welcome back this is Mrs Hansen once again going to wrap up our lessons in chapter 1 with two remaining topics the Vesper Theory and intermolecular attractions as we review the Vesper Theory we'll remember that it stands for the veence shell electron pair repulsion Theory the veence shell [Music] electron pair repulsion theory in other words we want to make sure that Bond angles are maximized so that we're not sterically hindered in an uh in a molecular geometric shape there are going to be review sessions here on the Vesper Theory and in your e-learning I have provided for you a a Vesper chart that you may choose to print and kind of study over as you work to remember some of these geometric shapes so when I talk about the Vesper Theory and molecular geometry we're remembering that uh the focus here on organic chemistry so we're looking at really just uh two three or four electron domains and sometimes electron domains can be referred to as a steric number if that's a word that you might have heard of before I use the terms electron domains an electron domain from a central atom can either be a a bond or a lone pair of electrons so how many things are you looking out from a central atom and the first I'd like to talk about is four electron domains in which all four of those electron domains are bonded and zero are unbonded and an example here might be a methane molecule CH4 on a piece of paper we're used to dry uh drawing methane CH4 with what appears to be 90° Bond angles by just creating a very um you know straight up and down kind of figure we understand that that is not correct because we can maximize those Bond angles by creating larger angles up to 109.5° by altering the shape if we place that Central atom and have three of the four domains kind of pointing down kind of like the base of a pyramid and one pointing straight up we have 109.5° Bond angles in all directions clearly that is a maximized Bond angles as if we pictured it instead of a 90° so a tetrahedral molecular geometry where we have four electron domains leading out to all four of them being bonded to another atom carbon tetrachloride would be another example ccl4 where we have the central atom has four electron domains in which all four are bonded zero or unbonded 440 is how you would read your vasper chart finding it as a tetrahedral in shape if we have excuse me if we have four electron domains but only three are bonded and one is unbonded the shape is now referred to as a trigonal pyramidal shape trigonal pyramidal for example the molecule ammonia ammonia has the central atom of a nitrogen leading down to three hydrogens but instead of another atom up here it has a lone pair of electrons we would read this as four electron domains of the four three are bonded and one is unbonded 431 on your Vesper chart and remember these electrons kind of are much larger they're electron cloud if you will and they end up kind of pushing those hydrogens closer together somewhat a little bit closer together so you have an optimal Bond angle that's actually less than 10 uh than the previous slide which was 10 9.5 here you're seeing them at about 107° they slightly pushed together based on the density of this Rich area of electron cloud so 107° Bond angles with a trigonal pyramid geometry if we have four electron domains two of which are bonded and two are unbonded as this example of a water molecule 422 is how we would read that on the Vesper chart you have what's called a molecular geometry of bent water is a bent molecule 422 a polar molecule and the bond angles are about 105° knowing that we have two electron rich areas that are going to decrease the bond angle between these two hydrogens just knowing that electrons take up more space if you will that electron cloud kind of needs more space and it pushes the hydrogens a little bit closer together so with four electron domains we can have a tetrahedral if all are bonding we can have a trigonal pyramidal if three are bonded and we could have a bent shape if only two are bonded let's recall three electron domains in which all three are bonded this is called trigonal planer just like an equilateral triangle the equilateral Al triangle has the bonds leading out to three other atoms and the bond angles between those is 120° the very important part of trigonal planer is that it is a very flat molecule it lies within the same plane as the paper and that's going to be very important when we talk about um mechanism if you will especially when we do um SN nucleophilic substitution reactions and elimination reactions if we have a carbo cat ion a carbo cat ion is electron deficient meaning that it has only three bonds missing one making that carbon have a plus one formal charge this is an example of a trigonal planer and because it's flat we can see that an attacking molecule can either come from the top side or from the bottom side we can have an inversion or retention of configuration so Bond angles of 120° SP2 hybridization just as in before when we had tetrahedral all of those tetrahedrals were sp3 four electron domains trigonal planer has three domains so it's SP2 hybridized and lies flat in the same plane a third characteristic of Vesper is a linear configuration in which we have simply two electron domains in all two are bonded here you can clearly see if this were uh be and then hydrogen on each side the bond angle would be a straight line of 180° a triple bond carbon is also an example of a 180° not only between the two carbons but also leading out to the next Branch as well so a linear geometry and here's a chart that kind of summarizes what we've went through Slide by slide noticing that we have um in terms of a steric number I called electron