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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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c3h8 this is called propane
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c4 h10 this is known as butane so alkanes generally follow this formula c n h two n plus two
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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
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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
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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
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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
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do not contain double bonds so a two carbon alkene is known as ethene
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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
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and the common name for this compound is known as ethylene
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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
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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
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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
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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
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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
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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
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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
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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
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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
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here's a question for you so let's say we have a lewis structure that looks like this
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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
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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
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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
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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
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to calculate the formal charge of an element so let's use carbon as an example
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calculate the formal charge of each carbon atom for these three situations
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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
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and it doesn't have any dots around it so 4 minus 3 is 1. so this particular carbon atom has a formal charge of plus one so we could say that it has a positive formal charge now moving on to the next one it's going to be four valence electrons minus three bonds and one dot so we got three bonds one dot that's four minus four so that's zero so this one is neutral it doesn't have a charge now for the last example on the right the valence electrons of carbon will still be the same
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we still have three bonds but this time we have two dots one lone pair is equivalent to two dots so three plus two is five four minus five is negative one so this particular carbon atom has a negative charge we said that positive let me say that again we said that positively charged ions are called cations and negatively charged ions are known as anion when you add a carbon to it this is called a carbocation and on the right this is called a carb anion carb anions have negative charges
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now when you have an odd number of electrons you have what is known as a radical so a one carbon radical is known as a methyl radical radicals tend to be neutral now let's work on some more examples go ahead and calculate the formal charge of the sulfur atom and also calculate the formal charge of the nitrogen atom go ahead and take a minute to work on that example for those two examples rather by the way for those of you who are looking for a specific topic but for
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some reason you're not finding it in this video when you get a chance do a youtube search type in organic chemistry playlist i have two playlists an older one and a newer one the newer one is going to be more helpful so in that playlist chances are you you could find a topic that you might be looking for so just feel free to take a look at that as well i'll be posting some links in the description section below of this video with some other topics that might help you with the first
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exam that you might be taking in organic chemistry so feel free to check that out as well so let's begin let's start with sulfur let's write the formula so the formal charge is going to be the number of valence electrons minus the sum of the bonds and dots around that element now how many valence electrons does sulfur have sulfur is found in group 6a of the periodic table if you don't have one you could do a google search you could find it in google images and then you could pull up
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the periodic table if you look for sulfur it's right under oxygen and it's in group 6a which means that sulfur has six valence electrons in this structure it only has one bond attached to it now how many dots does it have well we have three lone pairs which is equivalent to six dots so this is six minus seven so the sulfur atom has a negative one formal charge now for nitrogen in the ammonium ion nitrogen has five valence electrons it's in group 5a of the periodic table in
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this structure it has four bonds no dots so 5 minus four is one so this particular nitrogen atom has a plus one or some may say a one plus formal charge so that's how you could determine the formal charge of an element now let's go back to this structure because there's another question that i'm going to ask that teaches another concept how many bonding electrons and nonbonding electrons are present in this ion what would you say
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a lone pair
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represents a pair of non-bonding electrons because they're only attached to one atom so they're non-bonding electrons in a bond you have two bonding electrons
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so to count the number of bonding electrons this is going to be 2 4 6 eight so this particular ion has a total of eight bonding electrons because it has four bonds each bond contains two bonding electrons now how many nonbonding electrons do we have two four six there's a total of six nonbonding electrons which equates to three lone pairs so make sure you understand that constant one lone pair is equal to two nonbonding electrons and one bond equates to two bonding electrons
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now let's continue our discussion of drawing the lewis structures of organic compounds as well as considering the functional groups of those compounds so let's consider ch3ch2 oh and let's compare that with ch3 cho so feel free to pause the video and go ahead and draw the two lewis structures so for the one on the left we have a carbon that is connected to three hydrogen atoms and then it's attached to a ch2 that is a carbon with two hydrogen atoms and that's connected to an o h group
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keep in mind oxygen likes to have two bonds and it's going to have two lone pairs so that's the lewis structure for this molecule now whenever you see an oh group this is the functional group of an alcohol
