INTEC Chemistry Blog

1.7 Introductory organic chemistry

Posted on: July 13, 2011

1.7 has no content as yet, but feel free to leave any comments/questions

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3 Responses to "1.7 Introductory organic chemistry"

Assalamualaikum,
I want to ask about something, reaction between another subtances with alkene is more higher and faster than reaction with alkane. How we need to explain that? Thank you very much. From 10eg5 2013

Salam 🙂

Every single physical activity (including chemical reactions) takes place so the objects (or chemicals) total internal energy is lowered and energy is released.(*) The fundamental energy is called ENTROPY (You will study introductory entropy probably at the beginning of year two), and all things cause an INCREASE in entropy.

That’s it! It’s how the the physical universe works (with the exception of Divine intervention!)

When it comes to chemical reactions, Entropy increases and chemicals internal energy decreases by the re-distribution of electrons, by the forming of bonds. Bonds involve attractions of electrons to the nuclear charge of an atom (after taking into account shielding) and to a smaller extent the magnetic attraction of oppositely spinning electrons.

In alkanes, the electrons are quite evenly shared in covalent bonds (hence bonds are very poorly polar, so much so that we actually call them ‘non-polar’ bonds). The bonds in alkanes are (called) “sigma bonds”, and those bonds are quite strong. So the arrangement of e- in those bonds is relatively stable and difficult to break. {some kind of bond breaking proces is always necessary in typical chemical reactions}. Hence alkanes are relatively unreactive.

In alkenes, you have an area of high electron density between the two C=C atoms. You have 4e- between two atoms made up of 2 e- in the C-C sigma bond and 2 e- in the CC pi bond. So the C=C double bond (sigma + pi)) is relatively e- rich compared to alkanes which only contain sigma bonds.

The pi bonds are not along the internuclear axis hence are further away from the attracting nuclei, so are more able to break away from the attractive nuclear charge and react with other molecules which can accept the electron density that the alkene can provide (from the pi bond).

So alkenes are more reactive than alkanes. Less energy is needed to break the less strongly held pi electrons in the C=C, hence reactions with alkenes are faster i.e. the Activation energy minimum energy input needed for the reaction to take place for alkene uis lower than for alkanes.

(*) Note it is possibly to increase an objects or chemicals internal energy which may cause a decrease in entropy, BUT, something somewhere else would have to ‘pay’ for that to happen, and ITS energy would have to decrease more than the objects (or chemicals) energy increased, hence the TOTAL entropy would still increase. Some biological processes do this.

Does that help at all?

Thank u very much for a very good explanation. It really help at all.

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