Organic Chemistry – Some Basic Principles and Techniques - Tetravalence of Carbon: Shapes of Organic Compounds

Carbon has an atomic number of 6 with the electronic configuration $[He] 2s^2 2p^2$. To form four bonds, it undergoes excitation to $[He] 2s^1 2p_x^1 2p_y^1 2p_z^1$, demonstrating its tetravalence.

Hybridization is the process of mixing atomic orbitals to form new hybrid orbitals. In organic compounds, carbon undergoes three types of hybridization: $sp^3$, $sp^2$, and $sp$.

$sp^3$ Hybridization: Resulting in a tetrahedral shape with bond angles of $109.5^\circ$. It occurs when carbon is bonded to four other atoms via single bonds (e.g., $CH_4$).

$sp^2$ Hybridization: Resulting in a trigonal planar shape with bond angles of $120^\circ$. It occurs when carbon is bonded to three other atoms (e.g., $CH_2=CH_2$), involving one $\pi$ bond.

$sp$ Hybridization: Resulting in a linear shape with a bond angle of $180^\circ$. It occurs when carbon is bonded to two other atoms (e.g., $HC \equiv CH$), involving two $\pi$ bonds.

Bond Length and Strength: $sp$ hybridized carbons have the shortest bond length and highest bond strength due to higher s-character, whereas $sp^3$ has the longest bond length.

Electronegativity: The electronegativity of carbon increases with the increase in s-character. Therefore, the order is $sp > sp^2 > sp^3$.

Sigma ($\sigma$) bonds are formed by the axial (head-on) overlap of orbitals, while Pi ($\pi$) bonds are formed by the lateral (sideways) overlap of unhybridized p-orbitals.