domains if I have four electron domains all four are bonded none are unbonded we call that tetrahedral Bond angles of about 109.5 if we have four electron domains and only three are bonded we called it trigonal pyramidal and the bond angle is reduced slightly to about 107 if we have four electron domains and only two are bonded leaving two lone pairs we called it bent and roughly about 105 for its degrees of bond angle trigonal pyramidal or I'm sorry trigonal planer trigonal planer we would have three electron domains all bonded giving us 120° equilateral triangle and 220 gives us 180° as well and you'll be asked to practice determining Bond angles and recognizing what type of molecular geometry around the central atom so let's take a look at this example here's h3o+ this is what we called hydronium it is the acid polyatomic ion h3o pluses that look familiar from bronze dead Lowry acid base pairs the central atom oxygen has four electron domains three of which are bonded and one is unbonded now don't let that positive charge bother you for counting domains it's there to let you know oxygen has a formal charge of + one let's review that oxygen lives in group six it has six veence electrons it has 1 2 3 4 five assigned electrons giving it a formal charge of plus one that's why you need to see the plus charge because it's part of the leis Dot Structure but it does not matter when we're counting domains a domain is a bond I don't care if the domain is a single double or triple it still counts as one Dom Dom and in this example three are bonded one unbonded so according to your Vesper chart four domains three bonded one unbonded is a trigonal pyramidal shape so this is trigonal pyramidal and that means the bond angles are approximately 107° here we have a central atom of Boron and again the negative charge is there because it has a formal charge Boron lives in group three so it has three veence electrons it has four assigned electrons around it so that means it has a minus one formal charge but we don't worry about that when we're counting domains the Boron here has four electron domains all four are bonded zero unbonded so on your Vesper chart you read 440 and you see that that's tetrahedral Bond angles of about 109.5 around the central atom we have three electron domains all three are bonded zero are unbonded 33 0 is trigonal planer and here we have 4 40 four electron domains all four are bonded 440 is tetrahedral very significant in terms of organic chemistry we discussed the carbo cat Ion with three domains 330 is trigonal planer and if the carbon ended up having a ative formal charge notice here it has four domains only three are bonded and one is unbonded that's what we referred to as trigonal pyramidal these Bond angles are approximately 120° lying all in the same plane whereas this carbon has a set of lone pair electrons making the bond angle about 107° and the last major concept of our chapter is the polarity of molecules and intermolecular attractions and based on the polarity we can understand what type of attractions a molecule would have remember that a molecule is polar if it has unequal electron distribution so for example here is a carbon atom and we clearly see the polarity of the bond is making the car the chlorine very electron Rich so this is a negative area and the carbon's left with a net positive the more negative region we can see uh the more red it comes out in this diagram and the more electron deficient comes out with Blues so this is what we refer to as a polar molecule where we have an unequal distribution of charge when I look at some examples let's look at letter A I have a molec built of ch3 attached to a ch2 leading up to an oxygen and back down to a ch2 ch3 and I'll just draw those out and I did that on purpose to give us a visual remember that the octet there of oxygen is required iring two sets of alone pair of electrons and so here this is the polar bonds right so here I can clearly see that the electron density is being pulled towards the oxygen this region of oxygen is a very electron rich and these carbons are electron deficient this is what we refer to as a polar molecule it has two polar bonds and those polar bonds create an unequal distribution of electron charge so it's over over overall polar if I compare that to letter B by the time I'm done drawing a carbon dioxide molecule we have a double bond going to oxygen in either direction now these bonds are indeed polar but notice that the dipole itself cancels there is an equal distribution of electron charge so the dipole moments cancel and what that means is that I cannot point to any specific region in this molecule that's any more negative or positive the bond is polar but the molecule is nonpolar because overall these dipoles have can canell making the molecule very equal in electron distribution it's a very symmetrical molecule non-polar molecules are always symmetrical and just thinking about that how the dipole moments cancelled in carbon dioxide but in this dimethyl ether we had a very rich oxygen area making the mo uh polarity stand out where we had positive and negative regions in that same molecule so just thinking about if a molecule is polar or nonpolar think symmetry here is a carbon attached to three chlorines and one hydrogen clearly we can see this is not symmetrical and what I mean by that is to be symmetrical all four domain would have to be exactly the same and they're not so this electron distribution is being pulled away from the hydrogen and also away from the carbons towards all of the chlorines so this is a polar molecule with very rich electron areas on the chlorine and electron deficient areas on the hydrogen and carbon here we have in