2.1

Question 1

Why can cycloalkanes exhibit cis-trans isomerism?

Answer 1

Cycloalkanes exhibit cis-trans isomerism due to the cyclic structure preventing 360 degree rotation: groups are fixed in position relative to each other.
- Sometimes it is possible for either cis or trans to form, one might be more likely.

Question 2

Is cis-trans isomerism possible in alkanes?

Answer 2

No, because molecules can rotate around sigma bond. No pi bonds to prevent rotation.

In cycoalkanes, yes: ring prevents rotation.

Question 3

How to tell if an alkene is cis or trans?

Answer 3

• Cis if parent carbons on same side either side of double bond
• Trans if parent carbons different sides of double bond

Question 4

Consequence of a ring flip?

Answer 4

Axial and equatorial groups swap positions

• -> whatever is axial goes equatorial
• -> whatever is equatorial goes axial

Question 5

What are sterioisomers?

Answer 5

Sterioisomers have same attachment but different 3D structures: cis-trans isomers

Question 6

Heterolytic vs homolytic reactions?

Answer 6

Heterolytic: forms polar products (+ and -)
Homolytic: forms neutral/non-polar products (0 and 0).

Question 7

How to use arrows in representing organic reaction reaction-mechanisms?

Answer 7

• Double-headed: two electrons
• Half-headed: one electron

Direction of arrow is from -ve to +ve: tail at the e- rich centre. Thus, start by identifying the nucleophile.

Question 8

What is a conformer? vs conformation.

Answer 8

A conformer describes any 3D arrangement of atoms resulting from rotation around a single bond.

A conformation is its representation of position of atoms in the molecule.

Question 9

Torsional strain vs angle strain?

Answer 9

Torsional strain, aka eclipsed interaction strain, occurs when unfavoured interaction occurs when non-bonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed formation. In a flat pentagon structure, overcome by envelope conformation.

Angle strain: results when a bond angle in a molecule differs from the optimal tetrahedral angle of 109.5°.

Question 10

What is steric strain?

Answer 10

Steric strain or non-bonded interaction strain is the strain occurring when two parts of a molecule try to occupy the same space.

Question 11

If there is a larger substituent on a molecule, where will it preferentially exist to reduce steric interaction?

Answer 11

Large substituents will preferentially exist in an equatorial position.

Question 12

What is the bond angle observed in ethene?

Answer 12

• Predicted: 120°, may vary with strain + repulsion.

Thus, not tetrahedral.