Ch. 13.4 : Electrophilic Addition in Alkenes

Question 1

Define electrophile

Answer 1

An electron pair receptor

Question 2

What is electrophilic addition?

Answer 2

The mechanism of how alkanes undergo an addition reaction

Question 3

Describe electrophilic addition

Answer 3

• double bond = region of high electron density because of pi - electrons
• High electron density of pi electrons attracts electrophiles
• an electrophile is usually a positive ion or a molecule containing an atom with a partial positive charge.

Question 4

Describe the electrophilic addition of HBr with butene

Answer 4

Hydrogen bromide is polar and contains the dipole H( delta positive ) - Br ( delta negative )

The electron pair in the pi bond is attracted to the partially positive hydrogen atom, causing the double bond to break

A bond forms between the H atom of the HBr molecule and the carbon atom that took the electrons from the double bond

The H - Br bond breaks by heterolytic fission, with the electron pair going to the bromine atom

A bromide ion and a carbocation are formed

The br- ion reacts with the carbocation to form the final addition product.

Question 5

Describe how butene would react with Br2 ( a non - polar molecule )

Answer 5

When the bromine approaches the double bond, the pi electrons causes polarisation of the Br-Br bond, with one end becoming partially positive and the other partially negative ( induced dipole)

The reaction now carries on as it would normally with a polar molecule

Question 6

What is Markownikoff’s rule ?

Answer 6

It predicts the isomer produced in the largest amount.

When a hydrogen halide reacts with an asymmetrical alkene the halide ( or OH ) will bond to the C bonded to the most C’s.

This is because alkyl groups have an electron donating effect, and therefore they push electrons towards positive charges. This spreads out the positive charge amongst themselves.

Question 7

Describe stability in carbonations

Answer 7

The more carbons there are bonded to a carbocation, the more stable that carbocation will be.

Carbocations bonded to 3 other carbons / alkyl groups are more likely to for than carbocations bonded to 2 other carbons / alkyl groups.

Question 8

Describe the difference between primary and secondary carbocations

Answer 8

Most stable : tertiary

Least stable : primary