During the second test of the Chunin exams, Orochimaru goes for Sasuke and even gives him power. But why does he avoid cursing Naruto like he did to Sasuke? Wasn't Naruto as powerful as Sasuke? Or was he too powerful to be controlled?
Also, why does he use the Five Elements Seal on Naruto?
The main reason why Orochimaru wanted Sasuke rather than Naruto, was Itachi Uchiha.
Sasuke Uchiha, was developing into the prodigy that his elder brother Itachi was. Orochimaru had experienced Itachi's power first hand. And it proved too great for him.
Another reason, was the Uchiha clan's famed Sharingan. Itachi's prowess with his Sharingan was outstanding. Orochimaru expected the same from Sasuke.
Since Orochimaru couldn't take control of Itachi's body, he decided to settle for his younger brother's.
As to why Naruto wasn't chosen, Orochimaru favoured Sasuke over Naruto. Because at that time, Naruto was at the back of the class, and Sasuke was the Uchiha prodigy. Also, the fact that Sasuke is Sharingan abled and Itachi's brother, outweighed Naruto's overall.
The two reasons as to why Orochimaru used the Five Elements Seal:
Orochimaru was actually going after Itachi, Sasuke's older brother. After he failed on getting Itachi's body he decided to get his youngest brother, Sasuke, because he was going to be an easier target.
The main reason why he even targeted Itachi was because of his Sharingan. He was going after a Kekkei Genkai, which is a special ability that not a lot of people can have, that his next vessel will have to make him stronger. He did not target Naruto because he doesn't have a Kekkei Genkai.
He put the Five Elements Seal on Naruto like that he would not use the Kyuubi's chakra and his own.
I would say there are two main reasons why Orochimaru went after Sasuke.
Orochimaru wanted Sasuke or Itachi's eyes. After all Orochimaru was researching inherited chakra or called Kekkei Genkai. Meaning you must be born into that specific clan to gain the chakra or special features of it. Orochimaru was only looking for special Kekkei Genkai. Here's a few examples: Shikotsumyaku clan and their ability to using their bones in their bodies as weapons. User Kimimaru. Crystal release nature chakra users are able to materialize and use crystals. Finally Jugo's clan Kekkei Genkai. Allowing them to absorb natural energy into their own bodies and use it. This interest in Kekkei Genkai caused him to desire one for his vessel. So when Itachi showed him the potential in the Sharingan. He immediately desired to control and use it.
Orochimaru could not use him a vessel. A) Naruto is not easily controlled and brainwashed exceptionally when it involves someone close to him. He would have sliced off his own hand before intentionally hurting a loved one. B) He is the Jinchuriki. Kurama or the nine tails fox would have never allowed the snake to stay in Naruto's mind for long. He or she would have knocked some sense into the boy long ago. C) His loyalty to the village. Orochimaru needed someone who has nothing and no one holding him back. Sasuke had no loyalty to the village and the only thing holding him back was somewhat friend Naruto. While Naruto was very loyal to his village and was gaining friends. Causing Naruto to not need him unlike Sasuke who desired for more power which Orochimaru happily supplied for him.
