Reactions involving carbon-carbon bond formation:

A. Reimer-Tiemann's reaction:
The Reimer-Tiemann's reaction is a reaction in which a phenol is treated with chloroform (CHCl3) and a base, typically sodium hydroxide (NaOH). This reaction results in the formation of a carbonyl compound (aldehyde or ketone) and a hydroxybenzene (phenol) derivative. It involves the formation of a carbon-carbon bond between the carbonyl carbon and the carbon of the chloroform.

B. Hydroboration oxidation:
Hydroboration oxidation is a reaction in which an alkene is treated with borane (BH3) followed by oxidation with hydrogen peroxide (H2O2) and a base. This reaction results in the formation of an alcohol. The key step in this reaction involves the addition of boron to the alkene, forming a boron-carbon bond. The subsequent oxidation and base treatment then convert the boron-carbon bond to a carbon-carbon bond.

C. Cannizaro reaction:
The Cannizaro reaction is a disproportionation reaction in which an aldehyde is treated with a strong base, typically concentrated sodium hydroxide (NaOH). This reaction results in the formation of a carboxylic acid and an alcohol. Although the Cannizaro reaction does not directly involve carbon-carbon bond formation, it is an important reaction that provides a way to indirectly convert one aldehyde into two different products, one of which contains a new carbon-carbon bond.

D. Reaction of primary alcohol with PCC:
The reaction of a primary alcohol with pyridinium chlorochromate (PCC) is an oxidation reaction that converts the primary alcohol to an aldehyde. This reaction involves the formation of a carbon-oxygen bond between the alcohol and the chromium atom in PCC. Although this reaction does not directly involve carbon-carbon bond formation, it is an important step in the synthesis of aldehydes and can serve as a starting point for further reactions that lead to carbon-carbon bond formation.

In summary, out of the given reactions, the Reimer-Tiemann's reaction and the hydroboration oxidation reaction directly involve carbon-carbon bond formation. The Cannizaro reaction and the reaction of a primary alcohol with PCC do not directly involve carbon-carbon bond formation, but they are important reactions in organic chemistry that can lead to the formation of compounds containing carbon-carbon bonds.

Reimer teimer becz when phenol react with chloroform it form salicylicaldehyde