Download Introduction, Alkanes and Structural Isomers

http://fatmaelif.blogspot.hk/2012/05/how-to-succeed-in-organic-chemistry.html
Organic Chemistry 1
Introduction, Alkanes and Structural Isomers
IB Topic 10.1.1, 10.1.3 - 10.1.6
p. 366 - 376
1
Starter Activity 1
Give as many meanings
as you can for the
term “organic”.
Name 20 organic
substances you have
encountered, directly
or indirectly, today.
2
Starter Activity 2
THE FOUR CARBON CHALLENGE
Build as many molecules as you can that contain 4 carbon atoms. Draw the structure of each
one and give its molecular formula. State the name if you think you know it.
3
Starter Activity 2
THE FOUR CARBON CHALLENGE ... continued
What “rules” did you follow (if any)?
4
Organic Compounds
There are an estimated 100,000,000 or so
different “organic compounds”.
http://chem1180.blogspot.hk/2010/12/23-introduction-to-organic-chemistry.html
All of these compounds contain CARBON
... a very versatile element.
5
What’s So Special About Carbon?
Carbon can form either single,
double or triple bonds with other
carbon atoms or with atoms of
other elements.
The single carbon-carbon
bond is very stable.
It is more stable than
bonds formed between
other Group 14
elements.
6
What’s So Special About Carbon?
Carbon can form stable long straight chains,
branched chains and ring structures.
polyethylene
glucose
cholesterol
7
Homologous Series
A homologous series is a group of
organic chemicals with these features:
1. the same general formula
2. successive members
differ by CH2
e.g. alkenes = CnH2n
first 4 members of the alkenes
C2H4 C3H6 C4H8 C5H10
3. chemical properties are the same for all members
4. physical properties gradually change as you move along the
homologous series
8
Homologous Series
Example: Physical Properties of Alkanes
Use your knowledge of structure and bonding to explain
the boiling points for these successive alkanes:
CH4 = -164ºC
C2H6 = -89ºC
C3H8 = -42ºC
Notice the general formula
is the same (CnH2n+2)
and all formulas differ by CH2.
trend:
BP increases moving along the series.
explanation:
Alkanes are nonpolar molecules.
As the total number of electrons increases,
the strength of the dispersion forces
increases.
9
Homologous Series
Common Homologous Series
alkanes
alcohols
amines
amides
halogenoalkanes
aromatics
aldehydes
ketones
alkenes
esters
alkynes
carboxylic acids
alcohols
nitriles
10
Nomenclature and Structures for
Organic Compounds:
The Alkanes
11
Hydrocarbons
Hydrocarbons are
obtained from crude oil.
http://heartlandenergydevelopment.blogspot.hk
fuels*
uses
plastics
precursor for food additives,
dyes, pharmaceuticals, fabrics
and more!
12
Homologous Series of Hydrocarbons
alkanes
single carbon-carbon bonds
general formula = CnH2n+2
alkenes
at least one double carbon-carbon bond
general formula = CnH2n
(if one double bond present)
alkynes
at least one triple carbon-carbon bond
general formula = CnH2n-2
(if one triple bond present)
13
IUPAC Nomenclature for Hydrocarbons
Names of all hydrocarbons include a stem and a suffix.
methane
ethene
propyne
1. The stem tells you the number of carbons in the longest chain:
1 = meth
2 = eth
3 = prop
4 = but
5 = pent
6 = hex
7 = hept
8 = oct
9 = non
10 = dec
11 = undec
12 = dodec
Monkeys eat
peeled bananas!
(or peanut butter)
2. The suffix tells you the homologous series:
ane = alkane
ene = alkene
yne = alkyne
14
IUPAC Nomenclature for Hydrocarbons
Try these:
H H H
H C C C H
propane
H H H
H
H C C C H
propyne
H
H H
H C C H
ethene
15
IUPAC Nomenclature for Hydrocarbons
3. Many hydrocarbons contain branches of shorter carbon chains.
Branch names follow this pattern: stem + yl
methyl = 1 carbon branch
ethyl = 2 carbon branch
The branch name and position is added as a prefix to the name of the
longest carbon chain:
H H H H
4
3
2
1
H C C C C H
H H CH3 H
Add the branch name as a prefix to the
name of the longest chain. Use a number
to indicate the carbon with the branch.
