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November 8, 2021
Masinde Muliro University of Science and Technology
How many covalent bonds are predicted for each atom?
a. 0 b. Al c. Br d. Si
Which of the following is $not$ another representation for 2-methylbutane?
Classify each pair of compounds as isomers or resonance structures.
Label each bond in the following compounds as ionic or covalent.
a. F$_2$ b. LiBr c. CH$_3$CH$_3$ d. NaNH$_2$
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Hello, everyone. Welcome to our first lecture on structure and bonding in our interest to organic chemistry. Topic in this lecture will be talking about organic chemistry and what it means to have or to represent different types of structures and different types of bonds within a nor Ganic compound and even for the same organic compounds. So, for example, if I say let's represent benzene so we can represent benzene in multiple ways, one of them being the most common way to represent compounds in organic chemistry in the way that I'll be representing mostly all the compounds from this lecture onwards is with just a stick methodology. So each vertex each vertex that we created here represents a carbon atom so we can see that we have six. Vertex is so we have six carbon atoms. Well, we know that from high school chemistry that carbon needs to have four bonds to be neutral, and this molecule has no charge on it. So clearly this is a neutral carbon. So what does that mean? That means that each one of these carbon atoms here well, they have one bond to the carbon that's on the left of them. one bond to the carbon that's above them. And they have another pie bond here, which we'll talk about in future lectures. So that's only three bonds. Well, this means that this carbon is potentially missing one bond. But that's not the case. When we draw in this stick notation any bonds that air remaining. After all, the bonds that are illustrated by the stick are going to be automatically assumed to be hydrogen unless there is a charge. So, for example, if there was a negative charge on this carbon, then we know that they have this carbon has a lone pair, electrons and therefore will not have anything. 1/4 group bound to it. However, because this compound is neutral, we know that all of these carbon atoms are going to have hydrogen bound to it. This is one way of drawing a benzene ring. Well, what about some other ways of drawing benzene So wicked Drop benzene like this. Okay, Yeah. Benzene could also be drawn by this by drawing out literally the letters and the symbols for each atom that's involved in the compound. So, for example, carbon, hydrogen, carbon, hydrogen, and then showing the bonding in between those representing the sharing of electrons. And we'll talk about the type of bonds that this is a Covalin bond. Ionic bonds, Uh, in a few moments. Next, another way that we can represent. Benzene is just by the molecular formula. So we see that we have six carbons and six hydrogen. So C six and H six. So this is going to be what we call the molecular formula. Okay, Yeah, But then we have something called the empirical formula. So empirical formula is the lowest common denominators, and she is the lowest possible value whole integer for the combination off the atoms involved in a compound. So if we see that, for example, if we divide each side by three, then we will get our empirical formula being C two h two. And even we can go further than that going divided by two divided by two C. H. So this is the lowest reduced formula, the most reduced formal that we can generate that has that has a full positive integer each of these being to the subscript one so we can see that this is going to be our empirical formula. So now we see three different ways that we could represent the same structure of benzene. So what about if the molecule is not planner is not flat? How do we represent, uh, geometry and in the existence of molecules in three D space? Because molecules, not all of them are going to be flats. They will exist in some sort of three dimensional space. So how do we, for example, represent a carbon atom that has four substitutes in three dimensional space? Well, bonds that are in the same plane of your page are drawn as sticks just regular sticks, as these two shown here Bonds that air coming out of the page towards you are shown as wedges. So we'll draw our wedge here on bonds are going into the page. Hm. And bonds that are going into the page are shown as dashes like this. So here we represented stereo chemistry. So stereo chemistry is the existence of a molecule in three D space and understanding his interactions with different atoms in this three dimensional orientation. So, in any molecule, in any carbon atom has four bonds, we're gonna have to bonds in the plane of the page. So you have one bond here. One bond here, we're going to have one bond coming out of the page on the final bond going into the page. So this consist, Toots. The four bonds off our molecule. So what about, uh, molecule, for example, like benzene that has thes single straight lines in between each of these, uh, carbon atoms alternating with two lines. So what's our bond? Is this Well, when we have a carbon or any sort of Adam bounded to another Adam Well, the straight line, this is what we call a co valence bond. So generally, bonding joins two atoms in a stable arrangement. Through bonding, we can obtain the lowest energy level of these atoms involved. So for example, ch three minus and C H three minus would much rather be neutral and just be bounded to each other as this molecule here. So this bond that we draw in between this is what we call equivalence Bond. This is what we call co violence bond. So coagulant bond is the sharing of electrons between two nuclear. So if we look at our periodic trends and we'll actually talk about that in the future lecture, but we know from high school that carbon has four valence electrons. So if I were draws four valence electrons one to 34 on the same thing for the other carbon here, we can see that this bond is composed of two electrons, one from each carbon. So we have a bond that is formed by the sharing off the electrons from the atoms that are involved in the formation of the bond. So the carbon is involved information of bond. And we know that all bonds are composed of two electrons. So each bond and this benzene ring up here. Each of these have two electrons. Okay, but equivalent bond is the sharing of these two electrons between the players involved in the bond. So one from this carbon on left and one from the carbon on the right. Well, what about a compound that doesn't share electrons? What about something that takes away electron? So if you were to look at, for example, in a C l just salt, we can see that in a