C8 Stereochemistry 2

Question 1

what features of molecules can lead to cis and trans stereoisomers?

Answer 1

• restricted rotation about double bonds
• presence of ring systems

Question 2

describe the difference between cis and trans isomers

Answer 2

• cis: substituents are on the same side of the double bond or ring
• trans: substituents are on opposite sides

Question 3

describe the CIP rules for assigning priority of substituents

Answer 3

• consider atom you want to assign and look at atoms attached
• higher atomic number has higher priority
• for isotopes, higher atomic mass has higher priority
• if atoms have same priority, secondary atoms attached are considered
• double and triple bonds are considered as if the attached atoms are duplicated or triplicated

Question 4

what is a stereogenic centre?

Answer 4

• 4 different groups on tetrahedral carbon (have 2 arrangements, 2 enantiomers)
• chiral carbon

Question 5

how is configuration described at stereogenic centres?

Answer 5

• assign an order of priority to groups attached to stereogenic centre
• view the molecule from the side opposite the lowest priority (that group faces away from you)
• if order of priority of remaining groups is clockwise, the configuration is R, if it is anti-clockwise it is S

Question 6

describe the relationship between enantiomers and optical activity

Answer 6

• enantiomers are chemically identical except in optical properties (biological properties may be different)

Question 7

what is optical activity?

Answer 7

• the ability of a compound to rotate the plane of polarised light observed in a polarimeter
• enantiomers rotate the plane in opposite directions

Question 8

what do the words dextrorotatory and laevorotatory mean in regards to enantiomers?

Answer 8

• dextrorotatory isomers rotate light clockwise
• laevorotatory isomers rotate light anti-clockwise