B3 - Electrophilic Addition

Question 1

What is the name of the mechanism for addition reactions in alkenes to form saturated compounds?

Answer 1

Electrophilic addition.

Question 2

What is an electrophile?

Answer 2

An atom or groups of atoms that is attracted to an electron-rich centre and accepts an electron pair. An electrophile is usually a positive ion or molecule containing an atom with a partial positive (delta+) charge.

Question 3

What attracts electrophiles?

Answer 3

The high electron density of the pi-electrons.

Question 4

The double bond in an alkene represents a region of high or low electron density? Why?

Answer 4

High electron-density, due to the presence of the pi-electrons.

Question 6

Using the reaction between but-2-ene and hydrogen bromide as an example, describe the steps for the electrophilic addition mechanism:

Answer 6

1. Bromine is more electronegative than hydrogen, so hydrogen bromide is polar and contains the dipole Hδ+—Brδ-
2. The electron pair in the pi-bond is attracted to the partially positive hydrogen atom, causing the double bond to break
3. A bond forms between the hydrogen atom of the H—Br molecule and a carbon atom that was part of the double bond
4. The H—Br bond breaks by heterolytic fission, with the electron pair going to the bromine atom
5. A bromide ion (Br-) and a carbocation are formed
6. In the final step the Br- ion reacts with the carbocation to form the addition product

Question 7

What is a carbocation?

Answer 7

A carbocation contains a positively charged carbon atom.

Question 8

How does the electrophilic addition mechanism work when alkenes react with non-polar molecules? Use bromine and an alkene as an example:

Answer 8

1. When bromine approaches an alkene, the pi-electrons interact with the electrons in the Br—Br bond. This interaction causes polarisation of the Br—Br bond, with one end of the molecule becoming Brδ+ and the other end of the molecule becoming Brδ-
2. The electron pair in the pi-bond is attracted to the Brδ+ end of the molecule, causing the double bond to break
3. A bond has now been formed between one of the carbon atoms from the double bond and a bromine atom
4. The Br—Br bond breaks by heterolytic fission, with the electron pair going to the Brδ- and a carbocation are formed
5. In the final stage of the reaction mechanism the Br- ion reacts with the carbocation to form the addition product of the reaction

Question 9

What is step 1 called in the electrophilic addition mechanism of an alkene with a non-polar molecule?

Answer 9

An induced dipole.

Question 10

What is the difference between a primary and secondary carbocation?

Answer 10

A primary carbocation is when the positive charge is on a carbon atom at the end of chain. In a secondary carbocation the positive charge is on a carbon atom with two carbon chains attached.

Question 11

What is the trend with stability of carbocations and why?

Answer 11

The more alkyl groups attached to the carbocation, the more the charge is spread out, making the ion more stable. Therefore tertiary carbocations are more stable than secondary carbocations, which are more stable than primary carbocations.