Lesson video

Hello, I am Mrs. Adcock.

And welcome to today's lesson.

Today's lesson is on our alkenes.

We are going to be looking at what are alkenes and how do they react.

Today's lesson outcome is, I can describe and explain the reactions and properties of the first four alkenes.

Some of the keywords that we will be using in today's lesson include, alkenes, functional group, isomers, unsaturated, and saturated.

Here you can see each of those keywords written in a sentence.

It would be a good idea to pause the video now and read through those sentences and you might like to make some notes so that you can refer back to them later in the lesson if needed.

Today's lesson on alkenes is split into two main parts.

The first part of the lesson is an introduction to alkenes, and then we are going to move on to look at how alkenes react.

So the second part of the lesson is reactions of alkenes.

Let's get started on the first part of the lesson, introduction to alkenes.

The alkenes are a homologous series of hydrocarbons, and hydrocarbons are compounds that contain hydrogen and carbon atoms only.

We can see an example of an alkene shown.

And this alkenes has two carbon atoms and four hydrogen atoms. So it would have the molecular formula C2H4.

The functional group in alkenes is C double bond C, and we can see that highlighted on the molecule shown.

So that molecule has a carbon-carbon double bond and therefore, it is an alkenes.

Just a reminder that the functional group is the atom or group of atoms that is responsible for the way a chemical reacts.

The functional group in alkenes is a carbon-carbon double bond.

Shown in the diagram there is the simplest alkene.

It has the molecular formula C2H4.

Why can you not have an alkenes with only one carbon atom? You cannot get an aine with only one carbon atom because alkenes contain the functional group, which is a carbon-carbon double bond.

So you have to have at least two carbon atoms. Well done if you are able to work that out.

Time for a check for understanding.

Which of the following molecules is alkene.

Look carefully at the diagrams shown in a, b, and c and see if you can identify which of those is an alkene.

The alkene is shown on image b.

So well done if you identified that as an alkene.

And the reason that B is an alkene is because it contains a carbon-carbon double bond.

If you look carefully at the images shown in a and c, you will notice that both of these only contain carbon to carbon single covalent bonds.

To be an alkene, the molecule needs to contain a carbon to carbon double bond.

When naming kines, we use the suffix e, so that's an ene at the end of the name.

So for example, ethene is the name of alkene.

The first four alkenes in the alkene homologous series are, ethene, so ethene is our simplest alkene and that has the molecular formula C2H4, and you can see it contains that carbon-carbon double bond.

Next we have propene and propene contains three carbons.

It has the molecular formula C3H6, and you can see it contains the carbon-carbon double bond.

You'll notice that each carbon atom can form four covalent bonds.

So we can check we've got the correct number of hydrogen atoms attached by checking that each carbon atom has formed four covalent bonds.

So we have ethene, propene.

Next we have butene, and butene has four carbon atoms, so it the molecular formula is C4H8, and then, with five carbon atoms is pentene.

So pentene has the molecular formula, C5H10.

So just to quickly recap, the first four alkenes in the alkene homologous series are ethene, propene, butene, and pentene.

The longer the alkene chain, the higher the boiling point.

We've got a table here showing us the alkene and the boiling point.

We can see the boiling point of ethene is negative 104 degrees Celsius, the boiling point of propene is negative 47 degrees Celsius, the boiling point of butene is negative 6 degrees Celsius and the boiling point of pentene is 30 degrees Celsius.

And the boiling point is the point at which it changes state from a liquid to the gas state.

As the alkene carbon chain gets longer, so we go from et Ethan to propene to butene to pentene and obviously ethene has two carbons, propene has three carbons, butene has four carbons, and pentene has five carbons.

So as that carbon chain length increases and the alkene has become a longer alkene, then the boiling point has increased from negative 104 to negative 47 to negative 6, and then up to 30.

So the longer the aine chain, the higher the boiling point.

Time for another check for understanding.

Which of the following shows a molecule of pentene? Now, first of all, you can check if they are all alkenes by looking for the carbon-carbon double bond.

And then try to remember, how many carbons will there be in a molecule of pentene.

