Preparation of Alkanes

METHODS OF PREPARATION

Alkanes are prepared by the following methods

(1) Reductions Methods:

Alkanes can be prepared by the reduction of various organic compounds as follows:

(a) By hydrogenation of unsaturated hydrocarbons

Alkanes are obtained by hydrogenation of unsaturated hydrocarbons (alkenes & alkynes) in presence of finally divided catalyst e.g. Ni, Pt, Pd etc.

The following reaction is known as Sabatier & Senderen's reaction

\begin{align}R-CH=CH-R'+{{H}_{2}}\xrightarrow{Ni}R-C{{H}_{2}}-C{{H}_{2}}-R' \\ \\ \end{align}

$R-C\equiv C-R^{\prime }+2H_2\stackrel{Pt}{\to }R-C{H}_2-C{H}_2-R^{\prime }$

(b) By the reduction of Alkyl Halides:

Alkyl halides undergo reduction with nascent hydrogen to form alkanes.

$R-X+2H\to R-H+HX$

By using following methods alkanes can be prepared:

Note:

(i) LiAlH₄ is a very strong base and gives elimination reaction with 3⁰ alkyl halides.

(ii) NaBH₄ reduces only 2⁰ and 3⁰ alkyl halides

(c) By reduction of Alcohols: By following two methods, alkanes can be prepared from alcohols.

(d) By reduction of Carbonyl Compounds: By following three methods aldehydes & ketones are reduced to give alkanes.

(e) By reduction of Carboxylic acid: Reduction by HI/P/$\Delta$ gives alkane.

$\underset{\left(No.ofC=n\right)}{R-COOH}\stackrel{HI/P/\Delta }{\to }\underset{\left(No.ofC=n\right)}{R-C{H}_3}$

$C{H}_3-COOH\stackrel{HI/P/\Delta }{\to }C{H}_3-C{H}_3$

$C_6{H}_5-COOH\stackrel{HI/P/\Delta }{\to }{C}_6{H}_5-C{H}_3$

(2) From Organometallic Compounds

Grignard's reagents and alkyl lithium reacts with water and other compounds having acidic hydrogen to give hydrocarbon corresponding to the alkyl group of the organometallic compounds.

$\stackrel{-\delta }{R}-\stackrel{+\delta }{\mathrm{Mg}}X+\stackrel{+\delta }{H}-\stackrel{-\delta }{\mathrm{OH}}\to R-H+MgOHX$

$\stackrel{-\delta }{R}-\stackrel{+\delta }{Li+}\stackrel{+\delta }{H}-\stackrel{-\delta }{\mathrm{OH}}\to R-H+LiOH$

Grignard's reagents reacts with alkyl halides to give alkanes

$R-MgX+R^{\prime }X\to R-R^{\prime }$ (Alkanes with higher number of carbons atoms)

(3) From coupling reactions

(a) By Wurtz Synthesis:

Case 1: When both alkyl halides are same:

$R-X+R-X\underset{dryether}{\overset{Na}{\to }}R-R+2NaX$

Case 2: When both alkyl halides are different:

RX + R¢X $\stackrel{Na/dryether}{\to }$RR + R¢R¢ + RR¢

CH₃CH₂Br + CH₃CH₂CH₂CH₂Br $\stackrel{Na/dryether}{\to }$n butane + n – octane + n 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.

$R-X+R-X\stackrel{Zn/C_2{H}_{5}OH}{\to }R-R+Zn{X}_2$

$C{H}_3-Br+C{H}_{3}Br\to C{H}_3-C{H}_3+ZnB{r}_2$

(c) Corey – House Synthesis

Step (i): Alkyl halides react with lithium in dry ether to form alkyl lithium

$R-X+2Li\stackrel{Dryether}{\to }R-Li+LiX$

Step (ii): This alkyl lithium reacts with CuI to give dialkyl lithium cuprate known as Gillman reagent

$2R-Li+CuI\to R_{2}CuLi+LiI$

Step (iii): This further reacts with alkyl halides to give alkanes.

$R_{2}CuLi+R^{\prime }-X\to R-R^{\prime }+R-Cu+LiX$

Some examples are

This method is suitable for the preparation of unsymmetrical alkanes of the type $R-R^{\prime }$

(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.

$R^{1}C{O}_{2}H+R^{2}C{O}_{2}K+2H_{2}O\stackrel{Current}{\to }{R}^{\prime }-R^2+2C{O}_2+H_2+2KOH$

$2C{H}_{3}C{O}_{2}K\left(aq\right)+2H_{2}O\stackrel{Current}{\to }\underset{Atanode}{\underbrace{C{H}_3-C{H}_3+2C{O}_2}}+\underset{AtCathode}{\underbrace{2KOH+H_2}}$

Esters, lower alkanes and alkenes are also obtained as side products.

$C_2{H}_{5}C{O}_{2}K{C}_2{H}_{5}C{O_2}^{-}+K^{+}$

$C_2{H}_{5}C{O}_2^{-}\to \underset{\left(I\right)}{C_2{H}_{5}C{O}_2}+e^{-}$

$C_2{H}_{5}C{O_2}^{\bullet }\to \underset{\left(II\right)}{C_2{H_5}^{\bullet }}+C{O}_2\uparrow$

$\underset{\left(I\right)}{C_2{H}_{5}C{O_2}^{\bullet }}+\underset{\left(II\right)}{C_2{H_5}^{\bullet }}\to \underset{\left(Ester\right)}{C_2{H}_{5}C{O}_2}{C}_2{H}_5$

$C_2{H_5}^{\bullet }+C_2{H_5}^{\bullet }\to C_2{H}_5-C_2{H}_5$

(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 (B₂H₆) adds to an double bond forming trialkyl borane which on treatment with AgNO₃/NaOH or acetic acid yields corresponding alkane.

$R-CH=C{H}_2\stackrel{B_2{H}_6}{\to }{\left(R-C{H}_2-C{H}_2\right)}_3-B\stackrel{AgN{O}_3/NaOH}{\to }R-C{H}_2-C{H}_2-C{H}_2-C{H}_2-R$

(4) Decarboxylation of acids

Sodium salts of fatty acids undergo decarboxylation when heated with soda lime.

\begin{align}R-COONa\xrightarrow[\left( NaOH+CaO \right)]{Soda\lim e/\Delta }R-H+N{{a}_{2}}C{{O}_{3}} \\ C=nC=n-1 \\ \end{align}

This reaction takes place as follows

Note:

Sodium formate gives hydrogen gas instead of R – H.

(5) Miscellaneous Methods

(a) Metal carbides (aluminium and beryllium carbides) on hydrolysis gives methane.

$A{l}_4{C}_3\stackrel{H_{2}O}{\to }Al{\left(OH\right)}_3+C{H}_4$

$B{e}_{2}C\stackrel{H_{2}O}{\to }Be{\left(OH\right)}_2+C{H}_4$

(b) Berthelot Synthesis can be used for the preparation of methane and ethane.

$C+H_2\underset{{120}^{0}C}{\overset{Electricarc}{\to }}C{H}_4$

$2C+3H_2\underset{\Delta }{\overset{Electricarc}{\to }}C{H}_3-C{H}_3$

Illustration 1. (i) Prepare butane from chloro ethane using the Corey – House synthesis.

(ii) Deuteropropane from isopropyl bromide.

Solution: