Organic Chemistry - Basic Introduction

in this video we're going to talk about organic chemistry now this video is for those of you who are taking the first semester of organic chemistry in college organic chemistry focuses on organic compounds compounds that contain carbon atoms now carbon likes to form four bonds but you also need to know the number of bonds that the other elements like to form so elements in the first group of the periodic table like hydrogen they like to form one bond beryllium for example likes to form two bonds boron which has three valence electrons it likes to form three bonds carbon has four valence electrons carbon likes to form four bonds nitrogen nitrogen likes to form three bonds in organic chemistry oxygen likes to form two and elements like fluorine and the other halogens chlorine bromine iodine they like to form one bond now there are cases where chlorine bromine and iodine conform seven bonds but we're really not going to cover that in this video i do have another video on title lewis structures which goes into that detail for those of you who might be curious now understanding that this is important because it helps us to draw lewis structures so for instance let's say if we want to draw the lewis structure of water h2o knowing that oxygen likes to form two bonds and hydrogen likes to form one we can start with a structure that looks like this now oxygen likes to have eight electrons each bond represents two electrons so that's a total of four to get to eight we need to add two lone pairs so that is the lewis structure for water now the bond between hydrogen and oxygen is known as an h-bond or a hydrogen bond which is a special type of covalent bond hydrogen bonds occur whenever h is directly attached to nitrogen oxygen or fluorine hydrogen bonding explains why water has such a high boiling point now let's say if we have methyl fluoride how can we draw the lewis structure for that molecule so we know that carbon likes to form four bonds so we're going to put that in the middle hydrogen likes to form one bond fluorine also likes to form one bond so this is the best lewis structure that we can make and if you want you can add three lone pairs to fluorine so that it has eight electrons around it elements in the second row like carbon nitrogen oxygen and fluorine they like to have eight electrons they don't always have to have eight but ideally speaking they like to have eight so that they can be stable but that's the structure of methyl fluoride now the carbon fluorine bond is a polar bond carbon has an electronegativity value of 2. 5 and the electronegativity value of fluorine is 4. 0 whenever the electronegativity difference between two elements is 0.

5 or more the bond is said to be polar so this is a polar bond it's not a hydrogen bond because fluorine is not directly attached to hydrogen but it's a polar covalent bond now you might be wondering what does it mean for a bond to be polar a polarized object is an object that is neutral overall but has charge separation that is one side is positive and the other side is negative so this is a polarized object the bond is polar because fluorine carries a partial negative charge due to the fact that it's more electronegative than carbon so it pulls electrons toward itself and carbon being less electronegative has a partial positive charge so due to that charge separation we can say that the carbon fluorine bond is polar now the carbon hydrogen bond that's a non-polar bond the reason being is the electronegativity difference between carbon and hydrogen is less than 0. 5 carbon we said its electronegativity value is 2. 5 hydrogen is 2.

1 so the difference between the two is 0. 4 so understand this anytime you see a hydrocarbon or a bond that contains carbon and hydrogen those bonds are non-polar bonds because sometimes you need to determine if a molecule is polar or nonpolar so for instance methane ch4 that's a non-polar molecule because it only contains carbon and hydrogen so far we've considered two types of covalent bonds polar covalent bonds and another special type which are hydrogen bonds and we also talked about nonpolar covalent bonds like the carbon hydrogen bond that's a non-polar covalent bond we need to understand the difference between a covalent bond and an ionic bond in a covalent bond the electrons are being shared they can be shared equally or unequally so in the case of the hydrogen molecule h2 the electrons in this bond are being shared equally between these two atoms because the atoms are identical so that's known as a nonpolar covalent bond which i'll put cb for covalent bond in the case of hydrogen fluoride that is a polar covalent bond hydrogen has an electronegativity of 2. 1 fluorine has a value of 4.

