week 4

Question 1

diastereoisomerism.

rules

Answer 1

In general, a molecule with n stereocentres will have a maximum of 2n stereoisomers.

There are two pairs of enantiomers:
(a) and (b) are one pair, the other pair is (c) and (d).
(a) and (b) are enantiomers and are non-superimposable mirror images.
Likewise, (c) and (d) are enantiomers and are non-superimposable mirror images.

Question 2

the absolute configuration (R/S)
naming
rule

Answer 2

is denoted in the following way:
using (a) as an example (2R, 3R)-2,3,4-trihydroxybutanal
meaning the configuration at carbon 2 is R and R at carbon 3

if h point towards you do the oppostie raotion to what you got

Question 3

achiral disastereomer referred to + what is it

Answer 3

as a meso compound.

What?
These meso compounds (c) and (d) are the same molecule orientated differently and are superimposable.
Meso compounds are achiral compounds with two or more stereo centres and are optically inactive.

The cis-isomer possesses a plane of symmetry and hence an achiral meso compound because it is superimposable on its mirror image.

Question 4

Assigning configurations to stereogenic centres

Answer 4

A chiral centre can have two possible configurations R and S.
1. Locate the stereogenic centre and identify the four different substituents.
2. Assign priorities (1 to 4) to all substituents attached to the chiral centre (see below for steps in order to do this).
Rotate the group or atom with the lowest priority (priority 4) so it is orientated away from you. Next, draw a curve from the first-priority substituent through to the second, and then the third substituent. Determine if the direction of this priority curve is clockwise (R) or anticlockwise

Question 5

Rules to follow in order to assign priorities of substituents:

Answer 5

1. Assign priorities of substituents according to the Cahn-Ingold-Prelog prioritisation scheme which gives the highest priority to the atom with the highest atomic number.
   H, C, N, O, F, S, Cl, Br, I (increasing priority)
   
   2. If the two atoms or substituents are the same (for example if there are two CH2 groups attached to the stereogenic carbon), proceed to the next position of the chain. You may need to go even further until the first point of difference.
      Groups with Π-bonds are given single-bond equivalents by duplication or triplication of multiply bonded atoms

Question 6

OBSERVATIONS SURROUNDING CIRAILITY

Answer 6

The only physical property in which enantiomers differ is how they interact differently with plane-polarised light.

This property of a chiral compound is called optical activity. The rotation is called optical rotation.

When a beam of plane-polarised light is rotated counterclockwise the molecule is the (-, L) form.

When a beam of plane-polarised light is rotated clockwise the molecule is of the (+, D) form.

Question 7

optical activity

Answer 7

The +/- indicate the direction that an enantiomer rotates polarised light and R- and S- groups indicate the arrangement of groups at the stereogenic centre.

Thus optical activity sign (+/-) is independent of the R/S. For example, (R)-2-hydroxypropanoic acid and (S)-2,3-dihydroxypropanal are both negative.