Te Tikanga Tauhohenga SN1 me te SN2
Ko ngā tauhohenga whakakapinga nukleopiri he momo tauhohenga matū noa i roto i te matū waro. Kei roto i ēnei ko te whakakapinga o tētahi rōpū mahi ki tētahi atu i roto i te ngota kotahi. Ko ngā tikanga matua e rua e whakahaere ana i ngā tauhohenga whakakapinga nukleopiri ko te SN1 (ngā tauhohenga whakakapinga nukleopiri unimolecular) me te SN2 (ngā tauhohenga whakakapinga nukleopiri bimolecular). He mea nui te mārama ki ēnei tikanga rerekē i roto i te ako i te matū waro, nā te mea he rerekē ngā āhuatanga, ngā āhuatanga, me ngā hua o ia tikanga.
Tikanga SN1
Ko te tikanga o SN1 ko te "Unimolecular Nucleophilic Substitution." E rua ngā taahiraa o tēnei tikanga, arā, ko te hanganga o te carbocation hei taahiraa tuatahi, ā, muri iho ko te whakaekenga nucleophilic ki te carbocation.
Wāhanga 1: Te Hanganga Warokāti
Ko te taahiraa tuatahi o te tikanga SN1 ko te wehenga o te rōpū wehe mai i te ngota matua hei hanga i te warowaihā. Ko te rōpū wehe pai he mea e ngāwari ana te tango, pērā i te halide (Cl^-, Br^-, I^-) te tosylate rānei. He unimolecular tēnei tukanga nā te mea kotahi anake te ngota e uru ana ki te taahiraa whakatau tere, arā, ko te pūhui matua tonu.
\[
RL \rightarrow R^+ + L^-
\]
I konei, ko RL te ngota matua, ko R^+ te warohanga, ā, ko L^- te rōpū wehe.
Hipanga 2: Whakaekenga Nucleophilic
I te taahiraa tuarua, ka whakaekea e te nucleophile te carbocation kua hangaia hei whakaputa i tētahi hua whakakapinga.
\[
R^+ + Nuc^- \rightarrow R-Nuc
\]
I roto i ngā tauhohenga SN1, ko te angitu o te tauhohenga e whakawhirinaki nui ana ki te pumau o te kaporeihana i hangaia. Nō reira, he maha ake ngā tauhohenga SN1 e puta ana me ngā kaporeihana tuatoru pumau i ngā kaporeihana tuatahi, tuarua rānei.
Ngā Āhuatanga o te Tauhohenga SN1
1. Te Pūmautanga o te Warowaihā: Mēnā he pumau te warowaihā, ka māmā ake te tauhohenga SN1. Nō reira, he nui ake te kitea o te SN1 i ngā waro tuatoru.
2. Te matū stereochemical: He maha ngā wā ka puta he ranunga racemic i ngā tauhohenga SN1 nā te mea he papatahi te carbocation, ā, ka taea e te nucleophile te whakaeke mai i ngā taha e rua.
3. Ngā whakarewa: Ka tautokohia e ngā whakarewa protic pōro pērā i te wai me te waipiro ngā tauhohenga SN1 nā te pumau o ngā katote i hangaia.
Tikanga SN2
Ko te tikanga o SN2 ko te "Bimolecular Nucleophilic Substitution." I roto i tēnei tikanga, ka whakaekea e te nucleophile te waro e herea ana ki te rōpū wehe i te wā kotahi me te tango atu i te rōpū wehe.
Te Whakaekenga Nukleofilic me te Tangohanga Rōpū
Kotahi te taahiraa ka puta ai te tauhohenga SN2 (tikanga whakakotahi), ka whakaekea e te nucleophile te waro e piri ana ki te rōpū wehe mai i te taha o muri, i te wā e tukuna ana te rōpū wehe.
\[
Nuc^- + RL \rightarrow R-Nuc + L^-
\]
He tukanga bimolecular tēnei nā te mea e rua ngā ngota e uru ana ki te taahiraa whakatau tere, arā, ko te nucleophile me te pūhui matua.
