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SolutionsConstitution of Organic Compounds

1. Hybridization

  1. CH3–CH=CH2 — C1 (CH2=) and C2 (=CH–): sp2; C3 (CH3): sp3.
  2. CH3–C≡N — CH3: sp3; C of the C≡N group: sp; N: sp.
  3. CH3–CHO — CH3: sp3; C of the CHO: sp2; O: sp2.
  4. CH2=C=CH2 — Terminal carbons: sp2; central carbon (with two double bonds): sp.
  5. CH3–NH2 — C: sp3; N: sp3 (three bonds and one lone pair).
  6. O=C=O — C: sp (two double bonds); each O: sp2.

Rule of thumb: count the electron densities (bonded atoms + lone pairs). Four densities: sp3; three: sp2; two: sp. A double or triple bond counts as a single density.

2. Geometry and bond angles

MoleculeDensitiesHybridizationGeometryApproximate angle
CH44sp3tetrahedral109.5°
NH34 (3 bonds + 1 pair)sp3pyramidal107°
H2O4 (2 bonds + 2 pairs)sp3angular (bent)104.5°
H2C=O3sp2trigonal planar≈120°
HC≡CH2splinear180°
HC≡N2splinear180°

Lone pairs take up more space than bonding pairs and close the angles: that is why the angle falls from 109.5° (CH4) to 107° (NH3) and 104.5° (H2O).

3. σ and π bonds

Moleculeσ bondsπ bonds
CH2=CH–CH3 (propene)81
CH3–C≡N (acetonitrile)52
CH3–COOH (acetic acid)71
CH2=CH–C≡N (acrylonitrile)63

Every single bond is σ. A double bond is one σ plus one π, and a triple bond is one σ plus two π.

4. Where are the lone pairs?

  1. CH3–OH — Methanol: the O is sp3 hybridized; its two lone pairs are in sp3 orbitals.
  2. CH3–NH2 — Methylamine: the N is sp3 hybridized; its lone pair is in an sp3 orbital.
  3. H2C=O — Formaldehyde: the O is sp2 hybridized; its two lone pairs are in sp2 orbitals. The leftover p orbital forms the π bond.
  4. HC≡N — Hydrogen cyanide: the N is sp hybridized; its lone pair is in an sp orbital. The two p orbitals form the two π bonds.

5. Electron densities

Electron densitiesHybridizationOrientation of the hybrid orbitalsIdeal angle
4sp3towards the vertices of a tetrahedron109.5°
3sp2in a plane, towards the vertices of a triangle120°
2spin a straight line, in opposite directions180°

6. True or false?

  1. A carbon bonded to four atoms is always sp3 hybridized.
    True. Four electron densities point towards the vertices of a tetrahedron.
  2. In water, oxygen is sp hybridized because it is bonded to only two atoms.
    False. The oxygen of water has four electron densities (two bonds and two lone pairs): it is sp3, which is why the molecule is bent and not linear.
  3. A double bond consists of two σ bonds.
    False. A double bond consists of one σ bond and one π bond.
  4. The H–N–H angle in ammonia is smaller than the H–C–H angle in methane.
    True. The lone pair on nitrogen repels more than a bonding pair and closes the angle (107° versus 109.5°).
  5. In ethene, all six atoms lie in the same plane.
    True. The two sp2 carbons and the four hydrogens lie in one plane; the π bond prevents rotation and keeps that geometry.

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