Alkanes

Question 1

Alkanes

Answer 1

CnH2n+2

Acyclic
Cyclic

Question 2

Cyclic Alkanes

Answer 2

Ring structure
CnH2n

Cyclopropane-triangle
Cyclobutane- square
Cyclopentane-pentagon
Cyclohexane- hexagon

Question 3

Acyclic Alkanes

Answer 3

Straight chain
Branched chain
CnH2n+2

Question 4

Nomenclature

Answer 4

1. Prefix
2. Parent
3. Suffix

Question 5

Prefix

Answer 5

Substituents

Question 6

Parent

Answer 6

Longest continuous carbon chain bearing principal functional group

Question 7

Suffix

Answer 7

Principal functional group

Question 8

Isomerism

Answer 8

Alkanes with 4 or more carbons exhibit structural isomerism due to branching of the hydrocarbon chains.

Question 9

Physical properties

Answer 9

1. Increasing MP/BP
2. Increasing Density
3. Not soluble
4. Increasing viscosity

Question 10

Increasing MP/BP

Answer 10

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

Question 11

Increasing density

Answer 11

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

Question 12

Not soluble

Answer 12

Alkanes are non polar molecules hence insoluble in polar solvents but soluble in non-polar solvents

Question 13

Increasing viscosity

Answer 13

Increasing Mr as strength of id id increases down group increasing viscosity

Question 14

Chemical properties

Answer 14

Reactivity
Combustion
Free radical substitution

Question 15

Reactivity

Answer 15

generally unreactive (inert)

1. due to non polar C-H bonds as C and H have similar electronegativities
2. C-C and C-H bonds are strong

alkane do not have electron rich/poor centres to attract reactive charged species

Question 16

Combustion

Answer 16

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

Question 17

Free radical substitution

Answer 17

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

Question 18

Free radical mechanism

Answer 18

Initiation
Propagation
Termination

Question 19

Initiation

Answer 19

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

Question 20

Propagation

Answer 20

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

Question 21

Termination

Answer 21

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

Question 22

Poly substitution

Answer 22

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

Question 23

Minimising multiple substitutions

Answer 23

Excess alkane/limited halogen molecule
monosubstituted alkane predominates greater chance for ·Cl collide with alkane than monosubstituted alkane in propagation

Question 24

Reactivity of halogens with alkanes

Answer 24

increases increases in the order X-X bond energies
F2 (explosive)
Cl2/Br2 (250C-400C in UV light)
I (least reactive)

Question 25

Expected yield for monosubstituted of alkane

Answer 25

Assume that all H has an equal chance of being substituted

1. Draw out the isomers of alkane substituted
2. Count total no.of H atoms-y
3. Count possibility of each type of substitution from (1)-x
4. x/y or ratio to simplest

Question 26

Rate of substitution at 1,2,3 degree carbon atoms

Answer 26

1. 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

Question 27

The discrepancy in expected and experimental ratios of monosubstituted alkanes

Answer 27

1. How is monosubstituted alkane products formed (its is formed via a T/S/P radical
2. Which radical more stable hence which product more stable
3. Stable product formed more preferentially hence yield of that product greater

Question 28

Environmental pollutants

Answer 28

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

Question 29

Use of catalytic converter

Answer 29

containing rhodium,platinum,palladium to convert harmful gases to less harmful gases through redox reactions

Hydrocarbons+NOx—>CO2+H2O+N2

Question 30

Enhanced greenhouse effect

Answer 30

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