[13.4] electrophilic addition in alkenes

Question 1

what is an electrophile?

Answer 1

an electron pair acceptor

Question 2

what is an addition reaction?

Answer 2

a reaction in which a reactant is added to an unsaturated molecule to make a saturated molecule

Question 3

explanation of the mechanism for an electrophilic addition reaction with a polar molecule (example)

Answer 3

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. 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. Br- ion reacts with the carbocation to form the addition products

Question 4

draw the mechanism for the for an electrophilic addition reaction with a polar molecule

Answer 4

• draw delta signs
• draw reaction mechanism between H—Br
• arrows must touch the bond / products
• draw all (8) electrons on bromide ion
• make sure things are added to the correct carbon atom

Question 5

what is a carbocation?

Answer 5

a positively charged carbon atom

Question 6

explanation of the mechanism for an electrophilic addition reaction with a non-polar molecule (example)

Answer 6

1. when bromine approaches an alkene, the pi-electrons interact with the electrons in the Br—Br bond
2. this interaction causes the polarisation of the Br—Br bond, with one end of the molecule becoming Brδ+ and the other end of the molecule becoming Brδ-. this is known as an induced dipole
3. the electron pair in the pi-bond is attracted to the Brδ+ end of the molecule, causing the double bond to break
4. a bond has now been formed between one of the carbon atoms from the double bond and a bromine atom
5. the Br—Br bond breaks by heterolytic fission, with the electron pair going to the Brδ- end of the molecule
6. a bromide ion (Br-) and a carbocation are formed
7. Br- ions reacts with the carbocation to form the addition product of the reaction

Question 7

what is markovnikov’s rule?

Answer 7

when HX adds across an asymmetrical double bond, the major product formed is the molecule in which hydrogen adds to the carbon atom in the double with the greater number of hydrogen atoms already attached to it

Question 8

summary of major / minor products based on primary / secondary carbocations

Answer 8

• primary carbocation = minor product
• secondary carbocation = major product

Question 9

describe carbocation stability

Answer 9

• secondary carbocation (major product) is more stable
  > secondary carbocation is more stable than primary carbocation
• stability increases from primary -> secondary -> tertiary carbocation etc.