C. L is going to be actually existing as n a plus and C l minus. So what does that mean? That means that this chlorine has an extra two electrons or an extra lone pair electrons around it's nucleus while the sodium actually lost some electrons from its nuclear. So what does this mean? This means that these compounds are are formed through an ionic bonds. So an Ionic Bond and Ionic bond forms from when there's a transfer of electrons from one atom, for example, in the sodium to another, the chlorine atoms. So you can see that the chlorine atom took those two electrons from the nitrogen atom and brought it to itself to make it C l minus. So these were the most the common to bonding bonding types that you will see within a molecule. So intra molecular bonding within the same molecule. But there are several types of inter molecular bonding that can occur in organic compounds, one of them being your Vaillant bond. So, for example, in proteins, you're gonna have a guy sulfide bridge between different amino acid residues. Sistine residues on different secondary structures are in different monomers coming together and forming a tertiary structure through the formation of a co violent die sulfide bond. Just such as this, alternatively, you can have stabilization of those different monomers from hydrogen bonding. So hydrogen bonding occurs with the hydrogen with three different nucleus is one being nitrogen, oxygen and flooring. So if you look at the example here, if we have just alcohol here, so this is methanol or we could say it's actually ethanol. So if you have a method and ethanol here and then we just have acetone is a simple key tone. We can see that oxygen is involved, which is one of the players. And obviously hydrogen has to be involved to make a hydrogen bonding. So we can say that the hydrogen atom is going toe. Hydrogen, the hydrogen atom from the ethanol, is going to hydrogen bond with the auction item of this key tone, and we can represent that hydrogen bonding by, uh, dashes like this or more. Traditionally, you present hydrogen bonding with dashes like this, so hydrogen bonding always stabilizes your molecule, and it will, uh, really increase your boiling point of your molecules and it will stabilize it. And it can actually be involved in a lot of enzymatic reactions to kind of coordinate an orient your substrate in a specific way that the enzyme can attack are correctly so those air type of bond ings that we have intra molecular early. So we have a coiling bond and Ionic bond and then in terms molecular, uh, bonding, which is, for example, this hydrogen bonding. And we also have covalin bonds through this sulfide bridge that we saw here. Now we can talk about different types of structures on different ways that we can actually illustrate molecules. So we did the most common one of the most likely ones that you are going to see in your organic chemistry classes. But some other ones that you might see in the first few weeks of organic chemistry just to test your overall knowledge off different types off structures is the lewis dot structure. So lewis dot structure will represent your atoms in using their valence electrons. So once again, the next lecture we'll talk about the periodic trends and how we can determine the valence electrons. But the column number, uh, simple trip is trick is a column number represents a number of valence electrons. So we have column 1234 and then 5678 Actually, force on this side 45678 and then in the middle of the transition metals that actually don't follow this rule, uh, too much. But we can say that for the alkali metals, alkaline earth and the third, uh, column. And then we have the, um, noble gasses, the ha logins and the other carbon columns here that so we can see that if an atom is placed in, for example, hydrogen, it is in the first column. So we know that hydrogen is going to have only one valence, electrons and lewis dot structures they represent. There's they're bonding to different atoms through their valence electrons because that is what participates in the formation of your Covalin bonds, such as between these two carbons here. So if I wanted to draw methanol so I could draw methanol like this so there's a carbon with four hydrants. E could also draw methanol like this, or I could draw methanol using the lewis dot structure. So if I start with central carbon atom, I see that carbon is in column number four. So it has four valence electrons and has four hydrogen each having one valence electron. So actually, let me do this in a different color, just so it's really all clear. Okay, we see that now we satisfy the architect of carbon. So this is why this methanol forms because it satisfies octet. Rule on what is the architect rule means that a stable molecule is going to have eight valence electrons around it. And it will do as it will essentially do anything it can to achieve this configuration so we can see that carbon alone does not exist in nature because it only has four electrons is highly unstable. However, methanol will exist in nature because we now have eight valence electrons and we have four Covalin bonds on And remember that we learned that the coil bond is being formed from the sharing of these electrons for them coming from the hydrogen atom and for them coming from the carbon valence electron. And now what if we have multiple bonds? So we talked about a single bone and tell Obama we didn't give them a name. So if you look back at our Ben's an example here, we can say that a single bond is called the Sigma Bond. So it's one of those Greek letters sigma and this is called the Sigma Bond, so single bond is fairly stable, fairly strong. So what about now? When you have to bonds well, one of those bonds are still going to be our Sigma Bond, but now the second bond is going to be a pie bond and now a pie bond. What it does is it decreases the length between these two molecules such that the distance between this carbon and this carbon is going to be a lot shorter than the distance between this carbon and this carbon that only has one Sigma bond. So we ADM or number of bonds we decrease. Decrease the bond length between those atoms that are involved in the bond. Because we decrease the bond length, we increase its stability because we increase the stability. Now we have to use more energy to break that bond so you can see how it's advantageous for molecules. Toe have multiple bonds because it really increases their stability and prevents them from reacting with other things in biological systems such as free amino acids or enzymes and whatnot. And with that, that concludes our first lecture on structure and bonding. Please join me for the next lecture, which we will be talking about periodic table trends
Nuclear Magnetic Resonance Spectroscopy
Acids and Bases