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Alkanes.

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Presentation on theme: "Alkanes."— Presentation transcript:

1 Alkanes

2 Br CH3─CH─CH2─CH2─Cl STEP Longest chain has 4 carbon atoms. C─C─C─C STEP Number chain and add substituents. C ─ C ─ C ─ C ─ Cl STEP 3 Add hydrogen to complete 4 bonds to each C. CH3─CH─CH2─CH2─Cl bromo-4-chlorobutane

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

4 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 1/1/2019 Dr Seemal Jelani Chem-160

5 Structural Isomers: Pentane (C5H12)
2-methylbutane 2,2-dimethylpropane 1/1/2019 Dr Seemal Jelani Chem-160

6 Structural Isomers: Hexane (C6H14)
2,3-dimethylbutane 2-methylpentane 2,2-dimethylbutane 3-methylpentane 1/1/2019 Dr Seemal Jelani Chem-160

7 Structural Isomers: Heptane (C7H16)
2,2-dimethylpentane 2-methylhexane 2,3-dimethylpentane 3-methylhexane 1/1/2019 Dr Seemal Jelani Chem-160

8 Structural Isomers: Heptane (C7H16)
2,4-dimethylpentane 3-ethylpentane 3,3-dimethylpentane 2,2,3-trimethylbutane 1/1/2019 Dr Seemal Jelani Chem-160

9 Cyclic Hydrocarbon - Cycloalkane
Cyclobutane = = Cyclopentane = Cyclohexane

10 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)

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

12 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

13 Halogenation Free Radical reaction 1/1/2019 Dr Seemal Jelani Chem-160

14 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

15 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 1/1/2019 Dr Seemal Jelani Chem-160

16 General reaction heat or R-H + X2 R-X + H-X light R= Alkyl group
Halogenation means: X2 where X = F, Br, Cl, I 1/1/2019 Dr Seemal Jelani Chem-160

17 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 1/1/2019 Dr Seemal Jelani Chem-160

18 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” 1/1/2019 Dr Seemal Jelani Chem-160

19 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. 1/1/2019 Dr Seemal Jelani Chem-160

20 Initiation ​Homolytic fission forms radicals​
Radicals are very reactive atoms or groups. Cl​2​ ​2Cl​. Radicals react quickly with surrounding molecules. 1/1/2019 Dr Seemal Jelani Chem-160

21 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 1/1/2019 Dr Seemal Jelani Chem-160

22 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. 1/1/2019 Dr Seemal Jelani Chem-160

23 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 1/1/2019 Dr Seemal Jelani Chem-160

24 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 1/1/2019 Dr Seemal Jelani Chem-160

25 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.


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