The correct answer is b.

B shows a molecule of pentene.

It contains five carbons, which gives it the prefixed pent, and then it contains a carbon-carbon double bond, which means it's an alkene, so it ends in e.

So this is pentene.

A shows a molecule of ethene that contains two carbon atoms. C shows a molecule of propene, and the longest carbon chain length in propene is three carbons.

Well done if you identified b as a molecule of pentene.

Time for another question.

Which alkene will have the highest boiling point? Is it a, C2H4, b, C5H10, c, C9H18 or d, C12H24.

See if you can remember the relationship between alkene chain length and boiling point.

D, will have the highest boiling point because D contains the highest number of carbon atoms. And the longer the alkene chain or the larger the alkene molecule, the higher the boiling point.

Well done if you've got that correct.

You've clearly been focusing very well at the start of today's lesson.

We are going to look closely now at drawing alkene molecules and working out the molecular formula.

So first of all, I will have a go and then it'll be your turn to have a go.

We are going to work out the molecular formula of alkenes.

Number one, draw a molecule of butene.

Now, the prefix is but.

So I know that the longest carbon chain will be four carbon atoms and it ends in ene so I know I need to include a carbon-carbon double bond.

I have identified molecule of butene as shown there.

And you will notice that each carbon forms four covalent bonds.

So I can check that I've added the correct number of hydrogen atoms. If we now, in step two, work out the molecular formula, the molecular formula shows us the number of atoms of each element in the molecule.

So the molecular formula for butene, if we add the carbon atoms first of all, then we'll notice there are four carbon atoms, and if we add up the number of hydrogen atoms, we will notice we have eight hydrogen atoms. So the molecular formula for butene will be C4H8.

Your turn to have a go.

So you need to work out the molecular formula of alkenes and you are going to draw a molecule of propene and then work out the molecular formula of propene.

If you pause the video now, have a go at this question, then come back when you're ready to go over the answers.

Welcome back.

Let's see how you got on.

Propene contains three carbon atoms because it has the prefix, prop.

And then it has the suffix ene, so we need to make sure that our molecule also contains a carbon-carbon double covalent bond.

Your answer will hopefully look similar to this one.

You could have drawn your double bond at the other end of the molecule.

That would be absolutely fine.

And you may have placed your hydrogen atoms differently as well.

Just check though that each of your carbon atoms has only formed four covalent bonds and you haven't accidentally added any extra hydrogen atoms. Question two, work out the molecular formula.

We need to add the number of carbon atoms that we've got.

So in this case there's three carbon atoms, and then if we add up our hydrogen atoms, you should notice that you've got six hydrogen atoms. So the molecular formula for propene is C3H6.

Well done if you drew your molecule correctly and you've worked out the correct molecular formula.

Alkenes have the general formula, CnH2n.

Now n can be any number.

So however many carbon atoms we've got, we then have 2n, which is the number of hydrogen atoms. So if we know our carbon atoms, for example, are 3, then our hydrogen atoms will be 2 times 3.

So we would have six hydrogen atoms, so we always have double the number of hydrogen than there were carbon in alkene molecules.

Let's have a look at some examples.

So we've got ethene here.

And ethene has got two carbon atoms and therefore, it has got four hydrogen atoms. So the molecular formula is C2H4.

Propene has got three carbon atoms. So when we are working out the number of hydrogen atoms, we do 2 times n.

In this case, n is 3.

So we do 2 times 3, and we know we've got 6 hydrogen atoms in our propene molecule.

So the molecular formula for propene is C3H6.

And butene has got four carbon atoms. So in this case, n is 4.

So when we work out the number of hydrogen atoms, we do 2 times 4, and that gives us 8.

So the molecular formula for but is C4H8.

Overall alkenes have the general formula, CnH2n.

Alkenes with four or more carbon atoms are able to form structural isomers and isomers are structures with the same molecular formula, but different arrangement of atoms in space.

So let's have a look at an example.

Here we've got two molecules of pentene.

They have five carbon atoms in their longest carbon chain.

