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Alkenes-Alkynes - Concept
M.Ed., Columbia Teachers College
Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.
Alkenes and alkynes are unsaturated bonds that contain one or more double or triple bonds (e.g., ethene, propene, butene, etc.) and have different chemical compounds and properties. Their properties are similar to those of alkanes meaning that they are non-polar, have low solubility in water and have low melting and boiling points. Alkenes and alkynes are much more reactive than alkanes.
Alright. So we're going to talk about alkenes and alkynes. So we know that alkanes, alkanes are all singly bonded to each other and they're all saturated with the most amount of hydrogen that they can possible have. Well alkenes and alkynes are unsaturated meaning that they do not have the maximum amount of hydrogen that can, that it can have around carbon. And because they have they contain double if they're an alkene or triple bonds if they are an alkyne in the parent carbon chain. Okay. So let's talk about some of these guys.
So they're going to have the same actual prefixes as we've been doing with organic compounds. But we notice that there is nothing before ethene, maybe notice there's no methane. Why do you think there's no methane? Because hydrogen, sorry, the carbon is alone. It's by itself. The definition of alkene or alkyne meaning it, that it has a double or triple bond within it. But if it doesn't have another carbon it can't do that. So the smallest amount of carbons that an alkene or alkyne can contain, is two.
So we're starting with ethene. Ethene is C2H4. If it's an ene, meaning it's an alkene, it's double bonded. And this is used typically in like plant hormones or in grocery stores in making sure that plants look ripe and fresh. They make sure they put that they are like the fruits and vegetables, they're going to use ethene to make that happen.
Another one that you might come across is propene, having three carbons and this is used within gasoline to make sure our fuel in cars, our cars are running. Well, another one is butene. And here's when you come across a problem. Now, when we're done with butene we have four carbons within it. Bute means four. So we can either have the double bond between the first and the second carbon or we can have a double bond between the second and third carbon. This is used in gas refining and this guy is used in rubber manufacturing. Very different chemical compounds, very different chemical properties and so when you get the butene, the double bond and triple bond can actually move and change the way the these chemicals behave.
Okay. Triple bonds are the exact same way except they're named differently. To distinguish we're talking about triple bonding. We're going to use pre- the suffix y-n-e, yne, which is the same thing as you can [IB] is alkynes have y-n-e, so if you have two carbons of ethyne, three carbons of propyne, four carbons of butyne and then again when you get to betyne you're start dealing with the same issue that you had in alkenes, and betenes. Okay.
So what are some properties of alkenes and alkynes? Well, just like alkanes, very similarly to alkanes and we've talked, discussed in that, they're non-polar. Meaning that they do not have any distinguishing, one atom within it is not more electronegative than the other. So it's a non-polar compound, has a low solubility in water due to the fact that it's non-polar so it will not be attracted or repelled by water. It has a low melting point and boiling point because of the intermolecular forces that it contains because it's non-polar and it's London dispersion forces, they're not attracted to each other even and so it, they're easily put into, made into a gas. That's why most hydrocarbons are gaseous and however, the difference between alkenes and alkynes versus alkanes, is that they're much more reactive.
So why is that? So let's look at this carbon chain we have right here. Notice we have a double bond here. Now when you have a double bond it means that they're sharing four electrons other than just the simple two. So we have an enrichment of electrons right here. If a [IB] electrons going on around here. So it's to be called a good reaction site. So for a highly electronegative atom or compound comes up to it or polar compound comes close, it's going to seal the electrons and going to want to react with it. So just because this itself is non-polar, this area right here is very rich in electrons making it very attractive to other things coming close nearby, so it will be in many more reactions an it's not quite [IB] chemically stable as its friend the alkanes.
So this is basically the properties of alkanes, sorry, alkenes and alkynes.
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