Give the different combinations of RX and R'X that can be used to synthesize 3-methylpentane. Write the IUPAC name for rach of the RX and R'X?

1 Answer
Sep 28, 2015

You could use any of three combinations.

I presume that you are referring to the Wurtz reaction:

"R-X + 2Na + R'X" → "R-R' + 2NaX"

You want to make 3-methylpentane

3-Methylpentane3-Methylpentane

We must work backwards from the product to figure out the starting materials.

We must cut a C-C bond to get two alkyl groups for the starting material.

Scheme 1

We could cut off the methyl group:

Scheme 1Scheme 1

This gives us a "CH"_3"·" and a "CH"_3"CH"_2stackrelcolor(blue)(·)("C")"HCH"_2"CH"_3 group.

The reaction is then

underbrace("CH"_3"Br")_color(red)("bromomethane") + "2Na" + underbrace("CH"_3"CH"_2"CHBrCH"_2"CH"_3)_color(red)("3-bromopentane") → "CH"_3"CH"_2"CH"("CH"_3)"CH"_2"CH"_3+ "2NaBr"

Scheme 2

We could cut the bond between "C-2" and "C-3":

Scheme 2Scheme 2

This gives us a "CH"_3"CH"_2 stackrelcolor (blue)(·)("C")"HCH"_3 and a "·CH"_2"CH"_3 group.

The reaction is then

underbrace("CH"_3"CH"_2"CHBrCH"_3)_color(red)("2-bromobuthane") + "2Na" + underbrace("CH"_3"CH"_2"Br")_color(red)("bromoethane") → "CH"_3"CH"_2"CH"("CH"_3)"CH"_2"CH"_3 + "2NaBr"

Scheme 3

Finally, we could cut the bond between "C-1" and "C-2":

Scheme 3Scheme 3

This gives us a "CH"_3"CH"_2"CH"("CH"_3) "CH"_2"·" and a "·CH"_3 group.

The reaction is then

underbrace("CH"_3"CH"_2"CH"("CH"_3)"CH"_2"Br")_color(red)("2-bromo-3-methylbutane") + "2Na" + underbrace("CH"_3"Br")_color(red)("bromomethane") → "CH"_3"CH"_2"CH"("CH"_3)"CH"_2"CH"_3 + "2NaBr"

Note: None of these schemes will give you good yields of product.

Statistically, the product will consist of 50 % "R-R'", 25 % "R-R", and 25 % "R'-R'".