However, in the first molecule, you'll notice that the carbon-carbon double bond is by carbon atom 1.

If we numbered those carbon atoms from 1 to 5.

Whereas in the second molecule that carbon-carbon double bond is by carbon atom 2.

And therefore they've got the same molecular formula, but the atoms are arranged differently in the space.

These are isomers.

This is called pent-1-ene because of the position of that carbon-carbon double bond, and this is called pent-2-ene, again, because of the position of the carbon-carbon double bond.

These structures are isomers with the same molecular formula, C5H10, but a different arrangement of atoms in space.

Time for a question.

What is the general formula of alkenes? Is it a CnH2n plus 2, b, CnH2n plus 1, or c, CnH2n? Now just take a moment to think how do we work out the number of hydrogen atoms when we know the number of carbon atoms in an alkene molecule? Use this to help you decide which one shows the general formula of alkenes.

The correct answer is c.

So the general formula of alkenes is CnH2n.

You might recognise a.

A is CnH2n plus 2.

That is the general formula of alkenes.

Time for our first practise task of today's lesson.

You've got five questions here.

Question 1, what is the functional group of alkenes? Question 2, name the first four alkenes.

Question 3, draw the displayed structure of ethene.

Question 4, what is the molecular formula of ethene propene, butene and pentene? And the last question, question 5, you need to draw two isomers of molecules with the molecular formula C6H12.

You may be able to draw more than two isomers, which have the molecular formula C6H12.

So if you are able to draw any extra, then please do have a go.

If you pause the video now, have a go at answering these five questions and then come back when you're ready to go over the answers.

Question 1, what is the functional group of alkenes? That is a carbon-carbon double bond.

Question 2, name the first four alkenes.

Now remember the simplest alkene is ethene.

So we start with ethene then propene, then butene, then pentene.

Question 3, draw the displayed structure of ethene.

Your image may look similar to one of these, or you may have drawn it slightly differently.

But so long as you've got two carbon atoms, there's a double bond between them, and you should have two hydrogen atoms attached to each of your carbon atoms. Well done if you've got those first three questions correct.

You're doing very well.

Question 4, what is the molecular formula of: so ethene is C2H4, propene is C3H6, butene is C4H8, and pentene is C5H10.

So well done if you've got those correct.

Draw two isomers of molecules with the molecular formula C6H12.

So hopefully you've drawn molecules that do only contain six carbon atoms and 12 hydrogen atoms. It would be worth checking your molecules.

And your answer may include this molecule shown here where the carbon-carbon double bond is positioned by carbon 1, or you may have positioned it by carbon 2, and finally, you might have positioned it by carbon 3.

So well done if you were able to identify two of those images, and therefore, you've drawn two isomers with the molecular formula C6H12.

That's excellent work.

We have finished the first part of our lesson.

We've looked at alkenes and what alkenes are.

And we've identified the first four alkenes in the alkenes homologous series.

Now we're going to move on to have a look at the reactions of alkenes.

An unsaturated molecule contained one or more double covalent bonds.

Alkenes are unsaturated because of the presence of the double carbon-carbon bond.

We can see an alkene shown there.

That's ethene.

And because it contains a carbon-carbon double bond, it's an unsaturated molecule.

So all alkenes are unsaturated.

Here is an example of a saturated molecule.

So alkanes are saturated molecules because they do not contain one or more double covalent bonds.

When alkanes react, they change from unsaturated to saturated molecules.

The double carbon-carbon bond opens up and atoms can attach to the carbon atoms either side of the double covalent bond.

Here we can see the carbon-carbon double bond in our unsaturated alkene molecule.

This reacts with molecule X.

The product shows that the carbon-carbon double bond has opened up and atoms of X have attached to the carbon atoms on either side of the double covalent bond.

We have gone from a unsaturated molecule, and when we look at our product, we can see that we formed a saturated molecule.

Unsaturated alkene, and that's the molecule reacting with the alkene and then we form a saturated molecule as our product.

There you can see those two atoms of X and how they've bonded to the carbons on either side of that double covalent bond.

