Classification, Naming and Physical Properties

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Ethanol and dimethyl ether have the same molar mass, forty six, so their boiling points should be close, right? But ethanol boils at three hundred and fifty one kelvin, and the ether at only two hundred and forty eight kelvin, a gap of one hundred and three kelvin! So where does that gap come from? Keep that question with you; today we classify alcohols, name them, and see what decides their boiling points and solubility. These orders come up often in JEE, so let's begin.

0:35What counts as an alcohol?

So, what exactly is an alcohol? On screen you see R, dash, O H: the capital R is an alkyl group, the dash is a single bond, and O H is the hydroxyl group. It sits on an s p three carbon; the small raised three is a superscript. Okay? One O H group makes an alcohol monohydric, two make it dihydric, and three make it trihydric. Clear? The famous ones are glycol, ethane one two diol, which is dihydric, and glycerol, propane one two three triol, which is trihydric. With me? Now let me ask you this. Phenol also has a hydroxyl group. So, is phenol an alcohol? No, phenol is not an alcohol in this sense. In phenols, and in enols, the vinylic alcohols, the O H is on an s p two carbon, so they are not counted as alcohols here. Got it?

1:31Primary, secondary and tertiary

Next, let us sort monohydric alcohols into three classes. Look at the carbon bearing the O H, and count the carbons attached to it. One carbon, as in R, C H two, O H, means primary; the small two written low is a subscript. In the brackets, the digit with a small raised circle is a short way to write the class, so primary is written as one degree. Okay? Two carbons, as in R two C H, O H, means secondary, two degree; three carbons, as in R three C, O H, means tertiary, three degree. Clear? And if that carbon is next to a carbon carbon double bond, the C, two lines, C on screen, the alcohol is allylic; if it is next to a benzene ring, it is benzylic. Got it?

2:18IUPAC names

Now, the IUPAC names. Drop the final e of the alkane and add o l, so ethane becomes ethanol; the arrow means becomes. But polyhydric alcohols keep the e, as in ethane one two diol, okay? Number the chain from the end nearer the O H group. Here, C H three, then C H with O H in round brackets, then C H two, then C H three. Counting from the left, the O H hangs from carbon two, so this is butan two ol. Right? Now a small trap. Allyl alcohol, C H two, double bond, C H, C H two, O H, has a double bond and an O H group. Which one gets carbon number one? The O H group does! In numbering, it takes precedence over double bonds and halogens. So the double bond starts at carbon two, and the name is prop two en one ol. Clear?

3:19Structure around the oxygen

Quickly, the structure. The oxygen is s p three hybridised, with two lone pairs. In methanol, the angle COH, marked with the angle sign, is one hundred and eight point nine degrees, the small circle meaning degrees, and the C O bond is one hundred and forty two picometres, written p m. With me?

3:41Boiling points: the hydrogen bond

Now, back to our puzzle. Capital M is the molar mass, and capital K is kelvin. Ethanol and dimethyl ether both have M equal to forty six, yet ethanol boils at three hundred and fifty one kelvin and the ether at two hundred and forty eight. Propane, with M equal to forty four, boils at two hundred and thirty one. So what does ethanol have that the others do not? Yes, exactly, an O H group, and with it, hydrogen bonding! Ethanol molecules are held together by intermolecular hydrogen bonds, and these must be broken before it boils. Okay? The ether has only dipole dipole forces, and propane only van der Waals forces. So hydrogen bonding explains our gap of one hundred and three kelvin, and the puzzle is solved. Clear?

4:30Chain length and branching

Now, two trends from the course table. First, longer chains boil higher: methanol at three hundred and thirty seven kelvin, ethanol at three hundred and fifty one, and butan one ol at three hundred and ninety one. Right? Second, branching. These isomers all share the formula C four H ten O, okay? Primary butan one ol boils at three hundred and ninety one kelvin, and secondary butan two ol at three hundred and seventy two. Will the tertiary isomer boil higher or lower? It boils lower, very good! Two methyl propan two ol, the tertiary one, boils at only three hundred and fifty five kelvin. With me? The course gives the reason: branching reduces the surface area and hinders hydrogen bonding. So for isomers, the boiling point order is primary, greater than secondary, greater than tertiary; those are greater than signs on screen.

5:27Solubility in water

Okay? Now, solubility. The lower alcohols mix with water in all proportions, because they form hydrogen bonds with water. Clear? But as the hydrocarbon part grows, solubility falls; the double arrow means so. Now be careful with this next one. Branching lowered the boiling point. Does it lower the solubility too? No, it does the opposite, and this is the trap! Branching raises solubility, and so do more O H groups. Got it?

5:59Example 1: is each alcohol primary, secondary or tertiary?

Alright, Example one, an easy one from the course. Classify each of these four alcohols as primary, secondary or tertiary. In parts C and D, C H three in round brackets with a small three after them means three methyl groups on one carbon, okay? Pause the video and try it first. Done? Okay, let's check. Find the O H carbon, then count the carbons attached to it. In part A there is just one, so it is primary. With me? In part B, the O H carbon has two C H three groups, so it is secondary; in part C it has three, so it is tertiary. Okay? Now look at part D, which also has three methyl groups on one carbon. So it is tertiary too, right? Oh no, careful, this is the trap! The O H sits on the C H two, and that carbon has only one carbon attached. So part D is primary. Always find the O H carbon first, then count. Clear?

7:03Example 2: decreasing solubility of four alcohols in water

Now Example two, a little harder. Arrange in order of decreasing solubility in water: butan one ol, pentan one ol, ethanol, and two methyl propan two ol. Remember the trap we just saw, and pause the video to try it first. Done? Okay, let's check. Ethanol has the smallest hydrocarbon part, C two, so it is the most soluble; pentan one ol has the largest, C five, so it is the least soluble. Okay? Now the tricky pair. Both butan one ol and two methyl propan two ol have four carbons. Which one dissolves better? The branched one, two methyl propan two ol! Its C four part is compact, and branching raises solubility. If you chose butan one ol, you fell into the boiling point trap. Clear? So the order is ethanol, then two methyl propan two ol, then butan one ol, then pentan one ol, in decreasing solubility.

8:05Quick revision: class, naming, boiling point, solubility

Right? So, quick revision. To classify, count the carbons on the O H carbon; to name, use the o l ending and number from the end nearer the O H group. Hydrogen bonds make boiling points high, and boiling points rise with chain length and fall with branching. Solubility falls as the hydrocarbon part grows, but rises with branching and with more O H groups. Now go and try the practice problems on this page. All the best!