Textbook Question
Show how Wittig reactions might be used to synthesize the following compounds. In each case, start with an alkyl halide and a ketone or an aldehyde.
(a) Ph–CH=C(CH3)2
(b) Ph–C(CH3)=CH2
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Ch. 18 - Ketones and Aldehydes
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
Chapter 18, Problem 33c,d
Show how Wittig reactions might be used to synthesize the following compounds. In each case, start with an alkyl halide and a ketone or an aldehyde.
(c) Ph–CH=CH–CH=CH–Ph
(d) 
Verified step by step guidance1
Step 1: Understand the Wittig reaction. The Wittig reaction is used to form alkenes by reacting a phosphonium ylide with a carbonyl compound (ketone or aldehyde). The ylide is typically prepared from an alkyl halide and triphenylphosphine.
Step 2: For compound (i), identify the target structure. The compound contains two phenyl groups attached to a conjugated diene system. To synthesize this, you need two Wittig reactions to form the conjugated double bonds.
Step 3: For compound (i), choose the starting materials. Select an alkyl halide to prepare the ylide and a ketone or aldehyde for the carbonyl compound. For example, benzyl bromide (PhCH2Br) can be used to prepare the ylide, and acetophenone (PhCOCH3) can be used as the carbonyl compound.
Step 4: For compound (ii), identify the target structure. The compound contains a cyclopentyl group attached to an alkene with a methyl group. To synthesize this, use a Wittig reaction to form the double bond.
Step 5: For compound (ii), choose the starting materials. Select an alkyl halide to prepare the ylide and a ketone or aldehyde for the carbonyl compound. For example, cyclopentyl bromide can be used to prepare the ylide, and acetone (CH3COCH3) can be used as the carbonyl compound.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Wittig Reaction
The Wittig reaction is a chemical reaction that allows for the formation of alkenes through the reaction of an aldehyde or ketone with a phosphonium ylide. This reaction is significant in organic synthesis as it provides a method to create double bonds with high stereoselectivity. The ylide acts as a nucleophile, attacking the carbonyl carbon, leading to the formation of an alkene and a phosphine oxide as a byproduct.
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Phosphonium Ylide
A phosphonium ylide is a compound containing a positively charged phosphorus atom bonded to a negatively charged carbon atom. This unique structure allows the ylide to act as a nucleophile in the Wittig reaction. The stability and reactivity of the ylide are crucial for the successful formation of the desired alkene product, making the choice of ylide an important factor in synthetic applications.
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Stereochemistry in Alkenes
Stereochemistry refers to the spatial arrangement of atoms in molecules and is particularly important in the context of alkenes, which can exhibit geometric isomerism (cis/trans or E/Z isomerism). The Wittig reaction can yield different stereoisomers depending on the configuration of the starting materials and the ylide used. Understanding stereochemistry is essential for predicting the properties and reactivity of the synthesized alkenes.
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Related Practice
Textbook Question
Like other strong nucleophiles, triphenylphosphine attacks and opens epoxides. The initial product (a betaine) quickly cyclizes to an oxaphosphetane that collapses to an alkene and triphenylphosphine oxide.
(b) Show how this sequence might be used to convert cis-cyclooctene to trans-cyclooctene.
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Textbook Question
Predict the major products of the following reactions.
(a)
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Textbook Question
(a) Outline the syntheses indicated in Solved Problem 18-2, beginning with aldehydes and alkyl halides.
(b) Both of these syntheses of 1-phenylbuta-1,3-diene form the central double bond. Show how you would synthesize this target molecule by forming the terminal double bond.
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Textbook Question
Predict the major products of the following reactions.
(b)
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Textbook Question
Predict the major products of the following reactions.
(c)
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