A Level Chemistry Revision "Electrophilic Addition of a Halogen to Alkenes"

Summary

In this Freesciencelessons video, the process of electrophilic addition of halogens to alkenes is explained thoroughly. The video focuses on the chemical reaction mechanism between ethene and bromine, a specific case of electrophilic addition. The instructor breaks down the stages of the reaction and emphasizes the difference between a halogen addition and a hydrogen halide addition to alkenes. The process involves an induced dipole in the bromine molecule due to the high electron density of the alkene, leading to the formation of a carbocation intermediate and ultimately resulting in a dibromoethane product. Additionally, the video discusses using bromine water to test for unsaturation in molecules, where decolorization indicates the presence of a double bond.

Highlights

• The high electron density in alkenes causes an induced dipole in the bromine molecule, leading to electrophilic addition. 🌊
• Electrophilic addition with halogens creates a single dibromo product, regardless of the symmetry of the alkene. ⚖️
• The reaction with an alkene can be visibly tested using bromine water, which decolorizes in the presence of unsaturation. 👀
• In bromine addition, the reaction forms a carbocation intermediate before reaching the final product. 🔄

Key Takeaways

• Electrophilic addition is a reaction where a halogen molecule adds across the double bond in alkenes. 🌟
• Halogens, unlike hydrogen halides, do not have a permanent dipole; instead, they form an induced dipole in the presence of an alkene. ⚛️
• Using bromine water, an orange bromine solution, the presence of an alkene is indicated by decolorization, signifying a double bond. 🚰
• A major difference in halogen addition is that it results in a single product even with asymmetric alkenes. 🎯

Overview

Electrophilic addition reactions are an essential concept in organic chemistry, particularly when addressing how halogens interact with alkenes. In this reaction, halogens such as bromine add across the double bond of alkenes without having a permanent dipole, relying instead on the induced dipole created by the nearby electrons of the alkene. This is a departure from the action of hydrogen halides, which possess a natural dipole.

The video explains the step-by-step process of how ethene and bromine react. As the bromine molecule approaches the alkene, the electron-rich double bond in ethene induces a dipole in the bromine molecule. This interaction facilitates the addition, forming a carbocation intermediate that quickly bonds with the bromide ion, yielding a dibromo product. This entire mechanism reveals the elegance and predictability of such chemical processes.

Moreover, the practical application of this reaction is discussed, namely using bromine water as a qualitative test for unsaturated compounds. When bromine water is added to a sample, the presence of unsaturated compounds causes a color change from orange to colorless as bromine adds across the double bond. This provides a straightforward visual test to identify alkenes in a mixture, demonstrating both the utility and simplicity of this basic yet powerful chemical principle.

Chapters

• 00:00 - 00:30: Introduction to Electrophilic Addition This chapter introduces the concept of electrophilic addition in organic chemistry, focusing on how halogen molecules add to alkenes. By the end of the chapter, learners should understand this reaction mechanism and how to test for unsaturation in molecules using bromine water. It also builds on previous lessons regarding electrophilic addition of hydrogen halides, like hydrogen bromide, hydrogen chloride, and hydrogen iodide to alkenes.
• 00:30 - 01:00: Electrophilic Addition of Hydrogen Halides The chapter titled 'Electrophilic Addition of Hydrogen Halides' explains the reaction of hydrogen halides with alkenes. It describes how the hydrogen halide molecule, which has a permanent dipole due to the electronegativity difference between its hydrogen and halide components, reacts with alkenes. In the initial stage of this reaction, the electrons in the pi bond of the alkene are attracted to the positively charged hydrogen atom of the hydrogen halide. Consequently, the hydrogen acts as an electrophile. The electrons from the alkene form a covalent bond with the hydrogen atom, leading to the formation of a carbocation intermediate.
• 01:00 - 01:30: Mechanism of Electrophilic Addition of Hydrogen Halides The chapter discusses the mechanism of electrophilic addition of hydrogen halides, specifically focusing on the reaction between an alkene and a hydrogen halide like hydrogen bromide to form bromoethane. It explains the two stages of the reaction: first, the electron pair from the bromide ion is attracted to the positively charged carbon atom, forming a covalent bond. The result is the formation of bromoethane. The chapter promises to further examine reactions involving alkenes and halogen molecules.
• 01:30 - 02:00: Overview of Halogen Addition to Alkenes This chapter discusses the reaction between ethene and bromine, noting that a similar reaction can occur with other alkenes and halogens. During this reaction, the halogen molecule adds across the double bond, illustrating an example of electrophilic addition. However, the mechanism of this reaction is slightly different from previous ones because halogen molecules do not have a permanent dipole. The chapter encourages understanding and learning this reaction mechanism.
• 02:00 - 03:00: Mechanism of Electrophilic Addition of Halogens The chapter explores the mechanism of electrophilic addition of halogens, using the reaction between ethene and bromine as an example, while noting that the same concept applies to other alkenes and halogens. Initially, the bromine molecule, which lacks a permanent dipole, approaches the ethene molecule. The high electron density of the alkene's double bond repels the electron pair within the bromine molecule, resulting in an induced dipole in bromine.
• 03:00 - 04:00: Differences Between Halogen and Hydrogen Halide Addition This chapter explores the fundamental differences between halogen and hydrogen halide addition to alkenes, with a particular focus on the mechanism involving bromine. It explains how the pair of electrons in the pi bond of the alkene are drawn towards the positively charged bromine, which acts as an electrophile. This interaction leads to a new covalent bond formation with the bromine atom while the bromine-bromine bond breaks heterolytically, transferring both electrons to the other bromine atom. This highlights the concept of heterolytic bond dissociation.
• 04:00 - 05:00: Testing for Unsaturation with Bromine Water The chapter 'Testing for Unsaturation with Bromine Water' explains the process of how a carbocation intermediate and a bromide ion react. In stage three, the electron pair on the bromide ion is attracted to the positively charged carbon atom in the carbocation intermediate. This electron pair forms a covalent bond, resulting in the final product, which is 1,2-dibromoethane. The chapter emphasizes that during this reaction, the halogen molecule adds across the double bond, effectively testing for unsaturation.