0 so hydrogen is going to be partially positive fluorine will be partially negative and this is a special type of a polar covalent bond that's a hydrogen bond so those are some two examples of covalent bond you have non-polar covalent bonds and polar covalent bonds so make sure you understand the difference here the electrons are shared equally as the key word and in this case the electrons are shared unequally fluorine is going to have a tighter hold on those electrons now let's talk about ionic bonds so in an ionic bond the electrons are not shared they're transferred a good example is the reaction between sodium metal and chlorine sodium has one valence electron chlorine has seven sodium is a metal and chlorine is a non-metal metals like to give away the electrons because they're electropositive nonmetals like to accept or acquire electrons because they're electronegative so sodium is going to give one electron to chlorine a half arrow represents the flow of one electron a full arrow represents the flow of two electrons so in this reaction sodium is going to turn into the sodium plus ion which is known as a cation positively charged ions are called cations now chlorine when it acquires an electron it becomes chloride so now it has eight electrons it's gonna now have a negative charge negatively charged ions are known as anions now going back to physics you know that opposite charges attract one another so a positively charged ion will feel a force of attraction to a negatively charged ion so these two charges they're attracted to each other and that force of attraction that electrostatic force is what keeps the ions in an ionic crystal together so that creates that ionic bond the electrostatic force of attraction that pulls the sodium and the chloride ions together so now you know the difference between an ionic bond and a covalent bond so now we're going to spend some time drawing lewis structures of certain organic compounds but before we do let's talk about some common names of alkanes alkanes are saturated organic compounds meaning that the carbon atoms are filled with hydrogen atoms methane is a one carbon alkane it's m e t h a n e a two carbon alkane c two h six this is known as ethane c3h8 this is called propane c4 h10 this is known as butane so alkanes generally follow this formula c n h two n plus two so a five carbon alkane will have twelve hydrogen atoms if n is five then it's going to be h two times five plus two so that's ten plus two you get twelve a five carbon alkane is known as pentane a six carbon alkane is known as hexane you may want to take some notes by the way because you'll need to know this at least up to 10. the seven carbon alkane that's heptane next we have octane and then after that this is that's a 20. this is not a or if you want to call it no name and then c10h22 that's known as decade now let's talk about how to draw the lewis structure of c2h6 c2h6 we know that's ethane and you can write the condensed structure like this it's ch3ch3 so this tells you how many hydrogen atoms are on each carbon first carbon has three hydrogen atoms and that's attached to the second carbon which also has three hydrogen atoms so that's how you can draw the lewis structure of ethane now what about c2h4 how would you draw the lewis structure for that so this time each carbon atom is going to have two hydrogen atoms instead of three because there's a total of four and you want to make sure that the four hydrogen atoms are distributed equally so what bond do we need between the two carbon atoms well we know that carbon likes to form four bonds so the only way this is going to happen is if we put a double bond between the two carbon atoms and so this is known as an alkene alkenes contain at least one double bond alkanes do not contain double bonds so a two carbon alkene is known as ethene now what about this one c2h2 how can we draw the lewis structure for that well we have a total of two hydrogen atoms so we're going to put one hydrogen atom on each carbon and in order for the two carbon atoms to have four bonds we need to put a triple bond in the middle because carbon likes to form four bonds and so whenever you have a hydrocarbon with a triple bond you now have what is known as an alkyne a two carbon alkyne is known as ethyne the common name for this is acetylene and the common name for this compound is known as ethylene alkenes and alkynes are known as unsaturated compounds because they don't contain the maximum number of hydrogen atoms per carbon atom alkanes are known as saturated compounds so make sure you keep this in mind now let's focus on the carbon-carbon bonds what would you say which of these bonds is the longest bond the carbon-carbon single bond the double bond or the carbon-carbon triple bond which one is the longest and which one is the shortest you need to know that the carbon carbon single bond is longer than the carbon-carbon double bond and that's longer than the carbon-carbon triple bond the length of the carbon-carbon single bond is 154 picometers which is 1.

