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I'm not an expert teacher or lecturer of chemistry. I was only a student from SMA NEGERI 15 SURABAYA who had been one of the Bronze Medalist Participants of Olimpiade Sains Nasional X (2011) of Chemistry In Manado, North Sulawesi, 11 - 16 September 2011 and graduated in 2012. Now, I'm studying at Universitas Airlangga in Surabaya, Indonesia. I do love chemistry and I would like to help them who had difficulties in studying chemistry. That's why, please understand me if you found some misconcepts in my entries. Suggestions are always necessary in order to develop this blog. And I'm sorry because my English isn't so well.

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Friday, December 30, 2011

Nucleophilic Substitution Part I : Sn1 Reaction

Nucleophilic is any compound that tends to donate its electron pair to substitute another atom in another compound. Nucleo means Nucleues and Philic means like, so we can say that nucleophilic is a Lewis base. Based on its rate and mechanism reaction, nucleophilic substitution divided into two types: Unimolecular (SN1) and Bimolucular (SN2).

In SN1, carbocation is involved and also observed (First observed by George Andrew Olah via Superacids). The mechanism reaction of SN1 was first proposed by Christoper Ingold.

An example of a reaction taking place with an SN1 reaction mechanism is the hydrolysis of tert-butyl bromide with water forming tert-butyl alcohol:






Explanation:
Bromide is a good leaving group, when the compound is hydrolyzed, a stable carbocation will be formed. This stable carbocation can be formed because the methyl "gives" its electron. This is what we called as induction effect. There's no steric effect in this case so that the nucleophilic can attack the carbocation either from front or behind the compound. If SN1 happens in a chiral compound, the configuration is prevented, this is what we called as Retention (50% Retention, 50% Inversion). But the stereochemistry of the compound will be changed, for example see the picture below:



Protic polar solvent is a good solvent for this reaction because carbocation is a unstable compound, to facilitate this unstable one, of course a polar solvent needed to help ionization process and protic solvents like water are recommended because the hydrogen bond interaction keeps the stabilization of the carbocation. The hybridization is changed. See the picture above as the example, the compound is Sp3, but when carbocation formed, the hybridization becomes sp2 and makes the compound become planar. Because the rate-determining-step is the forming of the carbocation, the rate law of SN1 is r = k [Compound]. The picture below explain the rate of SN1.


(Source: http://guweb2.gonzaga.edu/faculty/cronk/CHEM240/images/reaction_coordinate_SN1.gif)

Further Reading . . .
Wikipedia


2 comments:

  1. kedua enantiomer tersebut bs di pisah ga?

    ReplyDelete
  2. bisa, tapi aku belum tau metode pemisahannya gimana..

    ReplyDelete