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Physical properties of Physical properties of alkanes & cycloalkanesalkanes & cycloalkanes
Physical Physical statestate
Boiling Point
Solubility
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Physical statePhysical state At room temperature (25At room temperature (25ooC) and atmospheric C) and atmospheric
pressure (1 atm), for unbranched alkanes,pressure (1 atm), for unbranched alkanes,
CC11 – C – C4 4 : gases: gases
CC55 – C – C1717 : liquids : liquids
CC18 18 - more : solids- more : solids
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Boiling pointsBoiling points
The boiling points of the straight alkanes The boiling points of the straight alkanes show a show a regular increase with increasing regular increase with increasing molecular weightmolecular weight. .
BranchingBranching of the alkanes chain, of the alkanes chain, lower the lower the boiling point.boiling point.
Mr ↑ boiling point ↑
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C-H is non polar bond C-H is non polar bond Intermolecular forces exist – Intermolecular forces exist – London London
dispersiondispersionThe The London dispersionLondon dispersion forces increase : forces increase : As molecular weight increases,As molecular weight increases, Molecular size increasesMolecular size increases Molecular surface area increaseMolecular surface area increase
Therefore, more energy is required to Therefore, more energy is required to separate molecules from one anotherseparate molecules from one another
Result - a higher boiling point.Result - a higher boiling point.
Mr ↑ boiling point ↑
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Chain branching Chain branching :: makes a molecule more compactmakes a molecule more compact Surface area reducesSurface area reduces The strength of the London dispersion The strength of the London dispersion
forces reduceforces reduce Lower boiling points.Lower boiling points.
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Isomeric Alkanes
Have different boiling point due to branching
PentanePentane
CHCH33CHCH22CHCH22CHCH22CHCH33
3737ooCC
2-methylbutane2-methylbutane
CHCH33CHCHCHCH22CHCH
CHCH33
28.528.5ooCC
2,2-2,2-dimethylpropanedimethylpropane
CHCH33
CHCH33CCHCCH33
CHCH33
99ooCC
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CH3CH2CH2CH3
CH3CH2CH2CH3
CH3CHCH3
CH3
CH3CHCH3
CH3
2-methylpropane
butane
For example, butane and 2-methylpropane both have a molecular formula C4H10, but the atoms are arranged differently
In butane the C atoms are arranged in a single chain, but 2-methylpropane is a shorter chain with a branch
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Name Molecular structureBoiling point
Butane has a higher boiling point because the dispersion forces are greater.
The molecules are longer (and so set up bigger temporary dipoles) and can lie closer together than the shorter, fatter 2-methylpropane molecules.
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Cycloalkanes
The boiling points of cycloalkanes are 10oC to 15oC higher than the corresponding straight chain alkanes.
CycloalkaneCycloalkane Boiling pointBoiling point alkanealkane Boiling Boiling pointpoint
CyclobutaneCyclobutane 1313ooCC ButaneButane -0.5-0.5ooCC
CyclopentaneCyclopentane 4949ooCC PentanePentane 36..336..3ooCC
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SolubilitySolubility
Alkanes – less dense than waterAlkanes – less dense than water
Alkanes and cycloalkanes are almost Alkanes and cycloalkanes are almost totally insoluble in watertotally insoluble in water
They are They are non-polar non-polar moleculemolecule Unable to form hydrogen bond with Unable to form hydrogen bond with
HH22OO
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Liquid alkanes and cycloalkanes are Liquid alkanes and cycloalkanes are soluble in one another, and they generally soluble in one another, and they generally dissolve in non-polar solvents.dissolve in non-polar solvents.
Good solvents for alkanes are benzene, Good solvents for alkanes are benzene, carbon tetrachloride, chloroform, and other carbon tetrachloride, chloroform, and other hydrocarbons.hydrocarbons.
