301: Addition Reactions

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Questions and Answers

What is the primary reason carbonyl compounds undergo nucleophilic addition reactions?

  • Carbon atoms have higher electronegativity than oxygen atoms.
  • Nucleophiles are attracted to negatively charged atoms.
  • The presence of acidic groups reduces nucleophilic strength.
  • The carbonyl carbon possesses a partial positive charge. (correct)

Which effect allows stabilization of the positive charge in a carbocation?

  • The presence of a neighboring functional group.
  • Hyperconjugation through adjacent C-H bonds. (correct)
  • Delocalization of electrons from pi bonds.
  • The inductive effect of nearby electronegative atoms.

How does the inductive effect influence a nucleophile's ability to react with a carbonyl compound?

  • It enhances the nucleophile's polarization towards the carbonyl carbon.
  • It affects the stability of the carbonyl electrophile. (correct)
  • It increases the nucleophile's charge density.
  • It decreases the overall charge of the nucleophile.

What role does delocalization play in stabilizing carbocations?

<p>It allows positive charges to be equally shared among multiple atoms. (A)</p> Signup and view all the answers

In a nucleophilic addition mechanism, what is the first step that occurs?

<p>Nucleophile attacks the carbonyl carbon. (C)</p> Signup and view all the answers

What is the primary difference between Markovnikov and Anti-Markovnikov addition in reactions with unsymmetrical alkenes?

<p>Markovnikov addition leads to the more stable carbocation. (D)</p> Signup and view all the answers

Which factor is most associated with stabilizing a positive charge in a carbocation?

<p>Increasing the number of adjacent alkyl groups. (D)</p> Signup and view all the answers

What type of stabilization involves the overlap of the carbocation's vacant 2p orbital with neighboring C-H bonds?

<p>Hyperconjugation (A)</p> Signup and view all the answers

Which of the following correctly describes the inductive effect in regard to carbocation stability?

<p>Alkyl groups can donate electron density towards the positively charged center. (C)</p> Signup and view all the answers

How does hyperconjugation differ from inductive effects in the context of carbocation stabilization?

<p>Inductive effects solely depend on the structure of the alkyl groups, while hyperconjugation is about orbital overlap. (A)</p> Signup and view all the answers

Which product is expected from a reaction of an unsymmetrical alkene with HX under Markovnikov's rule?

<p>Product with halide added to the more substituted carbon. (A)</p> Signup and view all the answers

What characteristic contributes to a carbocation being more stable?

<p>Greater hyperconjugation due to increased alkyl substituents. (A)</p> Signup and view all the answers

Which statement best represents the concept of delocalization in relation to carbocations?

<p>It involves spread of positive charge over several atoms or bonds. (B)</p> Signup and view all the answers

What characterizes electrophilic addition reactions?

<p>They involve the attack of an electrophile on a nucleophilic site. (C)</p> Signup and view all the answers

Which of the following statements is true regarding Markovnikov’s rule?

<p>It predicts the outcome of reactions involving unsymmetrical reagents. (A)</p> Signup and view all the answers

Which factor contributes most significantly to the stabilization of a carbocation?

<p>Resonance structures that delocalize the positive charge. (C)</p> Signup and view all the answers

Which of the following describes hyperconjugation in carbocations?

<p>It stabilizes the carbocation through overlapping of orbitals from adjacent sigma bonds. (C)</p> Signup and view all the answers

What effect does an inductive effect have on a carbocation?

<p>It decreases the stability by pulling electron density away from the carbocation. (C)</p> Signup and view all the answers

Which statement accurately reflects nucleophilic addition reactions?

<p>They generally occur with carbon-heteroatom double bonds. (A)</p> Signup and view all the answers

In which scenario would nucleophilic addition be favored over electrophilic addition?

<p>In the presence of a strong nucleophile. (B)</p> Signup and view all the answers

What is one effect of substituents on the stability of carbocations?

<p>Electron-donating groups enhance stability by providing electron density. (D)</p> Signup and view all the answers

Flashcards

Markovnikov's rule

In the addition of HX to an unsymmetrical alkene, the hydrogen atom adds to the carbon atom with more hydrogen atoms.

Anti-Markovnikov addition

Addition of HX to an unsymmetrical alkene, where hydrogen adds to the less substituted carbon atom.

Carbocation stability

More highly substituted carbocations are more stable than less substituted ones due to alkyl group electron donation.

Inductive effect

Electron-donating effect of alkyl groups towards a positive center (carbocation).

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Hyperconjugation

Stabilization of carbocation by overlap of vacant 2p orbital with neighboring C-H σ-bond.

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Regioselective product

A reaction producing one specific isomer from possible isomers.

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Unsymmetrical alkene

An alkene with different groups attached to the two carbons involved in the double bond.

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Carbocation

Positively charged carbon ion.

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Nucleophilic Addition

A reaction where a nucleophile attacks an electron-deficient double bond, forming an anion intermediate followed by protonation to create an addition product.

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Electrophile

A molecule or ion that is electron-deficient and attracts electrons, making it susceptible to attack by nucleophiles.

