Questions: Wittig Reaction: Phosphorus Ylides and Alkene Synthesis

Question 1

A chemist wants to synthesize (Z)-3-heptene (a cis-alkene) using a Wittig reaction. Which type of ylide should they use?

• A. A stabilized ylide (with an adjacent ester group), because it reacts faster and produces predominantly Z-alkene
• B. An unstabilized ylide (no electron-withdrawing groups on the carbanion), because it reacts quickly and irreversibly, favoring Z-alkene formation
• C. A stabilized ylide, because the extra stability slows the reaction and allows kinetic control toward the Z-isomer
• D. Either type — the geometric outcome is determined by the carbonyl compound, not the ylide

Answer: (B)

Unstabilized ylides (no electron-withdrawing groups on the carbanion carbon) react quickly and irreversibly via a kinetically controlled pathway that preferentially produces the Z (cis) alkene. Stabilized ylides (with EWGs like esters or nitriles) react more slowly and reversibly, allowing equilibration to the thermodynamically more stable E (trans) product. Option A is wrong on both counts — stabilized ylides are slower, not faster, and they favor E-alkenes. Option D is wrong — it is the ylide type, not the carbonyl, that determines stereoselectivity.

Question 2

What is the primary thermodynamic driving force that makes the Wittig reaction proceed to completion?

• A. The stability of the new alkene π bond formed in the product
• B. The formation of triphenylphosphine oxide (Ph₃P=O), which contains an exceptionally strong P=O bond (~540 kJ/mol)
• C. The release of strain from the four-membered oxaphosphetane ring
• D. The restoration of aromaticity in the triphenylphosphine leaving group

Answer: (B)

The extraordinary thermodynamic driving force is the formation of the P=O bond in triphenylphosphine oxide. At ~540 kJ/mol, this is one of the strongest bonds in organic chemistry and makes the overall reaction highly exothermic and essentially irreversible. While oxaphosphetane ring strain (option C) does contribute to the [2+2] cycloreversion step, it is the exceptional strength of the P=O bond that makes the overall equilibrium so strongly product-favored. The alkene π bond (option A) forms but is not the primary driver.

Question 3

A stabilized Wittig ylide (one with an electron-withdrawing group adjacent to the carbanion) predominantly produces the E-alkene because it reacts slowly enough to allow thermodynamic equilibration.

• T. True
• F. False

Answer: True

Stabilized ylides have the carbanion's charge delocalized by adjacent electron-withdrawing groups, making them less reactive. They react more slowly with carbonyl compounds, and the intermediate steps (including oxaphosphetane formation) are reversible, allowing the system to equilibrate. Since the E (trans) alkene is typically more thermodynamically stable than the Z (cis) alkene, equilibration produces E-alkene as the major product. This contrasts with unstabilized ylides, which react irreversibly under kinetic control and give Z-alkene.

Question 4

The Wittig reaction works with aldehydes but can seldom be used with ketones, because the additional alkyl group makes the carbonyl too hindered for ylide attack.

• T. True
• F. False

Answer: False

The Wittig reaction works with both aldehydes and ketones. Ketones are less reactive than aldehydes due to greater steric hindrance and electron donation from two alkyl groups, but the reaction still proceeds — sometimes requiring more forcing conditions or more reactive unstabilized ylides. The Core Idea explicitly states the reaction 'converts aldehydes or ketones to alkenes.' Using the Wittig reaction on ketones is routine in synthesis, particularly for making trisubstituted alkenes.

Question 5

Why is the formation of the oxaphosphetane ring a key intermediate step, and how does its decomposition give rise to stereocontrol in the Wittig reaction?

Model answer: The oxaphosphetane is a strained four-membered ring formed when the oxygen of the original carbonyl attacks the phosphorus after the initial C-C bond-forming step. This ring undergoes a concerted [2+2] cycloreversion, releasing the alkene and triphenylphosphine oxide. The stereochemistry of the alkene is set when the oxaphosphetane ring forms: with unstabilized ylides, ring formation is fast and irreversible, favoring a syn arrangement of substituents that leads to Z-alkene upon ring opening. With stabilized ylides, ring formation is reversible, and the more stable anti arrangement accumulates, leading to E-alkene. The oxaphosphetane acts as a stereochemical relay — the geometry established in ring formation is preserved in the alkene product.

Stereocontrol in the Wittig reaction is mechanistic, not imposed after the fact. The ylide type controls the reversibility of the ring-forming step, which controls the geometry of the ring, which determines the E/Z outcome. Understanding this mechanism is what distinguishes conceptual mastery from mere memorization of 'unstabilized = Z, stabilized = E.'