Draw a Lewis structure for a compound with molecular formula C4H11N in which three of the carbon atoms are bonded to the nitrogen atom. What is the geometry of the nitrogen atom in this compound? Does this compound exhibit a molecular dipole moment? If so, indicate the direction of the dipole moment.
The correct answer and explanation is:
Correct Answer:
- Lewis Structure:
The compound with formula C₄H₁₁N, where three carbon atoms are bonded to the nitrogen, is trimethylamine, also known as (CH₃)₃N.
H H H
\ | /
H3C — N — C — H
/
H3C
(Each CH₃ represents a methyl group: carbon bonded to three hydrogens.)
- Geometry around Nitrogen:
- Trigonal pyramidal
- Molecular dipole moment:
- Yes, there is a net dipole moment.
- Direction: From the center of the three methyl groups toward the nitrogen atom (since nitrogen is more electronegative).
Explanation (≈300 words):
The molecular formula C₄H₁₁N corresponds to several possible structural isomers, but the one where three carbon atoms are bonded to the nitrogen is trimethylamine. This compound consists of a nitrogen atom bonded to three methyl groups (CH₃) and one lone pair of electrons.
Each methyl group contains a carbon bonded to three hydrogen atoms. The nitrogen atom forms three sigma bonds with the carbon atoms and holds one lone pair. This arrangement leads to a trigonal pyramidal geometry around the nitrogen atom. This shape arises because of the lone pair-bond pair repulsion, which pushes the bonded atoms down, creating a pyramid-like structure. The geometry is predicted by VSEPR theory (AX₃E type).
Nitrogen is more electronegative than carbon and hydrogen, which means it pulls electron density towards itself. In trimethylamine, this causes a separation of charge — the nitrogen becomes slightly negative (δ⁻), while the methyl groups are slightly positive (δ⁺). The three C–N bonds are not arranged symmetrically in a plane, so their dipole moments do not cancel out.
As a result, trimethylamine has a net dipole moment, pointing from the methyl groups toward the nitrogen atom. This dipole moment influences the compound’s physical properties, such as its boiling point, solubility, and how it interacts with polar solvents like water.
In summary, the Lewis structure shows nitrogen bonded to three methyl groups with a lone pair, leading to a trigonal pyramidal shape and a net molecular dipole directed toward nitrogen.