Organic Chemistry I - Reactions of Alkenes - Quick Guides | Chemistry

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Study GuideOrganic Chemistry I–Reactions of Alkenes1.Alkenes: Hydration (Direct Addition of Water)The addition of water to an alkene in the presence of a catalytic amount of strong acid leads to theformation of alcohols (hydroxy-alkanes).Mechanism of Hydration (Direct Addition of Water):This reaction proceeds via a standard carbocation mechanism and follows the Markovnikov rule. Themechanism for the addition of water to ethene is as follows:1.Formation of a Carbocation:The hydrogen ion (H⁺) is attracted to theπbond of the alkene, which breaks to form aσbond withone of the double-bonded carbons. The second carbon of the original double bond becomes acarbocation.Reaction:(ethene to carbocation)2.Formation of Oxonium Ion:

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Study GuideAn acid-base reaction occurs between the water molecule and the carbocation, resulting in theformation of an oxonium ion.Reaction:(carbocation reacting with water to form oxonium ion)3.Formation of Alcohol:The oxonium ion stabilizes by losing a hydrogen ion (H⁺), resulting in the formation of ethanol (or analcohol).Reaction:(oxonium ion loses H⁺to form ethanol)Key Takeaway•Hydration of Alkenes:The addition of water to alkenes in the presence of acid formsalcohols. The reaction proceeds through a carbocation intermediate, following Markovnikov’srule.•Mechanism:1.The hydrogen ion adds to the alkene to form a carbocation.2.Water then reacts with the carbocation to form an oxonium ion.3.The oxonium ion loses a proton to yield an alcohol.

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Study Guide2.Alkenes: Hydroboration-OxidationWater can be added to an alkene in a special way, where the major product is different from what wemight expect using the Markovnikov rule. This reaction is calledhydroboration-oxidation.In this process, a special compound with boron is added to the alkene. The boron attaches to onecarbon of the double bond, and hydrogen is added to the other carbon. This leads to the formation ofa new compound calledorganoborane.1.The Hydroboration MechanismThe first step in this process is the formation of theorganoboranefrom the alkene. This happensvery quickly. The boron atom attaches to the less substituted carbon (the one with fewer hydrogenatoms). This step forms a new C-H bond using a unique interaction called afour-center interaction.In this interaction, bonds between four atoms are made and broken at the same time, which allowsthe reaction to proceed smoothly.

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Study Guide2.Oxidation of the OrganoboraneOnce the organoborane is formed, it undergoes a three-step oxidation process. In the first step, amolecule called ahydroperoxidereacts with the organoborane. The hydroperoxide has an oxygenatom that adds to the boron atom.This step is important because it begins the transformation of the organoborane into an alcohol.3.Formation of a Borate EsterThe next step involves the formation of aborate ester. The hydroperoxide group rearranges andloses a hydrogen ion, leading to the creation of the borate ester. This intermediate is not very stable,and it continues to change in the next steps.4.Final Step: Hydrolysis to Form AlcoholFinally, the borate ester reacts with water in a process calledhydrolysis. This is where the esterbreaks down using water, splitting into alcohol and borate ions. The alcohol formed is the final productof the reaction.

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Study GuideBy the end of the reaction, we have an alcohol as the major product, and no carbocation intermediateis formed during the process. This is a unique feature of the hydroboration-oxidation reactioncompared to other reactions involving alkenes.Key Takeaway•Hydroboration-oxidationadds water to alkenes in a way that doesn’t follow the Markovnikovrule.•The process involves adding a boron-containing compound (organoborane) to the alkene.•The organoborane undergoes a three-step oxidation to form an alcohol.•No carbocation intermediatesare involved in this reaction.•The final product is an alcohol, with borate ions as by-products.3.Alkenes: Catalytic Addition of HydrogenHydrogenationis the process of adding hydrogen to an alkene. While this reaction releases energy(exothermic), it happens very slowly on its own. To speed up the process, ametal catalystsuch asplatinum, palladium, nickel, or rhodium is used. This makes the reaction happen much faster.Although it seems simple, hydrogenation is actually quite complex and happens infour steps.Step 1: Reaction with the CatalystIn the first step, a hydrogen molecule reacts with the metal catalyst. This reaction breaks theσbondbetween the hydrogen atoms and forms weakhydrogen-metal bonds.

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Study GuideStep 2: Contact with the AlkeneNext, theπbondof the alkene molecule comes into contact with the metal catalyst. This bond isbroken, and two weakcarbon-metal single bondsare created.Step 3: Hydrogen TransferAfter that, the hydrogen atoms that are attached to the metal catalyst are transferred one by one tothe alkene. This results in the formation of a newalkane.Step 4: Alkane FormationOnce the two newcarbon-hydrogen bondsare formed, the alkane molecule can move away fromthe catalyst.Because both hydrogen atoms are added to the same side (orface) of the alkene, the reaction iscalled asyn addition.Anti AdditionIn some cases, the hydrogen atoms might approach the alkene molecules from opposite sides, ratherthan the same side. This is known as ananti addition.Anti addition happens when the double bond of the alkene isomerizes more quickly than the secondhydrogen is added in the hydrogenation reaction.4.Alkenes: Addition of CarbenesCarbenesare very reactive molecules with the general formulaR₂C:. In this form, the carbon atomhas only six electrons in its outer shell, making it highly reactive and electron-seeking (electrophilic).

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