Q13P-a - Organic Chemistry (Mcmurry)

Question

Assign E or Z configuration to the following alkenes:

Step-by-Step Solution

Answer

Z configuration

Step 1: E and Z designation

The cis-trans naming system works only for disubstituted alkenes having two substituents other than hydrogen on the double bond. But when trisubstituted and tetrasubstituted of the double bond is present then the E and Z system is used for describing alkene stereochemistry.

Step 2: Rules for E and Z designation

• Consider each of the double-bond carbons separately and ranks the two substituents according to the atomic number of the first atom in each. An atom with a higher atomic number will rank higher.

 

• If the first atom same then, look at the second, third, or fourth atoms away from the double-bond until the first difference is found.

 

• Multiple-bonded atoms are equivalent to the same number of single-bonded atoms.
• Once the groups attached have been ranked look at the entire molecule. If the higher ranked groups on each carbon are on the same side of the double bond then alkene is said to have Z geometry and if present on opposite sides then the alkene has E geometry. 

Step 3: Ranking of given groups

As ranking is atomic number dependent, more the atomic number higher will be ranking. So, 

In ${\text{-CH}}_3$ and ${\text{-CH}}_2{\text{CH}}_3$:

• In ${\text{-CH}}_3$ carbon is attached three hydrogens.
• In ${\text{-CH}}_2{\text{CH}}_3$ carbon is attached to two hydrogens and one carbon.

So, ${\text{-CH}}_2{\text{CH}}_3$ rank 1.

In ${\text{-CH}}_2\text{OH}$ and $-\text{Cl}$ :

• In ${\text{-CH}}_2\text{OH}$ carbon is attached to two hydrogens and one oxygen.
• In $-\text{Cl}$ has a higher atomic number than carbon.

So,$-\text{Cl}$  rank 1.

Step 4: Configuration

From the structure of the given compound, it can be seen both high ranking groups on the same side so the Z configuration.