SN1 thiab SN2 Cov Txheej Txheem Tshuaj Tiv Thaiv
Cov tshuaj hloov pauv nucleophilic yog ib hom tshuaj lom neeg uas siv ntau hauv cov organic chemistry. Lawv cuam tshuam nrog kev hloov pauv ntawm ib pawg ua haujlwm nrog lwm pawg hauv tib lub molecule. Ob lub tshuab tseem ceeb uas tswj cov tshuaj hloov pauv nucleophilic yog SN1 (unimolecular nucleophilic substitution reactions) thiab SN2 (bimolecular nucleophilic substitution reactions). Kev nkag siab txog cov tshuab sib txawv no yog qhov tseem ceeb hauv kev kawm txog organic chemistry, vim txhua lub tshuab muaj cov yam ntxwv sib txawv, cov xwm txheej, thiab cov khoom tsim tawm.
SN1 txoj kev
SN1 sawv cev rau "Unimolecular Nucleophilic Substitution." Lub tshuab no muaj ob kauj ruam, uas yog kev tsim cov carbocation ua thawj kauj ruam, ua raws li nucleophilic tawm tsam cov carbocation.
Theem 1: Kev Tsim Carbocation
Kauj ruam thawj zaug hauv lub tshuab SN1 yog kev sib cais ntawm pawg tawm ntawm lub molecule niam txiv los tsim ib qho carbocation. Ib pawg tawm zoo yog ib qho uas yooj yim tshem tawm, xws li halide (Cl^-, Br^-, I^-) lossis tosylate. Cov txheej txheem no yog unimolecular vim nws tsuas yog siv ib lub molecule hauv kauj ruam txiav txim siab tus nqi, uas yog cov tshuaj niam txiv nws tus kheej.
\[
RL \rightarrow R^+ + L^-
\]
Ntawm no, RL yog lub molecule niam txiv, R^+ yog carbocation, thiab L^- yog pawg tawm mus.
Kauj Ruam 2: Kev Tawm Tsam Nucleophilic
Hauv kauj ruam thib ob, tus nucleophile tawm tsam cov carbocation uas tsim los tsim cov khoom hloov pauv.
\[
R^+ + Nuc^- \rightarrow R-Nuc
\]
Hauv SN1 cov tshuaj tiv thaiv, qhov ua tiav ntawm cov tshuaj tiv thaiv nyob ntawm qhov ruaj khov ntawm cov carbocation tsim. Yog li ntawd, SN1 cov tshuaj tiv thaiv tshwm sim ntau zaus nrog cov carbocations ruaj khov dua li cov carbocations thawj zaug lossis theem nrab.
Cov yam ntxwv ntawm SN1 Reaction
1. Kev Ruaj Ntseg ntawm Carbocation: Qhov ruaj khov ntawm carbocation, qhov yooj yim dua rau SN1 tshwm sim. Yog li ntawd, SN1 feem ntau tshwm sim ntawm cov carbons tertiary.
2. Stereochemistry: SN1 cov tshuaj tiv thaiv feem ntau tsim cov sib xyaw racemic vim tias carbocation yog planar thiab nucleophile tuaj yeem tawm tsam los ntawm ob sab.
3. Cov kuab tshuaj: Cov kuab tshuaj polar protic xws li dej thiab cawv txhawb nqa SN1 cov tshuaj tiv thaiv vim yog qhov ruaj khov ntawm cov ions tsim.
SN2 txoj kev
SN2 sawv cev rau "Bimolecular Nucleophilic Substitution." Hauv txoj kev no, tus nucleophile tawm tsam cov pa roj carbon uas txuas nrog pawg tawm tib lub sijhawm nrog rau pawg tawm raug tshem tawm.
Kev Tawm Tsam Nucleophilic thiab Kev Tawm Ntawm Pawg Neeg
Qhov kev tshuaj tiv thaiv SN2 tshwm sim hauv ib kauj ruam (kev sib koom ua ke), qhov twg cov nucleophile tawm tsam cov pa roj carbon uas txuas nrog pawg tawm los ntawm sab nraub qaum, thaum pawg tawm raug tso tawm.
\[
Nuc^- + RL \rightarrow R-Nuc + L^-
\]
Cov txheej txheem no yog bimolecular vim nws cuam tshuam nrog ob lub molecule hauv kauj ruam txiav txim siab tus nqi, uas yog nucleophile thiab cov niam txiv compound.
Cov yam ntxwv ntawm SN2 Reaction
1. Kev cuam tshuam Steric: Vim tias tus nucleophile yuav tsum tawm tsam los ntawm tom qab, SN2 cov tshuaj tiv thaiv tshwm sim yooj yim dua ntawm cov pa roj carbon thawj thiab theem nrab. Cov pa roj carbon theem peb feem ntau raug cuam tshuam dhau los ua kom tsis txhob raug tawm tsam hauv txoj hauv kev no.
