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Alkanes and Structural Isomers Chemistry Overview

Explore constitutional isomers of alkanes, their properties, chemical reactions, and naming conventions. Learn about halogenation, radical reactions, and the structure of cycloalkanes. Includes an exercise on drawing isomers and understanding physical properties.

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Alkanes and Structural Isomers Chemistry Overview

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  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 Dr Seemal Jelani Chem-160

  4. Structural Isomers: Pentane (C5H12) pentane 2-methylbutane 2,2-dimethylpropane Dr Seemal Jelani Chem-160

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

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

  7. Structural Isomers: Heptane (C7H16) 2,4-dimethylpentane 3-ethylpentane 3,3-dimethylpentane 2,2,3-trimethylbutane 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 bp & mp ↑ Number of carbon atoms ↑ Number of branches ↑ bp & mp ↓ CH3 CH3CH2CH2CH3 CH3CHCH3

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

  12. Halogenation Free Radical reaction Dr Seemal Jelani Chem-160

  13. Chemical reactions of Alkanes Low reactivity 2- Halogenation: Alkanes react with Halogens. CH4 + Cl2CH3Cl + HCl Chloromethane Heat or light Dichloromethane CH3Cl+ Cl2CH2Cl2 + HCl Heat or light CH2Cl2+ Cl2CHCl3 + HCl Trichloromethane Heat or light CHCl3+ Cl2CCl4 + 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 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 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 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” 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. 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. Dr Seemal Jelani Chem-160

  20. Reactions 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 Initiation Propagation Propagation Termination h Radical Halogenation of Alkanes (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 Dr Seemal Jelani Chem-160

  21. CHLORINATION OF METHANE Further propagationIf excess chlorine is present, further substitution takes place The equations show the propagation steps for the formation of... dichloromethaneCl• + CH3Cl ——> CH2Cl• + HCl Cl2 + CH2Cl• ——> CH2Cl2 + Cl• trichloromethane Cl• + CH2Cl2 ——> CHCl2• + HCl Cl2 + CHCl2• ——> CHCl3 + Cl• tetrachloromethane Cl• + CHCl3 ——> CCl3• + HCl Cl2 + CCl3• ——> CCl4 + Cl• Mixtures Because of the many possible reactions there will be a mixture of products. Individual haloalkanes can be separated by fractional distillation. Dr Seemal Jelani Chem-160

  22. CRACKING Involves the breaking of C-C bonds in alkanes Converts heavy fractions into higher value products THERMALproceeds via a free radical mechanism CATALYTICproceeds via a carbocation (carbonium ion) mechanism CATALYTIC SLIGHT PRESSURE HIGH TEMPERATURE ... 450°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 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 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.

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