54 angstroms one angstrom is 100 picometers the cc double bond is 133 picometers in ethene and in ethyne i mean ethyne it's uh 120. so we need to understand are is that triple bonds are short bonds whereas single bonds are long bonds because sometimes you may get a test question that asks you something about bond length which of these bonds is the shortest or which of these bonds is the longest so single bonds are longer than triple bonds keep that in mind so now that we've talked about bottom left let's talk about bond strength which bond is the strongest the single bond the double bond or the triple bond what would you say the single bond is the weakest the triple bond is the strongest why is that well it's easier to break one bond instead of three bonds imagine trying to break a pencil it's easier breaking one pencil than trying to break three pencils at a time three bonds are stronger than one now let's talk about sigma and pi bonds a single bond contains one sigma bond all single bonds are sigma bonds a double bond contains one sigma and one pi bond the triple bond contains one sigma and two pi bonds now which bond is stronger a sigma bond or a pi bond you need to know that sigma bonds are stronger than pi bonds so it's harder to break this bond versus just one of the pi bonds in the triple bond not all three bonds but one of the pi bonds so in summary a triple bond is stronger than a single bond because you're comparing three bonds to one however a sigma bond is stronger than a pi bond when you're comparing one bond with one bond so i'm just going to say that one more time sigma bonds are stronger than pi bonds but triple bonds are stronger than single bonds now the next thing we're going to talk about is bond order is the bond order for a single bond a double bond and a triple bond this one is pretty straightforward for a single bond the bond order is one for a double bond the bond order is 2 and for triple bond the bond order is 3. so that's just something to know now let's talk about hybridization what is the hybridization of the carbon atoms highlighted in green would you say it's s sp2 sp3 sp dsp3 d2 sp3 what would you say a quick and simple way to determine the hybridization around a certain carbon atom is to count the number of atoms attached to that particular carbon atom and the number of lone pairs that it has so this particular carbon atom is attached to four other atoms so you could think of it as having four groups around it so the hybridization is going to be s1 p3 because the exponents add up to four now let's say if we want to determine the hybridization of this carbon atom that carbon is attached to three other atoms so it has three groups around it the hybridization is going to be s1 p2 or sp2 hybridized and then for this particular alkyne the carbon atom has two atoms attached to it so for two groups it's going to be sp hybridized so that's a quick and simple way to determine the hybridization of a carbon atom here's a question for you so let's say we have a lewis structure that looks like this what is the hybridization of not the atoms but the bond what is the hybridization of the ch bond and also of this particular ch bond what would you say if you were to get a test question that asks you that question how would you determine the hybridizations of this bond highlighted in red and this bond highlighted in blue what you need to do is determine the hybridization of the atoms that are connected to those bonds so what is the hybridization of this carbon atom that carbon is attached to four other atoms so it has four groups and so it's going to be sp3 hybridized now you need to look at the other atom the hydrogen hydrogen is only attached to one atom so what do you think the hybridization of hydrogen is going to be this is going to be s so that's the hybridization of the ch bond that's going to be sp3 dash s you simply just write both of these together now what about the ch bond highlighted in blue what is the hybridization of that ch bond feel free to pause the video if you want to try it so let's start with the carbon atom that carbon is attached to two other atoms so it's going to be sp hybridized and hydrogen is always s hybridized so that particular ch bond is going to be sp dash s hybridized now another question that you might be asking given this compound is how many sigma and pi bonds are in this compound feel free to pause the video and try so first let's count the number of sigma bonds every single bond is a sigma bond so we have one two three four five and there's one sigma bond in the triple bond so we have a total of six sigma bonds now how many pi bonds do we have we know that a double bond contains one pi bond a triple bond contains two pi bonds so this molecule have six sigma bonds and two pi bonds so that's how you can determine the number of sigma and pi bonds in an organic compound now the next topic of discussion is how to calculate the formal charge of an element so let's use carbon as an example calculate the formal charge of each carbon atom for these three situations if you know how to do it feel free to go ahead and try now there's a formula that will help you to calculate the formal charge of an atom and here it is the formal charge is going to be equal to the number of valence electrons of the element minus the number of bonds and dots attached to that element so for the first example carbon has four bonds i mean let me say it again carbon has four valence electrons it's in group 4a of the periodic table and in this example it has 3 bonds 1 2 3 and it doesn't have any dots around it so 4 minus 3 is 1.