Naming, Structure and Physical Properties

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle. Ethoxyethane and butan one ol have the same molecular formula. But ethoxyethane boils at three hundred and eight kelvin, and butan one ol at three hundred and ninety kelvin, more than eighty kelvin higher! And yet, in water, the two dissolve about equally well. So how can that be? Ether names and properties come up often in JEE, so let's find out!

0:29What is an ether?

So, what is an ether? Two hydrocarbon groups on one oxygen atom: R, O, R prime. Each R is a hydrocarbon group, the dashes are bonds, and the prime, the small mark on the second R, means this group may be different. Okay? If both groups are the same, the ether is simple, or symmetrical, like C two H five O C two H five; the small numbers below the letters count the atoms. If the groups differ, like C H three O C two H five, it is mixed, or unsymmetrical. Right? Ethers have the same general formula as alcohols, C n H two n plus two O, where the small n counts the carbons. So the two are functional isomers: same formula, different functional group. With me? Ethers also show chain and position isomers, and one more kind, metamers, where the groups on the two sides of the oxygen differ, as the brackets say. You will need this word in Example two. Clear?

1:27Naming: common and IUPAC names

Now, names. The common name is easy: name the two groups, then add the word ether. So, for C H three O C two H five, is it methyl ethyl ether, or ethyl methyl ether? It is ethyl methyl ether! The groups go in alphabetical order, and the letter e comes before the letter m. Many students write methyl ethyl ether, so don't fall for that, okay? Second, the IUPAC name, alkoxy alkane. The larger group is the parent alkane, and the smaller group, with the oxygen, becomes the alkoxy group. Right? Here ethyl is the larger group, so the parent is ethane, and methyl with the oxygen becomes methoxy. So the name is methoxyethane. Got it? Now a bigger one. C H three in brackets, with a small three after the bracket, means three methyl groups on one carbon, which holds the oxygen. Its longest chain has three carbons, so the parent is propane, with a methyl and the methoxy both on carbon two. The name is two methoxy two methylpropane, okay? And with a benzene ring, the ring is the parent. Anisole, C six H five OCH three, is methoxybenzene, and you will meet it often. Clear?

2:43Structure: a bent molecule

Now the shape. The oxygen is s p three hybridised, the three written high up, so the molecule is bent. In methoxymethane, the COC angle is one hundred and eleven point seven degrees, the small circle meaning degrees. That is more than the tetrahedral one hundred and nine point five degrees. So why is it bigger? Because the two bulky methyl groups repel each other, so the angle opens up a little. In water, two small hydrogen atoms repel much less, so the HOH angle is only one hundred and four point five degrees. Okay? Now a trap. Diethyl ether has an ethyl group on each side, so it is symmetrical. So its dipole moment must be zero, right? No, no, careful, it is not zero! The molecule is bent, so the two C O bond moments do not cancel. Diethyl ether has a small dipole moment, about one point two debye, the capital D on screen. With me?

3:41Boiling point and solubility

Now, back to our puzzle. For an alcohol, hydrogen bonding between molecules comes from its O H group, a hydrogen joined to an oxygen. So, can two ether molecules hydrogen bond with each other? No, they cannot! An ether has no O H group, so there are no hydrogen bonds, H bonds for short, between its molecules. Clear? So ethers boil low. Ethoxyethane boils at three hundred and eight kelvin, capital K for kelvin, close to pentane, an alkane of similar mass, at three hundred and nine kelvin; the wavy equals sign means about equal. But butan one ol has hydrogen bonds and boils far higher, at three hundred and ninety kelvin, as the less than sign shows. Half the puzzle is solved, okay? Now the other half. If ethers cannot hydrogen bond among themselves, why does ethoxyethane dissolve in water as well as butan one ol does? Because water brings its own hydrogens! The ether oxygen accepts hydrogen bonds from water. So ethoxyethane dissolves to about seven point five grams per one hundred millilitres, g for grams and m L for millilitres, just like butan one ol. Only the alcohol also bonds with itself, so only it boils high. Puzzle solved, got it?

5:03Example 1: naming 2-methoxypropane and ethoxybenzene

Now Example one, a warm up. Give the IUPAC names of C H three OCH, then C H three in brackets with a small two after the bracket, and of C six H five O C two H five. Pause the video and try it first. Done? Okay, let's check. In the first ether, the right side is a C H carrying two methyl groups: three carbons, larger than the methyl on the left. So the parent is propane. Right? Now, where does the methoxy group sit on that propane chain? Is it one methoxypropane, or two methoxypropane? The oxygen is joined to the C H, which is the middle carbon of the chain, carbon two. So the name is two methoxypropane. Clear? In the second ether, C six H five is a benzene ring, so the ring is the parent, as in anisole, and ethyl with the oxygen becomes ethoxy. So it is ethoxybenzene, with the common name phenetole. That was quick!

6:07Example 2: C4H10O ethers, metamers and position isomers

Now Example two, a harder one. Write all the ethers of formula C four H ten O with their IUPAC names, and say what kind of isomers they are of one another. Pause the video and try it first. Done? Okay, let's check. The four carbons are shared across the oxygen: two plus two, or one plus three. Two plus two puts an ethyl group on each side, which is ethoxyethane, from our puzzle. Okay? Now, one carbon on one side, and three on the other. How many different ethers does that give? It gives two of them! The three carbon chain can join the oxygen through an end carbon, as in C H three OCH two C H two C H three, which is one methoxypropane. With me? Or it can join through its middle carbon, which gives two methoxypropane, from Example one. And three plus one is just one plus three seen from the other side, so don't count it twice. So there are exactly three ethers, right? Now the second part. Ethoxyethane and one methoxypropane: what kind of isomers are they? They are metamers, yes, exactly! Same formula, but different groups on the two sides of the oxygen: ethyl and ethyl in one, methyl and propyl in the other. And ethoxyethane and two methoxypropane are metamers too, for the same reason. Clear? And one methoxypropane and two methoxypropane differ only in which carbon of the propane chain holds the oxygen. So they are position isomers, and that is the full answer!

7:44Quick revision: naming, shape, boiling point, solubility

So, quick revision. An ether is ROR prime, named alkoxy alkane, with the larger group as the parent. Its oxygen is s p three, so it is bent, with a COC angle of one hundred and eleven point seven degrees and a small dipole moment. It cannot hydrogen bond with itself, so it boils low, but it hydrogen bonds with water, so it dissolves about as well as an alcohol of similar mass. Now try the practice problems on this page. All the best!