Quiz 5 Study Guide Chemistry 231
Isomers: compounds that have the same chemical formula but different
structures
Number of Alkane Isomers
Formula
Number of Isomers
5
C6H14
C7H16
9
18
C8H18
35
C9H20
C10H22
75
C15H32
4,347
366,319
C20H42
C30H62
4,111,846,783
th
th
* Table 3.2 McMurry 7 ed. Pg 81, See McMurry 6 ed. Pg 76
Trends
* As number of Carbons increase in the same family the following occurs:
- Increase in Molar Mass
- Density as a liquid Increases (density overall increases)
- Increase in Melting Point
- Increase in Boiling Point
- Increase in Heating value (ex. Hexane will have a hotter flame that butane)
- Decrease in Solubility in aqueous system not very soluble anyway
Decrease in Solubility in non-aqueous system not very soluble
anyway
- Decrease in Vapor pressure
* Within the same family;
- Molar mass increases: states of matter change from gas, liquefiable gas,
liquid, and solid
* Some families start as a liquid (ex. Carboxylic acids e.g., Formic acid and
Acetic acid)
Branching
* When isomers form it is due to branching of the molecule
(ex. Butane has 2 isomers, Pentane has 3 isomers)
* Branching does not change vapor density (it is independent)
- Calculate vapor density by taking:
Molar Mass
22.4 L
- 1 mole of any ideal gas = 22.4 L @ STP (0 Celsius, 1atm)
- 1atm = 29.92 in. Hg = 760 mm Hg = 76 cm Hg = 760 torr = 14.69 psia
* Within the Alkane family, as branching increases the alkane’s boiling
point is lowered. Examples:
Alkane
- Pentane (no branches)
- Iso-pentane (one branch)
(2-methylbutane)
- Neo-pentane (two branches)
(2, 2-dimethylpropane)
- Octane (no branches)
- Iso-octane (three branches)
(2, 2, 4-trimethylpentane)
Boiling Point
36.1 degrees C
27.85 degrees C
9.5 degrees C
125.7 degrees C
99.3 degrees C
* Branched-chain alkanes are lower-boiling because they are more nearly
Spherical than straight chain alkanes, have smaller surface areas, and
consequently have smaller dispersion forces.
*See McMurry 6th ed. Pg 88, or McMurry 7th ed. Pg 52
* Alkanes show regular increases in both boiling point and melting point as
molecular weight increases. This affect is due to weak dispersion forces
between molecules. Only when sufficient energy is applied to overcome
these forces does the solid melt or liquid boil Dispersion forces increase as
molecule size increases, this accounts for the higher melting and boiling
points.
*See McMurry 6th ed. Pg 87, or McMurry 7th ed. Pg 92
Physical properties of n-alkanes
Name
Methane
Ethane
Propane
Butane
Pentane
Hexane
Heptane
Octane
Nonane
Decane
Boiling Pt (F)
-258.7
-127.5
- 43.7
31.1
96.9
155.7
209.2
258.2
303.4
345.5
Melting Pt (F)
-296.5
-297.9
-305.8
-217.0
-201.5
-139.6
-131.1
- 70.2
- 64.3
- 21.4
*See Properties of Petroleum fluids, McCain copyright 1973
* Alkanes are occasionally referred to aliphatic compounds.
This comes from the Greek word aleiphas, meaning “fat.”
In organic chemistry today it means loosely saturated.
Physical properties of alkenes
Name
Ethylene
Propylene
1-butene
1-pentene
1-hexene
1-heptene
1-octene
Boiling Pt (F)
-154.6
- 53.9
20.7
86
146
199
252
Melting Pt (F)
-272.5
-301.4
-301.6
-254.4
-216
-182
-155
*Hydrocarbons that contain two or more double bonds are known as
alkadienes, alkatrienes, and alkatetraenes. The suffix denotes the number
of double bonds.
Ex. CH2=C=C=CH-CH3
(1, 2 ,3-pentatriene)
*See Properties of Petroleum fluids, McCain copyright 1973
Physical properties of the butylenes
Name
Isobutylene
1-butene
Trans-2-butene
Cis-2-butene
Boiling Pt (F)
Melting Pt (F)
19.6
20.7
33.6
38.7
-220.6
-301.6
-158.0
-218.0
*See Properties of Petroleum fluids, McCain copyright 1973
Physical properties of cyclic aliphatic hydrocarbons
Name
Boiling Pt (F)
Cyclopropane
Cyclobutane
Cyclopentane
Cyclohexane
Cycloheptane
Cyclooctane
-27
55
121
177
244
300
Melting Pt (F)
-197
-112
-137
44
10
57
*See Properties of Petroleum fluids, McCain copyright 1973
Physical properties of aromatic (arenes) hydrocarbons
Name
Benzene
Toluene
o-xylene
m-xylene
p-xylene
Boiling Pt (F)
176
231
292
282
281
*See Properties of Petroleum fluids, McCain copyright 1973
Melting Pt (F)
42
-139
- 13
- 54
56
Physical properties of hexane isomers
Boiling Pt (F)
Name
n-hexane
3-methylpentane
2-methylpentane
(isohexane)
2, 3-dimethylbutane
2, 2-dimethylbutane
(neohexane)
Melting Pt (F)
155.7
145.9
140.5
-139.6
-180.4
-244.6
136.4
121.5
-199.4
-147.8
*See Properties of Petroleum fluids, McCain copyright 1973
Hydrocarbons
*See Properties of Petroleum fluids, McCain copyright 1973
Hydocarbons
Aromatics
Aliphatics
(arenes)
Alkanes
Alkenes
Alkynes
Cyclic
aliphatics
* Vapor Pressure is the pressure above a liquid at equilibrium.
* If the pressure on a liquid is reduced it will boil at lower temperature than
its boiling point.
* Vapor pressure increases with temperature non-linearly.
Gasoline
Fractional distillation is used to refine crude oil into three principle cuts
according to their boiling points.
-Straight run gasoline (boiling point 30-200 C)
-kerosene (boiling point 175-300 C)
-heating oil or diesel fuel (boiling point 275-400 C)
After reducing the pressure further distillation yields:
-lubricating oils and waxes
This leaves undistillable tarry residue of asphalt.
* Straight run gasoline contains a large number of straight chain alkanes.
Therefore it is a poor fuel because it causes engine knock.
In order to create a higher quality fuel catalytic cracking is used to break
down larger molecules of kerosene (with high boiling point) into smaller
molecules which are used for gasoline
* See McMurry 6th ed. Pgs 94-95 or McMurry 7th ed. Pgs. 99-100
*Octane number of fuel is the measure by which the antiknock properties
of fuel are judged.
Octane Rating
- Measure of the autoignition resistance of gasoline and other fuels used in
spark ignition internal combustion engines.
* In a gasoline engine a high octane rating is desired.
* In a diesel a low octane rating is desirable.
Examples of Octane Ratings:
- n-octane
-10
- n-heptane
0
- n-pentane
62
- iso-octane 100
- benzene
101
- E85 ethanol 105
* Interesting information: In winter months butane is added to gas to
increase the octane. This is not done in summer because it is too hot and
will cause vapor locking and it will evaporate out of the tank.
* Natural Gas liquid (NGL) from natural gas wells
Examples:
- Propane boiling pt -44 degrees F (used in wasp sprays)
- Isobutane boiling pt 10 degrees F (used in under arm deodorant)
- n-Butane boiling point 31 degrees F (used in glass perfume bottles)
• Liquefied petroleum Gas (LPG) comes from petroleum crude and is
cat cracked (catalyst)