comparing CH3Br (polar) and CH4(non-polar)
electronegativity
C-Br = 0.3 – no dipole moment
C-H = 0.4 – no dipole moment
how is CH3Br polar? do you add the dipoles facing thesame direction to get 1.5?
The Correct Answer and Explanation is :
Why is CH₃Br Polar While CH₄ is Non-Polar?
Methyl bromide (CH₃Br) is polar, whereas methane (CH₄) is non-polar due to differences in molecular shape and dipole moment summation.
Electronegativity and Dipole Moments
- Electronegativity values (Pauling scale):
- Carbon (C) = 2.55
- Hydrogen (H) = 2.20
- Bromine (Br) = 2.96
- Bond Dipoles:
- C-H bond: Difference = 2.55 – 2.20 = 0.35 (nearly non-polar)
- C-Br bond: Difference = 2.96 – 2.55 = 0.41 (small dipole, but important)
Despite both electronegativity differences being small (0.3–0.4), what matters is the molecular geometry and how these dipole moments sum up.
Why is CH₄ Non-Polar?
- CH₄ has a tetrahedral structure with four identical C-H bonds.
- The small dipoles in each C-H bond cancel out perfectly due to symmetry.
- This results in zero net dipole moment, making CH₄ non-polar.
Why is CH₃Br Polar?
- CH₃Br is also tetrahedral, but now one hydrogen is replaced by bromine.
- The C-Br bond is more polar than the C-H bonds.
- The dipole moments do not cancel out due to asymmetry.
- The C-Br dipole points towards bromine, while the weaker C-H dipoles point in different directions.
- The vector sum of all dipoles gives a net dipole moment towards Br, making CH₃Br polar.
Thus, CH₃Br is polar because the dipoles do not completely cancel, unlike CH₄ where symmetry ensures no net dipole.