Understanding Hybridization in Organic Molecules

Hybridization is a concept in chemistry where atomic orbitals mix to form new hybrid orbitals. The type of hybridization around a carbon atom depends on the number of sigma bonds it forms and the presence of lone pairs (which are usually absent on carbon in these types of molecules). For carbon atoms in organic compounds, we commonly encounter sp³, sp², and sp hybridization.

• sp³ Hybridization: Occurs when a carbon atom forms four sigma bonds. This is typical for carbon atoms involved only in single bonds. Each bond is a sigma bond.
• sp² Hybridization: Occurs when a carbon atom forms three sigma bonds and one pi bond. This happens when a carbon atom is involved in one double bond (one sigma, one pi) and two single bonds (two sigma).
• sp Hybridization: Occurs when a carbon atom forms two sigma bonds and two pi bonds. This happens when a carbon atom is involved in one triple bond (one sigma, two pi) and one single bond (one sigma), or two double bonds (each double bond is one sigma, one pi).

We need to find the molecule where the carbon atoms, read from left to right, have the hybridization sequence sp³, sp², sp², sp. Let's examine each option:

Analyzing Hybridization for Each Molecule

Option 1: CH ≡ C - C ≡ CH

Let's look at the hybridization of each carbon atom from left to right:

• First C (CH): Forms one C-H single bond and one C≡C triple bond. It forms 1 sigma (C-H) + 1 sigma (C≡C) = 2 sigma bonds. Hybridization is sp.
• Second C (-C-): Forms one C≡C triple bond and one C-C single bond. It forms 1 sigma (C≡C) + 1 sigma (C-C) = 2 sigma bonds. Hybridization is sp.
• Third C (-C-): Forms one C-C single bond and one C≡C triple bond. It forms 1 sigma (C-C) + 1 sigma (C≡C) = 2 sigma bonds. Hybridization is sp.
• Fourth C (CH): Forms one C≡C triple bond and one C-H single bond. It forms 1 sigma (C≡C) + 1 sigma (C-H) = 2 sigma bonds. Hybridization is sp.

The sequence of hybridization is sp, sp, sp, sp. This does not match sp³, sp², sp², sp.

Option 2: CH ₂= CH - C ≡ CH

Let's look at the hybridization of each carbon atom from left to right:

• First C (CH₂): Forms two C-H single bonds and one C=C double bond. It forms 2 sigma (C-H) + 1 sigma (C=C) = 3 sigma bonds. Hybridization is sp².
• Second C (-CH=): Forms one C=C double bond, one C-C single bond, and one C-H single bond. It forms 1 sigma (C=C) + 1 sigma (C-C) + 1 sigma (C-H) = 3 sigma bonds. Hybridization is sp².
• Third C (-C-): Forms one C-C single bond and one C≡C triple bond. It forms 1 sigma (C-C) + 1 sigma (C≡C) = 2 sigma bonds. Hybridization is sp.
• Fourth C (CH): Forms one C≡C triple bond and one C-H single bond. It forms 1 sigma (C≡C) + 1 sigma (C-H) = 2 sigma bonds. Hybridization is sp.

The sequence of hybridization is sp², sp², sp, sp. This does not match sp³, sp², sp², sp.

Option 3: CH ₃- CH = CH - CH ₃

Let's look at the hybridization of each carbon atom from left to right:

• First C (CH₃): Forms three C-H single bonds and one C-C single bond. It forms 3 sigma (C-H) + 1 sigma (C-C) = 4 sigma bonds. Hybridization is sp³.
• Second C (-CH=): Forms one C-C single bond, one C=C double bond, and one C-H single bond. It forms 1 sigma (C-C) + 1 sigma (C=C) + 1 sigma (C-H) = 3 sigma bonds. Hybridization is sp².
• Third C (=CH-): Forms one C=C double bond, one C-C single bond, and one C-H single bond. It forms 1 sigma (C=C) + 1 sigma (C-C) + 1 sigma (C-H) = 3 sigma bonds. Hybridization is sp².
• Fourth C (-CH₃): Forms one C-C single bond and three C-H single bonds. It forms 1 sigma (C-C) + 3 sigma (C-H) = 4 sigma bonds. Hybridization is sp³.

The sequence of hybridization is sp³, sp², sp², sp³. This does not match sp³, sp², sp², sp.

Option 4: CH ₃- CH = CH - CN

Let's look at the hybridization of each carbon atom from left to right:

• First C (CH₃): Forms three C-H single bonds and one C-C single bond. It forms 3 sigma (C-H) + 1 sigma (C-C) = 4 sigma bonds. Hybridization is sp³.
• Second C (-CH=): Forms one C-C single bond, one C=C double bond, and one C-H single bond. It forms 1 sigma (C-C) + 1 sigma (C=C) + 1 sigma (C-H) = 3 sigma bonds. Hybridization is sp².
• Third C (=CH-): Forms one C=C double bond, one C-C single bond, and one C-H single bond. It forms 1 sigma (C=C) + 1 sigma (C-C) + 1 sigma (C-H) = 3 sigma bonds. Hybridization is sp².
• Fourth C (-CN): Forms one C-C single bond and one C≡N triple bond. It forms 1 sigma (C-C) + 1 sigma (C≡N) = 2 sigma bonds. Hybridization is sp. (Note: The carbon is triple bonded to Nitrogen).

The sequence of hybridization is sp³, sp², sp², sp. This exactly matches the required sequence.

Conclusion

By analyzing the number of sigma bonds around each carbon atom in the given molecules, we determined their hybridization. The molecule CH ₃- CH = CH - CN shows the carbon hybridization sequence sp³, sp², sp², sp from left to right.