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SNi

SNi atau Substitusi Nukleofilik internal adalah sebuah reaksi substitusi dalam kimia organik, yang merupakan singkatan yang spesifik tapi tidak sering ditemui bagi mekanisme reaksi substitusi nukleofilik alifatik. Nama tersebut diperkenalkan oleh Cowdrey et al. pada tahun 1937 untuk memberi label reaksi nukleofilik yang terjadi dengan retensi konfigurasi,[1] Namun kemudian dinamakan untuk menggambarkan berbagai reaksi yang dilanjutkan dengan mekanisme serupa.

Mekanisme reaksi

Contoh khas reaksi organik yang menunjukkan mekanisme ini adalah klorinasi dengan tionil klorida, atau dekomposisi alkil kloroformat, di mana ciri utamanya adalah retensi pada konfigurasi stereokimia. Beberapa contoh untuk reaksi ini dilaporkan oleh Edward S. Lewis dan Charles E. Boozer pada tahun 1952.[2] Studi mekanistik dan kinetik dilaporkan beberapa tahun kemudian oleh berbagai peneliti.[3][4]

Mekanisme reaksi SNi
Mekanisme reaksi SNi
Mekanisme reaksi SNi

Tionil klorida pertama-tama bereaksi dengan alkohol untuk membentuk alkil kloro sulfit, yang sebenarnya membentuk pasangan ion intim. Langkah kedua adalah hilangnya molekul sulfur dioksida bersama penggantinya oleh klorida, yang dilekatkan pada gugus sulfit. Perbedaan antara SN1 dan SNi sebenarnya adalah bahwa pasangan ion tidak sepenuhnya terpisah, dan karena itu tidak ada karbokation nyata yang terbentuk, yang mana kembali akan menyebabkan rasemisasi.

Tipe reaksi ini terkait dengan banyak bentuk gugus tetangga, misalnya reaksi pasangan elektron sunyi belerang atau nitrogen pada sulfur mustard atau nitrogen mustard untuk membentuk zat antara kationik.

Mekanisme reaksi ini didukung oleh pengamatan bahwa penambahan piridin pada reaksi menyebabkan inversi. Alasan di balik temuan ini adalah bahwa piridin bereaksi dengan zat antara sulfit yang menggantikan klorin. Klorin yang lepas harus menggunakan serangan nukleofilik dari belakang seperti pada substitusi nukleofilik biasa.[3]

Dalam gambar lengkap untuk reaksi ini sulfit bereaksi dengan ion klorin dalam suatu reaksi standar SN2 dengan inversi konfigurasi. Bila pelarutnya juga merupakan nukleofil seperti dioksan dua reaksi SN2 berturut-turut berlangsung dan stereokimia kembali retensi. Dengan kondisi reaksi standar SN1 keluaran reaksi adalah retensi melalui kompetisi dengan mekanisme SNi dan bukan rasemisasi serta dengan penambahan piridin hasilnya akan kembali inversi.[5][3]

Lihat pula

Referensi

  1. ^ Hughes, Edward D.; Ingold, Christopher K.; Scott, Alan D. (1937). "258. The mechanism of elimination reactions. Part I. Unimolecular olefin formation from alkyl halides in sulphur dioxide and formic acid". Journal of the Chemical Society (Resumed): 1271. doi:10.1039/JR9370001271. 
  2. ^ Lewis, Edward S.; Boozer, Charles E. (January 1952). "The Kinetics and Stereochemistry of the Decomposition of Secondary Alkyl Chlorosulfites". Journal of the American Chemical Society. 74 (2): 308–311. doi:10.1021/ja01122a005. 
  3. ^ a b c Cram, Donald J. (January 1953). "Studies in Stereochemistry. XVI. Ionic Intermediates in the Decomposition of Certain Alkyl Chlorosulfites". Journal of the American Chemical Society. 75 (2): 332–338. doi:10.1021/ja01098a024. 
  4. ^ Lee, C.C.; Clayton, J.W.; Lee, D.G.; Finlayson, A.J. (January 1962). "Rearrangement studies with 14C—XIII". Tetrahedron. 18 (12): 1395–1402. doi:10.1016/S0040-4020(01)99294-4. 
  5. ^ March, Jerry (2007). Knipe, A.C., ed. March's Advanced Organic Chemistry Reactions, Mechanisms, and Structure (edisi ke-6th). Hoboken: John Wiley & Sons. hlm. 468-469. ISBN 9780470084946. 

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