-Structural isomers
-Naming Branched Alkanes
Mr. Shields
Regents Chemistry
U16 L02
1
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)
20 C (>303,000)
8C (18)
2
Structural Isomers


In order to determine if 2
molecules are isomers of
one another first determine
the molecular formula’s of
both
To the right we have



pentane (Top)
2 – methyl butane (bottom)
What is the mol. Formula
for each? Are they
structural isomers?
C5H12 – Yes, they
Are isomers
3
Isomers

Isomers have different structures and
different chemical and physical properties.
Formula
B.P.
M.P.
Density
Sol. In 100ml Alcohol
Butane
C4H10
0C
-138 C
0.622
1813 ml
2-methylpropane
C4H10
-12 C
-159 C
0.604
1320 ml
NOTE!
CH3CH2OH and CH3OCH3 are also isomers of one another
4
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
5
Branched-chain alkanes


Beginning with butane, C4H10, there is more than 1
way to arrange the atoms besides one carbon after
another (i.e. straight chain).
Can you figure out how many ways it can be drawn?
CH3-CH2-CH2-CH3
&
CH3
|
CH3-CH-CH3
6
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-CH3
*
CH
CH3
|
C4H10
4 10
CH3-CH-CH3
* having
Note that these are totally different compounds

there own unique Chemical & Physical Properties!
7
Note that what we have drawn for butane represents
Real structural changes and not just apparent changes
Resulting from 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 represent the SAME molecule!
8
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?
Remember, carbon
Must have 4 bonds.
No more, No less!
Straight chain
isomer
9
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
c

What’s the longest chain?
The base name is derived from the number of
C’s in the longest chain.
10 carbons would be decane
10
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.”
c-c-c-c-c-c-c-c-c-c
c
c
c
Methyl
CH3—
Ethyl
CH3CH2 –
Propyl
CH3CH2CH2
–
Butyl
CH3CH2CH2C
H2 –
11
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
1 2 3 4 5 6 7 8 9 10
c-c-c-c-c-c-c-c-c-c
c
c
7-ethyl-3-methyldecane
c
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 nos. by a comma &
12
separate the last no. from the name by a dash
Name this compound
CH3
|
H3C-C-CH2-CH3
|
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
13
Example:
H
H–C–H
H H
H–C–C–C–H
H H
H-C-H
|
H
Longest continuous chain has 3
carbon atoms – propane.
2 Branches each have 1 carbon –
dimethyl. And … both
Branches have to be at C-2. WHY??
So ….
Only specify the branch number if
necessary.
Ex: this is Dimethylpropane & not
2,2-dimethylpropane
14
Name this compound:
H
H–C–H
H H H
H– C–C–C–C–H
H
H
H–C–H H–C–H
H
H
Longest continuous chain has
6 carbon atoms: hexane
Branch is 1 carbon long: methyl
Branch is located at C-3
3-methylhexane or
C7H16 or
CH3CH2CH2CH(CH3)CH2CH3
15
More Naming Problems
H3C
CH2
CH
CH3
CH3
Methylbutane
CH3
CH3
CH
H3C
CH
CH3
Dimethylpropane
CH
CH2
CH2
CH2
CH3
2,3,7-Trimethyloctane
3-ethylpentane
CH3CH(CH3)CH(CH3)CH2CH2CH2CH(CH3)CH3
16
More Naming Problems
C
C
C
|
|
|
C-C-C-C-C-C-C
|
C
|
C
What is the name
Of this compound ?
4-ethyl-2,5-dimethyloctane
WOW !
17
Problems:

Draw the structural
formula for the
following:




methylpropane
3 – ethyl – 4 methylnonane
dimethylpropane
2,3,4 – trimethyldecane
3-ethyl-3-methylpentane
What name does this compound have?
18
Name this compound
9
C-C-C
|
1
C-C-C-C-C-C-C-C
| |
|
CC C
|
C
6,6-diethyl-3,5dimethylnonane
19