KEMORG Review 2 (Ch. 5 and 6)

(5.1 - 5.2) What is stereochemistry?

• 2D structure of a molecule

• 3D structure of molecule

• results in similar properties

• results in different properties

Answer explanation

e.g. digestion of starch but not cellulose.

starch is an alpha helix structure (rings of gluc. join equatorially and axially to oxygen atom) vs cellulose beta pleated sheet (two glucose rings both joined equatorially to oxygen atom)

(5.1 - 5.2) Isomers that have same IUPAC name, same functional groups but different 3D oritentation in space

• isotopes

• constitutional isomers

• stereoisomers

Answer explanation

constitutional: diff IUPAC name, same/diff FG, different physical and chemical properties

(5.3) What is/are examples of a superimposable object

• socks

• glasses

• hands

• feet

Answer explanation

superimposable = mirror images but are identical, can overlap one over the other perfectly (aka achiral H2O)

non superimposable = mirror images that are not identical, imperfect overlap (aka chiral CHBrClF)

(5.3) Characteristics of an enantiomer

• stereogenic center (atom connected to 4 distinct other atoms)

• have mirror images

• chiral

• achiral

Answer explanation

diastereomers: stereoisomers that are not mirror images and non-superimposable

with 1 stereogenic center = always chiral

with >1 stereogenic center = may be chiral or achiral

(5.3) what are differences between chiral and achiral molecules

• superimposability / non superimposability

• presence or absence of plane of symmetry

• mirror images / non mirror images

• high bond energy / low bond energy

Answer explanation

chiral : non superimposable, mirror images, no plane of symmetry

achiral: superimposable, mirror images, have plane of symmetry

(5.4 - 5.6) What is false when it comes to drawing enantiomers

• place 4 groups arbitrarily for 1st enantiomer

• draw mirror image for 2nd enantiomer

• seterogenic centers cannot occur in rings

• is a tetrahedral structure with C in the center

Answer explanation

stereogenic centers may also occur in ring structures (so long as C is connected to 4 distinct groups)

(5.9) How are 2 different enantiomers labelled?

• R (rectus)

  S (sinister)

• R (sinister)

  S (rectus)

• alpha (helix)

  beta (pleated sheets)

• alpha (pleated sheets)

  beta (helix)

Answer explanation

R = right or clockwise

S = sinister or counterclockwise

Refers to the direction by which each group is bonded to the stereogenic centers according to their priorities (molecule oriented opposite to 4 then see direction from 1 to 2 to 3)

(5.9) according to rule of atomic number, which is the highest priority

• C

• O

• H

• Cl

Answer explanation

Highest atomic number = highest priority

(5.9) According to rule of atomic number of adjacent molecules which group would be lowest priority

a = CH3

b = CH2OH

c = CCl2Br

d = OH

• a

• c

• b

• d

Answer explanation

Higher atomic number = higher priority

in this example: d > c > b > a

(5.9) How do we assign priorities among isotopes (highest priority to lowest)

• by adjacent atoms

• increasing mass number

• decreasing mass number

• charge density

Answer explanation

e.g. tritium takes priority over deuterium which takes priority over hydrogen