Alkanes Physical and Chemical Properties - Properties of Alkanes Alkanes are Nonpolar Only...

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Transcript of Alkanes Physical and Chemical Properties - Properties of Alkanes Alkanes are Nonpolar Only...

  • Alkanes

    Physical and Chemical Properties

  • Attractive Forces Acting Between Ions & MoleculesSecondary Forces

    Pure Electrostatic Attractions - chiefly ionic compounds

    Dipole-dipole attractions - chiefly between molecules

    + + +

    + + + +

    +

  • Dispersion Forces - Very weak, due to a temporary shift in electron distribution dependent on the size of the molecule

    Attractive Forces Acting Between Ions & Molecules

  • Physical Properties of Alkanes

    Alkanes are Nonpolar Only dispersion forces act between molecules.

    Most alkanes are insoluble in water.

    Boiling points, melting points, and densities generally increase with the size of the alkane:

    Decane > Heptane > Butane

  • Physical Properties of Alkanes

    Compound B.P. C

    CH3CH3 -89

    CH3(CH2)3CH3 36

    CH3(CH2)7CH3 151

    CH3(CH2)11CH3 235

    CH3(CH2)16CH3 316

  • Physical Properties of Alkanes

    Boiling points generally decrease with increased branching as a result of decreased surface area. Consider C9H20

    CH3CH2CH2CH2CH2CH2CH2CH2CH3

    H3C CHCH2CH2CH2CH

    CH3

    CH3CH3

    H3C C

    CH3

    CH3

    CH2 C

    CH3

    CH3

    CH3

  • Physical Properties of Alkanes

    3-D Structures of Isomeric Nonanes

    n-nonane, bp 151C

    2,6-dimethylheptane, bp 135C

    2,2,4,4-tetramethylpentane, bp 122C

  • Physical Properties of AlkanesThe Basis for Fractional Distillation

    Hot crude oil is fed in at t h e f o o t o f t h e fractionating column and mixed with steam. Various constituents, or fractions, of the crude oil separate at various points of the column.

  • # Cs1-4

    7

    13

    16

    19

    22

    25+

    Physical Properties of AlkanesThe Basis for Fractional Distillation

  • Physical Properties of Alkanes

    Comparison of Boiling Points for Alkanes and Alcohols

    Compound B.P. C Compound B.P. C

    CH3CH3 -89 CH3-OH 65

    CH3(CH2)3CH3 36 CH3(CH2)3-OH 118

    CH3(CH2)7CH3 151 CH3(CH2)7-OH 194

    CH3(CH2)11CH3 235 CH3(CH2)11-OH 255

    CH3(CH2)16CH3 316 CH3(CH2)16-OH 308

    Note: All organic molecules may have Alkane-like portions of their structures which confer hydrocarbon-like properties on the molecules

  • Boiling involves breakdown of

    dispersion forces + polar forces.

    Boiling involves breakdown of

    dispersion forces

    Physical Properties of AlkanesComparison of the Boiling of Ethane (a hydrocarbon) and Methanol (an alcohol)

    methanol, bp +65C

    ethane, bp -89C

  • Boiling involves breakdown of

    dispersion forcesBoiling involves

    breakdown of dispersion forces +

    polar forces.

    Physical Properties of Alkanes

    Comparison of the Boiling of a Hydrocarbon and an Alcohol

    Liquid Gas

    Liquid Gas

    However, the boiling points are very

    similar....Why ??

  • Physical Properties of Alkanes

    Comparison of Boiling Points for Alkanes and Alcohols

    Compound B.P. C Compound B.P. C

    CH3CH3 -89 CH3-OH 65

    CH3(CH2)3CH3 36 CH3(CH2)3-OH 118

    CH3(CH2)7CH3 151 CH3(CH2)7-OH 194

    CH3(CH2)11CH3 235 CH3(CH2)11-OH 255

    CH3(CH2)16CH3 316 CH3(CH2)16-OH 308

    Note: All organic molecules may have Alkane-like portions of their structures which confer hydrocarbon-like properties on the molecules

  • Chemical Reactions of

    Alkanes

  • Chemical Reactions of Alkanes

    Combustion - Burning in Oxygen

    Propane burns in oxygen to yield carbon dioxide and water.

