Halogen Derivatives - Unimolecular Nucleophilic Substitution.

Previously we have discussed about Bimolecular Nucleophilic Substitution, also known as S_N2. In this mechanism, the rate of the reaction is dependent on the concentration of the alkyl halide and the concentration of the nucleophile. Hence, this also implies that both alkyl halide and the nucleophile are involved in the slow step of the reaction.

However, sometimes the rate of nucleophilic substitution reaction between an alkyl halide and nucleophile (note: aryl halides are STILL unable to undergo this reaction. Click here to find out why.) can be dependent on the concentration of the alkyl halide and independent on the concentration of the nucleophile.

Therefore, the above paragraph suggested that there is an alternative mechanism (or you could call it reaction route) for the nucleophilic substitution reaction. This reaction is called unimolecular nucleophilic substitution, S_N1.

There are two steps in this mechansim. The first step is the breaking of the C-X bond to generate the carbocation.

While the second step, which is the fast step, is where the nucleophile is attracted to the carbocation.

The rationale to why S_N1 is possible is because there are reasons to support the formation of a carbocation. If a stable carbocation can be formed, the S_N1 mechanism would be likely to occur. (Click here to read how the carbocation can be stablised.)

As such, generally 2^o alkyl halide, 3^o alkyl halide and phenyl-halo-methane (e.g. a benzene ring with a -CH₂X substituent), will support a S_N1 mechanism because it can form a stable carbocation. How C₆H₅CH₂⁺ carbocation is stablise by the phenyl group follows the same principle as this.

In conclusion, if you need to describe the mechanism of Unimolecular Nucleohilic Substitution, S_N1, both the fast and slow steps must be drawn. Hence, the following illustration shows the requirements needed.

The video below provides the steps and some basic explanation to S_N1 mechanism.

^{-- -- -- -- --}

^{Article written by Kwok YL 2009. Updated in Feb 2010.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Ethene reacting with Br2 dissolved in ethanol (Poll Question)

In the reaction between Br₂ dissolved in ethanol (CH₃CH₂OH), the mechanism of the reaction can be described by the picture below.

When you review the mechanism of the reaction between ethene and aqueous Br₂, you can see that the solvent molecule (H₂O) contains an O with 2 pairs of lone pair of electrons. This electron pair is attracted to the carbocation which is an electrophile.

Hence, in the situation of Br₂, the similar situation occurs. Ethanol contains O which has 2 pairs of lone pair of electrons. A pair of electron can therefore be attracted to the electrophilic carbon of the carbocation, hence forming of CH₂BrCH₂OCH₂CH₃ as the major product.

^{-- -- -- -- --
Article written by Kwok YL 2010.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Effect of Alkyl Groups.

In organic chemistry, we consider alkyl groups to be electron donating. They are able to donate electrons via a means called hyperconjugation. When alkyl groups are linked to a carbon with a positive charge or a carbon with an unpaired electron, the alkyl groups with its electron donating properties stabilises the carbocation (or radical).

On the other hand, alkyl groups when attached to the benzene ring, donate electron density towards the ring and hence making the benzene ring more reactive toward electrophiles.

Hence, is the electron donating property of alkyl groups stabilising or destabilising?

Well, the answer is lies in the effect that the electron donating property causes. In the case of carbocations (and radicals), the alkyl groups donates their electron density towards the electron deficient carbon.

Hence, by donating the electron density, the alkyl group is no longer neutral in charge but rather carry some partial positive charge; inevitably sharing the positive charge (or the extent of electron deficiency) with the carbon that has the positive charge.

However, in the case of the methylbenzene, the alkyl groups donate electron density to the electron ring. This makes the pi electron ring richer in electron density and hence more susceptible to electrophilic attacks.

It is timely to remind ourselves that in the electrophilic substitution mechanism, the slow step is the reaction between the benzene ring and the electrophilc.

Hence, seeing the effect caused by the alkyl groups will lead us to making the correct choice to whether they stabilises or destabilises.

^{-- -- -- -- --
Article written by Kwok YL 2010.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Strong Oxidation of Alkenes

The following video discuss about the strong oxidation of alkenes; shedding information about balancing the chemical equation to explaining the type of products that are formed during the oxidation reaction.



In addition, it is important to note that the products from the strong oxidation of the alkenes provide very good information to the structure of the alkene. The following video illustrates some examples and skills that will enable us to elucidate (aka piece together) the structure of the alkenes when we are given the products of the oxidation reaction.



^{-- -- -- -- --
Article written by Kwok YL 2010.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Electrophilic Addition - Markovnikov's rule

The video below shows the description of the electrophilic addition reaction using propene and HBr. In this description, it applies Markovnikov's rule, and in addition explains why Markovnikov came up with his rule: The electrophile is added to the alkene in a manner to produce the more stable carbocation which will subsequently form the product.

Note: You may wish to read more about the stability of carbocation. Please click here to access that article.

Since, H is often the electrophile (since HX is a common addition reagent), hence by observing the products formed we notice that the H is added to the alkene carbon with more H attached to it. Hence, it popularises the phrase "the rich getting richer".



^{-- -- -- -- --
Article written by Kwok YL 2010.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}