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For more information, please see full course syllabus of Organic Chemistry Lab
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Lecture Comments (2)

1 answer

Last reply by: Professor Starkey
Sun Jul 26, 2015 9:10 AM

Post by Akilah Futch on July 23, 2015

Why would a melting point of an isomer be way off?
For instance, comparing melting points of ortho,meta,para bromonitrobenzene, ortho and meta melting points are much closer together in range vs para bromonitrobenzene. What could be causing or what would be an explanation to this?

Melting Point Lab

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Melting Point Tubes 0:40
  • Melting Point Apparatus 3:42
  • Recording a melting Point 5:50

Transcription: Melting Point Lab

Hi, welcome back to

Today, we are going to be talking about taking melting point, which is a great way to analyze a solid.0002

You might be using it to identify an unknown solid.0007

You might be using it to confirm the purity of solid that you have isolated and purified.0012

Melting point, obviously, is a physical property of a solid compound.0019

The more pure the solid is, the sharper the melting point is going to be.0024

That is why it makes a great tool to not only tell us whether or not the compound matches the literature value0029

so that helps with the identification of compound, but how pure your particular sample is, if I know the ranges of the melting point.0035

In order to prepare a melting point tube, we need to think about the different kinds of tubes we have.0043

In this one joint I have what are called capillary tubes.0052

Capillary tubes are very narrow tubes that are open on both ends.0055

We can use these to make TLC spotters or to draw liquids out but these are not melting point tubes.0059

Melting point tubes are capillary tubes that are sealed on one end.0065

They look like itty-bitty test tubes but are open on one end and sealed at the other.0070

What we want to do is we want to load the melting point tubes0076

so that our solid sample is packed tightly at the bottom, at the closed end, at the height of 1 – 3ml.0080

If you do not know what 1-3ml looks like, you should get a ruler and you should hold it up and you should measure it.0089

Once you know what a couple of ml looks like, then in the future you can go off your judgment.0097

Go ahead and measure it first to figure that out.0104

What we need, let us say on this watch glass I have my solid that, maybe I have just done the recrystallization,0107

I have just purified my compound, or I have isolated a solid, or I have one that I need to analyze.0113

I want to get some of that into my melting point tube.0119

The first thing I have to do is I have to make sure that this is a powder.0122

If it is not already a powder, a lot of times they are crystalline.0125

I have some nice crystals there.0129

What we want to do is grind that crystal up into a powder.0131

Take a small portion of your sample, represent a portion, and grind it up until it is a fine powder.0135

Because it is that fine powder, we need to take the melting point.0141

What we are going to do is we are going to scoop up just a little tiny bit of that powder into the top,0145

the open end of the melting point tube.0149

We can just tap it on the counter to get the solid to move from the open end all the way down to the closed end.0152

If you have jammed a lot of it up into the top and it is so packed up here, it is never going to move down.0161

You should throw out the glass tube and just get a new one and try.0167

You want to scoop out just the tiniest of that bit, tap it down to the bottom, watch it move.0171

If you are tapping it directly on the end, it is quite strong and it can withstand that tapping.0175

We watch it go to the end and then take a look.0181

If you are not at a couple of ml, scoop up a little more, tap and move it down,0183

until finally you have a sample size at the bottom that is between 1 and 3 ml and that is packed tightly.0188

We do not want air pockets in there because that is going to not give us even heating.0194

As we heat our samples, we want it nice and tightly packed and at the proper height.0199

If it is less than a ml, you cannot see the sample very well to make your analysis.0205

If it is more than 3ml, you are going to have a problem with uneven heating.0211

You are not going to get an accurate measurement there.0215

That is why it needs to be both tightly packed and somewhere around just a couple of ml.0217

What I have here is an example of melting point apparatus and it got a couple of things.0223

It got a heating block, it is going to heat our sample.0229

I slide this tin, it got a little slot to hold the melting point tubes.0232

I put the closed end down with the sample in it.0236

It got a heating block that the sample is now resting next to.0241

It got a thermometer to tell me what temperature the unit is at.0247

When I hit the power switch, it also turns on the light.0253

It has a little magnifying glass so I can look in this window and I can see my sample.0256

It is going to be magnified so I can see these crystals very clearly.0260

It also has a dial here to control how quickly I’m heating the sample.0264

This controls the rate of the temperature of heating.0268

You do not want to be heating it too quickly because if you are heating it too quickly,0272

you are going to have heat lag between what the melting point apparatus temperature is and what the crystals really are.0278

If it is heating too fast, then you are going to have a lag and your crystals are not going to be as hot as the thermometer is.0285

You need time, as it rises slowly, it equilibrates as it rises and you are going to get a much more accurate reading there.0291

Another problem with it heating too quickly, not only the heat lag but also you,0299

when you observe the melting, you have to step up and read the temperature.0305

If by the time you look up here and the temperature has already changed,0310

you are liable to record inaccurate temperature.0312

You want to heat it slowly, if you know the range of your melting point, approximately where it is going to melt,0316

you can heat it a little faster to get close to it, and then more slowly.0323

If you have no idea what your sample is, you can even do a quick melting point that is fast heating,0328

to get a ballpark range of where it is going to be.0333

And then, you can take a more careful approach with the second melting point to get a more accurate one.0336

This dial is going to tell you -- the reason there is a dial is0344

sometimes you do want to take a fast melting point, but most of the time you want it to be slower.0346

We are going to be watching our crystals and there are two things that we are going to be watching for,0351

there are two temperatures that we are going to be recording.0355

We are going to make sure that we always have a pen in hand and our notebook here,0357

so we are ready to record the data, as we are taking it.0360

We do not take little scraps of paper, paper towel, or something and then transfer it later.0362

This is your data, you need to record it live.0367

I’m going to be watching as it is heating up.0369

When we are watching the crystals, what I’m going to look for is the first sign of melting.0373

This is when your crystals start to look wet, significant amount of wetness.0379

You might notice that the crystals shrink a little bit, that is called centering.0385

That is not melting, we ignore that.0389

They are moving a little bit but you want to see when it starts to look wet, that is melting.0394

We are going to record that temperature, that is the first temperature of our melting point range.0399

We are going to continue watching our crystals.0404

Bit by bit, you are going to see the rest of the bulk of the crystals are melting.0406

Finally, you are going to see the very last crystal disappear.0412

Now, what you see in your little test tube is just a liquid.0415

Until, we have liquids and no crystals, that is your final temperature.0420

You take a quick look up at your thermometer and record that temperature.0424

That is going to be your melting point range, the point of the first melting and the point of the last crystal disappearing.0428

That is what you are going to report in your laboratory notebook.0435

You always record a range.0438

When we say take a melting point, we mean take a melting point range.0440

These are always need to be two numbers, even if it appears to just melt instantaneously, you can write the same numbers twice.0443

You will have 0° range or 1/2° range, that is okay.0449

The indication of purity is, a pure crystal or pure solid is going to have a sharp melting point.0454

We expect that range to be just 1 or 2°, hopefully.0461

When we have impurities being introduced, organic impurities,0466

that breaks up the crystal structure, makes it a lot easier for it to melt.0470

It will melt at a lower temperature than the literature value, typically.0474

It is also going to melt at a wider range.0479

That is going to be your evidence that we have not as pure as a crystal.0481

Melting point is a really great diagnostic tool.0486

Any time I have a solid product, it is most definitely one of the physical properties you want to record to characterize your product.0490

Good luck with your melting points.0496