C7 Stereochemistry 1

Question 1

what is stereochemistry?

Answer 1

• a subdiscipline of chemistry which studies the spatial arrangement of individual atoms in a molecule
• these arrangements give molecules their shape

Question 2

describe shape and bonding at 4-valent carbon

Answer 2

• tetrahedral
• sp3 hybridisation
• 109.5 degrees

Question 3

describe shape and bonding at 3-valent carbon

Answer 3

• trigonal
• planar structure with sp2 hybridisation around carbons
• 120 degrees
• overlap of p orbitals generates pi bond

Question 4

describe shape and bonding at 2-valent carbon

Answer 4

• linear
• sp hybridisation
• 180 degrees
• overlap of p orbits generates 2 pi bonds

Question 5

what are isomers and state the 2 general types

Answer 5

• compounds that have the same molecular formula
• structural / constitutional isomers
• stereoisomers

Question 6

what are structural / constitutional isomers?

Answer 6

• compounds with the same molecular formula but different structural formulae

Question 7

what are stereoisomers?

Answer 7

• compounds that are exactly the same (same molecular and structural formula)
• differ in the way that the various groups are arranged in space

Question 8

what are the 2 classes of stereoisomers?

Answer 8

• conformational
• configurational

Question 9

what are conformational isomers?

Answer 9

• different conformations of the same molecule
• interconvert easily through rotation around a single bond or ring flip (for cyclic structures)
• represent different conformations of the exact same molecule through bond rotation

Question 10

what are configurational isomers?

Answer 10

• identical groups differ in how they are arranged around the same central C atom
• do not easily interconvert
• need to break a bond to interconvert

Question 11

what is a Newman projection?

Answer 11

looking at a molecule (eg. ethane) from the end instead of the front

Question 12

what are the 2 conformers of Ethan in Newman projection?

Answer 12

• staggered (you can see all 6 hydrogen through gaps)
• eclipsed (the back hydrogens are hidden behind the ones at the front)

Question 13

what are the 3 conformers of a butane molecule in Newman projection?

Answer 13

• staggered conformer anti
• staggered conformer gauche
• eclipsed conformer

Question 14

starting with the front methyl at the bottom and the back methyl at the top and rotating the back group anti-clockwise by 1/6 of a turn, list the conformers of butane in the Newman projection

Answer 14

• staggered conformer anti (lowest energy)
• eclipsed conformer
• staggered conformer gauche
• eclipsed conformer (highest energy)
• staggered conformer gauche
• eclipsed conformer

Question 15

describe the structure of a staggered conformer anti of butane in Newman projection

Answer 15

• lowest energy
• carbons are linear and in plane
• methyls are as far apart as possible

Question 16

describe the structure of an eclipsed conformer of butane in Newman projection

Answer 16

• slightly less energetically favourable than the staggered conformer anti due to repulsion

Question 17

describe the structure of an eclipsed conformer after half a rotation of the back methyl group in butane in Newman projection (highest energy conformer)

Answer 17

• highest energy
• methyls are very close together and show lots of repulsion
• difficult to form this due to high energy required

Question 18

describe the chair conformation of cyclohexane

Answer 18

• 109.5 degree angles
• no ring strain (can’t force to be planar, would put too much strain on the bonds)
• no eclipsing in chair conformation

Question 19

which hydrogens in cyclohexane in chair conformation are axial and which are equatorial?

Answer 19

• axial: go straight up or down when you lay the molecule down
• equatorial: the hydrogens that stick out

Question 20

describe cyclohexane ring flip

Answer 20

• interconversion of conformers via ring flip changes the axial/equatorial relationship
• axial and equatorial groups swap

Question 21

describe cyclohexane boats

Answer 21

• no ring strain
• eclipsing in boat conformation
• flag-pole interaction in boat
• eclipsing and flag-pole interaction reduced in twist-boat form
• higher energy than chair