1. Alkanes

2. Constitutional Isomers
Have the same molecular formula.
Have different atom arrangements (different structural formula).
CH3CH2CH2CH3
Butane
C4H10
CH3
2-Methylpropane
CH3CHCH3
C4H10

3. Structural Isomer Practice
On piece of your own paper, draw AND name ALL of the isomers for the following alkanes:
Pentane
Hexane
Heptane
C5H12
C6H14
C7H16 3 5 9
Formulas
isomers
9/13/2018
Dr Seemal Jelani Chem-160

4. Structural Isomers: Pentane (C5H12)
2-methylbutane
2,2-dimethylpropane
9/13/2018
Dr Seemal Jelani Chem-160

5. Structural Isomers: Hexane (C6H14)
2,3-dimethylbutane
2-methylpentane
2,2-dimethylbutane
3-methylpentane
9/13/2018
Dr Seemal Jelani Chem-160

6. Structural Isomers: Heptane (C7H16)
2,2-dimethylpentane
2-methylhexane
2,3-dimethylpentane
3-methylhexane
9/13/2018
Dr Seemal Jelani Chem-160

7. Structural Isomers: Heptane (C7H16)
2,4-dimethylpentane
3-ethylpentane
3,3-dimethylpentane
2,2,3-trimethylbutane
9/13/2018
Dr Seemal Jelani Chem-160

8. Cyclic Hydrocarbon - Cycloalkane
Cyclobutane = =
Cyclopentane =
Cyclohexane

9. Physical Properties of Alkanes
Nonpolar
Insoluble in water.
Lower density than water.
Low boiling and melting points.
Gases with 1-4 carbon atoms.
(methane, propane, butane)
Liquids with 5-17 carbon atoms.
(kerosene, diesel, and jet fuels)
Solids with 18 or more carbon atoms.
(wax, paraffin, Vaseline)

10. Boiling & melting points of Alkanes
Number of carbon atoms ↑
bp & mp ↑
Number of branches ↑
bp & mp ↓
CH3
CH3CH2CH2CH3
CH3CHCH3

11. Chemical reactions of Alkanes
Low reactivity
1- Combustion:
Alkanes react with oxygen.
CO2, H2O, and energy are produced.
Alkane + O CO2 + H2O + heat
CH4 + 2O2 CO2 + 2H2O + energy

12. Halogenation
Free Radical reaction
9/13/2018
Dr Seemal Jelani Chem-160

13. Chemical reactions of Alkanes
Low reactivity
2- Halogenation:
Alkanes react with Halogens.
CH4 + Cl2 CH3Cl + HCl
Chloromethane
Heat or light
Dichloromethane
CH3Cl+ Cl2 CH2Cl2 + HCl
Heat or light
CH2Cl2+ Cl2 CHCl3 + HCl
Trichloromethane
Heat or light
CHCl3+ Cl2 CCl4 + HCl
Tetrachloromethane
Heat or light

14. Halogenation
Reaction between a substance and a halogen in which one or more halogens atoms are incorporated into molecules of the substance
Product is Hydrocarbon derivative
Halogenation is an example of Substitution reaction
It is a reaction in which part of a small reacting molecule replaces an atom or group of atoms on a hydrocarbons
9/13/2018
Dr Seemal Jelani Chem-160

15. General reaction heat or R-H + X2 R-X + H-X light R= Alkyl group
Halogenation means: X2 where X = F, Br, Cl, I
9/13/2018
Dr Seemal Jelani Chem-160

16. Radical Halogenation Terms
Mechanism
How the reaction occurs through multiple steps (most reactions actually occur in many steps)
Chain Reaction
Reactions that occur on their own after some initiating event
Free Radicals
Atoms that have one free electron—highly reactive
9/13/2018
Dr Seemal Jelani Chem-160

