Use valence bond theory to write the hybridization and bonding scheme for NCCH3. Sketch the model with the right geometry?

1 Answer
Jul 15, 2017

Warning! Long Answer. Here's what I get.

Explanation:

Step 1. Draw the Lewis structure

(a) Start with a skeleton structure.

The two C atoms (least electronegative) will be the central atoms, with the N attached to one of the carbons.

SkeletonSkeleton

(b) Attach the hydrogen atoms.

The question gives you a clue where they go.

The formula NCCH3 tells you that the three H atoms are attached to the terminal carbon atom.

ConnectivityConnectivity

(c) Add electrons so that every atom gets an octet.

There is only one good way to do it: put a triple bond between the C and the N, and a lone pair on the N.

archives.library.illinois.eduarchives.library.illinois.edu

Step 2. Use VSEPR theory to predict the geometry about each atom.

  • The terminal C atom: four electron domains (3 C-H bonds and a C-C bond).
    ∴ Tetrahedral.
  • The central C atom: two electron domains (a C-C bond and a C≡N bond).
    ∴ Linear.
  • The N atom: two electron domains (a C≡N bond and a lone pair)

Step 3. Assign hybridizations to each atom

  • The terminal C atom: tetrahedral. ∴ sp3 hybridized.
  • The central C atom: linear. ∴ sp hybridized.
  • The N atom: two electron domains. ∴ sp hybridized.

Step 4. Sketch the orbitals involved

Here is my drawing (apologies! I'm not a graphics artist).

OrbitalsOrbitals

My drawing may be confusing, so here is a description:

  • H atoms: unhybridized 1s orbitals
  • Terminal C atom: sp3 hybridized; bond angles 109.5°
  • C-H bonds: σ bonds formed by overlap of H 1s and C sp3 orbitals
  • Central C atom: sp hybridized
  • C-C bond: σ bond formed by overlap of C sp3 and C sp orbitals
  • N atom: sp hybridized.
  • C≡N bond: σ bond formed by overlap of C sp and N sp orbitals, plus two bonds formed by side-on overlap of unhybridized 2p orbitals
  • C-C-N bond angle = 180°

Here's a model of the molecule for comparison:

ModelModel