Alkanes Flashcards
Alkanes
CnH2n+2
Acyclic
Cyclic
Cyclic Alkanes
Ring structure
CnH2n
Cyclopropane-triangle
Cyclobutane- square
Cyclopentane-pentagon
Cyclohexane- hexagon
Acyclic Alkanes
Straight chain
Branched chain
CnH2n+2
Nomenclature
- Prefix
- Parent
- Suffix
Prefix
Substituents
Parent
Longest continuous carbon chain bearing principal functional group
Suffix
Principal functional group
Isomerism
Alkanes with 4 or more carbons exhibit structural isomerism due to branching of the hydrocarbon chains.
Physical properties
- Increasing MP/BP
- Increasing Density
- Not soluble
- Increasing viscosity
Increasing MP/BP
With increasing number of carbon, increasing Mr hence increasing electron cloud size.
Branched chain has lower MP/BP than straight chain due to lesser surface area of contact. Increasing degree of branching decreasing MP/BP
Increasing density
Increasing Mr down homologous series stronger id-id bringing alkane molecules closer together occupying smaller volume. Larger mass/smaller volume leads to increasing density down series (str8 chain)
density decreases with greater degree of branching
Not soluble
Alkanes are non polar molecules hence insoluble in polar solvents but soluble in non-polar solvents
Increasing viscosity
Increasing Mr as strength of id id increases down group increasing viscosity
Chemical properties
Reactivity
Combustion
Free radical substitution
Reactivity
generally unreactive (inert)
- due to non polar C-H bonds as C and H have similar electronegativities
- C-C and C-H bonds are strong
alkane do not have electron rich/poor centres to attract reactive charged species
Combustion
Alkanes (hydrocarbons) react with excess O2 to produce CO2 and H2O
if O2 supply limited incomplete combustion resulting in the formation of CO and/or soot-C and H2O
CxHy+ x+y/4O2 —–> xCO2 +y/2 H2O
Free radical substitution
Alkanes react with halogens such as Cl2 or Br2 in presence of UV light or high temperatures to prod. halogenoalkenes involving free radicals
a substitution reaction one or more of H atoms replaced by halogen atoms
Free radical mechanism
Initiation
Propagation
Termination
Initiation
Bond in Halogen molecule cleaved homolytically to produce 2 free halogen radicals in the presence of UV light/heat
Cl—–Cl (half arrow pointing from bond to each carbon)—-> 2Cl·
energy from UV sufficient to over Cl-Cl but not C-H hence bond in halogen molecule broken preferentially
Propagation
Halogen radicals undergo chain reaction (a repeated sequence of…)
e.g. 1Cl·+ CH4—–>CH3 + HCl
2. ·CH3 +Cl2 ——> Cl·+ CH3Cl
free Halogen radical used and regenerated and chain rxn continuous. Halogen radical behaves as a catalyst
Termination
Involves 2 rctive free radicals combine forming stable inert substance. Free radicals consumed
Any two
e. g. 1. 2Cl·—->Cl2
2. ·CH3+·CH3—->CH3CH3
3. ·CH3 + Cl· —–> CH3Cl
Poly substitution
multiple repetition of propagation steps according to how many halogen atoms substitute the H atoms in alkane
Di substitution- 2 pairs
Tri substitution - 3 pairs
Minimising multiple substitutions
Excess alkane/limited halogen molecule
monosubstituted alkane predominates greater chance for ·Cl collide with alkane than monosubstituted alkane in propagation
Reactivity of halogens with alkanes
increases increases in the order X-X bond energies
F2 (explosive)
Cl2/Br2 (250C-400C in UV light)
I (least reactive)
Expected yield for monosubstituted of alkane
Assume that all H has an equal chance of being substituted
- Draw out the isomers of alkane substituted
- Count total no.of H atoms-y
- Count possibility of each type of substitution from (1)-x
- x/y or ratio to simplest
Rate of substitution at 1,2,3 degree carbon atoms
- Alkyl groups (groups att.to C) are electron donating. Donating electrons to the electron deficient carbon atom that bears lone electron.
2. Tertiary radical (3 degree)---most stable Secondary radical (2 degree)---less stable Primary radical (1 degree)----least stable
The discrepancy in expected and experimental ratios of monosubstituted alkanes
- How is monosubstituted alkane products formed (its is formed via a T/S/P radical
- Which radical more stable hence which product more stable
- Stable product formed more preferentially hence yield of that product greater
Environmental pollutants
Smog
due to incomplete combustion of fuel/unburnt hydrocarbons in the presence of sunlight
CO
Due to incomplete combustion of hydrocarbon fuel
binds preferentially to Hb reducing O2 carrying capacity of RBC
NOx
Combustion of fuel in car engines due to rise in temp resulting in breakage of bond in N2, N atoms react with O2 to form oxides of nitrogen
contributes to smog
Hydrocarbons+O2+NO2+light—-> PAN
ozone at lower atm
NO2—>NO+O (atom)
O+O2—>O3
cause respiratory problems
catalyses oxidation of SO2 to SO3 which forms Acid rain
NO2+SO2—>SO3+NO
NO+1/2O2—>NO2 (regenerated)
SO3 dissolves in rain water so acid rain
H2O+ SO3—> H2SO4
Use of catalytic converter
containing rhodium,platinum,palladium to convert harmful gases to less harmful gases through redox reactions
Hydrocarbons+NOx—>CO2+H2O+N2
Enhanced greenhouse effect
Due to human activities such as deforestation + burning of fossil fuels greenhouse gases increases
greenhouse gases; CO2 and CH4 increase the average surface temperature of the earth
