L24 Functional Groups P3. Alkenes, Alkynes, and Addition

Summary

Highlights

• Alkenes and alkynes provide a playground for exciting reactions due to their unique bonds. 😯
• Pi bonds are the stars of the show in addition reactions, being weaker yet highly reactive. 🌟
• In bromination, each carbon bonds to a bromine atom following the breakage of a pi bond. 🧪
• Hydrogenation involves adding hydrogen atoms, turning hydrocarbons from unsaturated to saturated. 🔄
• Saturated vs unsaturated fats: a topic where chemistry meets nutrition in the real world. 🍽️

Key Takeaways

• Alkenes and alkynes are hydrocarbons with double and triple bonds, making them unique in chemical reactions. 🌟
• Pi bonds are weaker than sigma bonds and facilitate addition reactions where they break, allowing new atoms to attach. 🔧
• Bromination and hydrogenation are key types of addition reactions, useful in creating compounds like dibromoethane and ethane. 🧪
• Saturated hydrocarbons have the maximum number of hydrogen atoms, whereas unsaturated ones do not, a concept important in nutrition. 🥑
• The process of hydrogenation transforms unsaturated fats to saturated ones, used industrially to create margarine. 🧈

Overview

Chapters

• 00:00 - 01:00: Introduction to Alkenes, Alkynes, and Addition Reactions This chapter introduces alkenes and alkynes, which are hydrocarbons characterized by the presence of double and triple bonds between carbon atoms, respectively. The chapter also covers the concept of addition reactions, a fundamental reaction pattern for these molecules. The foundational role of carbon's ability to form four bonds is emphasized, setting up an understanding of how these structures influence chemical behavior.
• 01:00 - 03:00: Structural Characteristics of Alkenes and Alkynes The chapter focuses on the structural characteristics of alkenes and alkynes, which are types of hydrocarbons. An alkene contains a double bond between carbon atoms, whereas an alkyne contains a triple bond. In naming these compounds, the suffix '-ene' is used for alkenes, and '-yne' is used for alkynes. Each carbon involved in these bonds must also have additional bonds to other atoms.
• 03:00 - 04:30: Significance of Pi Bonds in Alkenes and Alkynes The chapter titled 'Significance of Pi Bonds in Alkenes and Alkynes' explains the importance of pi bonds in the structure of these hydrocarbons. The discussion starts with basic nomenclature differences in hydrocarbons based on the bonding between carbon atoms: ethane for single bonds, ethene for double bonds, and ethyne for triple bonds. The focus is on alkenes (double bonds) and alkynes (triple bonds), highlighting the concept of pi bonds. A double bond in an alkene is composed of a sigma bond and a pi bond, which significantly affects the reactivity and properties of the molecule.
• 04:30 - 05:30: Introduction to Pi Bond Reactivity and Addition Reactions The chapter introduces the concept of pi bond reactivity in alkynes, which contain two pi bonds and a sigma bond. The focus is on the pi bond due to its significance in bonding concepts. The text references the overlap of hybrid orbitals in double-bonded carbon.
• 05:30 - 07:00: Example of Bromination - An Addition Reaction This chapter discusses the concept of bromination as an addition reaction. It also covers the sigma bond structure of carbons, emphasizing the presence of sp2 hybrid orbitals on carbon atoms. Additionally, it explains the formation of pi bonds through the side-to-side overlap of p orbitals between adjacent carbon atoms.
• 07:00 - 09:30: Hydrogenation: Saturated vs. Unsaturated Compounds The chapter discusses the role of pi bonds in molecules, specifically how they prevent rotation between different parts of a molecule. This is due to the necessity of p orbitals remaining aligned to maintain the pi bond.
• 09:30 - 10:30: Applying Hydrogenation in Nutrition - Saturated and Unsaturated Fats Geometric or cis-trans isomers arise from restricted rotation around double bonds in molecules, such as in the context of fats. The p orbitals in the molecule line up in a way that breaking the pi bond would result in restricted rotational movement, leading to distinct isomers.

