Answer to Question #286979 in Organic Chemistry for Ian

Question

Show the Lewis Structure and geometric sketch for HCN.

The electronegativities of H, C, and N are 2.20, 2.55, and 3.04. Is the molecule Polar? Support your answer.

Answer 1

1) we should determine the shape of the molecule to see if the dipole moments cancel out or not.

Valence electrons = 1+4+5 = 10

Number of bonding pairs =2 (for central atom - carbon)

Number of lone pairs =0

General formula AB2

molecular geometry = linear

1) Electronegativity of H = 2.20, C = 2.55, N = 3.04

Difference |H-C|= |2.20-2.55|=|-0.35| = 0.35 (covalent nonpolar)

Difference |C-N|= |2.55-3.04|= |-0.49| = 0.49 (covalent polar)

Though H-C bond is considered to be nonpolar, electronegativity of carbon is more than that of hydrogen, that means this bond has very little dipole moment (yellow); the direction of this dipole moment is from hydrogen to carbon. Electronegativity of nitrogen is more than that of oxygen, the direction of dipole moment is from carbon to nitrogen.

The molecule is linear, but the dipole moments are in the same direction,i.e. they do not cancel out. The molecule is polar.