    C3H8 + O2 ---------------> CO2 + H2O

    Propane burns in oxygen to yield carbon dioxide and water.Propane burns in oxygen to yield carbon dioxide and water.

    C3H8 + O2 ---------------> CO2 + H2O

    C3H8 + ? O2 ---------------> 3 CO2 + 4 H2O

    C3H8 + 5 O2 ---------------> 3 CO2 + 4 H2O

  • Chemical Reactions of Alkanes

    Butane burns in oxygen to yield carbon dioxide and water.

    C4H10 + O2 ---------------> CO2 + H2O

    Butane burns in oxygen to yield carbon dioxide and water.Butane burns in oxygen to yield carbon dioxide and water.

    C4H10 + O2 ---------------> CO2 + H2O

    C4H10 + ? O2 ---------------> 4 CO2 + 5 H2O

    2 C4H10 + ? O2 ---------------> 8 CO2 + 10 H2O

    2 C4H10 + 13 O2 ---------------> 8 CO2 + 10 H2O

    Combustion - Burning in Oxygen

  • Chemical Reactions of AlkanesHalogenation - Reaction with Halogen + UV light

    Methane reacts with chlorine gas in the presence of ultraviolet light to yield chloromethane and hydrogen chloride gas.

    H C H

    H

    H

    Cl H C

    H

    H

    H+ +Cl Cl Cl

    uv light

    Methane reacts with chlorine gas in the presence of ultraviolet light to yield chloromethane and hydrogen chloride gas.

    CH4 + Cl2 CH3Cl + HCluv light

    CH4 CH3ClCl2

    uv light

  • Chemical Reactions of AlkanesHalogenation - Reaction with Halogen + UV light

    CH4 CH3ClCl2

    uv light

    H C H

    H

    H

    Cl2uv light H C

    H

    H

    Cl

    A SUBSTITUTION REACTION

  • Chemical Reactions of Alkanes

    Halogenation

    Ethane reacts with chlorine gas in the presence of ultraviolet light to yield chloroethane and hydrogen chloride gas.

    H C C

    H

    H

    Cl2uv light C C

    H

    H

    ClH

    H

    H

    H

    H

    H

    C2H6 C2H5ClCl2

    uv light

    A SUBSTITUTION REACTIONDoes it make any difference which hydrogen is exchanged in

    the substutution reaction ?

  • Chemical Reactions of Alkanes

    Note: It makes no difference which of the hydrogens in methane or ethane is replaced because all of the hydrogens within each compound

    are identical.

    Methane Ethane

  • Chemical Reactions of Alkanes

    Structures of Halogenated Alkanes

    1-chloropropane

    chloromethane

    2-chloropropane

    chloroethane

  • Chemical Reactions of Alkanes

    Propane reacts with chlorine gas in the presence of ultraviolet light to yield two different compounds with the formula C3H7Cl.

    Halogenation

    Cl2uv lightH C C

    H

    H

    C

    H

    H

    H

    H

    H

    H C C

    H

    H

    C

    H

    H

    H

    Cl

    Hor

    H C C

    H

    H

    C

    H

    Cl

    H

    H

    H

    1-chloropropane

    2-chloropropane

    bp 46C

    bp 36C

    45%

    55%

  • Chemical Reactions of Alkanes

    Structures of Halogenated Alkanes

    1-chloropropane

    chloromethane

    2-chloropropane

    chloroethane

  • Halogenation - Chlorination

    Chemical Reactions of Alkanes

  • Halogenation - Chlorination vs Bromination

    Chemical Reactions of Alkanes

    1-chloropropane

    2-chloropropane

    45%

    55%

    Cl2uv lightH C C

    H

    H

    C

    H

    H

    H

    H

    H

    H C C

    H

    H

    C

    H

    H

    H

    Hor

    H C C

    H

    H

    C

    H H

    H

    H

    Cl

    Cl

    Br2uv lightH C C

    H

    H

    C

    H

    H

    H

    H

    H

    H C C

    H

    H

    C

    H

    H

    H

    Hor

    H C C

    H

    H

    C

    H H

    H

    H

    Br

    Br

    1-bromopropane

    2-bromopropane

    3%

    97%Bromine is more selective !!

  • Halogenation - Bromination

    Chemical Reactions of Alkanes

    Substitution takes place at the carbon which has the fewest hydrogens in the

    starting material !!