HALOGENOALKANES

Oh my sweet oranges! I am starting to go crazy with all this revision. You too?? Too bad.

We have already covered many things about halogenoalkanes so here is a short summary.

• Electrophilic addition of hydrogen halides.
• Addition of halogens.

Now you will learn how to name them too. Isn't that epic??

Halogenoalkanes (geez, what a mouthful) contain only single bonds. We name them by first identifying where the halogen is attached. Count on which carbon it is on (smallest number counts). Write the number of the carbon then a relevant prefix. Finish off with the name of the alkane.

Halogen:           Prefix:

F                  flouro-

Cl                 chloro-

Br                 bromo- 

I                  iodo-

NUCLEOPHILE: An electron pair donor. 

NUCLEOPHILIC SUBSTITUTION/HYDROLYSIS of HALOGENOALKANES:

Because it's a substitution, you know that on top of the halogenoalkane you are going to have something else left over. That which has been replaced.

Think about it this way. In electrophilic addition we added the halides. Here we will remove them. See? Opposite.

You might come across questions where it will ask which hydrolysis reaction will take place faster. This rate is affected by two factors:

• Polarity - C-F bond is the most polar (greatest difference in electronegativity) so the C delta+ atom attracts the nucleophile more readily - So faster reaction.
• Bond enthalpy - The C-I bond is the weakest (I don't think you need to know why but I'll include it later in this post - probably at the end somewhere) so it is broken more easily. This means a faster reaction.

Faster as you go up. Then faster as you go down. One point contradicting the other? It's not a mistake, I assure you. The think is that in hydrolysis bond enthalpy is more effective than bond polarity. So you should keep the polarity in mind but focus on the bond enthalpy.

• Industries had a craze when it come to compounds containing chlorine. Such as, chloroethene and tetraflouroethene (these coming from radical substitution), especially CFC (ChloroFlouroCarbons).
• These were used for aerosols, air conditioners. This is because they were relatively unreactive, non-toxic and volatile. But every silver lining has a cloud to it (Sorry, I just had to). CFC's had a huge impact on the ozone layer (about that later - I promised).

There are now biodegradable alternatives to the killer CFC's, such as HCFC's and even CO2 as a blowing agent for expanding polymers.  

Now back to bond enthalpy. The C-I bond is the weakest because the outer electrons of Iodine are very far away, this means nuclear attraction is very weak and the electrons can be easily stolen away. You knew this was coming, didn't you?