Q49E - Organic Chemistry (Mcmurry)

Question

Occasionally, a chemist might need to invert the stereochemistry of an alkene—that is, to convert a cis alkene to a trans alkene, or vice versa. There is no one-step method for doing an alkene inversion, but the transformation can be carried out by combining several reactions in the proper sequence. How would you carry out the following reactions? 

(a) $\mathrm{tran}-5-\mathrm{decene}\stackrel{?}{\to }\mathbf{cis}\mathbf{-}\mathbf{5}\mathbf{-}\mathbf{decene}$

(b) $\mathrm{cis}-5-\mathrm{decene}\stackrel{?}{\to }\mathbf{trans}\mathbf{-}\mathbf{5}\mathbf{-}\mathbf{decene}$

Step-by-Step Solution

Answer

a,

b. 

Step-by-Step Solution Step 1: Conversion of trans isomer to the cis isomer

When two similar groups lie on the same side of the double bond then cis isomer.

If the similar groups lie on the opposite face of the double bond then trans isomer.

As rotation around a single bond is restricted. So the double cis isomer is converted to 

trans isomer using UV light, heat, etc.

Step 2: Conversion of trans-5-decene to cis-5-decene

In the first step, trans-5-decene is allowed to react with ${\mathrm{Br}}_2$ the in the presence of ${\text{CH}}_{\text{2}}{\text{Cl}}_{\text{2}}$ it gives trans addition of bromine. In the second step intermediate form is reacted with to give 5-decyne. This 5-decyne is reacted with Lindlar catalyst to give cis-5-decene.

Step 3: Mechanism of the reaction

The reaction is to proceed by following the mechanism:

Step 4: Conversion of cis-5-decene to trans-5-decene

In the first step, cis-5-decene is allowed to react with the $B{r}_2$ in the presence of  ${\text{CH}}_{\text{2}}{\text{Cl}}_{\text{2}}$it gives anti addition of bromine. In the second step intermediate form is reacted with  to give 5-decyne. This 5-decyne is reacted with $\mathrm{Na}$ in liquid ${\mathrm{NH}}_3$ to give trans-5-decene.

Step 5: Mechanism of the reaction

The reaction is to proceed by the following mechanism;