Question

What are the hybrid orbitals of the carbon atoms in these molecules? (a) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CH}_{3}\) (b) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CH}=\mathrm{CH}_{2}\) (c) \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{2} \mathrm{OH}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{O}\) (e) \(\mathrm{CH}_{3} \mathrm{COOH}\).

Short answer

(a) Both carbon atoms are sp3 hybridized. (b) All carbon atoms are sp2 hybridized. (c) The first and third carbon atoms are sp2 hybridized and the second carbon is sp hybridized. (d) The first carbon atom is sp3 hybridized and the second is sp2 hybridized. (e) The first carbon atom is sp3 hybridized and the second is sp2 hybridized.

Step-by-step solution

Analyze the first molecule H3C–CH3

Looking at the first molecule \( \mathrm{H}_{3} \mathrm{C}-\mathrm{CH}_{3} \), you will notice that both carbon atoms form four sigma bonds with hydrogen atoms or another carbon atom. Therefore they are both sp3 hybridized.

Analyze the second molecule H3C–CH=CH2

For \( \mathrm{H}_{3} \mathrm{C}-\mathrm{CH}=\mathrm{CH}_{2} \), the first carbon atom is forming three sigma bonds and is therefore sp2 hybridized. The other two carbon atoms are forming two sigma and one pi bonds which means they are also sp2 hybridized.

Analyze the third molecule CH3–C≡C–CH2OH

In the third compound, \( \mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{2} \mathrm{OH} \), the first and third carbon atoms form three sigma bonds each, hence they are sp2 hybridized. The second carbon atom forms two sigma bonds and one pi bond, which means it is sp hybridized.

Analyze the fourth molecule CH3CH=O

Moving to \( \mathrm{CH}_{3} \mathrm{CH}=\mathrm{O} \), the first carbon atom forms four sigma bonds, implying sp3 hybridization. The second carbon atom forms two sigma bonds and one pi bond, implying sp2 hybridization.

Analyze the fifth molecule CH3COOH

Lastly, for \( \mathrm{CH}_{3} \mathrm{COOH} \), the first carbon is forming four sigma bonds and is sp3 hybridized. The second carbon atom forms two sigma bonds and one pi bond which means it is sp2 hybridized.

Key concepts

Sigma Bonds

Sigma bonds are the primary type of covalent chemical bond. They occur when two atoms share a pair of electrons. This sharing happens along the axis connecting the two bonded nuclei. Imagine it like the handshake between two atoms, where their outermost electrons meet and form a strong connection.
These bonds are formed by the head-on overlap of atomic orbitals. This can include s-s, s-p, or p-p orbital interactions. One common characteristic of sigma bonds is their rotational symmetry. This means that the electron cloud is symmetrical around the bond axis, allowing parts of the molecule to rotate without breaking the bond.

• Example: In methane ( CH₄ ), every hydrogen atom forms a sigma bond with the central carbon atom.
• Sigma bonds are the first bonds to form between two atoms.
• They are generally stronger than pi bonds because of the direct overlap of orbitals.

Pi Bonds

Pi bonds are another form of covalent bond between atoms. They take shape when the orbitals overlap side-by-side instead of end-to-end. Pi bonds are usually found in double or triple bonds, along with a sigma bond.
In chemistry, the pi bond adds extra strength to the connection between two atoms but is weaker than the sigma bond. This is due to the sidelong overlap of the orbitals, which is less effective compared to the straightforward overlap in sigma bonds. They're notably found in unsaturated compounds like alkenes and alkynes.

• Example: In ethylene ( CH₄ ), the carbon atoms share a sigma bond and a pi bond, forming a double bond.
• Pi bonds restrict rotation; hence, parts of the molecule cannot rotate freely around the axis of the bond.
• These bonds are important for the complex structures in polymers and biological molecules.

sp3 Hybridization

sp3 hybridization is a component of molecular geometry and bonding. This occurs when one s orbital and three p orbitals become four equivalent hybrid orbitals. These orbitals are tied to a more tetrahedral shape, which spreads the bonds evenly in 3D space.
In molecules with sp3 hybridization, each hybrid orbital forms a sigma bond. This type of hybridization is typical in molecules like methane ( CH₄ ) where the carbon forms four sigma bonds.

• sp3 hybridized orbitals allow for tetrahedral molecular shapes.
• The bond angles in these structures are approximately 109.5°.
• Common with carbon atoms forming four single covalent bonds.

sp2 Hybridization

sp2 hybridization happens when one s orbital and two p orbitals mix to create three equivalent hybrid orbitals. These are flat or planar, giving a molecule a distinct trigonal planar shape.
In sp2 hybridized molecules, the atoms can form three sigma bonds and one pi bond. This kind of arrangement is prevalent in molecules with double bonds. For example, in ethylene ( C₂H₄ ), each carbon atom is sp2 hybridized.

• This type of hybridization results in a 120° bond angle.
• Molecules are often planar, laying flat on a plane.
• Common in alkenes and aromatics.

sp Hybridization

sp hybridization refers to when one s orbital combines with one p orbital to form two equivalent hybrid orbitals. These hybrid orbitals arrange themselves in a linear geometry. Molecules with sp hybridization, like acetylene ( C₂H₂ ), have atoms that form two sigma bonds and two pi bonds. This is due to the structure of the molecule, which often includes triple bonds.

• The resultant linear geometry has a bond angle of 180°.
• Molecules exhibit sp hybridization when involving triple bonds.
• Common in alkyne chains, like those seen in certain organic compounds.