Properties of Alkanes

PHYSICAL PROPERTIES

Principal intermolecular interactions between alkane molecules are London dispersion interactions. Due to weak forces, the C₁ to C₄ alkanes are gases, C₅ to C₁₇ are liquids, those with more than 18 carbon atoms are solids at 298K. All are colourless and odourless. Alkanes with straight carbon chains have higher boiling points than their isomers with branched chains.

Illustration 1. Arrange the following as per instructions given in parenthesis.

(a) 1-hexene, 2-hexene, 3-hexene (in increasing order of heat of hydrogenation

Solution: (a) 2-hexene < 3-hexene < 1-hexene

(b)

CHEMICAL PROPERTIES

Alkanes are relatively stable to most of the common reagents at room temperature. The $C-H$ and $C-C$ bonds in alkanes are almost non – polar and strong, and attacking reagents find no reaction sites to which they could be attached. Random collisions between them can occur but energy of collisions is too small to react. At high temperature energies of the collisions are sufficient for breaking $C-H$and $C-C$bonds resulting in the formation of free radicals.

Stability of free radicals is in order

Substitution reactions of Alkanes

(A) Halogenation

It involves the replacement of 1⁰, 2⁰ & 3⁰ H of alkanes by halogen (F, Cl, Br, I)

For a given type of abstraction of H (say 1⁰) reactivity of halogen is in order:

F₂ > Cl₂ > Br₂ > I₂

For a given halogen, abstraction of 1⁰, 2⁰ and 3⁰ H's is in order:

1⁰ < 2⁰ < 3⁰

For chlorination, reactivity of 1⁰, 2⁰, 3⁰ H's is in ratio:

1 : 2 : 3 : : 1 : 3.8 : 5

Bond energy of extraction of 1⁰ H > 2⁰ H > 3⁰ H, hence the reactivity order is 1⁰ < 2⁰ < 3⁰ H, hence, the stability order of alkyl radical free is

$C{H}_3\prec C{H}_{3}C{H}_2\prec (C{H}_3{)}_{2}CH\prec (C{H}_3{)}_{3}C$

Mechanism

$C{H}_4+C{l}_2\stackrel{hv}{\to }C{H}_{3}Cl+HCl$

(i) Initiation:

$C{l}_2\stackrel{h\nu }{\to }2C{l}^o$

(ii) Propagation:

$C{H}_4+C{l}^o\to HCl+C{H}_3^o$

$C{H}_3^o+C{l}_2^{}\to C{H}_{3}Cl+C{l}_{}^o$

(iii) Termination:

$C{H}_3^o+C{H}_3^o\to C{H}_3-C{H}_3^{}$

$C{l}_{}^o+C{l}_{}^o\to C{l}_2$

$C{H}_3^o+C_{}^o\to C{H}_3-Cl$

Based on relative reactivity of different types of H₂, percentage of each isomer in the product mixture can be calculated. Consider the case of n – butane.

Relative amount = No. of equivalent hydrogen x reactivity

Total amount = 21.2

% of A = $\frac{6.0}{21.2}\times 100=28.3$

% of B = $\frac{15.2}{21.2}\times 100=71.7$

Bromine is less reactive with alkanes in comparison to chlorine, but bromine is more selective in the site of attack when it does react

(B) Nitration

Replacement of H by a nitro group, $-N{O}_2$, is called nitration.

(C) Sulphonation

Replacement of H by a sulphonic acid group, SO₃H is called sulphonation. Oleum is used as a source of $(-S{O}_{3}H).$

RH + H₂SO₄/SO₃$\to$ RSO₃H + H₂SO₄

(CH₃)₃CH + H₂SO₄/SO₃ (CH₃)₃CSO₃H + H₂SO₄

Ease of replacement of H atoms is 3⁰ > 2⁰ > 1⁰.