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Chapter 25: Q28P. (page 1321)

When an extract of parsley seed is saponified and acidified, one of the fatty acids isolated is petroselenic acid, formula C18H34O2. Hydrogenation of petroselenic acid gives pure stearic acid. When petroselenic acid is treated with warm potassium permanganate followed by acidification, the only organic products are dodecanoic acid and adipic acid. The NMR spectrum shows absorptions of vinyl protons split by coupling constants of 7 Hz and 10 Hz. Propose a structure for petroselenic acid, and show how your structure is consistent with these observations.

Short Answer

Expert verified

The formula C18H34O2 has two degrees of unsaturation -- one is carbonyl and the other is alkene or ring. Catalytic hydrogenation of petroselenic acid gives stearic acid, so the carbon cannot include a ring. It must contain an alkene. Products from KMnO4 oxidation will determine the location of alkene.

HOOC(CH2)4COOH + HOOC(CH2)10CH3 => HOOC(CH2)4 CH=CH(CH2)10 CH3

Step by step solution

01

Petroselenic Acid

The formula C18H34O2 has two degrees of unsaturation -- one is carbonyl and the other is alkene or ring. Catalytic hydrogenation of petroselenic acid gives stearic acid, so the carbon cannot include a ring. It must contain an alkene. Products from KMnO4 oxidation will determine the location of alkene.

HOOC(CH2)4COOH + HOOC(CH2)10CH3 => HOOC(CH2)4 CH=CH(CH2)10 CH3

02

Explanation of coupling constants

If the alkene were trans, then the value of coupling constant for the vinyl protons would be about 15 Hz, and the coupling constant value of 10 Hz indicates cis-alkene. The 7 Hz coupling is from vinyl hydrogens to neighbouring CH2 groups.

Structure of Petroselenic acid

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Most popular questions from this chapter

Oils containing highly unsaturated acids like linolenic acid undergo oxidation in air. This reaction, called oxidative rancidity, is accelerated by heat, explaining why saturated fats are preferred for deep fat frying.

(a) Molecular oxygen is a diradical. What type of mechanism does a diradical suggest for this reaction?

(b) Why is the position shown (carbon-11) a likely site for attack?

(c) Propose a plausible mechanism for this reaction.

(d) BHA and BHT are antioxidants added to foods to interrupt the oxidation mechanism. Suggest how these molecules might work as antioxidants.

A triglyceride can be optically active if it contains two or more different fatty acids

  1. Draw the structure of an optically active triglyceride containing one equivalent of myristic acid and two equivalents of oleic acid.
  2. Draw the structure of an optically inactive triglyceride with the same fatty acid composition.

Draw the structure of an example of each of the following types of lipids:

(a) a saturated fat (b) a polyunsaturated oil (c) a wax (d) a soap (e) a detergent

(f) a phospholipid (g) a prostaglandin (h) a steroid (i) a sesquiterpene

Several commercial laundry soaps contain water-softening agents, usually sodium carbonate (Na2CO3) or sodium phosphate (Na3PO4or Na2HPO4). Explain how these water-softening agents allow soaps to be used in water that is hard by virtue of its-

(a) low pH

(b) dissolved Ca2+,Mg2+and Fe3+salts.

The synthesis of the alkylbenzenesulfonate detergent shown in figure 25-6 begins with the partial polymerization of propylene to give a pentamer.

Show how aromatic substitution reactions can convert this pentamer to the final synthetic detergent.
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