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Nature sources of alkanes;
Natural gas
oil
Natural gas contains primarily methane and ethane, with some propane and butane
Oil is a mixture of liquid alkanes and other hydrocarbons
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CHEMICAL REACTION OF CHEMICAL REACTION OF ALKANESALKANES
Non-reactivity of alkanes
Alkanes are generally inert towards many Alkanes are generally inert towards many chemical reagents chemical reagents (bases, acids, dehydrating (bases, acids, dehydrating agents and aqueous oxidizing agents)agents and aqueous oxidizing agents)
C and H has nearly the same electronegativity, C and H has nearly the same electronegativity, the C-H bonds of alkanes are only slightly the C-H bonds of alkanes are only slightly polarised (non polar)polarised (non polar)
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Reaction of alkanes
Combustion Halogenation
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Combustion of alkanes
Alkanes burnt in air ( oxygen ) to give carbon dioxide gas, water and heat.
CxHy+ O2 → x CO2 + H2O
+ Heat 4
yx
2
y
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Examples:
C4H8+ 3O2 → 4CO2 + 4H2O
+ Heat
Combustion of alkane in excess oxygen
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Examples:
C4H8+ 4O2 → 4CO + 4H2O
+ Heat
Combustion of alkane in limited oxygen
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Alkanes are unreactive towards polar or ionic reagents but can react with non-polar reagents such as oxygen and bromine.
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The low reactivity of alkanes toward The low reactivity of alkanes toward many reagents explain why alkanes many reagents explain why alkanes were originally called were originally called paraffinsparaffins. .
(Latin : parum affinis= low affinity)(Latin : parum affinis= low affinity)..
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Halogenation: a free radical Halogenation: a free radical substitution reactionsubstitution reaction
Alkanes react with Alkanes react with halogenhalogen (chlorine & bromine) (chlorine & bromine) to produce to produce haloalkanes haloalkanes in the presence of in the presence of light light or temperature >> 100 or temperature >> 100 ooC. C.
R–H + XR–H + X22 R–X + HX R–X + HX
With methane, the reaction produces a mixture of With methane, the reaction produces a mixture of halomethane and a hydrogen halide.halomethane and a hydrogen halide.
hv
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Examples:Examples:
i. CH4 + Cl2 hv CH3Cl + CH2Cl2
+ CHCl3 + CCl4
+ HCl
ii. CH3CH3 + Cl2 hv CH3CH2Cl + HCl
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CH3 CCH2Cl + HCl
| | CH3 CH3
CH3 CH3
| |iii. CH3 CCH3 + Cl2 hv
(ii) & (iii): 1 product only because all the hydrogen atoms are identical
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iv. CH3CH2CH3 + Br2 hv CH3CH2CH2Br
( minor)+ CH3CHCH3
| Br ( major)+ HBr
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CH4 + Cl2 CH3Cl + HCl
i. Chain initiation step
Reaction mechanism
hv
hvCl – Cl Cl – Cl 2Cl•2Cl•
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ii. Chain propagation steps
3HC• + Cl Cl_ CH3Cl + Cl•
HH-C H + •Cl •CH3 + HCl
H
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iii. Chain termination step.
•CH3 + •CH3 CH3CH3
•Cl + •CH3 CH3Cl
•Cl + •Cl Cl2
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Example Example
The bromination of 2-methylbutane yields a The bromination of 2-methylbutane yields a mixture of isomers.mixture of isomers.
CH3 C CH2 CH3
CH2Br
H
CH3 C CH3
H
CH3
CHBr CH3 C CH3
CH3
Br
CH2
Increasing % yield
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ExerciseExercise
Chlorination reaction of certain Chlorination reaction of certain alkanes can be used for laboratory alkanes can be used for laboratory preparations, for example in the preparations, for example in the preparation of chlorocyclopentane preparation of chlorocyclopentane from cyclopentane. Give the from cyclopentane. Give the mechanism for the reaction.mechanism for the reaction.