Q.What is a catalyst? What is an addition reaction?

Preparation of Alkanes

Although petroleum and natural gas remain the dominant real-world source, several named laboratory routes build alkanes from smaller/simpler precursors, each useful for a different synthetic situation:

• Catalytic hydrogenation of an alkene or alkyne (H2 over Pt/Pd, or over Ni at 298 K -- the Sabatier-Sendersens reaction) simply saturates the multiple bond.
• Decarboxylation of a sodium carboxylate with soda lime (NaOH + CaO) strips off -COONa as Na2CO3, shortening the chain by exactly one carbon relative to the parent acid.
• Kolbe's electrolytic method electrolyses a sodium/potassium carboxylate; decarboxylation at the anode dimerises two identical alkyl radicals into a symmetrical alkane R-R (so it can only build symmetrical products, and doubles the carbon count of the R group).
• Reduction of a haloalkane with nascent hydrogen (Zn/HCl, Zn/CH3COOH, Zn-Cu/ethanol, or LiAlH4) simply swaps the halogen for H; it works for all haloalkanes except fluorides.
• The Wurtz reaction couples two haloalkane molecules with sodium metal in dry ether, giving a symmetrical alkane with double the carbon count and an EVEN number of total carbons (2 R-X + 2Na --> R-R + 2NaX); it cannot cleanly make an alkane with an odd carbon count or an unsymmetrical alkane (a mixed R-X + R'-X gives a statistical mixture of R-R, R-R' and R'-R').
• The Corey-House synthesis (R-X + a lithium dialkylcuprate, R'2LiCu) is the general route to an UNSYMMETRICAL alkane, avoiding the Wurtz reaction's mixture problem.
• Grignard reagents (R-MgX, from R-X + Mg in dry ether) behave as a masked carbanion and are protonated by any source of an easily replaceable H (water is the simplest) straight to the alkane R-H.