1.3 Physical Properties of Alkanes :
1.3.1 Physical State :
The first four members (C1 to C4) are gases : the next thirteen members, (C5 to C17) are liquids while the higher members are waxy solids.
1.3.2 Boiling points :
The boiling points of n-alkanes increase regularly with the increase in the number of carbon atoms.
Among the isomeric alkanes, the branched chain isomers have relatively low boiling points as compared to their corresponding straight chain isomers. Greater the branching of the chain, lower is the boiling point. This is due to the fact that branching of the chain makes the molecules more compact and brings it close to a sphere, so the magnitude of Van der Waals force decreases.
1.3.3. Melting Points:
It is evident that the increase in melting point is relatively more in moving from an alkane having odd number of carbon atoms to the higher alkane with even no. of `C' while it is relatively less in moving from an alkane with even number of carbon atoms to the higher alkane.
Explanation : The alkanes with even no. of `C' atoms are more closely packed.
In keeping with the popular rule "like dissolves like" hydrocarbons are insoluble in polar solvent like water because they are predominantly non-polar in nature.
The densities of alkanes increase with increasing molecular weight but become constant at about 0.8 g cm-3. This means that all alkanes are lighter than water so they floats over water.
1.4.1 Chemical Reaction of Alkanes :
Characteristic reaction of alkanes are free radical substitution reaction, these reactions are generally chain reactions which are completed mainly in three steps.
(i) chain initiation (ii) chain propagation, (iii) chain termination
Examples of free radical substitution reaction →
R - H + X2 R - X + HX
When equimolar amount of methane and Cl2 are taken, a mixture of four possible products are formed, but if we take excess of CH4 then yield of CH3Cl will be the major product.
Reactivity of X2 : F2 > Cl2 > Br2 > I2
Reactivity of H : 3ºH > 2ºH > 1ºH
Alkanes reacts so vigorously with F2 that, even in the dark and at room temp, reactant diluted with an inert gas.
Iodination is a reversible reaction since HI formed as a by-product is a strong reducing agent and reduces alkyl iodide back to alkane. Hence iodination can be done only in presence of strong oxidizing agent like HIO3, HNO3 or HgO.
R - H + I2 R - I + HI
HI + HIO3 H2O + I2
Mechanism of halogenation of CH4 →
(i) Chain initiation → It is an endothermic step.
(ii) Chain propagation →
(iii) Chain termination → It is always exothermic.
Each photon of light leaves one chlorine molecule to form two chlorine radicals, each chlorine atom starts a chain and on an average each chain contains 5000 repetitions of the chain propagating cycle so about 10,000 molecules of CH3Cl are formed by one photon of light.
Some reagent affects the rate of halogenation:
Q.3 In the given ways which is feasible
Q.4 Which of the following reaction has zero activation energy
(B) Cl2 2 Cl
Q.5 If the Eact for a forward reaction is given
the Eact for backward reaction will be
(A) 1 kcal
(B) 4 kcal
(C) -4 kcal
(D) 3 kcal
Halogenations of higher alkanes:
Relative amounts of the various isomers differ remarkably depending upon the halogen used from the above reaction, it is observed that chlorination gives mixture in which no isomer greatly dominates while, bromination gives mixture in which one isomer dominates greatly (97% - 99%).
Factors determining the relative yields of the isomeric products.
(i) Probability factor → This factor is based on the number of each kind of H atom in the molecule.
(ii) Reactivity of hydrogen → The order of reactivity is 3º > 2º > 1º
1.4.3 Combustion : (i.e. complete oxidation)
O2 nCO2 + (n+1) H2O (DHcombustion = -ve)
O2 xCO2 + H2O
C5H12 + 8O2 5CO2 + 6H2O
Heat of combustion: Amount of heat i.e. liberated when 1 mole of hydrocarbon is completely burnt into CO2 & H2O.
Heat of combustion as a measure of stability of alkane :
Combustion is used as a measurement of stability.
More branched alkanes are more stable and have lower heat of combustion.
e.g. (I) CH3 - CH2 - CH2 - CH3
stability : II > I
DHcomb. : I > II
More branched alkane has more no. of primary C - H bonds. (therefore it has more bond energy).
Homologous : Higher homologous have higher heat of combustion.
Isomers : Branched isomer has lower heat of combustion.
(i) Initiators → They initiate the chain reaction. Initiators are R2O2, Perester's etc.
R - O - O - R
(ii) Inhibitors → A substance that slow down or stop the reaction are known as inhibitors. For example, O2 is a good inhibitor.
All reactive alkyl free radicals are consumed so reaction stops for a period of time.
Relative reactivity of halogen toward methane→
Order of reactivity is F2 > Cl2 > Br2 > I2 which can be explained by the value of ΔH (energy change).
Steps of halogenation, value of ΔH for each step. (Kcal/mole)
Ex.3 Explain why the chain initiating step in thermal chlorination of CH4 is
Cl2 and not CH4
Ans. Because Eact of Cl2 is less than Eact of CH4.
Ex.4 Chlorination of CH4 involves following steps :
Which of the following is rate determining?
(A) Step (i)
(B) Step (ii)
(C) Step (iii)
(D) Step (ii) and (iii) both
Reactivity of hydrogen 3º > 2º > 1º
Because formation of alkyl free radical is Rate determining step so, as H is more reactive which produce more stable free radical (less Eact).
order of stability of Free Radical.→