L24 Functional Groups P3. Alkenes, Alkynes, and Addition Transcription

• 00:00 - 00:30 So in this video we'll look at alkenes, alkynes,  and the reaction pattern known as addition   reactions. As I mentioned previously, an alkene  is simply a molecule that has a double bonded   carbon in it, and we know that since carbon  forms four bonds that each of those carbons
• 00:30 - 01:00 must have two additional bonds to something  else. An alkyne is a hydrocarbon that has a   triple bond in it to another carbon, and so each  of those carbons has one bond to something else. In terms of naming we saw previously in some  examples that really we just have to change the   ending of the name to “-ene” if there's a double  bond and “-yne” if there's a triple bond. So,
• 01:00 - 01:30 if we had two carbons, if they were all single  bonded it would be “ethane”, if there's a double   bond “ethene”, triple bond, “ethyne”. In terms  of what is significant about alkenes and alkynes,   really the main thing we want to focus on is this  idea we talked about previously where a double   bond really represents two different kinds of  bonds; a sigma bond and a pi bond. And of course
• 01:30 - 02:00 we recognize that in an alkyne we have two pi  bonds and a sigma bond. It's really the pi bond   that is of interest to us here, and the reason  is if we think about our bonding concepts we know   that if we had a double bonded carbon we would  have some kind of hybrid orbital overlap to form
• 02:00 - 02:30 the sigma bond. And of course, these carbons also  have some other sp2 hybrids going out to something   else, I'll just draw hydrogens here. But of  course we recognize that in addition to that sigma   bonding there is this p orbital on each of these  carbons that is overlapping side to side to form
• 02:30 - 03:00 the pi bond. And one of the things we talked about  previously is one thing that a pi bond contributes   to the molecule is that it prevents rotation of  one end of the molecule relative to the other and   that's because in order to maintain that pi bond  the p orbitals have to stay lined up and if you
• 03:00 - 03:30 were to rotate one end of the molecule relative to  the other, the p orbitals would no longer line up;   you would have to break the pi bond. And so it  tends to lead to this restricted rotation of the   molecule one end about the other, which, as we saw  previously in the isomer section leads to what we   call geometric or cis-trans isomers. And so for  example we saw in that case that you could have
• 03:30 - 04:00 let's say C2H2CL2 and that there are really two  different versions of this molecule depending on   which way those chlorine atoms and hydrogen atoms  are arranged in space. So we could have what we   call the cis version or the trans version of those  two molecules, 1,2-dichloroethane in this case.
• 04:00 - 04:30 But another important idea about pi bonds, second  idea, is that pi bonds are weaker than sigma bonds   and the reason that that's significant is because  they're weaker these pi bonds are chemically   reactive, and specifically it's possible to break  a pi bond without breaking the sigma bond because
• 04:30 - 05:00 it takes less energy to break a pi bond. And so  as a result the molecule can stay intact even when   the pi bond is broken which as we'll see leads  to a certain type of chemical reactivity what   is known as an addition reaction. So an addition  reaction, and we’ll use a simple example to start   with, we can start with just the two carbon  molecule with the double bond in the middle,
• 05:00 - 05:30 you'll remember that because it has two carbons  the base name is “eth-”, “ethane,” we switch the   ending to “-ene”, “ethene” because of the double  bond, and once again we recognize that one of   those bonds is a pi bond and that it's weaker  and it turns out if we react this ethene with
• 05:30 - 06:00 certain types of molecules, one example being  bromine, Br2, that what can happen is that the   pi bond breaks, which essentially frees up one  electron on each of the carbons and then those   electrons can pair up with the electrons that are  on each of the bromine atoms in the Br2 molecule
• 06:00 - 06:30 in order to add those bromine atoms on to the  carbon chain, one bromine on each carbon, and   as a result we have taken two molecules and added  them together to produce one larger, new molecule,   and hence the name “addition.” And you'll  notice that if we wanted to name this molecule,
• 06:30 - 07:00 this product molecule on the right, once again,  now the base name, two carbons so ethane, we've   got bromines on it so it's bromoethane. We have  two bromines so we would say dibromoethane, and of   course finally we need to number them we see that  one of the bromines is on number one, and one is   on number two, so this would be 1,2-dibromoethane  would be the name of this compound. Now this
• 07:00 - 07:30 particular version of addition since it involves  adding bromine to a molecule is often called   “bromination.” Another common example of addition  would be, instead of adding bromine, we could add   hydrogen H2 and as a result this addition reaction  is often known as “hydrogenation,” and so if we
• 07:30 - 08:00 did a hydrogenation on ethene (that is the same  molecule shown above) once again the idea here   is simply that we're going to break the pi bond,  keeping the sigma bond intact, and as a result   add two more atoms onto those carbons and so we  still have all of the original molecules, plus,
• 08:00 - 08:30 in addition we've added one hydrogen atom to each  of these carbons from that hydrogen molecule and   of course we've now gone from ethene to ethane,  and one important point that this brings up is   that we’ll notice that the alkene, that is the  ethene molecule that's on the left, because it
• 08:30 - 09:00 has a double bond does not have as many hydrogens  as the molecule could possibly have for a two   carbon compound and as a result we would say it is  “unsaturated,” whereas if we look at the product   on the right, ethane, the alkane, it has all the  hydrogens you can possibly put on two carbons   (that is six altogether) and so we would say that  ethane is a “saturated” hydrocarbon and so by
• 09:00 - 09:30 hydrogenating our unsaturated hydrocarbon we have  produced a saturated hydrocarbon. And in fact you   may recognize some of these words because they're  used in the nutritional industry, to some extent,   where later we'll talk about the structure of  fats and we can have unsaturated fats or saturated
• 09:30 - 10:00 fats. Unsaturated fats are those that have  carbon-carbon double bonds in them. Saturated fats   have all carbon-carbon single bonds in them. And  the process of converting an unsaturated fat to a   saturated fat is a process known as hydrogenation  where literally you can bubble hydrogen gas   through certain types of vegetable oils that are  unsaturated, as a result producing saturated,
• 10:00 - 10:30 more saturated fats within that product, creating  what was conventionally known as margarine, right?   Something more solid-like that people liked for  various reasons that involved baking and so forth.