Predict the approximate size of the following bond angles.
(b) the C—N—C bond angle in a secondary amine

Verified step by step guidance

1. Step 1: Recall the concept of hybridization and molecular geometry. In a secondary amine, the nitrogen atom is sp³ hybridized because it forms three sigma bonds (two C—N bonds and one N—H bond) and has one lone pair of electrons.
2. Step 2: Understand the effect of lone pairs on bond angles. Lone pairs occupy more space than bonding pairs due to their higher electron density, which causes bond angles to be slightly smaller than the ideal tetrahedral angle of 109.5°.
3. Step 3: Consider the geometry around the nitrogen atom. The molecular geometry is trigonal pyramidal due to the lone pair on nitrogen, which pushes the bonded atoms closer together.
4. Step 4: Predict the approximate bond angle. The C—N—C bond angle in a secondary amine will be slightly less than 109.5°, typically around 107°, due to the repulsion caused by the lone pair.
5. Step 5: Relate this prediction to real-world observations. Bond angles in molecules with lone pairs are consistently smaller than the ideal tetrahedral angle, and this trend is observed in secondary amines as well.

Key Concepts

Hybridization

Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals that can accommodate bonding. In the case of a secondary amine, the nitrogen atom typically undergoes sp3 hybridization, leading to a tetrahedral arrangement of electron pairs, which influences the bond angles around the nitrogen.

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) Theory is used to predict the geometry of molecules based on the repulsion between electron pairs. For a secondary amine, the presence of lone pairs on the nitrogen atom and the bonding pairs with carbon atoms will determine the bond angles, typically resulting in angles close to 109.5 degrees.

Bond Angles in Organic Molecules

Bond angles in organic molecules are influenced by the type of hybridization and the steric effects of surrounding atoms. In secondary amines, the C—N—C bond angle is generally expected to be around 109.5 degrees due to the tetrahedral geometry associated with sp3 hybridization, although it may be slightly affected by the presence of lone pairs.