Ngā Āhuatanga o te Tauhohenga SN2
1. Te Āraitanga Tūroa: Nā te mea me whakaeke te nucleophile mai i muri, ka tere ake te puta o ngā tauhohenga SN2 i ngā waro tuatahi me ngā waro tuarua. Ko ngā waro tuatoru he mea ārai rawa kia kore e taea te whakaeke i roto i tēnei tikanga.
2. Matū Whakairoiro: Ka puta he hurihanga whirihoranga (hurihanga Walden) i te pokapū whakairoiro, nā reira ka puta he hua rerekē i ngā matū whatu kaha e pāngia ana e te tauhohenga SN2.
3. Ngā Kaitiaki me ngā Whakarewa: Ko ngā whakarewa aprotic pōro pērā i te acetonitrile, te dimethyl sulfoxide (DMSO), te acetone rānei ka pai ake te tukanga SN2 nā te mea kāore e tino whakau i te nucleophile mā te whakarewatanga.
Te whakataurite i waenga i te SN1 me te SN2
I te wā e whiriwhiri ana i waenga i ngā tikanga SN1 me SN2, me whai whakaaro ki ētahi āhuatanga:
1. Hanganga Papa:
– SN1: He pai ake mō ngā pūhui me ngā warowaihā pumau (tuatoru > tuarua > tuatahi).
– SN2: He pai ake mō ngā matū he iti noa te aukati steric (tuatahi > tuarua > tuatoru).
2. Kaitautoko i te nukleo:
– SN1: Kāore i te tino whakawhirinaki ki te kaha o te nucleophile, nā te mea kāore te nucleophile e uru ki te taahiraa whakatau tere.
– SN2: Me whai i tētahi nucleophile kaha nā te mea ko te whakaeke nucleophilic te taahiraa e whakatau ana i te tere.
3. Te Wehenga atu i te Rōpū:
– E hiahiatia ana e ngā tikanga e rua tētahi rōpū wehe pai ka taea te wehe ngāwari i te rōpū matua.
4. Whakarewa:
– SN1 : He whakarewa pūmua porotaka e whakau ana i ngā katote.
– SN2: He whakarewa porotiki āporotiki e whakau ana i ngā katote engari kāore e whakau ana i ngā nucleophiles.
5. Te matū ā-ira:
– SN1: Ranunga racemic nā te mea ka taea e te nucleophile te whakaeke mai i ngā taha e rua o te carbocation.
– SN2: Hurihanga whirihoranga nā te whakaekenga nucleophilic mai i te taha ke atu ki te rōpū wehe.
Ngā Tauira o ngā Tauhohenga SN1 me SN2
Ngā Tauira o ngā Tauhohenga SN1
Te tauhohenga maroke o te Tert-Butyl alkyl halide me te wai:
\[
\kākau{(CH}_3\kākau{)}_3\kākau{C-Br} + H_2O \rightarrow \kākau{(CH}_3\kākau{)}_3\kākau{C-OH} + HBr
\]
I roto i tēnei tauhohenga, ka hangaia e te tert-butyl bromide he tert-butyl carbocation i muri i te wehenga o te rōpū bromide, ā, ka whai ake ko te whakaekenga nucleophilic a te wai.
Ngā Tauira o ngā Tauhohenga SN2
Te tauhohenga o te methyl bromide me ngā ion hydroxide:
\[
\text{CH}_3\text{Br} + OH^- \rightarrow \text{CH}_3\text{OH} + Br^-
\]
I konei, ka whakaekea e te katote hauwai te tuara o te waro methyl e piri ana ki te rōpū bromide i te wā kotahi me te tukunga o te rōpū bromide, ka puta he methanol.
Whakamutunga
Ko ngā tauhohenga SN1 me SN2 he tikanga matua e rua o ngā tauhohenga whakakapinga nucleophilic, me ngā rerekētanga taketake i roto i tā rāua tikanga mahi. Me hanga he carbocation mō te SN1, ā, ka puta i te nuinga o te wā me ngā matūriki nui ake, he manga ake, ko te SN2 ia ka haere tonu i roto i te taahiraa kotahi ka whakaekea tika e te nucleophile te here waro e piri ana ki te rōpū wehe, ka pai ake ki ngā matūriki iti ake, he iti ake te manga. He mea nui te māramatanga hōhonu ki ngā āhuatanga e pai ana ki ia tikanga me ngā hua ka puta i roto i te hanganga matū waro.
