The actual number of #C-C# bond forming reactions is very low. Grignards may be made fairly easily from hydrocarbyl halides and magnesium turnings, and usually react quantitatively with carbonyls species to make a #C-C# bond. The #C-C# bond is moreover functionalized with an hydroxyl group after workup.
The best way to make a carboxylic acid is to pour the Grignard onto dry ice (and step back):
#R-MgX +CO_2(s) rarr RC(=O)OMgX#
Here the alkyl (hydrocarbyl) chain has been extended by #1#. If an epoxide, i.e. ethylene oxide, a cyclic ether, is used the carbon chain is extended by #2#. Of course, some precautions have to be taken with the reaction; namely the exclusion of water and protic solvents.
