Question:
Given the following hydrocarbon:
$\text{CH}_3 - \text{C}(\text{CH}_3)_2 - \text{CH} = \text{CH} - \text{CH}(\text{CH}_3) - \text{C} \equiv \text{CH}$
The state of hybridisation of carbons marked 2, 3, 5 and 6 in the following hydrocarbon are, respectively :
Solution:
HINT: Count number of $\sigma$ bonds and then find hybridisation.
Explanation:
If number of $\sigma$ bonds = 2; hybridisation is $\text{sp}$.
If number of $\sigma$ bonds = 3; hybridisation is $\text{sp}^2$.
If number of $\sigma$ bonds = 4; hybridisation is $\text{sp}^3$.
Let's analyze the hybridization of the carbons marked 2, 3, 5, and 6 in the given hydrocarbon structure:
Original structure: $\text{CH}_3 - \text{C}(\text{CH}_3)_2 - \text{CH} = \text{CH} - \text{CH}(\text{CH}_3) - \text{C} \equiv \text{CH}$
Numbering the carbons from right to left as shown in the image:
Carbon 1: $\text{CH}$ (at the end of triple bond) - Forms 1 sigma bond with Carbon 2 and 1 sigma bond with Hydrogen. Also forms 2 pi bonds. So, 2 sigma bonds $\Rightarrow \text{sp}$ hybridized.
Carbon 2: $\text{C}$ (in triple bond) - Forms 1 sigma bond with Carbon 1 and 1 sigma bond with Carbon 3. Also forms 2 pi bonds. So, 2 sigma bonds $\Rightarrow \text{sp}$ hybridized.
Carbon 3: $\text{CH}(\text{CH}_3)$ - Forms 1 sigma bond with Carbon 2, 1 sigma bond with Carbon 4, 1 sigma bond with Hydrogen, and 1 sigma bond with the methyl group ($\text{CH}_3$). So, 4 sigma bonds $\Rightarrow \text{sp}^3$ hybridized.
Carbon 4: $\text{CH}$ (in double bond) - Forms 1 sigma bond with Carbon 3, 1 sigma bond with Carbon 5, and 1 sigma bond with Hydrogen. Also forms 1 pi bond. So, 3 sigma bonds $\Rightarrow \text{sp}^2$ hybridized.
Carbon 5: $\text{CH}$ (in double bond) - Forms 1 sigma bond with Carbon 4, 1 sigma bond with Carbon 6, and 1 sigma bond with Hydrogen. Also forms 1 pi bond. So, 3 sigma bonds $\Rightarrow \text{sp}^2$ hybridized.
Carbon 6: $\text{C}(\text{CH}_3)_2$ - Forms 1 sigma bond with Carbon 5 and 3 sigma bonds with methyl groups. So, 4 sigma bonds $\Rightarrow \text{sp}^3$ hybridized.
Carbon 7: $\text{CH}_3$ - Forms 1 sigma bond with Carbon 6 and 3 sigma bonds with Hydrogens. So, 4 sigma bonds $\Rightarrow \text{sp}^3$ hybridized.
We need the hybridization for carbons marked 2, 3, 5, and 6, respectively:
Carbon 2: $\text{sp}$
Carbon 3: $\text{sp}^3$
Carbon 5: $\text{sp}^2$
Carbon 6: $\text{sp}^3$
Therefore, the correct sequence is $\text{sp}$, $\text{sp}^3$, $\text{sp}^2$ and $\text{sp}^3$.