Alkanes

I intend to turn this blog into my study log.
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Alkanes are saturated alipathic hydrocarbons. All the carbon atoms are bonded to 4 atoms or groups.

Preparation of alkanes:
"The principal source of alkanes is petroleum. Alkanes from this source consist of a mixture of different alkanes according to the chain length." (Koay's organic chem. notes)

1. Hydrogenation of alkenes

C=C + H2 -----(Ni; Pt; Pd)-----> C-C Pressure = 3 atm, temperature = 100'C


2. Hydration of Grignard Reagent

Grignard Reagent can be prepared by the reaction of alkyl halide and Mg in ether. (Koay)
R-X + Mg -----> R-MgX

.....in diethylether. (Tan Yin Toon)

.....by heating alkyl halide, RX and Mg in ether under reflux. (Madamme How, 8/3/10)


The alkane can be obtained by adding the reagent to water or dilute acid. (Koay)

R-MgX + H2O -----> R-H + Mg(OH)X

X = I, Br, Cl (Koay)

This reaction is not useful for mass production. (Koay)


3. Wurtz Reaction (Koay)
"Wurtz Reaction involves the reaction of an alkyl halide with sodium metal, resulting in the lengthening of the carbon chain and formation of sodium halide salt.

2RX + 2Na -----> R-R + 2NaX

This reaction is especially useful in the synthesis of symmetrical alkanes*, i.e. not pentane.

If 2 different alkyl halides are used, a mixture of alkanes will be obtained.

3 CH3CH2Cl + 3 CH3Cl + 6Na -----> CH3CH3 + CH3CH2CH3 + CH3CH2CH2CH3 + 6NaCl


Reactions
1. Combustion
2. Halogenation, in the presence of light.

C-C + Cl2 ------> C-C-Cl + HCl

1. Cl2 ----> 2Cl (chain initiation)

2. Cl + C-C ---> HCl + C-C (radical) (chain propagation)

3. C-C (radical) + Cl2 ---> C-C-Cl + Cl (chain propagation)

4. Cl + Cl ----> Cl2

5. Cl + C-C (radical) ----> C-C-Cl

6. C-C (radical) + C-C(radical) -----> C-C-C-C (butane)

4,5,6 are chain termination steps.

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*It is worth noting that in the latter chapter, we will learn that halogenation of alkenes take place in the absence of light. Light causes the homolytic fission of halogen molecules, splitting them into free radicals, which hinders the addition reaction. (wwechampion, 8/3/10)

* This does not corroborates with YA and Success Chemistry SPM, page 342.

* Koay said, "The addition of halogens to the double bond is easy to be carried out. The reaction takes place in the absence of light."

The confusion arises from here, on whether presence of light will prevent the reaction from occuring.