Which of the following is an unsaturated molecule? Well done if you chose a.

A is an unsaturated molecule because it contains a carbon-carbon double bond.

If you look in molecules b and c, you'll notice that they only contain single covalent bonds.

alkenes react with hydrogen in the presence of a catalyst to produce alkanes.

Here we've got ethene, it's an alkene, it reacts with hydrogen, and we are going to produce an alkane.

Alkenes are unsaturated.

They can react with hydrogen in the presence of a catalyst to produce alkanes which are saturated molecules.

That carbon, carbon double bond opens up and hydrogen atoms attached to each of those carbon atoms. This reaction is a hydrogenation reaction, and hydrogenation reactions involve the addition of hydrogen.

Let's see if you can remember that.

What type of reaction is the reaction between alkenes and hydrogen in the presence of a catalyst? Is it a hydration, b, hydrogenation c, homologous series? Well done if you chose answer b.

So the reaction between alkene and hydrogen in the presence of a catalyst is known as a hydrogenation reaction because it involves the addition of hydrogen.

We've looked at how alkanes can react with hydrogen to form alkanes.

Alkanes can react with steam at high temperature and pressure to form alcohols, and this reaction also requires a catalyst.

And in this case, the catalyst could be concentrated acid.

Here we've got an alkene and it's gonna react with steam at high temperature and pressure.

We're going to use a concentrated acid as our catalyst and we will produce an alcohol.

Now you'll notice that alcohols contain the OH functional group.

The hydrogen and the OH from our steam have bonded either side of that carbon-carbon double bond.

So it's opened up and on one of the carbon atoms, we've got the hydrogen, And on the other carbon atom we've got the OH atoms. We have gone from alkene, we've reacted it with water, but because we've got high temperatures and pressure, this is actually steam, and then we've produced an alcohol.

Now this reaction, because it involves the addition of water, is known as a hydration reaction.

Time for another check for understanding.

What is the product when an alkene reacts with steam in the presence of concentrated acid.

Will you form A, an alkane, b, an alcohol, or c, a haloalkane.

When an alkene reacts with steam in the presence of a concentrated acid, you will form an alcohol.

So well done if you chose option b.

Alkanes can react to form alkanes, they can react to form alcohols.

Now we are going to have a look, how alkanes can react with halogens such as chlorine, bromine, and iodine to form haloalkanes.

Just a reminder that halogens are elements that are in Group 7.

Chlorine, bromine, and iodine are all in Group 7, and they form diatomic molecules.

So you'll notice when you look at the molecular formula that they have little twos because they always travel in pairs of atoms. So we've got Cl2, Br2 and I2.

Here is an example of a reaction between an alkene and a halogen.

There's our alkene.

It's going to react in this case with bromine, and there is our haloalkane.

So it looks similar to an alkane but we've got some halogen atoms bonded too.

So it's known as a haloalkane.

You will notice that those bromine atoms that reacted with the alkene have bonded to the carbon atoms on either side of that carbon-carbon double bond.

So the carbon-carbon double bonds opened up.

So we've only got a single bond now, and the bromine atoms have bonded either side.

We've got an al Keen reacting with a halogen to produce a haloalkane.

Let's check that we've understood that.

What is the product when an alkene reacts with a halogen? Is it a, an alkane, b, and alcohol or c, a haloalkane? The correct answer is c, a haloalkane.

So well done if you've got that one correct.

Alkanes can also react with oxygen in combustion reactions.

So our alkenes are very reactive.

Longer alkenes are less flammable than shorter alkenes.

And when alkenes combust fully, the general equation is alkene plus oxygen, and because we've got complete combustion here, it's going to be in a plentiful supply of oxygen and they will react to form our products, which are carbon dioxide and water.

An example of an equation for the incomplete combustion of alkenes is alkene plus, now this time, because it's incomplete combustion, we've got a limited supply of oxygen, so our alkene reacts in a limited supply of oxygen to produce carbon monoxide and water.

Now there are different equations that you can get for incomplete combustion.

You might form just carbon and water, or you could form some carbon and carbon monoxide and water.

