IIT-JEE Questions: Structure and Properties

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. On an old exam paper, there are two names: meta chlorobromobenzene, and meta bromochlorobenzene. Two different names, so two different compounds, right? Maybe even a pair of isomers? Hmm, hold that thought. Today we solve two short questions from old IITJEE papers, from nineteen eighty five and nineteen eighty seven: one on naming and isomers, and one on dipole moment. Each one needs only a few ideas from this chapter. So, let's begin.

0:35Idea 1: naming a halogen compound

So, idea number one is naming. In a halogen compound, the halogen is always written as a prefix: fluoro, chloro, bromo or iodo, in front of the parent hydrocarbon. For a simple halogen compound of benzene, the parent is benzene, as in chlorobenzene. Okay? Now suppose one benzene ring carries both a bromine and a chlorine. Which prefix do we write first, bromo, or chloro? We list them alphabetically, yes, exactly! The letter b comes before the letter c, so bromo is written before chloro. Right? And the numbers, the locants? We number to give the substituents the lowest locants. But if both directions give the same locants, alphabetical order decides once more: the prefix that comes first in the alphabet gets the lower number. So between bromo and chloro, bromo gets the lower number. Clear?

1:31Idea 2: positions on a benzene ring

Now idea number two. Put two groups on a benzene ring. In how many different ways can they sit, relative to each other? Three ways, very good! So a disubstituted benzene has three position isomers. Got it? The small italic letters on screen name them. Small o is ortho, the two groups on carbons one and two, side by side. Small m is meta, on carbons one and three, with one carbon between them. And small p is para, on carbons one and four, opposite each other. Okay? Now, what are isomers? Compounds with the same molecular formula, but different structures. So a pair of isomers must be two different compounds. Remember this; it is the key to our puzzle. With me?

2:20Idea 3: the polar C–Cl bond

Now idea number three: the carbon chlorine bond, written C, dash, C l in the heading. Carbon and chlorine share a pair of electrons. Which end of the bond becomes slightly negative? The chlorine end, yes, exactly! Chlorine is more electronegative than carbon. On screen, delta plus, the Greek letter delta with a small plus written high, sits on carbon: a partial positive charge. And delta minus on chlorine is a partial negative charge. This is the normal polarity of the bond. Okay? The dipole moment measures this polarity. Mu, the Greek letter on screen, equals q times d: the charge separated, times the distance between the charges. Its unit is the debye, the capital D in brackets. So less charge separation, or a shorter bond, means a smaller dipole moment. Clear?

3:10Idea 4: chlorine on an sp² carbon

Now idea number four, in the heading: chlorine on an s p two carbon; the small two written high goes with s p. Compare chlorobenzene, with chlorine on a benzene ring, and chlorocyclohexane, with chlorine on a cyclohexane ring. Which one has the larger dipole moment? It is chlorocyclohexane, at two point two debye! Chlorobenzene is lower, at one point six nine debye; the pointed sign between them means less than. So why is chlorobenzene less polar? There are two reasons. With me? Reason number one is hybridisation. The ring carbon in chlorobenzene is s p two. It is more electronegative than the s p three carbon of chlorocyclohexane, so the carbon chlorine bond is less polar. Right? Reason number two is resonance. Chlorine has lone pairs, and one lone pair is delocalised into the ring. So electron density goes from chlorine back into the ring, against the normal pull of the bond. Okay? This also gives the carbon chlorine bond partial double-bond character, so it is shorter. Less charge separation and a shorter bond: both make the dipole moment smaller. Got it? Now recall the vinylic halides. The halogen sits on a carbon of a C C double bond, the two short lines on screen, and that carbon is also s p two. Keep this in mind for our second question. Clear?

4:34Example 1: m-chlorobromobenzene vs m-bromochlorobenzene

Time for Example one, from the IITJEE paper of nineteen eighty five. True or false: meta chlorobromobenzene is an isomer of meta bromochlorobenzene. The small italic m in front of each name means meta. Pause the video and try it first. Done? Okay, let's check. Meta means the two groups sit on carbons one and three of the ring, with one carbon between them. Okay? Now read the first name: one chlorine and one bromine on a benzene ring, meta to each other. The second name: one bromine and one chlorine on a benzene ring, again meta. On screen, B r is bromine and C l is chlorine. Clear? So we have two different names. Two names, two compounds, so they are isomers, and the answer is true, right? No, no, wait, that is exactly the trap! Use idea number one: write the prefixes alphabetically, bromo first, and give bromo the lower number. Both names become one bromo three chlorobenzene. Same groups, same positions, so the same structure. With me? So it is one compound with two names. Isomers need different structures, so these are not isomers. Clear? So the statement is false: they are the same compound. And our opening puzzle is solved: two different names do not always mean two different compounds. Got it?

6:04Example 2: how resonance lowers vinyl chloride dipole moment

Now the harder one, Example two, from the nineteen eighty seven paper. Vinyl chloride, or chloroethene, can be written as a resonance hybrid of two structures. The first is C H two, double bond, C H, single bond, C l; the small two written low means two hydrogens. The double-headed arrow joins it to the second: C H two with a minus sign written high, single bond, C H, double bond, C l with a plus sign written high. What effect does this resonance have on the dipole moment? Pause the video and try it first. Done? Okay, let's check. Start with the normal polarity: carbon delta plus, chlorine delta minus. So the negative end of the bond is on chlorine. Okay? Now look at the second structure. It has full charges, a plus and a minus, not just partial ones. More charge, so a bigger dipole moment, right? Oh no, careful, look at the sign on chlorine! In the second structure, chlorine carries a positive charge: see the small plus written high on C l, the chlorine. But in the normal bond, chlorine is the negative end. With me? So the second structure opposes the normal polarity of the carbon chlorine bond. It pulls the other way. Right? The real molecule is a hybrid of both structures, so this opposing pull is part of it. The result is that the dipole moment is lowered. That is our answer. Clear? And the data agree. Vinyl chloride has a dipole moment of one point four debye, less than chloroethane, at two point zero debye. Just like chlorobenzene in idea four: chlorine on an s p two carbon, with resonance, gives a smaller dipole moment. Got it?

7:48Quick revision: alphabetical naming and sp2 C-Cl dipole

So, quick revision. If two names give the same structure, it is the same compound, not a pair of isomers. Write the prefixes alphabetically: meta bromochlorobenzene is one bromo three chlorobenzene. And when chlorine sits on an s p two carbon, resonance opposes the normal polarity, delta plus on carbon and delta minus on chlorine, so mu, the dipole moment, is smaller. Now go and try the practice problems of this chapter, and use these ideas there. All the best!