Chem 161
Sept 26, 2006
Groups (part of an alkane structure)
- in naming the particular group, drop the “ane” part and add “yl” to the name
- for example, methane  methyl
(i) Methane – CH4
H
C H
H
H
H C H
H
methane
methyl group
(ii) isopropyl group
CH3
H
CH3
isopropyl group
-
add “iso” to the common name of the alkane when this type of group is present in
the molecule
Eg:
H 3C
H2
C
H 3C
CH CH3
CH3
Isopentane
H2
C
normal (n) pentane
H 3C
H2
C
CH C
H
CH3 2
H2
C
isooctane
(iii) tert-butyl group
CH3
CH3
CH3
tert-butyl group
C
H2
CH3
C
H2
H2
C
CH3
- molecule with tert-butyl group is also called with the prefix “neo”
eg.
CH3
H3C C CH3
CH3
H C
CH3 2
H3C C
CH2
CH3
neoheptane
neopentane
CH3
Systematic Nomenclature (IUPAC)
RULES:
1. use common name for straight chain alkanes
2. find the longest chain with maximum number of branches
3. number from end of the chain, so 1st branch point has lowest number
4. name the chain, then add prefixes (for the groups attached) with number and name
the groups attached
eg.
n-butane
Number 2
Prefix
di
3
tri
isobutane (common name)
2-methyl propane (systematic name)
4
tetra
5
penta
6
hexa
7
hepta
eg.
methyl
methyl
6
7
5
2
4
1
3
H 3C
H 3C
CH
H 3C
CH2
H 2C
CH
CH3
CH
H 2C
CH3
3-ethyl-2,6-dimethylheptane
ethyl
8
octa
9
nona
Ring Structures and Naming:
- start with numbering at point of maximum branches
eg.
6
1
5
2
4
3
cyclohexane
1-ethylcyclohexane
11
12
7 6
9
10
8
5
1,1,2-trimethylcyclobutane
1
3
4
2
6-cyclopropyldodecane
Isomers – have same molecular formula but different compounds
1. structural/constitutional isomers
– compounds with same molecular formula and different names, numbers
2. Stereoisomers – have normally same name but different 3-D structure
(a) diastereomers/diastereoisomers (geometric isomers)
(b) enantiomers (mirror images of same molecule)
eg.
(i)
1,3-dimethylcyclobutane
diastereomers
cis-1,3-dimethylcyclobutane
- both CH3's up (same side)
trans-1,3-dimethylcyclobutane
- one CH3 down and one CH3 up (opposite side)
* 1,1-dimethylcyclobutane
* no stereoisomers
* structural isomer with respect to 1,3-dimethylcyclobutane
(ii) 1,3-dimethylcyclopentane
diastereomers
same molecules (drawn in different way)
same molecules (drawn in different way)
cis-1,3-dimethylcyclopentane
trans-1,3-dimethylcyclopentane
Sources of alkanes + Physical properties
-
sun: 99% of earth’s energy
30% of sun’s energy reflected from earth
47% energy – heating earth
23% energy – water cycle
0.2% energy – wind
0.02% energy – photosynthesis ( CO2 + H2O  C6H12O6 + O2)
-
1011 metric tons of CO2 fixed per year
-
C6H12O6  petroleum and natural gas (CH4)
Petroleum  >500 hydrocarbon
Distillation of petroleum
cold
hot
Physical Properties:
-
hydrocarbons – alkanes are non-polar – H and C have similar electronegativity
soluble in other organic solvents (like dissolves like)
immiscible with water (not infinitely soluble in water)
boiling point trend:
bp
300oC
-200oC
1
-
2
3 4
5 6
# of carbon atoms (n-alkane)
increasing the straight chain length increases the bp.
melting point trend:
mp
-70oC
-190oC
1
2
3 4
5 6
# of carbon atoms (n-alkane)
Density (ρ - RHO):
- hydrocarbons are less dense than water (1.0 g/cm3)
- density ranges from 0.65 – 0.75 g/cm3
eg.
•
•
mp (oC)
bp (oC)
-129
36
-160
28
-13
9
n-pentane has high bp due to multiple contacts of straight chains (London Forces)
neopentane has high mp due to round shape and well-packed crystal structure
Conformations
- different 3-D shapes a molecule can assume by rotation around single bonds
eg.
Ethane : H3C – CH3