17. Radical Halogenation Terms
Initiation Step
Step where a bond is split by heat/light, producing free radicals
Propagation Step
Step where free radicals react with non-radicals, producing more free radicals and continuing the “chain reaction”
Termination Step
Step where free radicals react with each other, producing non-radicals and terminating the “chain reaction”
9/13/2018
Dr Seemal Jelani Chem-160

18. Initiation​ ​Reaction starts with one ​photon of light.​ Must be UV​
Wavelength less than ​400nm​
Energy less than 300kJ ​per mole​
Reaction proceeds to ​completion after this.
Reason is the chlorine ​molecule​
Light breaks the Cl-Cl ​bond​
•​Homolytic fission​
​Each atom takes an ​electron
Forms radicals.
9/13/2018
Dr Seemal Jelani Chem-160

19. Initiation ​Homolytic fission forms radicals​
Radicals are very reactive atoms or groups.
Cl​2​ ​2Cl​.
Radicals react quickly with surrounding molecules.
9/13/2018
Dr Seemal Jelani Chem-160

20. Radical Halogenation of Alkanes
Reactions of Alkanes
Radical Halogenation of Alkanes
CH4 + Cl2  CH3Cl + HCl
Mechanism (chain reaction):
Step 1 Cl2 ⇌ Cl· + Cl ·
Step 2 Cl· + CH4  CH3· + HCl
Step 3 CH3· + Cl2  CH3Cl + Cl·
Step 4 Cl· + Cl·  Cl2
Type of Step h
Initiation
Propagation
Termination
(Free Radicals)
Overall reaction: CH4 + Cl2  CH3Cl + HCl
 chloromethane
Why not 1-chloromethane?
Halogenated product is a haloalkane
Naming: halogen atom is a substituent, replace –ine ending with –o
-F fluoro -Cl chloro -Br bromo -I iodo
9/13/2018
Dr Seemal Jelani Chem-160

21. CHLORINATION OF METHANE
Further
propagation If excess chlorine is present, further substitution takes place
The equations show the propagation steps for the formation of...
dichloromethane Cl• + CH3Cl ——> CH2Cl• HCl
Cl2 + CH2Cl• ——> CH2Cl Cl•
trichloromethane Cl• + CH2Cl2 ——> CHCl2• + HCl
Cl CHCl2• ——> CHCl Cl•
tetrachloromethane Cl• + CHCl3 ——> CCl3• + HCl
Cl CCl3• ——> CCl Cl•
Mixtures Because of the many possible reactions there will be a mixture of products.
Individual haloalkanes can be separated by fractional distillation.
9/13/2018
Dr Seemal Jelani Chem-160

22. CATALYTIC CRACKING Converts heavy fractions into higher value products
Involves the breaking of C-C bonds in alkanes
Converts heavy fractions into higher value products
THERMAL proceeds via a free radical mechanism
CATALYTIC proceeds via a carbocation (carbonium ion) mechanism
CATALYTIC
SLIGHT PRESSURE
HIGH TEMPERATURE °C
ZEOLITE CATALYST
CARBOCATION (IONIC) MECHANISM
HETEROLYTIC FISSION
PRODUCES BRANCHED AND CYCLIC ALKANES, AROMATIC HYDROCARBONS
USED FOR MOTOR FUELS
ZEOLITES are crystalline aluminosilicates; clay like substances
9/13/2018
Dr Seemal Jelani Chem-160

23. At each level a mixture of compounds in a similar boiling range is taken off
Rough fractions can then be distilled further to obtain narrower boiling ranges
Some fractions are more important - usually the lower boiling point ones, high boiling fractions may be broken down into useful lower boiling ones - CRACKING
9/13/2018
Dr Seemal Jelani Chem-160

24. Sources of Alkanes Natural gas 90 to 95 percent methane
5 to 10 percent ethane, and
a mixture of other low-boiling alkanes, chiefly propane, butane, and 2-methylpropane.
Petroleum
A thick liquid mixture of thousands of compounds, most of them hydrocarbons formed from the decomposition of marine plants and animals.