🥼Organic Chemistry Unit 9 Review

Alkynes are versatile building blocks in organic synthesis because their electron-rich triple bond can be transformed into alkenes, alcohols, and carbonyl compounds. Retrosynthetic analysis ties these reactions together by letting you plan a synthesis backward from your target molecule to simple starting materials.

Steps for Alkyne-Based Synthesis

Alkynes are more reactive than alkenes because the sp-hybridized carbons have greater s-character, concentrating electron density in the triple bond. This makes alkynes excellent precursors for building more complex molecules.

Synthesis of alkenes from alkynes:

Lindlar catalyst ( $Pd/CaCO_3$ poisoned with $PbO$ ) + $H_2$ gives cis-alkenes. Both hydrogens add from the same face of the triple bond (syn addition).
Dissolving metal reduction ( $Na$ in liquid $NH_3$ ) gives trans-alkenes. The hydrogens add from opposite faces (anti addition).

This is one of the most testable distinctions in this unit: same reagent class (reduction), but different stereochemical outcomes depending on which method you choose.

Synthesis of carbonyl compounds from alkynes (hydration reactions):

Acid-catalyzed hydration ( $H_2SO_4$ , $H_2O$ , with $HgSO_4$ catalyst) follows Markovnikov addition. For terminal alkynes, the $OH$ adds to the internal carbon, and tautomerization produces a methyl ketone.
Hydroboration-oxidation ( $BH_3 \cdot THF$ , then $H_2O_2/NaOH$ ) follows anti-Markovnikov addition. For terminal alkynes, the oxygen ends up on the terminal carbon, producing an aldehyde after tautomerization.
Oxymercuration ( $Hg(OAc)_2$ , $H_2O$ , then $NaBH_4$ ) also follows Markovnikov addition, giving ketones from terminal alkynes.

The key pattern: Markovnikov hydration of terminal alkynes → ketones. Anti-Markovnikov hydration of terminal alkynes → aldehydes.

Steps for alkyne-based synthesis, Organic chemistry 24: Alkynes - reactions, synthesis and protecting groups

Application of Retrosynthetic Analysis

Retrosynthetic analysis means working backward from your target molecule to find simpler precursors. Instead of asking "what can I make from this starting material?", you ask "what could I make this target from?"

Steps in retrosynthetic analysis:

Identify the target molecule and its key functional groups.
Determine strategic bond disconnections that simplify the target. Look for bonds that could have been formed by reactions you know.
Propose synthons (the hypothetical fragments that result from each disconnection). These represent the reactive pieces you'd need.
Match each synthon to a real, available reagent or starting material.
Write the synthesis forward from starting materials to target, confirming that each step is chemically feasible and gives the correct regiochemistry and stereochemistry.

Practical considerations:

Minimize the total number of steps (fewer steps = higher overall yield)
Choose reactions with high selectivity and predictable stereochemical outcomes
Consider the availability and cost of starting materials
Watch for functional group compatibility (will a reagent in step 3 destroy something you made in step 1?)

Steps for alkyne-based synthesis, Organic chemistry 20: Alkenes - oxymercuration, hydroboration

Conversion of Alkynes to Other Groups

Alkynes → Alkenes:

Lindlar catalyst with $H_2$ $H_{2}$ :
- Stereoselective formation of cis-alkenes
- Example: $HC \equiv C-CH_2CH_3 \xrightarrow{H_2, \text{ Lindlar}}$ cis- $CH_3CH=CHCH_2CH_3$ ... but note that for a terminal alkyne like 1-butyne, the product is simply 1-butene. Stereoselectivity matters most for internal alkynes.
Dissolving metal reduction ( $Na/NH_3$ $N a / N H_{3}$ ):
- Formation of trans-alkenes
- Example: An internal alkyne like 2-butyne gives trans-2-butene.

Alkynes → Carbonyl Compounds:

Acid-catalyzed hydration ( $HgSO_4$ $H g S O_{4}$ , $H_2SO_4$ $H_{2} S O_{4}$ , $H_2O$ $H_{2} O$ ):
- Markovnikov addition → ketone
- Example: $HC \equiv C-CH_2CH_3 \xrightarrow{HgSO_4, H_2SO_4, H_2O} CH_3COCH_2CH_3$
Hydroboration-oxidation ( $BH_3 \cdot THF$ $B H_{3} \cdot T H F$ , then $H_2O_2/NaOH$ $H_{2} O_{2} / N a O H$ ):
- Anti-Markovnikov addition → aldehyde (from terminal alkynes)
- Example: $HC \equiv C-CH_2CH_3 \xrightarrow{1.\, BH_3 \cdot THF \quad 2.\, H_2O_2, NaOH} CH_3CH_2CH_2CHO$
Oxymercuration-reduction ( $Hg(OAc)_2$ $H g (O A c)_{2}$ , $H_2O$ $H_{2} O$ , then $NaBH_4$ $N a B H_{4}$ ):
- Markovnikov addition → ketone
- Example: $HC \equiv C-CH_2CH_3 \xrightarrow{1.\, Hg(OAc)_2, H_2O \quad 2.\, NaBH_4} CH_3COCH_2CH_3$

Common Reaction Types in Organic Synthesis

Understanding these broad categories helps you classify new reactions and predict products:

Reduction: Gain of hydrogen or electrons (e.g., alkyne → alkene → alkane). In organic chemistry, think "adding $H$ " or "removing $O$ ."
Oxidation: Loss of hydrogen or electrons (e.g., alcohol → aldehyde → carboxylic acid). Think "adding $O$ " or "removing $H$ ."
Addition reactions: Atoms or groups add across a double or triple bond, breaking $\pi$ bonds and forming new $\sigma$ bonds. Most alkyne reactions in this unit are additions.
Elimination reactions: Atoms or groups are removed, forming a new $\pi$ bond (e.g., dehydrohalogenation to form an alkyne from a dihalide).
Substitution reactions: One atom or group replaces another (e.g., $S_N2$ alkylation of an acetylide anion to extend a carbon chain).

Stereochemistry runs through all of these. For synthesis problems, always ask: does this reaction give a specific stereochemical outcome (syn vs. anti addition, retention vs. inversion), and does that match what the target molecule requires?

🥼Organic Chemistry Unit 9 Review