Unit 2 Module 1 Basic concepts and hydrocarbons
Alkenes and bromine
Learning outcomes
•
Describe how heterolytic fission leads to electrophilic addition in alkenes, using
bromine as an example.
•
Recall the reactions of alkenes.
Starter
Complete the reaction mechanism worksheet
Addition reactions of the hydrogen halides
-write a step-by-step guide for the addition reactions of the hydrogen halides.
Key terms to include;
 Electronegativity
 Dipole

π bond
 Double bond
 Heterolytic fission
Unit 2 Module 1 Basic concepts and hydrocarbons
Addition of bromine
Bromine is a non-polar molecule and yet it is used as a test for unsaturation because it
reacts with alkenes.
Using what you know about alkenes, bromine and electrons see if you can come up with a
reaction mechanism.
Unit 2 Module 1 Basic concepts and hydrocarbons
1.
(a)
Hex-3-ene reacts with Br2 to produce 3,4-dibromohexane.
Describe, with the aid of curly arrows, the movement of the electrons in the
mechanism.
Show the intermediate, any relevant dipoles and lone pairs of electrons.
H
H
C
C2H5
C
C2H5
C2H5
H
H
C
C
Br
Br
C2H5
Br
Br
Intermediate
3,4dibromohexane
[4]
(b)
The mechanism in (a) shows cis-hex-3-ene reacting with Br2. Trans-hex-3-ene
also reacts with Br2 to produce 3,4-dibromohexane.
(i)
How does the structure of trans-hex-3-ene differ from that of cis-hex-3ene?
...............................................................................................................
................................................................................................................
[1]
(ii)
Explain why both cis and trans hex-3-ene react with Br2 to produce the
same structural isomer.
................................................................................................................
................................................................................................................
................................................................................................................
[1]
[Total 6 marks]
1.
2-Methylbut-1-ene can be formed by the partial hydrogenation of isoprene.
2-Methylbut-1-ene reacts with Br2 to produce 1,2-dibromo-2-methylbutane by an
electrophilic addition mechanism. The mechanism for the reaction is shown below.
Unit 2 Module 1 Basic concepts and hydrocarbons
H3C
CH3
H3C
C
CH2
step 1
CH3CH2
CH2Br
C
step 2
CH3CH2
CH3CH2
Br
CH2Br
C
Br
Br
Br
(i)
In step 1, Br2 behaves as an electrophile. Explain what is meant by the term
electrophile.
.........................................................................................................................
[1]
(ii)
Label any relevant dipoles and add ‘curly arrows’ to the mechanism to show the
movement of electron pairs in step 1 and in step 2.
[3]
[Total 4 marks]
Peer mark your answers.
What do you need to do to improve?
What about alkenes with more than one double bond?
 Myrcene (C10H16) is a member of the terpene family of chemicals. These are alkenes
built from whole numbers of isoprene (C5H6) molecules.
 These isoprene molecules can be linked together head to tail forming chains or can
be formed into rings.
 Tarpenes contain carbon-carbon double bonds.
 If reacted with excess hydrogen in the presence of a nickel catalyst, all the double
bonds will react and form a saturated molecule
Complete the flow chart