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Current time:0:00Total duration:11:17

Video transcript

let's start with physical properties of alcohols and so we're going to compare in this case alcohols to alkanes in this alkane on the left here two carbons so this is of course ethane on the right if we take off one of those hydrogen's and replace it with an O H right we of course have ethanol right here so let's let's start with boiling point so the boiling point of ethane is approximately negative 89 degrees Celsius and since room temperature is somewhere around 20 to 25 degrees Celsius we're at room temperature we are much higher than the boiling point of ethane which means it's already it's already boil it's already turned into a gas so at room temperature and room pressure ethane is a gas ethanol however has a much higher boiling point so we're around 78 degrees Celsius and once again sits room temperature some around 20 to 25 the boiling point of ethanol is much higher it is much higher than room temperature so at room temperature and pressure ethanol is a liquid it hasn't boiled yet and these large differences in boiling points between these two molecules can be attributed to the intermolecular forces that are present so if two molecules of ethane are interacting really the only intermolecular force is holding those molecules together be London dispersion forces which are the weakest of the intermolecular forces so it's relatively easy to pull apart two ethane molecules and that accounts for the very low boiling point doesn't take a lot of energy to pull them apart so it's easy for it to turn into a guess ethanol however is a much higher boiling point which means it's much harder to pull those molecules apart it takes more energy so let's look at why why ethanol has such a higher boiling point so if I show two ethanol molecules interacting right so here is one ethanol molecule and let's go ahead and draw another ethanol molecule right here and if I think about the oxygen hydrogen bond I know that's a polarized covalent bond I know that there's a large difference in electronegativity between the oxygen and the hydrogen oxygen is much more electronegative which means the electrons in the bond between oxygen and hydrogen are going to be much closer to the oxygen atom giving the oxygen atom a partial negative charge so these electrons in this bond between oxygen hydrogen are going to move away from the hydrogen hydrogen is going to lose a little bit of electron density leaving it relatively positive so we give it a partial positive charge the same thing for the other ethanol molecule right partially negative oxygen partially positive hydrogen and we know that opposite charges trapped right so the partially positive hydrogen is attracted to the partially negative oxygen and so there's a strong intermolecular force that holds those two molecules together and that of course is hydrogen bonding so there's some hydrogen bonding so there's hydrogen bonding between alcohol molecules so hydrogen bonding and since hydrogen bonding is the strongest intermolecular force it's relatively difficult to pull those molecules apart it takes a lot of energy takes a lot of heat and that's why the boiling point of ethanol is so much higher than the boiling point of ethane and also counts for the state of matter what about solubility so is ethanol soluble in water and of course it is and the reason why is is a hydrogen bonding once again so if we if we draw a water molecule in here all right I know that the water molecule is polarized in the same way that the alcohol molecule is right so the hydrogen is partially positive and the oxygen is right over here is partially negative and so once again opposite charges attract the hydrogen is attracted to this oxygen here and so and so because of hydrogen bonding right there's interaction between the water molecule between in between the alcohol molecule so the water molecule is polar all right so if you want to think about it in terms of polarity right because of the difference in electronegativity water is a polar molecule ethanol is a is a polar molecule and like dissolves like so these two molecules will be soluble in each other so if I look at the structure of ethanol right the reason why it is soluble in water is because of this portion of the molecule this this hydroxyl group this Oh H it's the differences in electronegativity that allow the hydrogen bonding so this portion of the molecule is the polar portion of the molecule and this portion of the molecule is the part that loves water which is why it is soluble so if it loves water we say it's hydrophilic hydrometer water fill meaning love so hydrophilic whereas this portion over here on the left alright this is more of an alkane type environment a non-polar type of environment so this part of the molecule is scared of water so it's hydrophobic so we have the hydrophobic portion of our alcohol molecule we have the hydrophilic portion of the alcohol molecule now we know that like dissolves like so nonpolar will not dissolve in polar but as long as we have in a relatively small number of carbon atoms in our alkyl group the O H group is is enough is polar enough for the alcohol to be soluble in water now if you have a large number of carbon atoms you