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2/12/2012
Alkanes
Alkanes: Compounds with C-C single bonds and C-H
bonds only (no functional groups)
SIMPLE HYDROCARBONS
Alkanes are saturated with hydrogen (no more can be
added). They are also called aliphatic compounds
The formula for an alkane with no rings in it must be
CnH2n+2
“LINEAR” HYDROCARBONS
SIMPLE HYDROCARBONS
Hydrocarbons
Benzene
ring
Aliphatic
Alkanes
Single bond
Aromatic
Alkenes
Alkynes
LEARN
THESE
CH 4
CH 4
methane
C 2H 6
CH 3CH 3
ethane
C 3H 8
CH 3CH 2CH 3
propane
C 4H 10
CH 3CH 2CH 2CH 3
butane
C 5H 12
CH 3CH 2CH 2CH 2CH 3
pentane
C 6H 14
CH 3(CH 2) 4CH 3
hexane
C 7H 16
CH 3(CH 2) 5CH 3
heptane
C 8H 18
CH 3(CH 2) 6CH 3
octane
C 9H 20
CH 3(CH 2) 7CH 3
nonane
C 10H 22
CH 3(CH 2) 8CH 3
decane
CnH2n+2
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HYDROCARBONS ARE NOT LINEAR
CHAINS CAN BEND
(make a model and see for yourself)
compare
109o
109o
tetrahedral
PROPANE
ETHANE
[ animation ]
DECANE
H
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
ALL OF THESE ARE THE SAME MOLECULE
H
CONDENSED STRUCTURES
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3
BALL AND
STICK
MODEL
The atom-to-atom connection sequence matters
but not the orientation in space.
It is important for you to be able to recognize the
same molecule when drawn in different orientations.
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HYDROCARBONS
LET’S START WITH THE HYDROCARBONS
CONSTITUTIONAL ISOMERS
C1
point
C2H6 no isomers
C2
line
C3 H8
C3
plane
(angle)
Molecules having the same molecular formula,
but a different bonding arrangement for the atoms.
CONSTITUTIONAL ISOMERS
.
CH4
The first three linear hydrocarbons (C1 - C3)
have no isomers. No other arrangements of
the atoms are possible.
At C4, however, we begin to see isomers.
( STRUCTURAL ISOMERS )
C4H10
EXAMPLE
C 2 H6 O
..
CH3-O-CH
3
..
..
H-O-CH
2-CH3
..
C4
molecular
formula
constitutional isomers
- same molecular formula
- different structure
2 isomers
C3 + C1
C5H12
3 isomers
C5
C4+C1
C3+2(C1)
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BE SYSTEMATIC WHEN DRAWING ISOMERS
C6H14
5 isomers
1. Draw the “linear” chain first ( no branching ).
C6
2. Remove one carbon and place it at different
positions on the shortened chain, making sure
not to repeat any structure already drawn.
C5+C1
3. Remove two carbons from the original chain,
place them on the chain, both individually (two
one carbon units) and together (one two carbon
unit), taking care not to repeat any structure
already drawn.
C4+2(C1)
With increasing numbers of carbon, the number
of isomers goes up rapidly.
ORIENTATION AND BENDING DO NOT MATTER
These are all the same isomer:
4. Continue removing carbons and replacing them
individually and in groups until it is no longer
possible to create new isomers.
CONSTITUTIONAL ISOMERS OF HEPTANE
all of these branch at
carbon number two
C7
C6 + C1
These are all the same isomer:
all of these
branch at carbon
number three
The atom-to-atom sequence determines the isomer
……………….. not the shape.
C5 + 2 (C1)
C5 + C2
C4 + 3 (C1)
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CONSTITUTIONAL ISOMERS
EXAMPLE
Both are C7H16 ,
but they are
different molecules.
- They are different molecules. They have the same
formula, but they are connected together differently.
- You cannot convert one to the other without taking
CONSTITUTIONAL ISOMERS WITH A GIVEN
FUNCTIONAL GROUP
PROBLEM ONE
DRAW ALL THE ISOMERS OF C4H10O
a) that are alcohols
b) that are ethers
it apart and rebuilding it
( i.e., without breaking and remaking bonds).
- All the physical properties of constitutional
As we shall see, there can be isomers both
within a class of compounds AND across
two or more classes of compounds.
isomers are different
different melting points and boiling points,
different solubilities and densities, etc.
Types of Constitutional
Isomerism
Different carbon
skeletons
C4H10O CONSTITUTIONAL ISOMERS
OH
alcohols
OH
OH
C4H10
Different
functional groups
C2H6O
Different position of
functional groups
hydrogens
omitted
OH
2-2
O
O
3-1
ethers
3-1
O
C3H9N
TWO DIFFERENT FUNCTIONAL GROUPS - SAME FORMULA
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Chapter 2
#10, #11, #12,#13
Chapter 3
#1,
#2b. Some of the questions need information from next section.
Please remember to come back later.
#3. this question is to ask you to draw one bromine-substituted
isobutane and two bromine-substituted isobutane. Hint: draw
the structure of isobutane; for monobromination derivatives you
should replace one hydrogen in the molecule with a Br and you
should get two different structures; for dibromination
derivatives, you should replace two hydrogens of isobutane
with two Br and you may get 3 different structures.
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