Alkanes
Br CH3─CH─CH2─CH2─Cl STEP 1 Longest chain has 4 carbon atoms. C─C─C─C STEP 2 Number chain and add substituents. C ─ C ─ C ─ C ─ Cl 1 2 3 4 STEP 3 Add hydrogen to complete 4 bonds to each C. CH3─CH─CH2─CH2─Cl 2-bromo-4-chlorobutane
Constitutional Isomers Have the same molecular formula. Have different atom arrangements (different structural formula). CH3CH2CH2CH3 Butane C4H10 CH3 2-Methylpropane CH3CHCH3 C4H10
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
Structural Isomers: Pentane (C5H12) 2-methylbutane 2,2-dimethylpropane 1/1/2019 Dr Seemal Jelani Chem-160
Structural Isomers: Hexane (C6H14) 2,3-dimethylbutane 2-methylpentane 2,2-dimethylbutane 3-methylpentane 1/1/2019 Dr Seemal Jelani Chem-160
Structural Isomers: Heptane (C7H16) 2,2-dimethylpentane 2-methylhexane 2,3-dimethylpentane 3-methylhexane 1/1/2019 Dr Seemal Jelani Chem-160
Structural Isomers: Heptane (C7H16) 2,4-dimethylpentane 3-ethylpentane 3,3-dimethylpentane 2,2,3-trimethylbutane 1/1/2019 Dr Seemal Jelani Chem-160
Cyclic Hydrocarbon - Cycloalkane Cyclobutane = = Cyclopentane = Cyclohexane
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)
Boiling & melting points of Alkanes Number of carbon atoms ↑ bp & mp ↑ Number of branches ↑ bp & mp ↓ CH3 CH3CH2CH2CH3 CH3CHCH3
Chemical reactions of Alkanes Low reactivity 1- Combustion: Alkanes react with oxygen. CO2, H2O, and energy are produced. Alkane + O2 CO2 + H2O + heat CH4 + 2O2 CO2 + 2H2O + energy
Halogenation Free Radical reaction 1/1/2019 Dr Seemal Jelani Chem-160
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
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
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
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
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
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
Initiation Homolytic fission forms radicals Radicals are very reactive atoms or groups. Cl2 2Cl. Radicals react quickly with surrounding molecules. 1/1/2019 Dr Seemal Jelani Chem-160
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
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• ——> 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. 1/1/2019 Dr Seemal Jelani Chem-160
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 ... 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 1/1/2019 Dr Seemal Jelani Chem-160
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
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.