2.6 HALOGENOALKANES- REACTIONS AND USES

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Hi boys and girls. If you're here with me, that probably means you are having some issues with your organic chemistry and halogenoalkanes in particular. Or perhaps you're here because you enjoy chemistry just like me. A fun fact about me, I started reading about all elements in the periodic table and write down a couple of facts about each every day. In this unit, we are going to explore halogenoalkanes. There are several reactions we will work on but there are also some uses of halogenoalkanes that you need to be aware of. As always, let's start with definitions because that just makes life so, so much easier. Since this unit is on halogenoalkanes, we might as well start with that definition.

Halogenoalkanes - It is a homologous series. They are alkanes in which one or more of the hydrogen atoms are replaced with a halogen (bromine, chlorine or iodine). This also means that the bond is polar in halogenoalkanes (between a carbon and the halogen) because the electronegativity difference is high. Halogens are more electronegative than carbon, therefore they will become slightly negative and carbon will be slightly positive. Due to this polar bond, they're susceptible to nucleophilic attack. And another definition: Nucleophile - species with a lone pair of electrons that donate these electrons. So, when we have this one atom being slightly positive and the other slightly negative (such as between a carbon and halogen) and we happen to have a nucleophile 'flying' around, which needs to donate a pair of electrons, it will do so to the carbon (slightly positive) and halogen will be left by itself completely negative now. This whole process basically means that nucleophile will substitute the halogen group, e.g. it will change places with bromine, chlorine or iodine (depending on which one is in the compound). The following reaction is called a nucleophilic substitution reaction.

Some questions in the exam may ask you to draw a mechanism for this reaction. And I prepared one for you as well (look at the picture above). Of course, if you want to practice more of them you can visit my youtube channel. Let's start with it then:

• You have 1-bromopropane. The part 'bromo' tells you there is a halogen in this compound and it is bromine. It is connected to carbon and therefore the bond is polar. Carbon is slightly positive and bromine is slightly negative (remember to show that clearly in your mechanism).

• Our nucleophile (OH-) is also flying around. As we said before, it wants to donate electrons and it does so to the carbon. This is shown by the arrow (the arrow shows the movement of electrons).

• Finally, you are left with an alcohol and bromine flying by itself.

There are also some reagents and conditions which you definitely need to remember.

1. NaOH - this is the nucleophile and it has to be in aqueous form.

2. Heat

3. Reflux - constant evaporation and condensation.

Reflux is needed because a lot of liquids would evaporate otherwise. By using reflux, whatever liquid evaporates, it is condensed back into liquid, so nothing is lost.

Here we go, this is another reaction for halogenoalkanes that you are required to know for your as-level chemistry. It is an elimination reaction. Elimination reaction - a reaction where a small molecule is lost to produce a double bond C=C. On the photo above I included an equation for this reaction. I tried to make it as clear as possible. What you need to know from it, is that whatever compound they give you, the double bond will always be on the carbon where the halogen was before. Moreover, note that in products you have this small molecule that has been lost, HBr. The reagents are similar to the ones in a nucleophilic substitution reaction, however, this time the sodium hydroxide needs to be dissolved in ethanol. Be sure you include this statement 'dissolved in ethanol', if it wasn't, it would just be another substitution reaction.

In the reaction above we used bromine, but bromine, chlorine or iodine could be used. They will all have a different effect on the compound. Electronegativity: The electronegativity difference between carbon and halogen decides how polar a bond is. Chlorine is the most electronegative out of chlorine, bromine and iodine, therefore its bond will be most polar. Also, because chlorine is the most electronegative, it will be the most negative which makes carbon in this bond the most positive. As carbon is very positive when it's next to chlorine, it will be even more susceptible to attack by nucleophiles. Bond strength: The bond between chlorine and carbon requires the biggest amount of energy to break because it is the strongest. As you can see these two properties work in opposite directions. The C-Cl bond is most polar, so it should undergo the nucleophilic substitution fastest. But the bond strength between chlorine and carbo takes priority and holds the bond strongly together. Therefore, this is actually the order in which the compounds would react: iodo> bromo> chloro fastest slowest

There is a way of testing for a halogen in an organic compound. There are exactly 4 steps that are needed to take. The first one is the same as in the nucleophilic substitutions reaction.

1. Add sodium hydroxide, NaOH, to the halogenoalkane.

2. Add dilute nitric acid, to the solution. This step is taken in order to neutralise any excess sodium hydroxide from the previous step. Acid (nitric acid) will neutralise the alkali (sodium hydroxide).

3. Add silver nitrate to the solution. Now, after you do this something will happen. The colour of the solution will change to either white -> for chlorine, cream--> for bromine or yellow --> for iodine. Obviously, if the colour doesn't change at all, it wasn't a halogenoalkane. If they ask you about an equation for this step in the exam, I wrote it down for you above.

4. Add ammonia to the solution. This step is necessary if you are not entirely sure what colour you saw when silver nitrate was added. If the solution dissolves in ammonia --> it was chlorine, if it dissolves in concentrated ammonia --> it was bromine and if it doesn't dissolve at all --> it was iodine.

Lastly, for this post, we are going to explore the uses of halogenoalkanes. They are used widely and we will just focus on a couple of them. So, 1. Solvents - halogenoalkanes are used as solvents. They can be used as solvents because they have both a polar part (between carbon and halogen) and the non-polar part (the remaining part of the chain). This means they easily mix with both types of solutions. (Because as you know, polar compounds dissolve in polar compounds and non-polar compounds dissolve in non-polar compounds). 2. Refrigerants - halogenoalkanes and more exactly CFC's have been used as refrigerants. This is because they're stable and non-flammable. However, there is a very big issue regarding CFC's (chlorofluorocarbons) and they have been banned. The reason being, they were found to be toxic and causing damage to our ozone layer. This is dangerous for life on Earth because holes in the ozone layer cause UV light to reach people and may cause skin cancer. The CFC's are toxic as a result of UV rays (from Sun) breaking the bond between C and Cl when CFC's diffuse into the higher atmosphere. When the bond breaks, a free radical is formed. We did this in the previous post, the initiation, propagation and termination stages, where chlorine radical was produced. Here again, radicals are formed however, you don't need to know all these equations, just the fact that in the propagation stage O3 is being converted to O2. Thankfully, in tody's world, as our awareness increased, we don't use CFC's anymore. However, there is a replacement for them, which is HFC's (hydrofluorocarbons). There is no chlorine here for the radicals to be formed when UV falls on it and the C-H bond is too strong for the radicals to form as well.

PS. Please remember, I am only a student, and as anyone, I can make mistakes. If you think you can see one, don't hesitate and comment (either here on on my youtube channel) Thank you!