letter b ch3 o ch3 and remember when I think about the geometry the oxygen has lone pairs of electrons sometimes drawing that out helps us see that and I can see that the dipole moment is being pulled towards the oxygen making it very rich in electron density so this is also a polar molecule NH3 when I draw its leis do Str structure I can see a very rich area here of electrons making this very electron rich and the hydrogen's Very electron deficient so this is a polar molecule in terms of bond polarity nitrogen being the more electr negative element is pulling the electron density towards itself and being pulled up towards the electron pair and the last one I have carbon attached to two chlorines attached to two bromines clearly we see that these are polar bonds and being pulled towards the chlorines towards the bromine but this is not the same electro negativity chlorine and bromine are not the same chlorine is much more electron more Electro negative than bromine so this is a more Dynamic uh pull towards the electrons as compared to this so this is also a polar molecule and I had drawn those out just to help emphasize where we see the vector arrow pointing to electron rich areas and electron deficient areas you may certainly pause and make sure your drawings match these now that we understand polarity of molecules we can apply that to the inmolecular forces and physical properties that accompany those inter those polarity so knowing that these properties such as solubility boiling point density states of matter melting point all of these things are affected by the attractions between molecules inter molecular for forces are attractions between compounds two molecules remember neutral molecules being polar and nonpolar are attracted to one another and we have three different types of uh intermolecular forces and I'm going to abbreviate those by the way as IMF intermolecular forces the first being dispersion the second being dipole dipole and the third being hydrogen bonding and let's kind of review what each one of those imfs talk about the weakest is known as a London dispersion force or simply dispersion force so keep in mind that all molecules have kinetic energy they are in constant random motion and that means electrons in the molecule sometimes produce an electron distribution that's not evenly balanced with the positive charge in in the nucleus so a very very temporary induced dipole will occur when two molecules become so close together they can influence the distribution of electron charge the kicker here is to understand that it's so temporary it's an instant in time and then the electrons themselves redistribute to a non-polar configuration but it's the interaction the closer that two molecules become where the positive charge on this particular nucleus or this particular nucleus these are atoms can influence the electron cloud of a neighboring atom you can see closer they are the more influential one molecule is on the other so a dispersion force is a temporary induced dipole all molecules can behave such as this all molecules can exhibit dis dispersion forces now a dispersion force can increase with increasing molar mass larger surface areas now keep in mind it's how we can influence one molecule to the next so the longer the molecule is the more surface area it has the longer molecules means that it can interact over a greater surface area to create these temporary dipoles if you have a shorter Sur surface area there is not as much area to interact to create these temporarily induced dipoles so the longer the carbon chain the heavier it is the stronger the London dispersion forces are the more energy it would take to raise the boiling point right boiling points are affected by dispersion forces meaning the heavier the molecule the longer the molecule the more energy it's going to take to force that liquid to turn to gas so you can see increasing the molar mass increasing the surface level has increased the boiling point now if dispersion forces are influenced by the ability of a molecule to interact we can have a greater surface area for interactions if we have straight chains right here I just have five carbons in a row all in a straight chain plenty of opportunity to have a large surface area for interaction but if I start branching here I have the same number of carbons but they're just attached in a branched form those molecules have less of a surface area and they actually start to roll past each other and have an a less of an opportunity to interact and set up temporary dipoles and again here's the same number of carbons but this is just now one big giant ball of a molecule right it looks like a ball instead of a straight chain these molecules are certainly going to roll past each other having little opportunity to interact and set up dispersion forces so low dispersion forces with high branching High dispersion forces with straight chains and you can see the higher the inter molecular attraction the higher the boiling point butane or pentane hexane all these straight chain carbons can interact with one another through this large surface area and not roll past each other with those balls at the end of the chains now a second intermolecular attraction is known as a dipole dipole interaction two polar molecules interact with each other to create very strong intermolecular attractions this is a molecule kind of written up due to electron charge known as acetone it has a central art uh carbon atom leading out to a double bond oxygen and we can see that the double bond oxygen makes this area very electron rich and the carbon would be electron deficient so positively charged the negative region on one molecule is very attracted to the