SNS - Organic Chemistry - Bonding

Question 1

Molecular Orbital

Answer 1

Created from the combination of two atomic orbitals

Obtained mathematically by adding the wave functions of the atomic orbitals. If the signs are the same, a lower energy bonding orbital is produced. If opposite, a higher energy anti-bonding orbital is produced

Question 2

Sigma Bond

Answer 2

Formed when there is head-to-head overlap of molecular orbitals.

Accomodate two electrons

Question 3

Pi Bonds

Answer 3

Bonding molecular orbitals formed by the parallel overlap of two p-orbitals

When both a sigma and a pi bond exist between two atoms, a double bond is formed. When a sigma and two pi bonds exist, a triple bond is formed.

The overlap of p-orbitals involved in a pi bond hinders rotation about double and triple bonds

Question 4

Sigma vs Pi Bonds

Answer 4

Pi bonds can’t exist independently of sigma bonds. Only after the formation of a sigma bond will the orbitals of adjacent p-orbitals be parallel.

Pi bonds generally weaker than sigma bonds - can be broken whilst stil leaving sigma bonds intact

Sigma bonds can rotate, whilst pi bonds can’t

Question 5

Hybridisation

Answer 5

A carbon atom has the electron configuration

1s2, 2s2, 2p2

A typical compound formed by carbon is CH4. Experimentation shows the four sigma bonds in this compound are equal. This is inconsistent with an assymetrical distribution of the four valence electrons: two in 2s, one in px and one in py.

The theory of orbital hybridisation developed to account for this discrepancy

Question 6

Hybridisation

sp3

Answer 6

If one s-orbital and three p-orbitals are mathematically combined, the result is four sp3 orbitals that have a new shape

These four orbitals will point towards the four vertices of a tetrahedron minimising repulsion

Accomplished by promoting one of the 2s electrons into the 2pz orbital to produce four valence orbitals each with one electron

Question 7

Hybridisation

sp2

Answer 7

If one s-orbital and two p-orbitals are mixed three sp2 orbitals are formed

Occurs, for example, in ethylene. The third p-orbital of each carbon atom is left unhybridised and participates in the pi bond. The three sp2 orbitals are 120° apart allowing for maximum separation

These orbitals participate in the formation of the C=C and C-H single bonds

Question 8

Hybridisation

sp

Answer 8

If two p-orbitals are used to form a triple bond, and the remaining p-orbital is mixed with an s-orbital, results in the formation of two sp orbitals.

Orientated 180° apart

Question 9

Single Bond

1. Component Bonds
2. Hybridisation
3. Angles
4. Examples

Answer 9

1. Sigma
2. sp3
3. 109.5°
4. C-C, C-H

Question 10

Double Bond

1. Component Bonds
2. Hybridisation
3. Angles
4. Examples

Answer 10

1. Sigma, Pi
2. sp2
3. 120°
4. C=C, C=O

Question 11

Triple Bond

1. Component Bonds
2. Hybridisation
3. Angles
4. Examples

Answer 11

1. Sigma, Pi, Pi
2. sp
3. 180°
4. C≡C, C≡N