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Carbonyl Group

A functional group consisting of a carbon atom double-bonded to an oxygen atom (C=O).

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Partial Positive Charge on Carbonyl Carbon

The oxygen atom in a carbonyl group is more electronegative than the carbon atom, resulting in the carbon atom having a partial positive charge.

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Stabilization of Oxygen's Negative Charge

The negative charge on the oxygen atom in a carbonyl group can be stabilized by protonation (addition of a proton) from an acid.

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Electrophilic Addition

A reaction where an electrophile attacks a double or triple bond, breaking the bond and forming two new single bonds.

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Regioselective Reaction

A reaction that produces one specific isomer as the major product from multiple possible isomers.

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Study Notes

Organic Pharmaceutical Chemistry (PCH 301)

  • Course focus: Organic Reactions and Mechanisms

Addition Reactions

  • Addition reactions: Chemical reactions where small molecules add to compounds to form adducts.
  • Occur in compounds with pi (Ï€) electrons.
  • Product contains all elements of the reacting species.
  • Types:
    • Electrophilic addition: Electrophile accepts electron pair from a double or triple bond, forming a carbocation intermediate, followed by nucleophile attack to form the addition product.
    • Nucleophilic addition: Nucleophile attacks the electron-deficient double bond forming an anion intermediate, followed by protonation to form the addition product.
    • Carbon-carbon double bond (C=C) and carbon-carbon triple bond (C≡C) are common sites for electrophilic addition.
    • Carbon-hetero atom double bonds (such as C=O) and triple bonds (such as C≡N) are sites for nucleophilic addition.

Electrophilic Addition Reactions

  • Electrophile: Electron-seeking species.
  • Accepts electron pair from the double or triple bond.
  • Forms carbocation intermediate.
  • Nucleophile attacks the positive species to form the addition product.

General Reaction Mechanisms (Electrophilic Addition)

  • Step 1 (Slow): Electrophile attacks the Ï€ bond, forming a carbocation intermediate.
  • Step 2 (Fast): Nucleophile attacks the carbocation, forming the final product.

General Reaction Mechanisms of Hydrogenation of Alkenes and Alkynes

  • Alkene or alkyne + hydrogen (H2) + Metal Catalyst (Pt, Pd or Ni ) gives Alkanes.
  • Two new C—H bonds are simultaneously formed from H atoms.
  • The catalyst surface attaches the alkene and a hydrogen atom is transferred to the alkene. A second hydrogen atom is transferred to the same face of double bond.
  • Hydrogen atoms add to the same face of the double bond (syn addition).

Hydrogenation Reaction Energy Diagram

  • A catalyst lowers the activation energy needed to reach the transition state.

Addition to Symmetrical and Unsymmetrical π bonds

  • Symmetrical alkenes/alkynes have the same substituents at each end of the bond.
  • Unsymmetrical alkenes/alkynes have different substituents at each end of the bond.

Electrophilic Addition to Unsymmetrical Alkenes

  • Markovnikov's rule: Addition of unsymmetrical reagents to an unsymmetrical alkene, where the hydrogen adds to the carbon with more hydrogen atoms.

Carbocation Stabilization

  • Factors that stabilize carbocation:
    • Inductive effects: Alkyl groups donate electrons to the positive center, stabilizing the carbocation.
    • Hyperconjugation: Overlap of the vacant 2p orbital of the carbocation with a neighboring C-H σ-bond orbital, stabilization the carbocation.
    • Delocalization: Distribution of electron density beyond a fixed place stabilizing the carbocation.

Halogenation of Alkenes and Alkynes

  • Addition of halogen (Cl2 or Br2) across the double bond in an alkene to yield a vicinal dihalide.
  • Used as a test for unsaturation (Ï€ bonds). The red colour of the bromine reagent disappears when an alkene or alkyne is present.
  • Halides add to neighboring carbons from opposite faces of the molecule.
  • Reaction occurs in the presence of inert and non-nucleophilic solvents (e.g., methylene chloride, chloroform, or carbon tetrachloride).

Hydration of Alkenes and Alkynes

  • Addition of water (H2O) to an alkene in the presence of an acid catalyst (H2SO4) forms alcohols.
  • Follows Markovnikov's rule.
  • Highly regioselective reaction.

Nucleophilic Addition Reactions

  • Nucleophile: Electron-rich species.
  • Attacks the electron-deficient carbonyl carbon to form anion intermediate.
  • Second step is protonation of the anion to form the addition product.
  • Oxygen is more electronegative than carbon, causing carbonyl carbons to have partial positive charge and act as electrophiles.

Addition of Organometallic Reagents to Carbony Compounds

  • Aldehydes and ketones react with organometallic reagents, such as Grignard reagents, to yield alcohols.
  • Grignard reagent reacts with CO2 to give magnesium salts of carboxylic acids, followed by acidification to form the carboxylic acid.

Reaction of Ammonia Derivatives with Carbonyl Compounds

  • Primary amines, hydroxylamine, hydrazine, and semicarbazide react with aldehydes and ketones in the presence of an acid catalyst to generate imines or substituted imines.

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