Posted by: Mark Foreman | February 8, 2012

Alkenes and the joy of organic chemistry

Dear Reader,

Like a graceful multicoloured song bird our organic chemistry continues to soar into the air, I sincerely hope as well as helping you to pass your exams my organic chemistry lessons bring you an inner joy which comes from transforming your self into a person who understands the world around them in a new and deeper way. While I will never be able to entertain you in the way that the Basshunter, Natalie Horler or Andy Bell / Vince Clarke (Erasure), it is important to know that many people obtain a deep satisfaction from their learning which is long lasting. I hope that you will come to know the joy of organic chemistry which comes to those who understand it.

OK pep talk is over for a moment lets look again at my slides which are about alkene chemistry.

What are alkenes, learn about cis and trans. Note the steric effect normally favours the trans. But in the case of small and medium cycloalkenes the cis is more stable. If you want to work out why try to make a model or draw trans cyclohexane.

Sometimes cis and trans does not allow us to express ourselves, for these cases we have E and Z

Use Hess's law to work out which of these alkenes is more stable, this should help you understand what makes a alkene more or less stable

A rule which is based on the energy of the TS, this is the rule to use when the base is nice and small. Note that small bases tend to make the alkene which is more thermodynamically favoured.

This is what happens when the base is big and hindered, it attacks the acidic hydrogen which is less hindered.

The rate of the dehydration of alcohols is largely controlled by the stability of the carbocations which are one of the stages in the E1 elimination. For all these alcohols it is possible to do the E2 as well.

Here is a problem for you do try

Rearrangement of carbocations, this is important and will pop up again in chemistry many times

This reaction is best thought of as being the reverse of the dehydration of an alcohol by the E1.

Try this problem at home, it will help your understanding

The hydride shift which is an alternative to the alkyl shift you saw earlier

A reminder from earlier, please study this reaction. Look at the driving force of going from secondary to another type (what type ?) of carbocation.


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