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Chapter 14: Q 38P (page 747)

Question: (a) Predict the values of m/zand the structures of the most abundant fragments you would observe in the mass spectrum of di-n-propyl-ether.

(b) Give logical fragmentations to account for the following ions observed in the mass spectrum of 2-methoxypentane: 102,87,71,59,31.

Short Answer

Expert verified

(a)

fragmentation equations of di-n-propyl-ether

(b)

fragmentation equations of 2-methoxypentane

Step by step solution

01

Molecular ion

When high-energy electron hits an organic compound, it removes one of the electrons from the molecule and hence produces positively charged ions known as the molecular ions.

formation of molecular ion

The molecular ion is not only acting as a cation but also as a free radical because of the presence of an odd number of electrons. Hence, it can be called as enol radical cation.

02

McLafferty rearrangement

The characteristic fragmentation of the molecular ion of a carbonyl compound (ketones and aldehydes) having at least one gamma hydrogen is known as the McLafferty rearrangement.

03

Equations for fragmentation with different m/z values.

(a) For di-n-propyl-ether, m/z values of most abundant fragments are found to be at 73 and 43.

fragmentation equations of di-n-propyl-ether

(b)

fragmentation equations of 2-methoxypentane

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Most popular questions from this chapter

Question. The 2001 Nobel Prize in Chemistry was awarded to three organic chemists who have developed methods for catalytic asymmetric synthesis. An asymmetric (or enantioselective) synthesis is one that converts an achiral starting material into mostly one enantiomer of a chiral product. K. Barry Sharpless (The Scripps Research Institute) developed an asymmetric epoxidation of allylic alcohols that gives excellent chemical yields and greater than 90% enantiomeric excess.

The Sharpless epoxidation uses tert-butyl hydroperoxide, titanium(IV) isopropoxide, and a dialkyl tartarate ester as the reagents. The following epoxidation of geraniol is typical.

  1. Which of these reagents is most likely to be the actual oxidizing agent? That is, which reagent is reduced in the reaction? What is the likely function of the other reagents?
  2. When achiral reagents react to give a chiral product, that product is normally formed as a racemic mixture of enantiomers. How can the Sharpless epoxidation give just one nearly pure enantiomer of the product?
  3. Draw the other enantiomer of the product. What reagents would you use if you wanted to epoxidize geraniol to give this other enantiomer?

Show how you would synthesize the following ethers in good yields from the indicated starting materials and any additional reagents needed.

(a) Cyclopentyl n propyl etherfromcyclopentanolandpropan-1-ol.

(b) n-butyl phenyl etherfromphenolandbutan-1-ol.

(c) 2-ethoxyoctanefrom anoctene

(d) 1-methoxydecanefrom adecene

(e) 1-ethoxy-1-methylcyclohexanefrom 2-methylcyclohexanol.

(f) trans-2,3-epoxyoctane from octane-2-ol.

(a) When ethylene oxide is treated with anhydrous HBr gas, the major product is 1,2-dibromoethane. When ethylene oxide is treated with concentrated aqueous HBr, the major product is ethylene glycol. Use mechanisms to explain these results.

(b) Under base-catalyzed conditions, several molecules of propylene oxide can react to give short polymers. Propose a mechanism for the base-catalyzed formation of the following trimer.

Show how you would convert 2-bromocyclohexanol to the following diol. You may use any additional reagents you need.

Question. (A true story.) An inexperienced graduate student moved into a laboratory and began work. He needed some diethyl ether for a reaction, so he opened an old, rusty 1-gallon can marked 鈥渆thyl ether鈥� and found there was half a gallon left. To purify the ether, the student set up a distillation apparatus, started a careful distillation, and went to the stockroom for the other reagents he needed. While he was at the stockroom, the student heard a muffled 鈥渂oom鈥�. He quickly returned to his lab to find a worker from another laboratory putting out a fire. Most of the distillation apparatus was embedded in the ceiling.

(a) Explain what probably happened.

(b) Explain how this near disaster might have been prevented.
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