A relative clause can be introduced with a prepositional phrase + relative pronoun in English, as follows:
The tree close to which you are sitting is an oak tree.
The tree (which) you are sitting close to is an oak tree.
How to use "duquel / de laquelle" with prepositionals phrases with de in French
Now let's look at such cases in French:
Here are the friends we are worried about.
Note that these pronouns agree in gender and number with the object they refer to.
The tree (which) you are sitting close to is an oak tree
-> You cannot say any of these:
L'arbre tu es assis près duquel est un chêne.
L'arbre duquel tu es assis près est un chêne.
Can we use de qui / dont here?
L'arbre près de quoi tu es assis...
au début de - at the beginning of
au milieu de - in the middle of
avec l'aide de - with the help of
à cause de - due to
au sujet de - about
au-dessus de - above
à partir de - from
À côté de, en face de, à l'extérieur de, à l'intérieur de, près de, loin de, au coin de (French Prepositions)
etc
See also De qui/dont/duquel = of/about whom, of/about which - with prepositional verbs with "de" (French Relative Pronouns), Dont = Whose (French Relative Pronouns) and Dont = Including/[quantity] of which (French Relative Pronouns)
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Examples and resources
The boy you're sitting next to has beautiful eyes.
Thanks to Léa and Julie, without whose help nothing would have been possible.
Here are the friends we are worried about.