2.13 Sources of Alkanes and Cycloalkanes Crude oil

2.13 Sources of Alkanes and Cycloalkanes Crude oil

2.13 Sources of Alkanes and Cycloalkanes Crude oil Naphtha Naphtha (bp (bp95-150 95-150C) C) Kerosene Kerosene (bp:

(bp: 150-230 150-230C) C) C5-C12 Light Lightgasoline gasoline (bp: (bp:25-95 25-95C) C) C12-C15 Crude oil

Gas Gasoil oil (bp: (bp: 230-340 230-340C) C) Refinery Refinerygas gas C1-C4 C15-C25

Residue Residue Petroleum Petroleumrefining refining Cracking converts high molecular weight hydrocarbons to more useful, low molecular weight ones Reforming increases branching of hydrocarbon chains branched hydrocarbons have better burning characteristics for automobile engines 2.14 Physical Properties of

Alkanes and Cycloalkanes Boiling BoilingPoints Pointsof ofAlkanes Alkanes governed by strength of intermolecular attractive forces alkanes are nonpolar, so dipole-dipole and dipole-induced dipole forces are absent only forces of intermolecular attraction are induced dipole-induced dipole forces Induced Induceddipole-Induced

dipole-Induceddipole dipoleattractive attractiveforces forces + + two nonpolar molecules center of positive charge and center of negative charge coincide in each Induced Induceddipole-Induced dipole-Induceddipole

dipoleattractive attractiveforces forces + + movement of electrons creates an instantaneous dipole in one molecule (left) Induced Induceddipole-Induced dipole-Induceddipole dipoleattractive attractiveforces

forces + + temporary dipole in one molecule (left) induces a complementary dipole in other molecule (right) Induced Induceddipole-Induced dipole-Induceddipole dipoleattractive

attractiveforces forces + + temporary dipole in one molecule (left) induces a complementary dipole in other molecule (right) Induced

Induceddipole-Induced dipole-Induceddipole dipoleattractive attractiveforces forces + + the result is a small attractive force between the two molecules

Induced Induceddipole-Induced dipole-Induceddipole dipoleattractive attractiveforces forces + +

the result is a small attractive force between the two molecules Boiling BoilingPoints Points increase with increasing number of carbons more atoms, more electrons, more opportunities for induced dipole-induced dipole forces decrease with chain branching branched molecules are more compact with smaller surface areafewer points of contact with other molecules Boiling

BoilingPoints Points increase with increasing number of carbons more atoms, more electrons, more opportunities for induced dipole-induced dipole forces Heptane bp 98C Octane bp 125C Nonane bp 150C

Boiling BoilingPoints Points decrease with chain branching branched molecules are more compact with smaller surface areafewer points of contact with other molecules Octane: bp 125C 2-Methylheptane: bp 118C 2,2,3,3-Tetramethylbutane: bp 107C 2.15 Chemical Properties. Combustion of Alkanes All alkanes burn in air to give

carbon dioxide and water. Heats Heatsof ofCombustion Combustion increase with increasing number of carbons more moles of O2 consumed, more moles of CO2 and H2O formed Heats Heatsof ofCombustion Combustion Heptane

4817 kJ/mol 654 kJ/mol Octane 5471 kJ/mol 654 kJ/mol Nonane 6125 kJ/mol Heats Heatsof ofCombustion Combustion

increase with increasing number of carbons more moles of O2 consumed, more moles of CO2 and H2O formed decrease with chain branching branched molecules are more stable (have less potential energy) than their unbranched isomers Heats Heatsof ofCombustion Combustion 5471 kJ/mol 5 kJ/mol 5466 kJ/mol 8 kJ/mol

5458 kJ/mol 6 kJ/mol 5452 kJ/mol Important ImportantPoint Point Isomers can differ in respect to their stability. Equivalent statement: Isomers differ in respect to their potential energy. Differences in potential energy can be measured by comparing heats of combustion. Figure 2.5 5471 kJ/mol

25 O2 + 2 5466 kJ/mol 5458 kJ/mol + 25 O2 2 5452 kJ/mol +

8CO2 + 9H2O 25 O2 2 25 O2 + 2 2.16 Oxidation-Reduction in Organic Chemistry Oxidation of carbon corresponds to an increase in the number of bonds between

carbon and oxygen and/or a decrease in the number of carbon-hydrogen bonds. O O increasing oxidation state of carbon O H H H C H -4

H H C H C H C HO

C OH H OH H -2 0 +2 +4

OH HC increasing oxidation state of carbon H H H C C H

H -3 H C H H CH C H

H -2 -1 But most compounds contain several (or many) carbons, and these can be in different oxidation states. Working from the molecular formula gives the average oxidation state. CH3CH2OH -3 -1

C2H6O Average oxidation state of C = -2 Fortunately, we rarely need to calculate the oxidation state of individual carbons in a molecule . We often have to decide whether a process is an oxidation or a reduction. Generalization Generalization Oxidation of carbon occurs when a bond between carbon and an atom which is less electronegative than carbon is replaced by a bond to an atom that is more electronegative than carbon. The reverse

process is reduction. C X oxidation C reduction X less electronegative than carbon Y more electronegative than carbon Y

Examples Examples Oxidation CH4 + Cl2 CH3Cl + HCl Reduction CH3Cl + 2Li CH3Li + LiCl

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