2. Stereochemistry: Qhov kev tshuaj tiv thaiv SN2 tsim tawm qhov kev hloov pauv ntawm qhov chaw (Walden inversion) ntawm qhov chaw stereogenic, yog li cov tshuaj sib xyaw ua haujlwm optically uas dhau los ntawm qhov kev tshuaj tiv thaiv SN2 yuav tsim cov khoom nrog qhov kev teeb tsa sib txawv.
3. Cov Tshuaj Tiv Thaiv thiab Cov Tshuaj Yaj: Cov tshuaj yaj polar aprotic xws li acetonitrile, dimethyl sulfoxide (DMSO), lossis acetone nyiam cov txheej txheem SN2 vim tias lawv tsis ua kom lub nucleophile ruaj khov ntau dhau los ntawm kev yaj.
Kev sib piv ntawm SN1 thiab SN2
Thaum xaiv ntawm SN1 thiab SN2 cov txheej txheem, ntau yam yuav tsum tau xav txog:
1. Cov Qauv Txheej Txheem:
– SN1: Nyiam rau cov tshuaj uas muaj cov carbocations ruaj khov (tertiary > secondary > primary).
– SN2: Nyiam rau cov tshuaj uas muaj kev cuam tshuam me ntsis ntawm steric (thawj > thib ob > thib peb).
2. Cov Nucleophile:
– SN1: Tsis tshua nyob ntawm lub zog ntawm nucleophile, vim tias cov kauj ruam txiav txim siab tus nqi tsis koom nrog nucleophile.
– SN2: Yuav tsum muaj nucleophile muaj zog vim tias kev tawm tsam nucleophilic yog kauj ruam txiav txim siab qhov nrawm.
3. Tawm ntawm Pawg:
- Ob txoj kev no xav tau ib pab pawg neeg tawm mus zoo uas tuaj yeem tawm hauv pab pawg niam txiv tau yooj yim.
4. Cov kuab tshuaj:
– SN1: Cov kuab tshuaj polar protic uas ua kom cov ions ruaj khov.
– SN2: Aprotic polar solvent uas ua rau cov ions ruaj khov tab sis tsis ua rau cov nucleophiles ruaj khov.
5. Kev sib xyaw ua ke ntawm cov tshuaj lom neeg:
– SN1: Kev sib xyaw ua ke ntawm cov kab mob sib kis vim tias cov nucleophile tuaj yeem tawm tsam los ntawm ob sab ntawm carbocation.
– SN2: Kev hloov pauv ntawm qhov teeb tsa vim yog kev tawm tsam ntawm nucleophilic los ntawm sab uas tig mus rau pawg neeg tawm mus.
Piv txwv ntawm SN1 thiab SN2 Cov Kev Tshwm Sim
Piv txwv ntawm SN1 Reactions
Kev ua kom qhuav ntawm Tert-Butyl alkyl halide nrog dej:
\[
\text{(CH}_3\text{)}_3\text{C-Br} + H_2O \rightarrow \text{(CH}_3\text{)}_3\text{C-OH} + HBr
\]
Hauv qhov kev tshuaj tiv thaiv no, tert-butyl bromide tsim cov tert-butyl carbocation tom qab kev sib cais ntawm pawg bromide, ua raws li kev tawm tsam nucleophilic los ntawm dej.
Piv txwv ntawm SN2 Reactions
Kev sib xyaw ntawm methyl bromide nrog hydroxide ions:
\[
\text{CH}_3\text{Br} + OH^- \rightarrow \text{CH}_3\text{OH} + Br^-
\]
Ntawm no, cov hydroxide ion tawm tsam sab nraub qaum ntawm cov methyl carbon txuas nrog pawg bromide tib lub sijhawm nrog kev tso tawm ntawm pawg bromide, tsim cov methanol.
Xaus
Cov tshuaj tiv thaiv SN1 thiab SN2 yog ob txoj hauv kev tseem ceeb ntawm cov tshuaj tiv thaiv nucleophilic, nrog rau qhov sib txawv tseem ceeb hauv lawv txoj kev ua haujlwm. SN1 xav tau kev tsim cov carbocation thiab feem ntau tshwm sim nrog cov substrates loj dua, ntau ceg, thaum SN2 mus rau hauv ib kauj ruam uas nucleophile ncaj qha tawm tsam cov pa roj carbon txuas nrog pawg tawm, nyiam cov substrates me me, tsawg ceg. Kev nkag siab zoo txog cov xwm txheej uas nyiam txhua txoj hauv kev thiab cov khoom tshwm sim yog qhov tseem ceeb hauv kev tsim cov tshuaj lom neeg organic.
