Methanol (CH3-OH) is the simplest of the organic alcohols and contains a C-O single (σ) bond. The electronegativity of carbon is 2.5, and the electronegativity of oxygen is 3.5. Draw the structure of methanol, including all lone pairs where appropriate (2) Calculate ΔEN and describe the electron distribution (polar, non-polar, ionic) within this bond (1.5) Draw the net dipole moment for this molecule (0.5)
The Correct Answer and Explanation is:
Structure of Methanol (CH₃OH):
Here’s the Lewis structure for methanol, including lone pairs:
H
|
H — C — O — H
|
H
- Carbon (C) forms 4 single bonds: three with hydrogen atoms and one with oxygen.
- Oxygen (O) forms two bonds: one with carbon and one with hydrogen, and it has two lone pairs of electrons.
Electronegativity Difference (ΔEN) and Bond Type:
- Electronegativity of Carbon (C) = 2.5
- Electronegativity of Oxygen (O) = 3.5
- ΔEN = |3.5 – 2.5| = 1.0
Since the difference in electronegativity is between 0.5 and 1.9, this bond is classified as polar covalent. The electrons in the C–O bond are more strongly attracted to the oxygen atom, making it partially negative (δ⁻), while carbon becomes partially positive (δ⁺). This uneven electron distribution creates a bond dipole pointing from carbon to oxygen.
Net Dipole Moment:
Methanol is not a symmetric molecule; the electronegative oxygen atom pulls electron density toward itself. In addition to the C–O bond, the O–H bond is also polar due to a ΔEN of 1.4 (O = 3.5, H = 2.1), contributing further to an overall dipole.
📍 Net dipole moment direction: From the partially positive region (carbon/hydrogens) toward the oxygen atom.
📐 Representation:
δ−
↑
H–C–O–H
|
H
δ+
Explanation:
Methanol (CH₃OH) is the simplest alcohol, consisting of a methyl group (CH₃–) bonded to a hydroxyl group (–OH). Its structure includes a carbon atom centrally bonded to three hydrogen atoms and one oxygen atom. The oxygen is further bonded to a hydrogen, completing the alcohol functional group. Importantly, oxygen has two lone pairs, which influence both molecular geometry and polarity.
To understand the electron distribution in the carbon–oxygen (C–O) bond, we calculate the difference in electronegativity (ΔEN). Carbon has an electronegativity of 2.5, and oxygen has 3.5. This gives a ΔEN of 1.0, indicating a polar covalent bond. The electrons in this bond are not shared equally; instead, they are drawn more toward the oxygen atom due to its higher electronegativity. This uneven sharing creates a partial negative charge (δ⁻) on the oxygen and a partial positive charge (δ⁺) on the carbon.
The molecule as a whole exhibits polarity because it is asymmetrical and contains multiple polar bonds (C–O and O–H). The net dipole moment is directed from the electron-deficient carbon and hydrogen atoms toward the more electronegative oxygen atom. This results in methanol being a polar molecule, which explains many of its physical properties, such as its high solubility in water and its relatively high boiling point compared to non-polar molecules of similar molar mass.
In conclusion, the structure of methanol, its bond polarity (ΔEN = 1.0), and its net dipole moment all point to a polar covalent character. The molecule’s asymmetrical shape and lone pairs on oxygen enhance this polarity, making methanol a prototypical example of a polar organic compound.
