Q12P-a - Organic Chemistry (Mcmurry)

Question

Rank the substituents in each of the following sets according to the sequence rules:

$-{\mathrm{CH}}_3,-\mathrm{OH},-\mathrm{H},-\mathrm{Cl}$

Step-by-Step Solution

Answer

Higher to lower-ranking: $-\mathrm{Cl}>-\mathrm{OH}>-{\mathrm{CH}}_3>-\mathrm{H}$

Step 1: E and Z designation

This naming scheme only applies to substituted alkenes that have two substituents other than hydrogen on the double bond. But when trisubstituted and tetrasubstituted of the double bond are present then the E and Z system is used for describing alkene stereochemistry.

Step 2: Rule for ranking substituents

• 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.

Step 3: Ranking of given groups

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

• Chlorine ranks higher than oxygen.
• Oxygen ranks higher than carbon.
• Carbon ranks higher than hydrogen.
• Higher to lower ranking: $-\mathrm{Cl}>-\mathrm{OH}>-{\mathrm{CH}}_3>-\mathrm{H}$