1. Organic Chemistry
Structural isomers
Naming Branched Alkanes

2. Isomers
Compounds with the same molecular formula but different structural arrangement are known as structural isomers.
The more C atoms there are in the formula, the more structural isomers there will be.
4C (1) 5C(3) 6C(5) 8C(18)
C20(>303,000)

3. Structural Isomers C5H12 – Yes, they Are isomers
In order to determine if something is an isomer determine the molecular formula
To the right we have
pentane (Top)
2 – methyl butane
What is the mol. Formula for each? Are they
structural isomers?
C5H12 – Yes, they
Are isomers

4. Isomers
Isomers have different structures and different chemical and physical properties.
Butane 2-methylpropane
Formula C4H10 C4H10
B.P C -12 C
M.P C C
Density
Sol. In 100ml Alcohol 1813 ml 1320 ml

5. Drawing simple alkanes
Recall that the members of the group of alkanes
Forms a homologous series and each member of
This series differs from the last by 1 –CH2- unit
When we draw the structural formulas of the 1st
Three members of this group there is only one
Way each can be drawn.
CH4
CH3-CH3
CH3-CH2-CH3

6. Branched-chain alkanes
Beginning with butane, C4H10, there is more than 1 way to arrange the atoms besides one carbon after another.
CH3-CH2-CH2-CH & CH3 |
CH3-CH-CH3

7. Branched-chain alkanes
In both butane structures we have the same numbers and kind of atoms namely, C4H10
The general formula for each is also CnH2n+2 so each represents the alkane “Butane”
Yet there is a difference. The difference lies in what atoms are joined to what atoms
CH3-CH2-CH2-CH & CH3
* |
CH3-CH-CH3 *
Nope
Are there any other ways to draw 4 carbon butane?

8. Note that what we have drawn for butane represents
Real structural changes and not just apparent changes
As a result of the rotation of a C-C single bond.
Let’s Look a pentane to see what we mean by this.
Rotation of this bond
Leads to this configuration
Of pentane
And rotation of this
Bond leads to this
All of these structures however represent the SAME molecule!

9. Pentane isomers To find new structural isomers of the straight chain
Alkanes we need to move the point of attachment of
The various carbon atoms.
So how many structural
Isomers does pentane
Have?
Straight chain
isomer
Remember, carbon
Must have 4 bonds.
No more, No less!

10. Naming branched-chain alkanes
Find the longest continuous chain or backbone of C atoms.
c-c-c-c-c-c-c-c-c-c
c c What’s the longest chain? c
The base name is derived from the number of C’s in the longest chain.
10 carbons would be decane

11. Naming branched-chain alkanes
Branches are added as a prefix and are named by counting the number of C atoms.
The “branch” alkane name ends in “yl.”
Methyl
CH3—
Ethyl
CH3CH2 –
Propyl
CH3CH2CH2 –
Butyl
CH3CH2CH2CH2 –
c-c-c-c-c-c-c-c-c-c
c c c

12. Naming branched-chain alkanes
The location of the branch (or substituent group) is shown by assigning numbers to the C’s in the backbone.
Number from the end that gives the lowest number for the first branch.
Substituent groups (branches) are listed alphabetically
c-c-c-c-c-c-c-c-c-c
c c c
7-ethyl-3-methyldecane
There may be more than 1 of the same type of branch.
Use di, tri, tetra etc. for 2, 3, and 4
Number the locations and separate the # by a comma &
separate the last # from the name by a dash

13. H H–C–H H H H–C–C–C–H Example: H-C-H | Longest continuous chain has 3
carbon atoms – propane.
2 Branches each have 1 carbon – dimethyl.
Branches have to be at C-2. WHY??
(Only give the number if necessary.)
Dimethylpropane or
C5H12 or
CH3C(CH3)2CH3

14. Name this compound CH3 | H3C-C-CH2-CH3 Longest continuous chain has 4
carbon atoms – butane.
2 Branches each have 1 carbon – dimethyl.
Branches are at C-2. WHY??
2,2-Dimethylbutane or
C6H14 or
CH3C(CH3)2CH2CH3
Name this compound
CH3 |
H3C-C-CH2-CH3

15. Name this compound: Longest continuous chain has H
6 carbon atoms: hexane
Branch is 1 carbon long: methyl
Branch is located at C-3
3-methylhexane or
C7H16 or
CH3CH2CH2CH(CH3)CH2CH3 or
CH3(CH2)2CH(CH3)CH2CH3 H
H – C – H
H H H
H – C – C – C – C – H
H H
H–C–H H–C–H
H H

16. More Naming Problems

17. Problems: Draw the following: 2 – methylpropane
3 – ethyl – 4 - methylnonane
2,2 – dimethylpropane
2,3,4 – trimethyldecane
What name does this compound have?