Preparation of Alkanes

From Alkenes

Catalytic Hydrogenation

Hydrogenation reactions that involve insoluble catalysts ⇒ heterogeneous catalysis. ($Ni$, $Pd$, $Pt$) Hydrogenation reactions that involve soluble catalysts ⇒ homogeneous catalysis. ($Rh[(C_6{H}_5{)}_{3}P{]}_{3}Cl$ - Wilkinson’s catalyst.)

Unsaturated compounds can be reduced to saturated compounds by catalytic hydrogenation.

• Support is used to prevent the catalyst from destroying its catalytic activity.

Refer:

• Catalytic Hydrogenation

Hydroboration Reduction or Dimerisation

• Syn-addition.
• Boron is added to that carbon which is less sterically hindered.

Refer:

• Hydroboration-Oxidation

Reduction

Dimerisation

From Alkynes

Catalytic Hydrogenation takes place twice.

From Alkyl Halides

Wurtz Reaction

Formation of higher alkanes from alkyl halides in the presence of sodium and dry ether is named Wurtz reaction. It can proceed with ionic as well as radical mechanisms.

Disproportionation products can also be formed. Cyclic products can also be formed.

If the substrate is tertiary, addition reaction is not preferred due to the bulky nature (steric hindrance.) Instead, the disproportionation reaction becomes exclusive.

Corey-Posner Whitesides-House Synthesis

Refer to Corey-Posner, Whitesides-House Synthesis.

Reduction with Metal Hydride

Metal Hydride	1$°$	2$°$	3$°$
LAH / $THF$	Y	Alkene	Alkene
$NaB{H}_4$ / $ROH$	N	Y	Y
$P{h}_{3}SnH$ / $THF$	Y	Y	Y

• Y → $H^{-}$ acts as a nucleophile and substitution takes place.
• Alkene → $H^{-}$ acts as a base and elimination might take place.
• N → No reaction.

LAH → provides $H^{-}$ in higher concentrations, so for reactive substances it starts acting as a base. $NaB{H}_4$ → Due to the high covalent nature of $B{H}_4^{-}$ bonds, reactions fail to take place for primary alkylhalides.

Refer to:

• LAH and Sodium Borohydride.

Reduction with Metal Acid

Reduction with Zinc Alcohol

$X^{-}$ is more stable than $Et{O}^{-}$.

From Alcohols

It is not possible to synthesize alkanes from alcohols.

From Carbonyl Compounds

Clemmensen Reduction

This reaction is not effective for those substrates which contain acid-sensitive groups. Zinc and Hydrochloric acid also reduces nitro groups to amino groups.

Wolff-Kishner Reduction

It can be thought to complement Clemmensen Reduction as that occurs under acidic condition, while this in basic.

Note that the hydrogen is not from the base, nor from the hydrazine but rather from the solvent, (in this case $H_{2}O$). Another solvent that commonly is used is Ethylene Glycol ($HOC{H}_{2}C{H}_{2}OH$).

Mozingo Reduction

Desulfurization with Hydrogen and Raney Nickel.

Carboxylation

Kolbe’s Electrolysis Method

Soda-lime Distillation

Reduction using red P. and HI