Number the carbons in the
longest chain starting from the
end closest to the branch.
2-methylbutane
Always separate numbers
from words with hyphens.
16
IUPAC Nomenclature for Hydrocarbons
4. If there are more than two branches of the same length, add a
prefix before the branch name and indicate the position of both
branches:
H H CH3 H
4
3
2
1
H C C C C H
2,2-dimethylbutane
H H CH3 H
Always number the C atoms from the end closest to a branch.
H CH3 H H
4
3
2
1
H C C C C H
2,3-dimethylbutane
H H CH3 H
17
IUPAC Nomenclature for Hydrocarbons
Try these:
H H CH3 H H
H C C C C C H
3-methylpentane
H H H H H
5-ethyl-2-methylheptane
18
IUPAC Nomenclature for Hydrocarbons
5. The carbon atoms in an alkane may be arranged in a ring structure:
Name the alkane by adding “cyclo” as a prefix.
cyclobutane
methylcyclopentane
19
Molecular, Empirical & Structural Formulas
molecular formula
shows the actual number of each atom
empirical formula
shows the lowest ratio of atoms
structural formula
shows all atoms and bonds
C4H10
C2H5
H H H H
H C C C C H
H H H H
20
Drawing Structural Formulas
Complete vs Condensed Structural Formulas
Make sure all carbon atoms have 4 bonds!
propane
pentane
CH3CH2CH3
CH3CH2CH2CH2CH3
21
Drawing Structural Formulas
Complete vs Condensed Structural Formulas
H H CH3 H H
3-methylpentane
H C C C C C H
H H H H H
CH3CH2CH(CH3)CH2 CH3
Branches are shown in parentheses.
22
Drawing Structural Formulas
Try these:
name
condensed structural formula
butane
CH3CH2CH2CH3
2-methylhexane
CH3CH2CH2CH2CH(CH3)CH3
3-ethylpentane
CH3CH2CH(C2H5)CH2CH3
2,2-dimethylbutane
CH3C(CH3)2CH2CH3
23
Structural Isomers
24
Same or Different?
What is similar about these molecules?
What is different?
same # C and same # H
∴ same molecular formula
C5H12
length of longest chain
# of branches
∴ different structural formula
different names
25
Same or Different?
Molecules with the same molecular formula but
different structural formulas are called ISOMERS.
More specifically, they are called STRUCTURAL isomers when
the structural formula is different due to changes in bonding patterns.
structural isomers of C5H12
structural isomers of C4H8
structural isomers of C3H6
26
Same or Different?
A student draws these structures as examples of structural isomers of C7H16.
How many unique structural isomers are shown?
(In other words, which structures are the same as another structure?)
(Tip: If they have the same name, the structure is the same!)
B
A
C
D
E
F
G
27
Same or Different?
A student draws these structures as examples of structural isomers of C7H16.
How many unique structural isomers are shown?
ANSWER = 3
A
D
=
heptane
C
F
B
1
2
3
4
5
6
=
6
3-methylhexane
5
4
3
2
3-methylhexane
E
1
=
1
2
3
4
5
6
3-methylhexane
G
=
3,3-dimethylpentane
28
Properties of Structural Isomers
Physical properties of structural isomers are slightly different.
C4H10: butane
BP = -1.0ºC
C4H10: 2-methylpropane
BP = -11.7ºC
In general, unbranched linear hydrocarbons have higher boiling points
(and melting points) than branched hydrocarbons.
Linear hydrocarbons are able to “pack together” more tightly than
branched hydrocarbons.
Because the intermolecular distance is less for the linear molecules, the
strength of the intermolecular forces is greater. This increases the BP.
29
Properties of Structural Isomers
The greater the number of branches, the lower the BP or MP.
C6H14
hexane
BP = 68.7ºC
2-methylpentane
BP = 60.3ºC
2,2-dimethylbutane
BP = 49.7ºC
The greater the number of branches, the less closely the
molecules can pack together. The strength of the dispersion
forces decreases if the molecules are further apart.
30