Chemistry > Hydrocarbons > 3.0 Methods of Preparation Alkanes
Hydrocarbons
1.0 Introduction
2.0 Alkanes
3.0 Methods of Preparation Alkanes
4.0 Physical Proparties
5.0 Chemical Properties
6.0 Alkenes
7.0 Methods of Preparation Alkenes
7.1 Dehydrohalgoenation
7.2 Dehydration of Alcohols
7.3 Dehalogenation
7.4 Thermal elimination reaction
7.5 By partial reduction of alkynes:
7.6 Wittig Reaction
7.7 Kolbe hydrocarbon synthesis
8.0 Physical Proparties
9.0 Chemical Properties
10.0 Mechanism Of Some Important Reaction Of Alkenes
10.1 Mechanism of halogen addition
10.2 Mechanism of halohydrin formation
10.3 Syn - hydroxylation
10.4 Oxidation reactions of alkenes
11.0 Alkynes
12.0 Methods of Preparation Alkynes
12.1 Industrial source
12.2 Kolbe’s method
12.3 Dehydrohalogenation of 1, 2 – dihalides
12.4 Dehydrohalogenation of 1, 1 – dihalides
12.5 Dehalogenation of tetrahalides or trihalides
12.6 Alkylation of acetylene and terminal alkynes
13.0 Physical Properties
14.0 Chemical Properteis
14.1 Electrophilic addition reactions
14.2 Acidity of Alkynes
14.3 Aromatic Hydrocarbons
14.4 Structure of Benzene
15.0 Modern Concept
15.1 Aromaticity in Benzene and Related Systems
15.2 Huckel’s rule or $\left( {{\bf{4n}}{\text{ }} + {\text{ }}{\bf{2}}} \right)\pi $ electron rule
16.0 Properteis
17.0 Mechanism of Electrophilic Substitution Reactions
17.1 Nitration
17.2 Friedel – Craft Alkylation
17.3 Friedel – Craft Acylation
17.4 Reactions of side chains
18.0 Toluene
19.0 Alkenyl Benzene
3.2 From coupling reactions
7.2 Dehydration of Alcohols
7.3 Dehalogenation
7.4 Thermal elimination reaction
7.5 By partial reduction of alkynes:
7.6 Wittig Reaction
7.7 Kolbe hydrocarbon synthesis
10.2 Mechanism of halohydrin formation
10.3 Syn - hydroxylation
10.4 Oxidation reactions of alkenes
12.2 Kolbe’s method
12.3 Dehydrohalogenation of 1, 2 – dihalides
12.4 Dehydrohalogenation of 1, 1 – dihalides
12.5 Dehalogenation of tetrahalides or trihalides
12.6 Alkylation of acetylene and terminal alkynes
14.2 Acidity of Alkynes
14.3 Aromatic Hydrocarbons
14.4 Structure of Benzene
15.2 Huckel’s rule or $\left( {{\bf{4n}}{\text{ }} + {\text{ }}{\bf{2}}} \right)\pi $ electron rule
17.2 Friedel – Craft Alkylation
17.3 Friedel – Craft Acylation
17.4 Reactions of side chains
(a) By Wurtz Synthesis:
Case 1: When both alkyl halides are same:
\[R - X + R - X\xrightarrow[{dry\,\,ether}]{{Na}}R - R + 2NaX\]
Case 2: When both alkyl halides are different:
\[R - X + R - X\xrightarrow[{{\text{dry ether}}}]{{Na}}R - R + 2NaX\]
\[C{H_3}C{H_2}Br{\text{ }} + {\text{ }}C{H_3}C{H_2}C{H_2}C{H_2}Br\xrightarrow{{Na/dry ether}}n{\text{ }}butane{\text{ }} + {\text{ }}n{\text{ }}-{\text{ }}octane{\text{ }} + {\text{ }}n{\text{ }}hexane\]
In this case a mixture of three alkanes is obtained & the separation of the mixture into individual member is not always easy, so it’s not a good method for the preparation of alkanes if both alkyl halides are different.
Note:
(i) CH4 can not be prepared by this method.
(ii) 30 alkyl halides do not give this reaction.
(iii) Alkenes are produced as by products.
(iv) This method is suitable for preparation of alkanes having even number of carbons.
