Q 7P - Organic Chemistry (Mcmurry)

Question

Question: Which member in each of the following sets ranks higher?  

$$\begin{gathered} \mathbf{a}\mathbf{)}\mathbf{ }\mathbf{-}\mathbf{H}\mathbf{ }\mathbf{or}\mathbf{ }\mathbf{-}\mathbf{Br} \\ \mathbf{b}\mathbf{)}\mathbf{ }\mathbf{-}\mathbf{CI}\mathbf{ }\mathbf{or}\mathbf{ }\mathbf{-}\mathbf{Br} \\ \mathbf{c}\mathbf{)}\mathbf{ }\mathbf{-}{\mathbf{CH}}_{\mathbf{3}}\mathbf{ }\mathbf{or}\mathbf{ }\mathbf{-}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{3}} \\ \mathbf{d}\mathbf{)}\mathbf{ }\mathbf{-}{\mathbf{NH}}_{\mathbf{2}}\mathbf{ }\mathbf{or}\mathbf{ }\mathbf{-}\mathbf{OH} \\ \mathbf{e}\mathbf{)}\mathbf{ }\mathbf{-}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}\mathbf{ }\mathbf{or}\mathbf{ }\mathbf{-}{\mathbf{CH}}_{\mathbf{2}} \\ \mathbf{f}\mathbf{)}\mathbf{ }\mathbf{-}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}\mathbf{ }\mathbf{or}\mathbf{ }\mathbf{-}\mathbf{CH}\mathbf{=}\mathbf{O} \end{gathered}$$

Step-by-Step Solution

Answer

$$\begin{gathered} \mathrm{a})-\mathrm{Br} \\ \mathrm{b}) -\mathrm{Br} \\ \mathrm{c}) -{\mathrm{CH}}_2{\mathrm{CH}}_3 \\ \mathrm{d}) -\mathrm{OH} \\ \mathrm{e}) -{\mathrm{CH}}_2\mathrm{OH} \\ \mathrm{f}) -\mathrm{CH}=\mathrm{O} \end{gathered}$$

Step 1: Sequence rule for specifying rankings

Rule 1

Numbering of all the four atoms directly attached to the chiral center done according to their atomic number. The atom with the highest atomic number has the highest ranking (first), and the atom with the lowest atomic number has the lowest ranking (fourth). 

Rule 2

If the first atoms are the same in the substituent then look at the second, third, or fourth atoms away from the chiral center until able to find the first difference.

Rule 3

If two atoms are bonded by multiple-bonded then it is counted as equivalent to the same number of single-bonded atoms.

Step 2: Given substituents ranking of -H or –Br

H has atomic number 1 and Br has atomic number 35. Here it can be seen that -Br will have a higher ranking as it has a higher atomic number than -H.

Step 3: Given substituents ranking of -Cl or -Br

Cl has atomic number 17 and Br has atomic number 35. Here it can be seen that -Br will have a higher ranking as it has a higher atomic number than -Cl. 

Step 4: Given substituents ranking of - CH 3   or   - CH 2 CH 3

Here first atom of substituents is same –C. In ${\mathrm{CH}}_3$ the carbon is attached to 3 H and in  $-{\mathrm{CH}}_2{\mathrm{CH}}_3$ the carbon is attached to 2 H and 1 C. Carbon has atomic number 12 and H has atomic number 1. So, $-{\mathrm{CH}}_2{\mathrm{CH}}_3$ will have a higher ranking than ${\mathrm{CH}}_3$ . 

Step 5: Given substituents ranking of - NH 2   or   - OH

N has atomic number 14 and oxygen has atomic number 16. So, $\mathrm{OH}$ has a higher ranking than ${\mathrm{NH}}_2$ . 

Step 6: Given substituents ranking of - CH 2 OH   or   - CH 3

Here the first atom of substituents is the same –C. In ${\mathrm{CH}}_3$ the carbon is attached to 3 H and in  ${\mathrm{CH}}_2\mathrm{OH}$ the carbon is attached to 2 H and 1 O. Oxygen has atomic number 16 and H has atomic number 1. So,$-{\mathrm{CH}}_2\mathrm{OH}$ will have a higher ranking than ${\mathrm{CH}}_3$. 

Step 7: Given substituents ranking of - CH 2 OH   or   - CH = O

Here the first atom of substituents is the same –C. In $-\mathrm{CH}=\mathrm{O}$ the carbon is attached to 1 H and 2 oxygen as a double bond indicates carbon is bonded to oxygen two times while in the carbon is attached to 2 H and 1 O. Oxygen has atomic number 16 and H  has atomic number 1. So,$-\mathrm{CH}=\mathrm{O}$ has a higher ranking than ${\mathrm{CH}}_2\mathrm{OH}$.