Chapter 25, Problem 17P

Interpretation Introduction

Interpretation:

In the structures of the given compounds, the carbon atoms that are expected to be labeled with ${}^{\text{14}}\text{C}$ in its methyl group are to be identified.

Concept introduction:

The isotopically labeled carbons in various compounds that are synthesized biologically enriched with ${}^{\text{14}}\text{C}$ in its methyl group are labeled at carbons $2,4,6,8$ and so on, also at the carbon atoms of the branching methyl groups in the structure.

The organic material are dated using radioactive isotope of carbon ${}^{\text{14}}\text{C}$, whose decay process allows to find the age of an object.

Answer to Problem 17P

Solution:

a) There are $8$ carbon atoms that are expected to be labeled with ${}^{\text{14}}\text{C}$ in the methyl groups of palmitic acid. They are shown below.

b) There are $10$ carbon atoms that are expected to be labeled with ${}^{\text{14}}\text{C}$ in the methyl groups of ${\text{PGE}}_{\text{2}}$. They are shown below.

c) There are $10$ carbon atoms that are expected to be labeled with ${}^{\text{14}}\text{C}$ in the methyl groups of ${\text{PGI}}_{\text{2}}$. They are shown below.

d) There are $3$ carbon atoms that are expected to be labeled with ${}^{\text{14}}\text{C}$ in the methyl groups of limonene. They are shown below.

e) There are $12$ carbon atoms that are expected to be labeled with ${}^{\text{14}}\text{C}$ in the methyl groups of $\text{β-Carotene}$. They are shown below.

Explanation of Solution

a) The given compound is Palmitic acid.

The isotopically labeled carbon atoms in palmitic acid synthesized from the acetate enriched with ${}^{\text{14}}\text{C}$ in its methyl group are labeled at the even carbon atoms, $2,4,6,8$ and so on as shown below.

b) The given compound is ${\text{PGE}}_{\text{2}}$, prostaglandin ${\text{E}}_{\text{2}}$.

The isotopically labeled carbon atoms in ${\text{PGE}}_{\text{2}}$ enriched with ${}^{\text{14}}\text{C}$ in its methyl group are shown below.

c) The given compound is ${\text{PGI}}_{\text{2}}$, prostaglandin ${\text{I}}_{\text{2}}$.

The isotopically labeled carbon atoms in ${\text{PGI}}_{\text{2}}$ enriched with ${}^{\text{14}}\text{C}$ in its methyl group are shown below.

d) The given compound is limonene.

It is synthesized by the geranyl diphosphate that is further synthesized from dimethylallyl phosphate and isopentenyl diphosphate. The isotopically labeled carbon atoms in limonene enriched with ${}^{\text{14}}\text{C}$ in its methyl group are shown below.

e) The given compound is $\text{β-Carotene}$.

The isotopically labeled carbon atoms in $\text{β-Carotene}$ enriched with ${}^{\text{14}}\text{C}$ in its methyl group are shown below.