Chem 3/21

Starting Material and Reaction Context

• Reactant: Unidentified compound reacted with bromine and carbon tetrachloride.

• Preliminary conclusion: Starting material likely contains a double bond (alkene).

• Bromine adds across double bonds to create a dibromide.

Stereochemistry of Bromine Addition

• Bromine undergoes anti addition to alkenes; however, structural observations indicate that original bromine addition did not appear anti.

• Resolution: The compound must have formed in a conformation that permitted bromines to be in an anti relationship upon structure formation.

• Outcome: The final stereochemistry was identified as Z stereochemistry of 3,3-hexane due to the original confirmation during reaction.

Addition Reactions Overview

• Focus will be on various addition reactions of alkenes and their specific stereochemical outcomes without detailing each mechanism.

Epoxide Reactions

• Formation of epoxides followed by opening with strong nucleophiles resulted in anti addition.

• Example: Strong nucleophiles like methoxide anion favor substitution at the less hindered positions (secondary over tertiary) when reacting with neutral epoxides.

• In contrast, if the epoxide opens in a positively charged state, regioselectivity shifts and prefers highly substituted centers due to charge distribution.

Carbocations and Carbanions

• Carbocations (positively charged intermediates) and carbanions (negatively charged intermediates) appear in various addition reactions.

• Carbenes:

  • Characterized as neutral species with incomplete octets that are highly reactive.

  • Can react with alkenes to generate cyclopropanes via simultaneous bond formation.

  • Generated in situ as they cannot be easily stored.

Hydrogenation Reaction

• Hydrogenation (adding H\u2082) is different from hydration (adding water).

• Requires a catalyst, typically heavy transition metals (like Pt, Pd) to cleave the H-H bond.

• Reaction exhibits syn addition, where both hydrogen atoms add on the same side of a double bond.

• Saturated vs. Unsaturated Fats:

  • Saturated fats (solid, e.g., butter) versus unsaturated fats (liquid, e.g., oils) transformed via hydrogenation (to make margarine).

Trans Fat Formation

• Incomplete hydrogenation can lead to the formation of trans fats through unintended isomerization of double bonds.

Addition of Hydroxy Groups

• Hydroxy group addition can proceed via epoxide formation followed by hydrolysis (anti addition overall).

• Osmium Tetroxide Mechanism:

  • Diol synthesis via osmium tetroxide leads to syn or anti addition outcomes depending on substrate configuration.

Ozonolysis

• Cutting double bonds using ozone (O\u2083) creates ozonides, followed by reduction reactions under specific conditions (e.g., Zn/H\u2082O) to yield aldehydes or ketones.

• Conceptual mnemonic: Cut double bond and add O to each side.

Industrial Applications of Catalysts

• Transition metals (like Pd, Pt) are often utilized in finely divided forms on solid supports for practical industrial reactions; this allows easy recovery post-reaction.

Summary and Practice

• Recap content learned, focusing on syn and anti addition in various reactions, emphasizing the significance of understanding conditions and substrate structures to predict outcomes.