when des becomes de, d'

Question

If des becomes de in front of an adjective that precedes a noun, why is 
this correct:  je suis jaloux des nouvelles bottes ....? Here des is used preceding the adjective...

Maarten Kamp · Accepted Answer

Claudia, 
The link attached to your post doesn’t seem to be connected to the question. 
However, the reason is that this is not the partitive ‘ des ‘, but the obligatory contraction of ‘ de les ‘ - that is the preposition ‘ de ‘ followed by the definite article ‘ les ‘. The expression is “ jaloux de qqc “ 
https://www.wordreference.com/fren/jaloux
This is covered in lesson attached below as : 
ATTENTION: 
This rule doesn't apply when des is the contraction of "de + les" (= of/from/to the) :
J'ai acheté de nouvelles bottes.I bought [some] new boots.
BUT
Je suis jalouse des nouvelles bottes que tu as achetées.I'm jealous of the new boots you bought. https://progress.lawlessfrench.com/revision/grammar/des-becomes-de-d-in-front-of-adjectives-preceding-nouns-french-articles

Anne Davies · Answer

It is confusing, Claudia B. But worth noting that this also applies to other situations where the preposition de + noun (J'ai envie de bottes/ J'ai un kilo de poires) becomes des when you’re  talking about a specific noun (J'ai envie des bottes que tu as achetées/ J'ai un kilo des poires qu'il m'a données. Also that it matches the English well - "some" boots versus "the" boots.