Draw the Lewis structure for the conjugate acid of methylamine, CH3NH2. Include the formal charge on the appropriate atom in the Lewis structure
The Correct Answer and Explanation is:
Correct Answer:
The conjugate acid of methylamine (CH₃NH₂) is CH₃NH₃⁺. Here is its Lewis structure:
markdownCopyEdit H H
\ /
H - N⁺ - CH₃
|
H
Alternatively, written with lone pairs:
- Carbon (C) in CH₃ has 4 bonds: 3 with H and 1 with N.
- Nitrogen (N) in NH₃⁺ has 4 bonds: 3 with H atoms and 1 with C. It has no lone pairs and carries a formal positive charge (⁺).
- Hydrogen (H) atoms each have one bond.
Explanation
To determine the Lewis structure for the conjugate acid of methylamine (CH₃NH₂), we first identify the structure of the original compound. Methylamine consists of a methyl group (CH₃) attached to an amino group (NH₂). The nitrogen in methylamine has a lone pair of electrons, which makes it a Brønsted-Lowry base capable of accepting a proton (H⁺).
When methylamine accepts a proton, the lone pair on nitrogen forms a bond with the H⁺, resulting in CH₃NH₃⁺, the conjugate acid. In this new structure, nitrogen forms four covalent bonds: three with hydrogen atoms and one with the carbon from the methyl group. Since nitrogen normally forms three bonds and has one lone pair, adding a fourth bond results in the loss of that lone pair and gives nitrogen a formal positive charge.
To calculate the formal charge on nitrogen, we use the formula:
Formal charge = Valence electrons – (Lone pair electrons + ½ Bonding electrons)
For nitrogen:
- Valence electrons = 5
- Lone pair electrons = 0 (after bonding with H⁺)
- Bonding electrons = 8 (4 bonds)
So:
Formal charge = 5 – (0 + 8/2) = 5 – 4 = +1
Thus, nitrogen carries a +1 charge, making it the formal site of the positive charge in the conjugate acid. The structure remains stable due to the full octet on all atoms and proper placement of the charge. This conjugate acid plays an important role in acid-base chemistry, particularly in aqueous or biological environments.