Alkenes normally undergo incomplete combustion, so they normally form carbon monoxide carbon, which is soot and water.

And the production of soot means that the incomplete combustion of alkenes produces smoky flames.

And we can see this here in this video clip.

We can see an alkene burning.

And because we have incomplete combustion, we can see it's producing a smoky flame, and that's because of those soot or carbon particles.

Which of the following alkene is the most flammable? Is it a C2H4, b, C7H14, c, C9H18, or d, C12H24? The correct answer is a, because the shorter the aine molecules generally the more flammable they are.

So well done If you chose answer a and remembered that longer alkenes are less flammable and shorter alkenes are more flammable.

Another question for you to have a go at here.

Alkenes burned with smoky flames due to complete combustion.

Is that statement true or false? That statement is false.

Now, can you justify your answer? So can you explain why this statement is false? Is it because a, alkenes burn with smoky flames due to incomplete combustion? Or is it b, alkenes burn with orange flames due to complete combustion? The correct answer is a, alkenes burn with smoky flames due to incomplete combustion.

So rather than complete combustion, it's the incomplete combustion that causes them to have smoky flames.

It's time for our final practise task of today's lesson, and this is all about the reactions of alkenes.

Question 1, for the following reactions show the displayed formula of the product.

You'll draw out the products showing all of the covalent bonds in our displayed formula for the following reactions: so a, propene and hydrogen reacting together in the presence of a catalyst, b, butene and steam react together at high temperature and pressure in the presence of concentrated acid, c, ethene, and chlorine, and D, you've got pentene and iodine.

Pause the video now, have a go answering this question, and then come back when you've drawn those products and you're ready to go over the answers.

Well done for having a go at question 1.

Now let's move on to question 2.

Each of the following products were made from alkenes for each product decide what the reactants would've been and if there were any conditions and reagents needed to produce these products.

Pause the video now, have a go answering this question and then come back when you're ready to go over the answers.

Okay, let's see how you've got on with those two questions.

So question one, drawing the displayed formula.

First of all, we had propene and hydrogen, and they would react together to form an alkane with three carbon atoms. So you should have drawn a propene molecule similar to that one shown there.

Part b, butene and steam would react together to form an alcohol.

And you may have drawn your oh in a different position, but there is a possible answer, we've got four carbon atoms and we've got an OH and we've got no double bond anymore.

So that alcohol there is butanol.

For c, you've got ethene and chlorine.

And there, you can see we've got the two carbon atoms from the ethene.

The double bond has opened up and we've got chlorine atoms bonded to each of the carbon atoms on either side of the double bond.

We've now formed a haloalkane.

And finally, pentene plus iodine.

Again, we're going to form another haloalkane.

Yours may look slightly different to this one if your double bond was in a different place, but you should have two iodine atoms attached, and you should have a carbon chain of five carbons long.

Well done if you answered question one correctly.

Let's move on to question two.

Here we have an image of an alcohol that contains a carbon chain length of four carbon atoms, So therefore, the reactants would've been butene and steam.

And this reaction requires high temperature and pressure along with a concentrated acid catalyst.

So well done if you identified the conditions and reagents there too.

To form this haloalkane, we would've needed to start with propene and chlorine, and there were no additional conditions or reagents needed.

And to form this alkane, we would've started with ethene and hydrogen, so they would've been the reactants, and these react in the presence of a catalyst.

Well done if you answered question two correctly.

We've covered a lot today on the reactions of alkenes.

We have reached the end of today's lesson.

Let's just summarise some of the key points that we've covered in today's lesson.

Reactions of an aine happen at its functional group and its functional group is the carbon carbon double bond.

Isomers are structures with the same molecular formula, but different arrangement of atoms in space, reactions of an aine change it from an unsaturated to a saturated molecule, larger alkenes have higher boiling points and are less flammable, and alkene tend to burn with smoky flames due to incomplete combustion.

Well done.

You've worked really hard in today's lesson.

I've really enjoyed the lesson on alkenes and I hope you have too.

And I hope you able to join me for another lesson soon.