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Ch. 21 - Carboxylic Acid Derivatives
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 21 - Carboxylic Acid DerivativesProblem 56
Chapter 21, Problem 56

One mole of acetyl chloride is added to a liter of triethylamine, resulting in a vigorous exothermic reaction. Once the reaction mixture has cooled, 1 mole of ethanol is added. Another vigorous exothermic reaction results. The mixture is analyzed and found to contain triethylamine, ethyl acetate, and triethylammonium chloride. Propose mechanisms for the two exothermic reactions.

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Step 1: Understand the first reaction. Acetyl chloride (CH3COCl) reacts with triethylamine ((C2H5)3N). Triethylamine acts as a base and abstracts the proton from the HCl byproduct of the reaction, forming triethylammonium chloride ((C2H5)3NH+Cl−). The acetyl chloride undergoes nucleophilic acyl substitution, where the chloride ion (Cl−) is displaced. This reaction is highly exothermic due to the formation of strong ionic bonds in triethylammonium chloride.
Step 2: Write the mechanism for the first reaction. The carbonyl carbon of acetyl chloride is electrophilic due to the electron-withdrawing effect of the chlorine atom. Triethylamine does not directly attack the carbonyl carbon but instead neutralizes the HCl formed during the reaction. The mechanism involves the nucleophilic attack of a base (possibly trace water or another nucleophile) on the carbonyl carbon, followed by the elimination of Cl−.
Step 3: Understand the second reaction. Ethanol (CH3CH2OH) is added to the reaction mixture. Ethanol reacts with the acetyl chloride (if any remains) or with the acyl intermediate to form ethyl acetate (CH3COOCH2CH3) via a nucleophilic acyl substitution mechanism. This reaction is also exothermic due to the formation of a stable ester bond and the release of HCl.
Step 4: Write the mechanism for the second reaction. Ethanol acts as a nucleophile and attacks the electrophilic carbonyl carbon of acetyl chloride. This forms a tetrahedral intermediate, which then collapses to expel the chloride ion (Cl−) and form ethyl acetate. The HCl produced in this reaction is neutralized by triethylamine, forming triethylammonium chloride.
Step 5: Summarize the products. After both reactions, the mixture contains triethylamine (if in excess), ethyl acetate (the ester product), and triethylammonium chloride (formed from the neutralization of HCl by triethylamine). The exothermic nature of both reactions is due to the formation of strong ionic bonds and stable covalent bonds in the products.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Nucleophilic Acyl Substitution

Nucleophilic acyl substitution is a fundamental reaction mechanism in organic chemistry where a nucleophile attacks an electrophilic carbonyl carbon, leading to the substitution of a leaving group. In the case of acetyl chloride, triethylamine acts as a nucleophile, attacking the carbonyl carbon and resulting in the formation of ethyl acetate and triethylammonium chloride.
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Nucleophiles and Electrophiles can react in Substitution Reactions.

Exothermic Reactions

Exothermic reactions are chemical processes that release energy, usually in the form of heat, to the surroundings. The vigorous nature of the reactions described indicates that the formation of products from reactants is energetically favorable, often due to the stability of the products compared to the reactants, as seen in the reactions involving acetyl chloride and ethanol.
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Role of Triethylamine

Triethylamine is a strong base and a nucleophile that plays a crucial role in the reactions described. It not only facilitates the nucleophilic attack on acetyl chloride but also helps in the deprotonation of the resulting triethylammonium ion, thus driving the reaction forward and stabilizing the products formed during the exothermic processes.
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Related Practice
Textbook Question

Grignard reagents add to carbonate esters as they add to other esters.

(a) Predict the major product of the following reaction.

(b) Show how you would synthesize 3-ethylpentan-3-ol using diethyl carbonate and ethyl bromide as your only organic reagents

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Textbook Question

Show how you would accomplish the following multistep syntheses, using the indicated starting material and any necessary reagents.

(c)

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Textbook Question

Show how you would accomplish the following multistep syntheses, using the indicated starting material and any necessary reagents.

(a) hept-6-en-1-ol → ε-caprolactone

(b) methoxybenzene → p-methoxybenzamide

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Textbook Question

Show how you would accomplish the following multistep syntheses, using the indicated starting material and any necessary reagents.

(d)

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Textbook Question

Grignard reagents add to carbonate esters as they add to other esters.

(c) Diethyl carbonate is a liquid reagent that is easy to handle. Also show how you might use diethyl carbonate instead of methyl isocyanate to make Sevin® insecticide.

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Textbook Question

Grignard reagents add to carbonate esters as they add to other esters.

(c) Diethyl carbonate is a liquid reagent that is easy to handle. In contrast, phosgene is a highly toxic and corrosive gas. Show how you might use diethyl carbonate instead of phosgene to make Lexan®.

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