your molecule is more nonpolar than polar and so alcohols will stop being soluble in water if they have a lot of carbon atoms on them so let's let's look at now the the preparation of alkoxides so let's let's look at an alcohol alright so here we have our alcohol and if we react our alcohol with a strong base so we'll give it a lone pair of electron to negative one formal charge alright so we have a strong base here and our strong base is going to take this proton and leave these electrons behind on this oxygen alright so now we have now we have an oxygen that used to have two lone pairs electrons now has three lone pairs that gives it a negative one formal charge and the base alright the base is going to form a bond alright with that proton like that so this is an acid-base reaction so if we react an alcohol with a strong base so this is a strong base here we're going to form the conjugate base to an alcohol which is called an alkoxide so this is an alkoxide ion right here so it's a chemical property of alcohols they they are acidic if you use a strong enough base and the conjugate base to an alcohol is called an alkoxide let's look at an example alright so let's take an awl so here I have my ethical molecule and we'll react that with a strong base something like sodium hydride so na H so na plus and H with two hydrogen with two electrons around it which makes it a negatively charged ion so that's called the hydride anion so we have the basic portion right the negatively charged hydrogen it's going to function as a base it's going to take these two electrons it's going to take that proton right there so the acidic proton on alcohols is the one on the oxygen and the electrons in here are going to kick off onto our oxygen like that so we're going to get for our product and alkoxide with three lone pairs of electrons around it giving it a negative 1 formal charge the sodium is floating around right positively charged so it's going to electrostatically ionically interact with our alkoxide anion and the the hydride anion picked up a proton so those two hydrogen's combine to form hydrogen gas which will of course bubble out of your solutions though so the formation of hydrogen gas will be observed in this reaction and this is how you form an alkoxide this this molecule is called sodium ethoxide right so we have sodium ethoxide over here on the right sodium ethoxide which is a relatively strong base that is used in a lot of organic chemistry reactions and let's see we use a strong base to form it we used a sodium hydride over here sodium hydride to form that molecule from ethanol so there's another way to form alkoxides so let's take a look at a way to form alcohols from Group one alkali metals so here we have our alcohol like that and if we react our alcohol with a group one metal so an alkali metal those all have one valence electron being in Group one on the periodic table so something like lithium or sodium or potassium we are going to form an alkoxide so we're going to form let's see three lone pairs of electrons and negative one formal charge and the mechanism the metal is going to donate it's one valence electron leaving it with a plus one charge so it's going to interact with your alkoxide like that and this also releases hydrogen gas like that so that's your general reaction let's look at an example where we react let's use let's use cyclohexanol so we're going to react cyclo hexanol with sodium so let's actually let's go ahead and redraw that cyclohexanol molecule here because i want to show a little bit of the mechanism so let's go ahead and draw it like that and put our lone pairs of electrons on the oxygen so sodium metal has 1 valence electron like that so if we think about what happens right sodium is is sodium will donate its one valence electron very easily because it will then have the stable electron configuration of a noble gas so the first step in the mechanism is donation of this one valence electrons we're going to show the movement of one electron right so we're going to use 1/2 headed arrow and then the two electrons in the bond between oxygen and hydrogen we're going to use a two-headed arrow to shoot to show the movement of those two electrons off on to that oxygen so let's go ahead and draw what we have now we have our cyclohexane ring and we now have three lone pairs of electrons around my oxygen which makes it negatively charged and the sodium right donated it's one valence electron so now it has a plus one charge so it's going to interact with that negatively charged oxygen and what happened to the hydrogen right that hydrogen right that hydrogen there is going to pick up one electron so now we have hydrogen with one electron around it that that is extremely reactive right hydrogen prefers to have two electrons around it so when two of those hydrogen atoms right get get close to each other right they're going to course react and share their electrons to form hydrogen gas so I could not get see I could draw it like that right so that's where those two electrons are right the two electrons are here so each right one from one hydrogen one from the other hydrogen so hydrogen gas is going to be produced in this reaction as well so that's an overview of physical properties and also and also the preparation of alkoxide anions