A Level Chemistry Revision "Electrophilic Addition of a Halogen to Alkenes" Transcription

• 00:00 - 00:30 [Music] hi and welcome back to precise lessons by the end of this video you should be able to describe the electrophilic addition of a halogen molecule to an alkene you should then be able to describe how to test for unsaturated molecules using bromine water in the last couple of videos we looked at the electrophilic addition of a hydrogen halide molecule to alkenes we looked at the addition of hydrogen bromide but this also applies to hydrogen chloride and hydrogen iodide because halogen atoms are more
• 00:30 - 01:00 electronegative than hydrogen hydrogen halide molecules have a permanent dipole in the first stage of this reaction the pair of electrons in the pi bond of the alkene are attracted to the positive hydrogen atom of the hydrogen bromide and in this case the positive hydrogen atom is acting as an electrophile the pair of electrons from the alkene now forms a covalent bond to the hydrogen atom at the end of this stage we've formed a carbocation intermediate with a positive
• 01:00 - 01:30 charge on a carbon atom we've also formed a bromide ion in the second stage the electron pair on the bromide ion are now attracted to the positive carbon atom forming a covalent bond and we've now made our product which in this case is bromoethane scientists call this reaction mechanism electrophilic addition in this video we're going to look at the reaction between an alkene and a halogen molecule i'm showing you here the equation for
• 01:30 - 02:00 the reaction between ethene and bromine but i should point out that we'd see a similar reaction with other alkenes and other halogens in this reaction the halogen molecule adds across the double bond this is another example of electrophilic addition but as you'll see the mechanism is slightly different to the one we've seen before and that difference is due to the fact that halogen molecules do not have a permanent dipole so let's take a look at the mechanism and again i'd recommend that you learn it we look at the mechanism for the
• 02:00 - 02:30 reaction between ethene and bromine but this also applies to other alkenes and other halogens in the first stage of the reaction the bromine molecule approaches the ethene molecule as we've said the bromine molecule does not have a permanent dipole however the double bond of the alkene is a region of high electron density and this high electron density repels the electron pair of the covalent bond in the bromine molecule and this means that the bromine molecule now has an induced
• 02:30 - 03:00 dipole in stage two the pair of electrons in the pi bond of the alkene are attracted to the positive bromine so in this case the positive bromine is acting as an electrophile the electron pair now forms a covalent bond to this bromine atom at the same time the covalent bond in the bromine molecule now breaks and the pair of electrons move on to the other bromine atom when a covalent bond breaks like this with both electrons going to the same atom scientists call this heterolytic
• 03:00 - 03:30 fission so at the end of stage two we've got a carbocation intermediate and a bromide ion in stage three the electron pair on the bromide ion are attracted to the positive carbon atom in the carbocation intermediate this electron pair now forms a covalent bond and we have our final product in this case the product is 1 2 dibromo ethane notice that in this reaction the halogen molecule adds across the double bond this means that the two halogen atoms
• 03:30 - 04:00 end up on two adjacent carbon atoms we cannot get both halogen atoms on the same carbon atom now you'll notice that there are two key differences between adding a halogen to an alkene and adding a hydrogen halide to an alkene firstly hydrogen halide molecules have a permanent dipole whereas in halogen molecules the dipole is induced secondly when we add a hydrogen halide to an asymmetric alkene we make a major and a minor product
• 04:00 - 04:30 however when we add a halogen to an asymmetric alkene we only make one product for example if we add bromine to butane we can only make one two dibromobutane okay now we can use the reaction with a halogen to test for the presence of an unsaturated molecule such as an alkene to do this we use bromine water which has an orange brown colour to test if a substance is unsaturated we add drops of bromine water and gently shake the test tube if our substance is unsaturated then the bromine will add
• 04:30 - 05:00 across a double bond and the product of the reaction will be colorless so we'll see the orange bromine water decolorize however if our test substance is saturated then the bromine will not react and the bromine water will remain orange okay so hopefully now you can describe the electrophilic addition of a halogen molecule to an alkene and describe how to test for the presence of unsaturated molecule [Music] you