Bond Parameters and Physical Properties

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Benzene is usually drawn as a ring of six carbons with three single and three double bonds, one after the other. Now, single and double bonds have different lengths. So benzene should have three long bonds and three short bonds, right? But in real benzene, all six carbon carbon bonds have exactly the same length! So how long are they? Keep that question in mind; we will answer it in our first example. Orders of bond length and boiling point come up often in JEE questions, so let's begin.

0:35Bond length: more bonds, shorter bond

So, first, bond length. Here is the carbon carbon single bond, C, dash, C, on screen. Its length is one hundred and fifty four picometres; the letters p m are short for picometres. Now think of a double bond between the same two carbons. Will it be longer or shorter than this single bond? Yes, it is shorter! The double bond, C, two lines, C, is one hundred and thirty four picometres. More bonds between the same two carbons, and they sit closer together. Okay? And the triple bond, C, three lines, C, is the shortest of all: one hundred and twenty picometres. With me? Now remember this line: a shorter bond is a stronger bond. So the triple bond is the shortest, and also the strongest, of these three. Clear? One more fact about strength. The Greek letters on screen are sigma and pi, the two kinds of bond. The pi bond is formed by sideways overlap, so it is weaker than the sigma bond. That is why addition reactions attack the pi bond. Right?

1:41C–H bonds follow the s-character

Now the C H bonds. The figure shows methane with an s p three carbon, ethene with s p two, and ethyne with s p. The bond angles, in degrees, are in orange, and in methane the solid wedge points towards you while the dashed bond goes away from you. The small three and two written high are superscripts, part of the names. The s p three carbon has twenty five percent s-character, and its C H bond is about one hundred and ten picometres. Okay? The s p two carbon has more s-character, thirty three point three percent. So here is my question: as the s-character goes up, does the C H bond get longer or shorter? It gets shorter, very good! The s p carbon has fifty percent s-character, and its C H bond is only about one hundred and six picometres. So the C H bond of s p three is the longest, and of s p the shortest. Right? So, more s-character gives a shorter bond, and a shorter bond is a stronger bond. Got it? And the reason is simple: an s orbital is closer to the nucleus than a p orbital. So as the s-character rises, the hybrid orbital holds its electrons closer to the nucleus, and the bond it forms is shorter and stronger.

2:55Boiling point: compare similar molar masses

Clear? Alright, now the physical properties, starting with boiling point. Take compounds of similar molar mass. Some form hydrogen bonds, some are polar but cannot, and some are non-polar. Which kind do you think boils the highest? Yes, exactly, the compounds with hydrogen bonding! Among them, acids boil higher than alcohols, and alcohols higher than amines. The sign on screen is greater than; here it means boils higher than. Okay? Next come the polar compounds that cannot form hydrogen bonds: aldehydes, ketones and halides. Right? And lowest of all, the non-polar compounds, the alkanes. So, hydrogen bonding on top, then polar, then non-polar. We will use this order in Example two. Clear?

3:43Within a family: molar mass and branching

Now, inside one family, like the alkanes. The boiling point rises with molar mass, so the heavier member boils higher. With me? But look at these two: pentane, and two two dimethyl propane. Both are C five H twelve; the small five and twelve written low are subscripts, the numbers of atoms. Same molar mass, so the same boiling point, right? Oh no, careful, this is the trap! Branching lowers the boiling point. Pentane is a straight chain, and two two dimethyl propane is branched, so pentane boils higher. Got it? And the reason is this: a branched molecule is more compact, so neighbouring molecules have less surface in contact. The double arrow on screen means therefore. Less surface contact, lower boiling point. Okay?

4:29Solubility and dipole moment

Next, solubility in water. Here is the rule: a compound dissolves in water if it can form hydrogen bonds with it, and its hydrocarbon part is small, up to about four carbons per polar group. So a long hydrocarbon chain gets in the way. Right? Now, dipole moment. Each polar bond has a bond moment, with a direction. The dipole moment of the molecule is the vector sum of these bond moments, so the directions matter. Got it? Now carbon tetrachloride, CCL four on screen. It has four polar carbon chlorine bonds. So it must have a dipole moment, right? No, no, wait, that is the trap! Carbon tetrachloride is symmetrical, so its four bond moments cancel. The vector sum is zero, so its dipole moment is zero. Clear? The same holds for other symmetrical molecules: carbon dioxide; CHCL, double bond, CHCL, in its trans form, with the two chlorines on opposite sides; and para dichloro benzene, the small p meaning para. All have zero dipole moment. With me?

5:32Example 1: bond length of ethane, ethene, ethyne, benzene

Now, Example one. Arrange in order of increasing carbon carbon bond length: ethane, ethene, ethyne and benzene. Pause the video and try it first. Done? Okay, let's check. Ethane has a single bond, one hundred and fifty four picometres. Ethene has a double bond, one hundred and thirty four. And ethyne has a triple bond, one hundred and twenty. Now benzene, our puzzle from the start. Where does it fit in? Here is the answer to our puzzle. All six carbon carbon bonds of benzene are one hundred and thirty nine picometres, between a single and a double bond. Longer than the double bond, but shorter than the single bond. Okay? So the increasing order is ethyne, less than ethene, less than benzene, less than ethane; the sign here is less than. And careful: increasing means the shortest comes first. Writing it the other way round is a common slip. Got it?

6:31Example 2: boiling point of propane, dimethyl ether, ethanol

Now a harder one, Example two. Arrange in order of increasing boiling point: propane, dimethyl ether and ethanol. Propane has molar mass forty four, and the other two have forty six each. Pause the video and try it first. Done? Okay, let's check. The molar masses are nearly equal, so mass cannot decide. We use the order we just learnt: hydrogen bonding, then polar, then non-polar. Okay? Propane, C H three, C H two, C H three, has only carbon and hydrogen. It is an alkane, non-polar, so it boils the lowest. Now, dimethyl ether has an oxygen atom, just like ethanol. So does it form hydrogen bonds too? No, it does not, and this is where students slip. Look: C H three, O, C H three. There is no hydrogen on the oxygen, no O H group. So its molecules cannot hydrogen bond with each other, but the ether is polar, so it goes in the middle. Right? And ethanol, C H three, C H two, O H, has the O H group, so its molecules form hydrogen bonds with each other. It boils the highest. Clear? So the answer is propane, less than dimethyl ether, less than ethanol. Notice that the ether and ethanol have the same molar mass, forty six, but ethanol boils higher. Mass alone does not decide; hydrogen bonding does. Got it?

7:54Quick revision: bond length, boiling point, dipole moment

So, quick revision. More bonds, or more s-character, give a shorter, stronger bond. Boiling point: hydrogen bonding, then polar, then non-polar, and branching lowers it. In symmetrical molecules the bond moments cancel, so the dipole moment is zero. Now go and try the practice problems on this page. All the best!