(a) Suggest a synthesis of the following alkene (A) using a Wittig reaction strategy. Draw the starting material(s), key reagent and a full reaction mechanism including an explanation of the observed geometry.
[15 marks]
(b) Which of the following (B) and (C) will favour the enol form? Briefly explain your reasoning.
B
C
[5 marks]
(c) Predict the product(s) and provide a mechanism for each of the following transformations:
(i)
(ii)
$[2 imes 10$ marks $]$

Question

(a) Suggest a synthesis of the following alkene (A) using a Wittig reaction strategy. Draw the starting material(s), key reagent and a full reaction mechanism including an explanation of the observed geometry.
[15 marks]
(b) Which of the following (B) and (C) will favour the enol form? Briefly explain your reasoning.
B
C
[5 marks]
(c) Predict the product(s) and provide a mechanism for each of the following transformations:
(i)
(ii)
$[2 	imes 10$ marks $]$

Accepted Answer

∻Solution∻ ‖ 1 ⋮ **Identify the Wittig reagent**: To synthesize alkene (A) using a Wittig reaction, we need to identify the Wittig reagent (phosphonium ylide) and the carbonyl compound that will be used. ‖ ‖ 2 ⋮ **Synthesis of the Wittig reagent**: The Wittig reagent can be synthesized from a phosphonium salt, which is formed by the reaction of an alkyl halide with triphenylphosphine. The alkyl halide should have the same carbon chain as alkene (A) minus one carbon and without the double bond. ‖ ‖ 3 ⋮ **Reaction mechanism**: The Wittig reaction mechanism involves the attack of the phosphonium ylide on the carbonyl carbon of the ester, followed by a [2+2] cycloaddition to form a four-membered oxaphosphetane intermediate, which then collapses to form the desired alkene (A) and triphenylphosphine oxide. ‖ ‖ 4 ⋮ **Geometry explanation**: The geometry of the alkene is determined by the stereochemistry of the ylide and the stability of the transition state during the cycloaddition step. Steric and electronic factors influence whether the reaction proceeds through a syn or anti transition state, leading to the observed E or Z alkene geometry. ‖ ∻1 Answer∻ ⚹ The synthesis of alkene (A) using a Wittig reaction involves the preparation of a phosphonium ylide from an alkyl halide and triphenylphosphine, followed by its reaction with an ester to form the alkene with the observed geometry. ⚹ ∻Key Concept∻ ⚹ Wittig reaction for alkene synthesis ⚹ ∻Explanation∻ ⚹ The Wittig reaction allows the synthesis of alkenes from carbonyl compounds and phosphonium ylides, with control over the alkene geometry based on the ylide and reaction conditions. ⚹ ‖ 1 ⋮ **Analyze the stability of enol forms**: To determine which compound (B or C) favors the enol form, we need to consider the stability of the enol tautomer, which is influenced by the ability to form hydrogen bonds and the degree of conjugation with other pi systems. ‖ ‖ 2 ⋮ **Compound comparison**: Compound B likely has a more stable enol form due to the possibility of intramolecular hydrogen bonding and extended conjugation with the aromatic ring, which is not as favorable in compound C. ‖ ∻2 Answer∻ ⚹ Compound B will favor the enol form due to the increased stability from intramolecular hydrogen bonding and conjugation. ⚹ ∻Key Concept∻ ⚹ Enol form stability ⚹ ∻Explanation∻ ⚹ The stability of the enol form is enhanced by intramolecular hydrogen bonding and conjugation with aromatic systems, leading to a preference for the enol tautomer in certain compounds. ⚹ ‖ 1 ⋮ **Predict the product for transformation (i)**: The cyclic ketone with an alpha hydrogen will undergo deprotonation by the base to form an enolate ion, which can then act as a nucleophile in various reactions. ‖ ‖ 2 ⋮ **Predict the product for transformation (ii)**: The reaction between an ester with a benzene ring and another ester under the influence of NaOEt will likely involve a Claisen condensation, where the enolate of one ester attacks the carbonyl carbon of the other, leading to a beta-keto ester product. ‖ ∻3 Answer∻ ⚹ For transformation (i), the product will be an enolate or a subsequent reaction product depending on the conditions. For transformation (ii), the product will be a beta-keto ester resulting from a Claisen condensation reaction. ⚹ ∻Key Concept∻ ⚹ Enolate chemistry and Claisen condensation ⚹ ∻Explanation∻ ⚹ Enolates are versatile nucleophiles that can participate in various reactions, including Claisen condensation, which forms beta-keto esters from two ester molecules. ⚹◊What are the key differences between the enol and keto forms of a compound in organic chemistry?⍭ Generate me a similar question◊

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