positive region on the second molecule and these intermolecular attractions based on opposite charges are known as dipole dipole now the difference is these are not temporary charges that's the difference between the dispersion force because it's just so temporary where these are permanent based on the polarity of the structure so polar molecules line up to create dipole dipole interactions notice that these attractions are between the acetone molecule and it's a much stronger interaction than the dispersion force and a third and strongest of all intermolecular attraction is known as hydrogen bonding this is a very special type and very strong type of dipole bipole interaction the hydrogen bonding is so attractive it meets the criteria of being the strongest intermolecular attraction known for example if I have in a molecule an oxygen attached to a hydrogen a nitrogen attached to a hydrogen or a Florine attached to a hydrogen n o or F remember these are the three most electronegative elements if you see those attached to a hydrogen which is not very Electro negative we set up a very strong intermolecular attraction so between molecules here's an O here's an O the hydrogen bond is between those two molecules so the hydrogen of one attracted to the oxygen of the other here's an example of ammonia ammonia has an NH Bond and therefore the lone pair on the nitrogen which is partially negative is very attractive to the hydrogen and this attraction Here is known as a hydrogen bond so this is the strongest inmolecular attraction but it's very special in that the oxygen nitrogen or Florine in the mo Ule must be attracted to a attached to a hydrogen itself so to apply these we could practice by just asking ourselves which has the highest boiling point and really to interpret that we're asking which has the strongest intermolecular forces so to do that I'd like to just remind us what kinds of forces there are so first of all all molecules have dispersion for forces I don't care if they're polar or non-polar all molecules can align themselves to influence the uh electron distribution of the mo of the molecular compound they're the weakest intermolecular attraction but they all have them remember dispersion forces are the only in intermolecular attractions that non-polar molecules have it's the only one nonpolar have this molecule is nonpolar po the central carbon is leading out to four identical methyl groups ch3s that is a completely equal electron distribution around the central atom so the only intermolecular force neopentane contains is a dispersion however in this other molecule called three hexanol 1 2 on the third carbon we see this hydroxy group O we know that this is a polar bond and we know that this is a polar bond making the oxygen very electron Rich the hydrogen electron deficient and the carbon electron deficient those are partial positive signs on the carbon and on the hydrogen so not only does this have dispersion this means that it is a polar molecule so it exhibits dipole dipole interactions and notice also that it meets the criteria for hydrogen bonding it has an o attached to an H so it exhibits hydrogen bonding as well all three intermolecular attractions are present in three hexanol the highest intermolecular attraction creates the highest boiling point so therefore strong IMF means higher boiling point three hexanol has a higher boiling point than neopentane let's try another in this example the central oxygen atom has a molecular geometry of four electron domains two are bonded two are not bonded 422 means that it's a bent molecule so think about that in terms of its geometry bent means that these R groups these carbon groups are really coming down at an angle of about 105° making this a very rich are area of electrons and the carbons themselves are electron deficient so this exhibits di I'm sorry this exhibits dispersion forces because all molecules do and since it is polar it exhibits dipole dipole interactions as well it does not meet the criteria for hydrogen bonding so two of the three are present here in this compound we still have the oxygen bonded to R groups this R group contains three carbons this our group contains three carbons so did these over here so we can see that this exhibit exhibits dispersion and dipole dipole so we need to consider other things for instance the larger the surface area the greater the dispersion force can be to set up notice that this is a straight chain carbon all in a row is a larger surface area for those molecules to interact this is balled up so since it's branched those molecules have less of an opportunity to interact with one another so therefore the higher boiling point is the linear ether that's the functional group here this would be dipropyl ether and therefore this would have the higher boiling point because it has the longer carbon chain on either side of the oxygen now in this example we just talked about uh this particular molecule being an ether so I have a a bent structure around the oxygen making this very electron Rich all molecules have dispersion and this would also have dipole dipole over here this is polar so I know that it has dispersion it has dipole dipole and it also beats the criteria for hydrogen bonding since all three are present these have greater intermolecular attractions and therefore this molecule would have the highest boiling point and that concludes our lessons from chapter 1 it's time to go to work practicing you have your Orion a non-graded practice you have your Wy plus assignment which is a graded assignment and finally your proctored multiple choice quiz keep in mind you have learning assistant sessions to keep practicing with help and I'm only an email away