Write the structure of Hex-1,3-dien-5-yne

The name tells us:

• Hex $\rightarrow$ 6 carbon atoms
• 1,3-dien $\rightarrow$ double bonds at positions 1 and 3
• 5-yne $\rightarrow$ triple bond at position 5

So the carbon chain is: $\mathrm{C_1=C_2-C_3=C_4-C_5\equiv C_6}$

Now add hydrogens to satisfy valency: $\mathrm{CH_2=CH-CH=CH-C\equiv CH}$

Count the $\sigma$ bonds

(a) Carbon-carbon $\sigma$ bonds

Every bond between two carbons contributes one $\sigma$ bond, whether it is single, double, or triple.

There are 5 C-C connections in a 6-carbon chain: $C_1-C_2,\; C_2-C_3,\; C_3-C_4,\; C_4-C_5,\; C_5-C_6$ So, $\text{C-C }\sigma\text{ bonds} = 5$

(b) Carbon-hydrogen $\sigma$ bonds

From the structure $\mathrm{CH_2=CH-CH=CH-C\equiv CH}$:

• $\mathrm{C_1}$ has 2 H
• $\mathrm{C_2}$ has 1 H
• $\mathrm{C_3}$ has 1 H
• $\mathrm{C_4}$ has 1 H
• $\mathrm{C_5}$ has 0 H
• $\mathrm{C_6}$ has 1 H

Total H atoms: $2+1+1+1+0+1=6$ Hence, $\text{C-H }\sigma\text{ bonds} = 6$

Therefore total $\sigma$ bonds: $\sigma = 5+6=11$

Count the $\pi$ bonds

• Each double bond has 1 $\pi$ bond
• Each triple bond has 2 $\pi$ bonds

Here we have:

• 2 double bonds $\rightarrow 2\pi$
• 1 triple bond $\rightarrow 2\pi$

So, $\pi = 2+2=4$

Total number of $\sigma + \pi$ bonds

$\sigma + \pi = 11+4=15$

Comparison with stored answer

Stored correct answer = 15

Our derived answer = 15

Hence, the derived answer matches the stored correct answer.