3. Drawing
Organic Structures
-
Drawing ‘displayed formula’ or ‘full structural formula’ is
often tested. Be sensitive to these terms.
Students often forget to illustrate bonds in substituents
(e.g. –OH and –NH2) when asked to draw displayed
formula:
wrong
correct
4. Drawing
Organic Structures
-
When drawing structural formulae, it is a good practice to
illustrate the ‘trigonal planar’ arrangement about sp 2 C
(i.e. C=C):
H3C
Cl
C
H
C
CH3
5.
6. Isomerism
-
An easy way to distinguish between structural and
stereoisomers is:
Stereoisomers have the same name
Structural isomers have different names
7. Structural Isomers
Q: Is there a mathematical formula to calculate the
number of structural isomers?
A: No. You have to draw out all possible structures.
But there is a general approach to doing that.
8. Drawing
Structural Isomers
1. Draw the structure with the longest possible chain.
2. If a functional group is present, shift the position of
the functional group to get all the different isomers
9. Drawing
Structural Isomers
3. After all positional isomers have been identified, draw
the structure of the next longest chain (minus 1 C)
4. Shift the position of the functional group and/ or
substituent to get all the different isomers
5. Repeat steps 3 & 4 until no more variations
10. Stereoisomers
- When using the formula 2n to determine the number of
stereoisomers, note that n = number of stereogenic
centers (i.e. chiral C + C=C that can exhibit cis/trans)
- Students make mistakes when they blindly count the
total number of C=C
13. Stereoisomers
A: no. of chiral C = 0
no. of C=C that can exhibit cis/trans = 4
no. of stereoisomers = 24 = 16
C=C in a ring cannot exhibit cis/trans
14. Identifying
Chiral Carbons
Tip 1: Ignore Cs in double bonds
- they have only 3 neighbours
Tip 2: Ignore Cs in benzene ring
- they are all C=C
Tip 3: Ignore “V-shaped Cs”
- they are bonded to 2H (same groups)
18. Identifying
Chiral Carbons
- Draw a line bisecting the C and the two groups in
question.
- If the structure is symmetrical about the line, the two
groups are the same and vice versa.
22. Drawing
Optical Isomers
1. Draw the tetrahedral structure about the chiral C
2. Fill in the four different groups with the most
‘complicated group’ at the top
- this will help to avoid issues with reflecting about the
mirror plane.
27. Functional Groups
- When identifying functional groups, it is safer to name the
homologous series (i.e. alkene, benzene, aldehyde, ketone
etc).
- Using terms like ‘carbonyl’ may sometimes be ambiguous.
- ‘Alkane’ is not a functional group
28. Functional Groups
- When ‘smaller’ functional groups make up a ‘larger’
functional group, you should name the ‘larger’ one:
i.e. phenol (instead of benzene or alcohol)
carboxylic acid (instead of alcohol)
Example
carboxylic acid
correct
alcohol
wrong
