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Chemistry 2131:
Organic Chemistry for the Life Sciences (3)

Electrophilic Addition Reactions

• last day we looked at our first example of a reaction mechanism, that was the addition of HCl to ethene to give chloroethane. To review, the pair of electrons from the pi bond (of the double bond, not the sigma bond) attack the electrophile, H⁺. This forms a new C-H sigma bond, but leaves the other carbon electron-deficient and bearing a positive charge (we call this a carbocation). This is the reaction intermediate. In the next step, the nucleophile, Cl^- attacks the carbocation to make a C-Cl bond. Chloroethane is the product.
• this is an example of the general reaction type which is the addition of hydrogen halides to alkenes.
• ethene is the simplest possible case for electrophilic addition. Let's look at a slightly more complex situation, the addition of HBr to propene. There are two ways this reaction can go. The difference is in the first step, which we call the protonation step. If the number 1 carbon is protonated, then the number 2 carbon becomes positively charged. If on the other hand the number 2 carbon is protonated, then the number one carbon becomes positively charged. The result of these two possibilities is two products, 1-bromopropane and 2-bromopropane.
• strikingly, very little 1-bromopropane is formed in this reaction. This is the basis of Markovnikov's Rule, which states that in the addition of a hydrogen halide to an alkene, the proton goes to the less substituted carbon and the halide goes to the carbon with the greater number of alkyl substituents.
• this is an example of a regioselective reaction. In a regioselective reaction only one of several possible products are actually formed.
• let's look at another example, the addition of HBr to 2-methylpropene. The only observed product is 2-bromo-2-methylpropane.
• why does this happen? It is because of the stability of the carbocation intermediates. Carbocations are more stable (or more easily formed, same thing) the more substituted the carbon atom is. So the order of stability is:

  tertiary > secondary > primary> methyl

• why should this be so? Let's look in a little more detail at the geometry of the carbocation to approach this. The carbocation is still sp² hybridized, so it is planar and has 120 degree bond angles. The p orbital that formerly participated in the pi bond is empty
• the electronegativity of this carbon atom is greater than that of its neighbours because of the electron deficient state. So it pulls electrons from neighbouring sigma bonds towards it. This is called an inductive effect. It stands to reason that the more carbon atoms around it the more electrons that can participate in this sharing of electrons. Looked at another way, one can appreciate that the positive charge is spread over a larger volume of space, and is therefore less extreme. It has been shown even for methyl cations that even there the carbon bears a charge of only about +0.645, the rest of the charge is born by the surrounding hydrogen atoms. This is obviously more extreme for substituted cations.

2. Carbocation Rearrangements:

• things can get even more complicated. Let's look at the addition of HCl to 3,3-dimethyl-1-butene. From what we already know about Markovnikov's rule we would predict the formation of mostly 2-chloro-3,3-dimethylbutane. In actual fact only 17% of the product is this, the rest is 3-chloro-2,3-dimethylbutane.
• how can we possibly get this product? What happens here is a carbocation rearrangement. The first step is the protonation of the terminal carbon to give the carbocation on carbon 2. Attack of this intermediate by Cl^- would give the 2-chloro product.
• the other thing that can happen to this carbocation is that the electrons from the C-CH₃ sigma bond can attack the carbocation giving a new carbocation intermediate which is methylated at the 2 position. Which of these carbocations is more stable? Clearly the tertiary carbocation is more stable. If the chloride ion attacks this carbocation the product is 3-chloro-2,3-dimethylbutane.
• rearrangement is favoured by increased stability of the carbocation
• another kind of rearrangement that can occur is called a hydride shift. In this case, the pair of electrons from a C-H sigma bond attack the carbocation. In fact when 3-methyl-1-butene reacts with HBr the products are 45% 2-bromo-3-methylbutane and 55% 3-bromo-3-methylbutane.
• rearrangements are in competition with addition of the nucleophile

3. Hydration of Alkenes:

• another kind of electrophilic addition reaction is called hydration. As the name suggests, this reaction is the addition of water to a molecule with a double bond.
• hydration requires the presence of a strong acid catalyst. Remember that a catalyst is a substance that accelarates a reaction, but is not actually consumed in the reaction).
• like the addition of hydrogen halides to alkenes, the addition of water is regioselective, with the less substituted carbon becoming protonated.
• in the reaction of 2-methylpropene with water in the presence of sulfuric acid, the first step is the attack of the electrons of the pi bond on a proton (usually shown on a hydronium ion).
• this gives a tertiary carbocation. In the next step of the reaction, a lone pair of electrons from the oxygen of water attack the carbocation to give another intermediate which bears a positive charge on oxygen, this is called an oxonium ion
• in the final step of the reaction, a proton is removed from the oxygen by water to regenerate the hydronium ion, and to give the alcohol product.
• rearrangements are possible here as well
• a lot of ethanol is produced in this way

4. Halogenation of Alkenes:

• another electrophilic addition reaction of alkenes is the halogenation of them. In this reaction Cl₂ or Br₂ is added. The product is called a vicinal dihalide.
• the reaction is similar to the others we have seen. The first step is the attack of the pi bond electrons on a slightly positively charged Br from Br₂ to give Br- and a bromonium ion. In this case, one of the lone pairs on Br attack the carbocation to give a bridged bromonium ion. This serves to share the positive charge over the two carbon atoms and the bromine atom, the charge is said to be delocalized.
• the next step of the reaction is the attack of the bromide ion on one of the carbon atoms. This attack comes from the opposite face as the bromonium ion is on. This is called anti stereochemistry.
• look at the reaction with cyclopentene to appreciate this.

5. Catalytic Hydrogenation:

• the last reaction that is an electrophilic reaction is the addition of H₂ to an alkene to generate an alkane.
• the mechanism of this reaction is not clear, it involves carbon-metal and hydrogen-metal bonds. the catalyst involved is usually palladium or platinum. The reaction occurs on the surface.
• the interesting thing about this reaction is that it adds both hydrogens to the same face of the molecule. This is called syn stereochemistry. This is most easily understood of one looks at a substituted cycloalkene such as 1,2-dimethylcyclohexene. The product of catalytic hydrogenation is cis-1,2-dimethlycyclohexane.
• the reaction can be useful synthetically.