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Ch. 19 - Aldehydes and Ketones: Nucleophilic AdditionWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins
Ch. 26 - Transition Metals
Naming Aldehydes
Naming Ketones
Oxidizing and Reducing Agents
Oxidation of Alcohols
Alkyne Hydration
Nucleophilic Addition
Organometallics on Ketones
Overview of Nucleophilic Addition of Solvents
Acetal Protecting Group
Imine vs Enamine
Addition of Amine Derivatives
Wolff Kishner Reduction
Baeyer-Villiger Oxidation
Acid Chloride to Ketone
Nitrile to Ketone
Wittig Reaction
Ketone and Aldehyde Synthesis Reactions
Additional Guides
Acetal and Hemiacetal
Johnny Betancourt

The Wittig reaction, also known as Wittig olefination, is a great way to turn aldehydes and ketones into alkenes. 

The box-out method:

Before we get into the mechanism, let’s look at a really quick way to get the right answer on an exam. If you see an aldehyde or ketone and an ylide, you can actually use something called the box-out method to predict the product.

Carbonyl and ylide yield alkeneCarbonyl and ylide yield alkene

On the reactant side, we’ve got an aldehyde on the left and an ylide on the right; on the product side, we’ve got an alkene. There is a complicated mechanism, but let’s see how we can skip it to just figure out what that product is:

Box-out methodBox-out methodBasically, you can just draw a box around the carbonyl oxygen and the triphenylphosphine (Ph3P). From there, imagine joining the two double bonds together through a double bond.  

The arrow-pushing mechanism:

The box-out method is great and all, but there’s nothing like a good mechanism to help understand exactly what’s going on. The first thing we need to do is get preparation going on the ylide. The best way to do it is to use a primary alkyl bromide (or other alkyl halide) but secondary will do.

Triphenylphosphine SN2Triphenylphosphine SN2

Now we’ve got that triphenylphosphonium ion, we’re one step away from forming our ylide! All we need to do is add a strong base to form a carbanion. Notice below that the ylide is zwitterionic; that is, it’s got adjacent opposite charges. It’s stabilized by the resonance shown.

Deprotonation to form the ylideDeprotonation to form the ylide

Great, now all that’s left is to react the ylide with the carbonyl. The ylide’s carbon is a pretty good nucleophile, and it can participate in nucleophilic addition. Let’s see how it’s done:

Wittig-full-mechanismWittig Full Mechanism

The carbonyl acts as an electrophile as the anionic carbon attacks it to form a betaine (pronounced beta-ene). The oxide attacks the cationic phosphorus to form an oxaphosphetane, which undergoes rearrangement to produce an alkene and phosphine oxide.

The Wittig doesn’t have selectivity for any particular stereochemistry without modification. It yields both the E-alkene and Z-alkene without preference. Modifications like the Horner-Wadsworth-Emmons and Schlosser preferentially form the E-alkene.

Johnny Betancourt

Johnny got his start tutoring Organic in 2006 when he was a Teaching Assistant. He graduated in Chemistry from FIU and finished up his UF Doctor of Pharmacy last year. He now enjoys helping thousands of students crush mechanisms, while moonlighting as a clinical pharmacist on weekends.