alkanes physical and chemical properties -...
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Alkanes
Physical and Chemical Properties
Attractive Forces Acting Between Ions & Molecules“Secondary Forces”
Pure Electrostatic Attractions - chiefly ionic compounds
Dipole-dipole attractions - chiefly between molecules
δ−δ+ δ−δ+ δ−δ+
δ−δ+ δ−δ+ δ−δ+ δ−δ+
δ−δ+
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. ºC
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
CH3
CH3
H3C C
CH3
CH3
CH2 C
CH3
CH3
CH3
Physical Properties of Alkanes
3-D Structures of Isomeric Nonanes
n-nonane, bp 151ºC
2,6-dimethylheptane, bp 135ºC
2,2,4,4-tetramethylpentane, bp 122ºC
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.
# C’s1-4
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. ºC Compound B.P. ºC
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 +65ºC
ethane, bp -89ºC
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. ºC Compound B.P. ºC
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
Cl2
uv 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
Cl2
uv 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
Cl2
uv 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 46ºC
bp 36ºC
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%
Cl2
uv 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
Br2
uv 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 !!