Question 16b44

Dec 9, 2016

The order is ("CH"_3)_3"C-" > "CH"_2"=CH-" > ("CH"_3)_2"CH-" > "CH"_3"CH"_2"-"

Explanation:

The Cahn-Ingold-Prelog order is based on the atomic numbers of the atoms attached to the chiral centre.

If there is a tie, we go one atom further out and keep going until the tie is broken.

In this problem, the atoms attached to the chiral centre are all carbon atoms, for
a 4-way tie.

To break the tie, we look at the atoms that are directly attached to these carbons.

First trial

$\text{C1}$ of the vinyl group is attached to another carbon by a $\text{C=C}$ double bond.

Each carbon in the double bond counts as two carbons.

So, $\text{C1}$ is attached to $\text{C, C}$, and $\text{H}$, written as ($\text{C,C,H}$) with the atoms listed in order of decreasing atomic number.

$\text{C1}$ of the isopropyl group is attached to $\text{C, C}$, and $\text{H}$ ($\text{C,C,H}$).

$\text{C1}$ of the tert-butyl group is attached to ($\text{C,C,C}$).

$\text{C1}$ of the ethyl group is attached to ($\text{C,H,H}$).

We now compare the atoms in parentheses and look at the first point of difference.

We find tert-butyl ($\text{C,C,C}$) > vinyl ($\text{C,C,H}$) = isopropyl ($\text{C,C,H}$) > ethyl ($\text{C,H,H}$).

So, tert-butyl is Priority 1 and ethyl is Priority 4, but vinyl and isopropyl are tied in the middle.

Second trial

To break the tie, we look at the atoms directly attached to $\text{C2}$.

$\text{C2}$ in the vinyl group is attached to ($\text{C,H,H}$).

$\text{C2}$ in the isopropyl group is attached to ($\text{H,H,H}$).

So the vinyl group wins and gets Priority 2, and the isopropyl group is Priority 3.

The order is ("CH"_3)_3"C-" > "CH"_2"=CH-" > ("CH"_3)_2"CH-" > "CH"_3"CH"_2"-"#