Question

$\mathbf{\alpha }$-Terpinene,${\mathbf{C}}_{\mathbf{10}}{\mathbf{H}}_{\mathbf{16}}$is a pleasant-smelling hydrocarbon that has been isolated from oil of marjoram. On hydrogenation over a palladium catalyst,$\mathbf{\alpha }\mathbf{-}$terpinene reacts with two molar equivalents${\mathbf{H}}_{\mathbf{2}}$to yield a hydrocarbon, ${\mathbf{C}}_{\mathbf{10}}{\mathbf{H}}_{\mathbf{20}}$.On ozonolysis, followed by reduction with zinc and acetic acid,$\mathbf{\alpha }\mathbf{-}$terpinene delivers two products, glyoxal, and 6-methyl-2,5- heptane dione.

(a) How many degrees of unsaturation does$\mathbf{\alpha }\mathbf{-}$terpinene have?

(b) How many double bonds and how many rings does it have?

(c) Propose a structure for$\mathbf{\alpha }\mathbf{-}$terpinene

Short answer

a. $\mathrm{\alpha }-$Terpinene ${\mathrm{C}}_{10}{\mathrm{H}}_{16}$has three degrees of unsaturation.

b. It has two double bonds and one six-membered ring.

c.

Step-by-step solution

Degree of unsaturation

It is also known as the Index of Hydrogen Efficiency (IHD). It determines the number of rings and bonds.

Terpinene has three degrees of unsaturation due to 2 double bonds and one ring residue.

$\mathrm{Degree}\mathrm{of}\mathrm{unsaturation}:\mathrm{ring}+\mathrm{\pi }\mathrm{bond}=\mathrm{C}-\frac{\mathrm{H}}{2}-\frac{\mathrm{X}}{2}+\frac{\mathrm{N}}{2}$

Number of double bonds and rings

It has one six-membered ring and two double bonds.

Structure of α-Terpinene

On hydrogenation over a palladium catalyst,$\mathrm{\alpha }-$terpinene reacts with two molar equivalents${\mathrm{H}}_2$to yield a hydrocarbon—${\mathrm{C}}_{10}{\mathrm{H}}_{20}$. On ozonolysis, followed by reduction with zinc and acetic acid, $\mathrm{\alpha }-$terpinene delivers two products, glyoxal, and 6-methyl-2,5- heptane dione.