Structure and Bonding

What is valence bond theory?

A chemical bond forms when two atomic orbitals overlap, and the electrons in those orbitals pair up.

Sometimes, atoms "mix" their orbitals (like an s orbital and a p orbital) to form new, equivalent orbitals better suited for bonding.

In VBT, orbital orientation tries to minimize electron-electron repulsion.

• That’s why, when orbitals hybridize (like in sp³), the resulting orbitals arrange themselves as far apart as possible — leading to specific molecular shapes.

  • Example: In methane (CH₄), the sp³ orbitals point to the corners of a tetrahedron (109.5° angles) to minimize repulsion.

SEARCH UP HYBRIDISATION.

What are constituional isomers (structural isomers)?

Molecules with the same molecular formular and molar mass, but their atoms are linked differently, and they differe in physical properties.

What are conformational isomers?

Conformational isomers are different shapes that a molecule can adopt by rotation around single bonds, without breaking any bonds.

They are the same molecule, but they look different in 3D space because parts of the molecule have rotated around single (σ) bonds.

Two basic types of conformations:
Staggered conformers is when hydrogen atoms are as far away from each other. The molecule is most stable here, due to minimal electron repulsion. Describes as having low energy.

Eclipsed conformers is when hydrogen atoms are positioned directly in front of each other. The molecule is less stable, but has higher energy.

What are the Sawhorse representations and Newman projection?

Sawhorse: the carbon-carbon bonds are tilted.

Newman: looking directly into the carbon-carbon bond. The circle represents the carbon-carbon bond.

What does staggered and eclipsed conformation look like for ethane?

These bonds are constantly moving (assuming not frozen), thus any conformation is possible at a point in time, but ofc the molecule spends more time in the staggered conformation.

What are the possible conformers of butane?

What are cycloalkanes?

Cyclopropane has the most stressed bonds.

What is ring strain in cycloalkanes?

The ideal bond angle for a sp3 hybridised C atom (4 single bonds) is 109.5º

This isn’t possible with the small rings, thus they will suffer ring strain.

Describe cyclopropane and butane.

Must be familiar with why it is flat.

Describe cyclopentane and cyclohexane.

https://qrchem.net/viewer?cid=8078&name=cyclohexane&desc=false&ballStick=false

look at the cyclohexane morecule, and how it is folded like a chair.

What are the hydrogen environments in the cyclohexane chair conformations?

Axial and equatorial positions.

Bulky substituents prefer to be in the equatorial position, as if they were in the axial position, they would clash.

Draw cyclohexane chair conformation.

All are slightly tilted.

Describe ring flipping.

Axial positions can turn into equatorial positions, vice versa.

This is because the molecule is not static, and is rotating.

What are substituted cyclohexanes?

What are disubstituted cycloalkanes?

Rings prevent free rotation around single bonds.

Isomers can exist when two or

more substituents are attached.

▪ The isomers do not interconvert

(would require breaking of C–C

bonds).

▪ These isomers are stereoisomers

- they differ by the arrangement

of atoms in space

▪ Also called cis/trans isomers,

diastereomers

WHEN BOLD LINE, THE BOND IS POINTING UP, WHEN IT IS DASHED IT IS POINTING DOWN (can be axial or equatorial).

Draw the two chair conformations of

cis-1-bromo-4-tert-butylcyclohexane.

Which is the most stable?

t-But is always equatorial because it is very big