Question

The IUPAC name of the structure

$C{H}_3-CH=\underset{C{H}_3}{\underset{|}{C}}-C\equiv C-C{H}_3$

• A. 4-Methylhex-4-en-2-yne
• B. 3-Methylhex-4-yn-2-ene
• C. 3-Methylhex-2-en-4-yne
• D. None of the above

Answer 1

The correct option is C

3-Methylhex-2-en-4-yne

The given compound has both a double bond as well as a triple bond.

$C{H}_3-CH=\underset{C{H}_3}{\underset{|}{C}}-C\equiv C-C{H}_3$

When we try to number from either side we see that both the bonds could be accommodated between

$C_2\&C_3$

Case I

$\stackrel{1}{C}{H}_3-\stackrel{2}{C}H=\stackrel{3}{\underset{C{H}_3}{\underset{|}{C}}}-\stackrel{4}{C}\equiv \stackrel{5}{C}-\stackrel{6}{C}{H}_3$

Double bond at $C_2-C_3$

Case II

$\stackrel{6}{C}{H}_3-\stackrel{5}{C}H=\stackrel{4}{\underset{C{H}_3}{\underset{|}{C}}}-\stackrel{3}{C}\equiv \stackrel{2}{C}-\stackrel{1}{C}{H}_3$

Trible bond at $C_2-C_3$

So both these bonds, via different numbering schemes, could be accommodated at $C_2-C_3$.

When we have a choice, the double bond always gets a higher priority (or lower numbering)

$\stackrel{1}{C}{H}_3-\stackrel{2}{C}H=\stackrel{3}{\underset{C{H}_3}{\underset{|}{C}}}-\stackrel{4}{C}\equiv \stackrel{5}{C}-\stackrel{6}{C}{H}_3$

So, looking at the longest chain our word root would be hex

We have double bond at 2 and triple bond at 4, so 2-en and 4-yn

Final name would be 3-Methylhex-2-en-4-yne