Question

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?

Answer 1

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

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:

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"`:

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"`:

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'"`.