Since initial electrophilic attack on the double bond may occur equally well from either side, it is in the second step (or stage) of the reaction (bonding of the nucleophile) that stereoselectivity may be imposed. Syn and Anti Additions to Alkenes. Legal. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. We see then that addition reactions to this function might occur in three different ways, depending on the relative orientation of the atoms or groups that add to the carbons of the double bond: (i) they may bond from the same side, (ii) they may bond from opposite sides, or (iii) they may bond randomly from both sides. The first two possibilities are examples of stereoselectivity, the first being termed syn-addition, and the second anti-addition. Makes a syn addition of OH creating a diol 2. Cuts the double bond and produces two carbonyl compounds. As illustrated in the drawing below, the pi-bond fixes the carbon-carbon double bond in a planar configuration, and does not permit free rotation about the double bond itself. Markovnikov’s rule is less about memorizing what goes where and more about understanding that if there’s a carbocation intermediate it will form on the most substituted carbon atom. The subsequent reaction in this series involves removal of the boron and replacement with a hydroxyl group. Missed the LibreFest? 1. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. Anti-Hydroxylation Syn-Hydroxylation Oxidative Cleavage via Ozonolysis Oxidative Cleavage via KMnO 4 Oxidative Cleavage Addition Reactivity of the Carbon-Carbon Double Bond A. HIO4. Addition is the most common reaction of alkenes, and in this chapter, we consider additions to alkenes in detail. This is a stereospecific reaction. The first two possibilities are examples of stereoselectivity, the first being termed syn-addition, and the second anti-addition. Since initial electrophilic attack on the double bond may occur equally well from either side, it is in the second step (or stage) of the reaction (bonding of the nucleophile) that stereoselectivity may be imposed. Addition of Br 2 to an alkene is an anti addition because the two new carbon-bromine bonds are formed on opposite faces of the alkene. Hydroboration also gives syn addition of the boron and hydrogen (and therefore the –OH and –H). 10.11: Stereochemistry of Electrophilic Addition of HX, 10.12: Hydration—Electrophilic Addition of Water. The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Watch the recordings here on Youtube! On the other hand, if the intermediate is short-lived and factors such as steric hindrance or neighboring group interactions favor one side in the second step, then stereoselectivity in product formation is likely. Labeling the addition as “Above” and “Below’ is not scientific and not accurate either as the direction depends on the viewer. L'iodure de triméthylsilyle est un réactif très utile permettant la coupure des éthers dans des conditions douces. The mechanism of this reaction may not be worth memorizing becuase it doesn't fit well within categories we have looked at so far. This Diels-Alder reaction is a syn cycloaddition reaction because the two new carbon-carbon sigma bonds are formed on the same face of the diene or dienophile. Since σ bonds are stronger and more stable than π bonds, double bonds tend to react to convert the double bond into σ bonds. If the two-step mechanism described above is correct, and if the carbocation intermediate is sufficiently long-lived to freely-rotate about the sigma-bond component of the original double bond, we would expect to find random or non-stereoselective addition in the products. On sait en effet que les réactions de substitutions nucléophiles en série aromatique obéissent à des mécanismes d'addition-élimination ou d'élimination-addition. The di- and tri-alkyl boranes behave just as the alkylboranes, but for simplicity they are often written as mono alkylboranes. These reagents can produce close to 100% anti-Markovnikov addition. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. Cuts the diol into two carbonyl … Notons que dans cette réaction on n'obtient pas PhI et CH 3 OH. Mark or AntiMark? Cuts the double bond and produces two carboxylic acids. KMnO4(aq), H+. 1. 1. A wide variety of functional groups can be formed by adding suitable reagents to the double bonds of alkenes. The following table summarizes the results obtained from many studies, the formula HX refers to all the strong Brønsted acids. The interesting differences in stereoselectivity noted here provide further insight into the mechanisms of these addition reactions. Zn, HOAc. Anti addition of X and OH, goes through a halonium ion intermediate, no rearrangement, OH goes on most hindered carbon of double bond . Therefore, a more universal approach is used to describe the stereochemistry of additions to the double bond. O3 2. Explain how with the help of drawings. Ch08 Reacns of Alkenes (landscape) Page 18 Stoichiometry of the Reaction In reality, each B-H bond can add across a double bond. Have questions or comments? Markovnikov vs Anti-Markovnikov in Alkene Addition Reactions Tutorial for Organic Chemistry Students -Step by step how to decide which products will form! KMnO4, OH- 2.