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Nucleophilic Substitution at a Vinylic Carbon, Reactivity Effects of Substrate

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Nucleophilic Substitution at a Vinylic Carbon, Reactivity Effects of Substrate KNOW MORE Weblinks • https://en.wikipedia.org/wiki/Allyl • http://courses.chem.psu.edu/chem210/mol-gallery/allyl/allyl.html • https://en.wikipedia.org/wiki/Nucleophilic_substitution • http://polymer.zju.edu.cn/attachments/2012-11/01-1352193505- 80382.pdf • http://www.masterorganicchemistry.com/2014/02/10/socl2-and-the-sni- mechanism/ • https://en.wikipedia.org/wiki/SNi • http://pdf.easechem.com/pdf/32/3c095c45-d886-4e49-8f97- 5fdcea6a87b9.pdf Suggested Readings CHEMISTRY 5, Organic chemistry-II 17, Nucleophilic Substitution at an Allylic, Aliphatic Trigonal and SNi Reactions and Nucleophilic Substitution at a Vinylic Carbon, Reactivity Effects of Substrate March’s Advanced Organic Chemistry: Reaction, mechanisms and structure By Michael B. Smith and Jerry March Advanced Organic Chemistry (Part A: Structure and mechanisms) By Francis A. Carey, Richard J. Sunberg Organic Chemistry By Jonathan Clayden, Nick Greeves and Stuar Warren CHEMISTRY 5, Organic chemistry-II 17, Nucleophilic Substitution at an Allylic, Aliphatic Trigonal and SNi Reactions and Nucleophilic Substitution at a Vinylic Carbon, Reactivity Effects of Substrate CHEMISTRY 5, Organic chemistry-II 17, Nucleophilic Substitution at an Allylic, Aliphatic Trigonal and SNi Reactions and Nucleophilic Substitution at a Vinylic Carbon, Reactivity Effects of Substrate Glossary A Allyl- An allyl group is a substituent with the structural formula H2C=CH- CH2R, where R is the rest of the molecule. It consists of the methylene bridge (- CH2-) attached to a vinyl group (-CH=CH2). Allylic rearrangement- It is an organic reaction in which the double bond in an allyl chemical compound shifts to the next carbon atom. It is encountered in nucleophilic substitution. N NGP- Neighbouring group participation(NGP) or anchimeric assistance in organic chemistry is defined as the interaction of a reaction centre with a lone pair of electrons present in a sigma bond or pi bond.. R Racemization- It refers to the conversion of an enantiomerically pure mixture into a mixture where more than one of the enantiomers are present. If the racemisation results in a mixture where the enantiomers are present in equal amount, the resulting sample is described as racemic or racemate. S SN1- SN1 reaction is a substitution reaction in organic chemistry. SN stands for nucleophilic substitution and the “1” represents the fact that the rate determining step is unimolecular. The reaction involves a carbocation intermediate, formed by separation of a leaving group from the carbon atom, followed by the attack of nucleophile and then deprotonation. CHEMISTRY 5, Organic chemistry-II 17, Nucleophilic Substitution at an Allylic, Aliphatic Trigonal and SNi Reactions and Nucleophilic Substitution at a Vinylic Carbon, Reactivity Effects of Substrate SN2- SN2 is a type of reaction mechanism that is common in organic chemistry. In this mechanism, one bond is broken and one bond is formed synchronously, i.e, in one step. Since two reacting species are involved in the slow (rate determining) step, this leads to the term substitution nucleophilic (bimolecular) or SN2. SNi- SNi or substitution nucleophilic internal stands for a specific but not often encountered nucleophilic aliphatic substitution reaction mechanism. A typical representative organic reaction displaying this mechanism is the chlorination of alcohols with thionyl chloride, or the decomposition of alkyl chloroformates, the main feature is retention of stereochemical configuration. V Vinylic carbon- Vinyl is the functional group –CH=CH2, namely the ethylene molecule (H2C=CH2) minus one hydrogen atom. Eg- R-CH=CH2. On a carbon skeleton, sp2 –hybridized carbons or positions are often called vinylic. Do you know? Nucleophilic Substitution at a Vinylic Carbon It is not surprising that with vinylic substrates addition and substitution often compete. For chloroquinones, where the charge is spread by resonance, tetrahedral intermediates have been isolated. In the case of Ph(MeO)C=C(NO2)Ph+RS-, the intermediate lived long enough to be detected by UV spectroscopy. Since both the tetrahedral and addition–elimination mechanisms begin the same way, it is usually difficult to tell them apart, and often no attempt is made to do so. The strongest kind of evidence for the addition–elimination sequence is the occurrence of a ‘‘rearrangement,’’ but of course the mechanism could still take place even if no rearrangement is found. Evidence that a tetrahedral or an addition–elimination mechanism takes place in certain cases (as opposed, e.g., to an SN1 or SN2 mechanism) is that the reaction rate increases when the leaving group is changed from Br to Cl to F (this is called the element effect). This clearly demonstrates that the carbon–halogen bond does not break in the rate-determining step (as it would in both the SN1 and SN2 mechanisms), because fluorine is by far the poorest leaving group among the halogens in both the SN1 and SN2 reactions. The rate is faster with fluorides in the cases CHEMISTRY 5, Organic chemistry-II 17, Nucleophilic Substitution at an Allylic, Aliphatic Trigonal and SNi Reactions and Nucleophilic Substitution at a Vinylic Carbon, Reactivity Effects of Substrate cited, because the superior electron withdrawing character of the fluorine makes the carbon of the C_F bond more positive, and hence more susceptible to nucleophilic attack. Ordinary vinylic substrates react very poorly if at all by these mechanisms, but substitution is greatly enhanced in substrates of the type ZCH=CHX, where Z is an electron-withdrawing group, such as HCO, RCO, EtOOC, ArSO2, NC, and F, since these b groups stabilize the carbanion. CHEMISTRY 5, Organic chemistry-II 17, Nucleophilic Substitution at an Allylic, Aliphatic Trigonal and SNi Reactions and Nucleophilic Substitution at a Vinylic Carbon, Reactivity Effects of Substrate .
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