Question

(a) How many constitutional isomers are possible for a triglyceride containing one molecule each of palmitic acid, oleic acid, and stearic acid? (b) Which of these constitutional isomers are chiral?

Short answer

Answer: There are six possible constitutional isomers for a triglyceride containing one molecule each of palmitic acid, oleic acid, and stearic acid. None of these isomers are chiral.

Step-by-step solution

Review the structures of palmitic, oleic, and stearic acids

Palmitic acid has 16 carbon atoms, oleic acid has 18 carbon atoms and one double bond, and stearic acid has 18 carbon atoms. They all have a carboxylic acid group at one end. To form a triglyceride, each acid will lose its OH group and bind to the glycerol backbone.

Understand the structure of triglycerides

A triglyceride molecule is composed of a glycerol backbone and three fatty acid molecules. Glycerol has three carbon atoms, each bearing an OH group. The fatty acids will react with the glycerol and form ester bonds, replacing the OH groups on the glycerol with fatty acid tails.

Count the possible isomers

To form the triglyceride, we have three positions on the glycerol backbone to which the fatty acids can bind. Since we have three different fatty acids (palmitic, oleic, and stearic acids), we can use the permutations formula to determine the number of constitutional isomers without considering chiral centers: Number of isomers = 3! = 3 × 2 × 1 = 6 There are six possible constitutional isomers for this triglyceride.

Determine chirality

To determine which isomers are chiral, we need to look at the central carbon atom on the glycerol backbone and see if it's attached to four different chemical groups. In this case, the central carbon atom is always attached to two identical ester bonds, and two fatty acids on the other side, so it cannot be a chiral center. Since the central carbon atom is not a chiral center, none of the isomers are chiral.

Summarize the results

There are six possible constitutional isomers for a triglyceride containing one molecule each of palmitic acid, oleic acid, and stearic acid. None of these isomers are chiral.