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and the fact that we have two carbons this is associated with the alkane ethane but because we have an alcohol we can name it ethanol so alcohols have the suffix o-l so this is ethanol
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now what about the structure on the right so we have a methyl group that is a ch3 and that is attached to a carbon atom now it's cho instead of o-h when you see c-h-o this is the functional group of an aldehyde if we were to write o-h this is not going to work because carbon doesn't have four bonds so that tells you that we need a different structure an aldehyde contains a carbonyl group which is basically a carbon double bonded to an oxygen and then here's the hydrogen so in this
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case every carbon atom has four bonds every hydrogen atom has one bond and the oxygen atom has two bonds which is what they usually have in neutral molecules and of course the oxygen is going to have two lone pairs as it did here so this is a carbonyl and this particular carbonyl which is attached to a hydrogen that is known as an aldehyde
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so that's the name of the functional group a two carbon aldehyde is called ethanol instead of ethanol so it ends in a l as opposed to ol the common name for this aldehyde is acetaldehyde
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now let's consider these two examples ch3och3 and ch3 co ch3 notice the difference so go ahead and draw the lewis structure for these two organic compounds for the first one we have a methyl group that's a carbon with three hydrogen atoms and then we have an oxygen in the middle as we said before oxygen likes to form two bonds and then another method group to the right of that so this particular compound has a functional group known as an ether an ether is basically
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an oxygen atom between two carbon atoms that's an ether now because this particular ether has two methyl groups one to the left one to the right this is known as a dimethyl ether the prefix die means two tri means three tetra means four penta means five now how would you draw the lewis structure for the compound on the right so we know that we have a methyl group on the left and then there's a carbon atom now where do we put the oxygen atom if we put it in the middle
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this carbon atom is going to have two bonds so to fix that problem we need to put it up here we need to make another carbonyl functional group and then write the methyl group so in this case every carbon atom has four bonds the oxygen atom has two bonds and every hydrogen atom has one bond that's why it's important to know the number of bonds each element like to make because it makes drawing the lewis structures a lot easier now even though we have a carbonyl functional group
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the name of the functional group for this particular molecule is different than an aldehyde this is known as a ketone when the carbonyl group is in the middle of the chain is the ketone when it's at the end of a chain it's an aldehyde so to name this particular ketone it's called propanone think of propane for a three carbon alkane so a three carbon ketone is going to be propanone ketones have the suffix o and e so notice the difference between a ketone and aldehyde
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a ketone will have the carbonyl group in the middle and aldehyde will have the carbonyl group at the end
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so an aldehyde has a hydrogen that's attached to the carbonyl group a ketone does not have the hydrogen attached to the carbonyl group and that's how you could distinguish a ketone from an aldehyde even though they both have the carbonyl functional group now here's the next one ch3 co2 ch3 versus ch3 and then ch2 times 3 co2h go ahead and draw the lewis structures for those two molecules so i'm not going to completely draw it out i'm going to expand the condensed structures
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so instead of writing c with three hydrogens i'm going to leave it as ch3 i'm going to expand this part it's not going to be like this because carbon doesn't have four bonds instead the ch3 is going to be attached to a carbon we're going to have a carbonyl group and then an oxygen attached to a ch3 so this functional group is known as an ester an ester has two oxygen atoms and it has a carbonyl group on the right we have a ch3 that's a methyl group and then we have three methylene groups
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so ch2 ch2 ch2 and then we have co2h which you can write as cooh but we're going to expand that further so we're going to have a carbonyl group and the hydroxyl group so when you combine a carbonyl group and a hydroxyl group like that you get a functional group called a carboxylic acid
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now that particular carboxylic acid has a total of five carbon atoms a five carbon alkane is known as hexane so a five carbon carboxylic acid is going to be called wait let me take that back a five carbon alkane is not hexane but pentane hexane has six carbons so a five carbon carboxylic acid is going to be called pentanoic acid so that's how you can name that particular carboxylic acid now as for the ester we have a methyl group
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and then here we have a two carbon group with two oxygens which is called ethanoate so combined this is called methyl ethyno8 if you want more examples on how to name esters just type in ester nomenclature in the youtube search bar and you'll see some videos come up that has more examples on how to name esters but that's how you can name this particular one it's called methyl ethanoate now here's a challenge problem go ahead and expand this particular structure
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you don't have to worry about naming it but go ahead and expand it what i would do is i would start with the ch group there so we have a carbon attached to a hydrogen and then that carbon has two methyl groups attached to it so we have one over here and then another one on top so this ch is the ch that we see here and then to the right there is another ch next to it so i'm going to put carbon with a hydrogen so that's the ch of this group and then attached to that is an oh which we can put here
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and then a ch3 the ch trees or the methyl groups they're always at the end of a structure never in the middle the ch groups tend to branch out and ch2 groups tend to be in the middle of a structure so let me give you another example so let's say we have ch3 times 3 and then the carbon and then a ch with a br and then a ch2 and then another ch2 times two followed by a ch string go ahead and expand that structure so for this one i'm going to start with a carbon atom
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that carbon has three methyl groups connected to it so here's the first one here is the second one and here is the third one next we have a ch group so i'm going to put a carbon and then the hydrogen and then there's a bromine atom which is going to be attached to this carbon because carbon likes to form four bonds next we have a ch2 group which tends to be in the middle so that's a carbon with two hydrogen atoms and then we have two more ch2 groups here's the first one
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and here is the second one and then we have a ch3 group at the end so that's how you can quickly expand a structure that looks like that so keep in mind the methyl groups are at the end of a structure like this the ch2 groups they're in the middle and the ch groups tend to branch off they're also in the middle but they tend to have other special elements attached to it like a bromine or an oh group
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