(b) By Frankland’s reaction
Alkyl halides undergo coupling in the presence of Zn metal and ethyl alcohol.
\[R - X + R - X\xrightarrow{{Zn/{C_2}{H_5}OH}}R - R + Zn{X_2}\]
\[C{H_3} - Br + C{H_3}Br\xrightarrow{{}}C{H_3} - C{H_3} + ZnB{r_2}\]
Step (i): Alkyl halides react with lithium in dry ether to form alkyl lithium
\[R - X + 2Li\xrightarrow{{Dry\,\,ether}}R - Li + LiX\]
Step (ii): This alkyl lithium reacts with CuI to give dialkyl lithium cuprate known as
Gillman reagent
\[R - X + 2Li\xrightarrow{{Dry\,\,ether}}R - Li + LiX\]Step (iii): This further reacts with alkyl halides to give alkanes.
\[{R_2}CuLi + R' - X\xrightarrow{{}}R - R' + R - Cu + LiX\]
Some examples are
This method is suitable for the preparation of unsymmetrical alkanes of the type R-R-
(d) By electrolysis of sodium or potassium salts of fatty acids (Kolbe’s electrolysis)
A concentrated solution of the sodium or potassium salt of a carboxylic acid or mixture of carboxylic acids is electrolysed.
\[{R^1}C{O_2}H + {R^2}C{O_2}K + 2{H_2}O\xrightarrow{{Current}}R' - {R^2} + 2C{O_2} + {H_2} + 2KOH\]
\[2C{H_3}C{O_2}K\left( {aq} \right) + 2{H_2}O\xrightarrow{{Current}}\underbrace {C{H_3} - C{H_3} + 2C{O_2}}_{At\,\,anode} + \underbrace {2KOH + {H_2}}_{At\,\,Cathode}\]
Esters, lower alkanes and alkenes are also obtained as side products.
\[{C_2}{H_5}C{O_2}K{C_2}{H_5}C{O_2}^ - + {K^ + }\]
\[{C_2}{H_5}CO_2^ - \xrightarrow{{}}\mathop {{C_2}{H_5}C{O_2}}\limits_{\left( I \right)} + {e^ - }\]
\[{C_2}{H_5}C{O_2}^ \bullet \xrightarrow{{}}\mathop {{C_2}{H_5}^ \bullet }\limits_{\left( {II} \right)} + C{O_2} \uparrow \]
\[\mathop {{C_2}{H_5}C{O_2}^ \bullet }\limits_{\left( I \right)} + \mathop {{C_2}{H_5}^ \bullet }\limits_{\left( {II} \right)} \xrightarrow{{}}\mathop {{C_2}{H_5}C{O_2}}\limits_{\left( {Ester} \right)} {C_2}{H_5}\]
\[\mathop {{C_2}{H_5}^ \bullet }\limits_{} + \mathop {{C_2}{H_5}^ \bullet }\limits_{} \xrightarrow{{}}\mathop {{C_2}{H_5} - }\limits_{} {C_2}{H_5}\]
(e) Hydroboronation
This process is used for preparation of long chain alkanes by coupling of alkyl boranes by means of silver nitrate in presence of $NaOH$ at 250C. Diborane $\left( {{B_2}{H_6}} \right)$ adds to an double bond forming trialkyl borane which on treatment with $AgN{O_3}/NaOH$ or acetic acid yields corresponding alkane. (4) Decarboxylation of acids
Sodium salts of fatty acids undergo decarboxylation when heated with soda lime.
Note:
Sodium formate gives hydrogen gas instead of R – H.
(5) Miscellaneous Methods
(a) Metal carbides (aluminium and beryllium carbides) on hydrolysis gives methane.
\[A{l_4}{C_3}\xrightarrow{{{H_2}O}}Al{\left( {OH} \right)_3} + C{H_4}\]
\[B{e_2}C\xrightarrow{{{H_2}O}}Be{\left( {OH} \right)_2} + C{H_4}\]
(b) Berthelot Synthesis can be used for the preparation of methane and ethane.
\[C + {H_2}\xrightarrow[{{{120}^0}C}]{{Electric\,\,arc}}C{H_4}\]
\[2C + 3{H_2}\xrightarrow[\Delta ]{{Electric\,\,arc}}C{H_3} - C{H_3}\]
