How can a functional group be added to a primary carbon?
There are many answers to this question, but I'll give you a few quick ones.
Assuming your primary carbon is an alkene you will want to do an anti-markovnikov alkene addition reaction.
The 2 common ones you likely learned are adding an OH or halogen
Adding and OH
The hydroboration oxidation reaction will add an OH to the primary carbon. If you want an alcohol, stop here
If you want an aldehyde react this with PCC
If you want a carboxylic acid, react this with a stronger oxidizing agent.
If you want a carboxylic acid derivative - start by turning it into a carboxylic acid, then react with SOCl2 turning it into an acyl chloride.
Now you have a reactive group that can be turned into an ester, amide, anhydride and more
Adding a Halogen
Another route is to add a halogen under radical conditions such as H-Br in the presence of peroxide.
This adds anti-markovnikov to the primary carbon.
Now that you have a primary bromide, you can do an SN2 reaction to replace the Br with a wide variety of functional groups
I cover many examples of SN2 reactions on my website here: