1. The Reactivity of Halo alkanes City and Islington College
David Martin
City and Islington College
Students Name: …………………………………………………………………..

2. Topic Aims Method Equipment Duration Chemistry
This practical work contains theory that would let the learner consider the factors that might affect the reactivity of halo-alkanes – the learner can make predictions about which is the most important factors
Level
Level 2 & Level 3 (16-19 year old)
Method
PowerPoint slide 3 to 12 are question based, then slides 14 to 23 are answers.
Equipment needed:
2cm3 of ethanol
test tubes
1-chlorobutane
1-bromobutane
1-iodobutane
1cm3 of silver nitrate solution
water bath.
Equipment
Laptop & Projector
Pens
Hand-Outs
Duration
>30 Minutes

3. The Reactivity of Halo alkanes
All halo alkanes have a carbon halogen bond:
C-F C-Cl C-Br C-I
2.5 – 4.0 = 1.5
Idea 1
As all group 7 elements are all more electronegative than carbon, this bond is polar  
Element
Electronegativity
value F
4.0 Cl
3.0 Br
2.8 I
2.5 C
Which bond is most polar? ____________
Which bond is least polar? ____________
One of the main reactions halo alkanes undergo is species attacking the positive carbon.
Which type of halo alkane would be the most reactive using the above idea?
___________________________________________________________________

4. The Reactivity of Halo alkanes
Idea 2
The other factor is bond strength.
The harder it is to break a bond the more/less reactive the halo alkane is.
Below is a table of bond strengths
Bond
Energy in KJmol-1
C-F
467
C-Cl
346
C-Br
290
C-I
228
Which type of halo alkane would be the most reactive using the above idea?
__________________________________________________________

5. The Reactivity of Halo alkanes
Practical Work
The reaction of halo alkanes with water
For example:
CH3CH2Br + H2O CH3CH2OH + H+ + Br-
In this practical we are going to investigate which halo alkane reacts the fastest with water. The reaction can be slow so it is going to be performed in a hot water bath.
Procedure:
Place 2cm3 of ethanol in three test tubes (halo alkanes are not very soluble in water therefore ethanol is used to help the halo alkane dissolve)
Add 5 drops of the 1-chlorobutane to test tube 1
Add 5 drops of the 1-bromobutane to test tube 2
Add 5 drops of the 1-iodobutane to test tube 3
Then add 1cm3 of silver nitrate solution to each test tube.
Place the test tubes in a water bath at 60oC.
Note your observations in the table below:
Halo alkane
Observation – speed of reaction
Tube 1
1-chlorobutane
Tube 2
1-bromobutane
Tube 3
1-iodobutane

6. The Reactivity of Halo alkanes
Which halo alkane reacts fastest under these conditions?
_________________________________________________________
Which factor is more important – bond polarity or bond strength in the reaction of halo alkane.
_________________________________________________________

7. The Reactivity of Halo alkanes
Write balanced equations for the hydrolysis reactions carried out in this practical
The hydrolysis of 1-chlorobutane
_________________________________________________________
The hydrolysis of 1-bromobutane
The hydrolysis of 1-iodobutane
Write the formula of the product formed between silver ions and the halide ions in these reactions. Silver forms ions of formula Ag+
The hydrolysis of 1-chlorobutane _____________________________
The hydrolysis of 1-bromobutane _____________________________
The hydrolysis of 1-iodobutane _______________________________

8. The Reactivity of Halo alkanes
What is a hydrolysis reaction?
_________________________________________________________
The hydrolysis of 1-bromobutane

9. Br- The Reactivity of Halo alkanes The formation of carbocation’s
Draw the structures of the 3 halo alkanes to be used in the spaces below.
1-bromo-butane 2-bromo-butane 2-bromo 2-methyl propane
________________ ________________ ______________________
Classify each of them as either primary, secondary or tertiary
When the C-Br bond is broken in these compounds the Br takes both the electrons from the bond forming a Br-ion
Br-
Draw the electrons round a Br- ion.
Is a Br- ion stable or not? Explain your answer.
________________________________________________________

10. The Reactivity of Halo alkanes
The other species formed is called a carbocation.
When the C-Br bond breaks in 1-bromo-butane the carbocation formed looks like this:
H H H H
H-C- C- C- C +
This is called a primary carbocation as the number of carbons directly attached to the carbon with the positive charge is one.
Draw the two carbocation’s formed by the other halo alkanes and classify them:

11. The Reactivity of Halo alkanes
Practical work
2) The reaction of bromoalkanes with water
For example:
CH3CH2CH2CH2Br H2O CH3CH2CH2CH2OH + H+ + Br-
In this practical we are going to investigate which bromoalkane (primary, secondary or tertiary) reacts the fastest with water. The reaction can be slow so it is going to be performed in a hot water bath.
Procedure:
Place 2cm3 of ethanol in three test tubes (halo alkanes are not very soluble in water therefore ethanol is used to help the halo alkane dissolve)
Add 5 drops of the 1-bromobutane to test tube 1
Add 5 drops of the 2-bromobutane to test tube 2
Add 5 drops of 2-bromo-2-methyl propane the to test tube 3
Then add 1cm3 of silver nitrate solution to each test tube
Place the test tubes in a water bath at 60oC
Note your observations in the table below:
Halo alkane
Type
Observation – speed of reaction
Tube 1
1-bromobutane
Tube 2
2-bromobutane
Tube 3
2-bromo,2methylpropane

12. The Reactivity of Halo alkanes
Practical work
Which bromoalkane reacts fastest under these conditions?
________________________________________________________
Chemical fact – hydrocarbon groups such as methyl groups donate electron density.
How does this fact help explain our practical observations?

13. The Reactivity
of Halo alkanes
Answers

14. The Reactivity of Halo alkanes
Answers
All halo alkanes have a carbon halogen bond:
C-F C-Cl C-Br C-I
2.5 – 4.0 = 1.5
Idea 1
As all group 7 elements are all more electronegative than carbon, this bond is polar  
Element
Electronegativity
value F
4.0 Cl
3.0 Br
2.8 I
2.5 C
Which bond is most polar? C-F
Which bond is least polar? C-I
One of the main reactions halo alkanes undergo is species attacking the positive carbon.
Which type of halo alkane would be the most reactive using the above idea?
Fluor alkanes

15. The Reactivity of Halo alkanes
Idea 2
The other factor is bond strength.
The harder it is to break a bond the more/less reactive the haloaklkane is.
Below is a table of bond strengths
Bond
Energy in KJmol-1
C-F
467
C-Cl
346
C-Br
290
C-I
228
Which type of halo alkane would be the most reactive using the above idea?
Iodoalkanes

16. The Reactivity of Halo alkanes
Practical Work
The reaction of halo alkanes with water
For example:
CH3CH2Br + H2O CH3CH2OH + H+ + Br-
In this practical we are going to investigate which halo alkane reacts the fastest with water. The reaction can be slow so it is going to be performed in a hot water bath.
Procedure:
Place 2cm3 of ethanol in three test tubes (halo alkanes are not very soluble in water therefore ethanol is used to help the halo alkane dissolve)
Add 5 drops of the 1-chlorobutane to test tube 1
Add 5 drops of the 1-bromobutane to test tube 2
Add 5 drops of the 1-iodobutane to test tube 3
Then add 1cm3 of silver nitrate solution to each test tube.
Place the test tubes in a water bath at 60oC.
Note your observations in the table below:
Halo alkane
Observation – speed of reaction
Tube 1
1-chlorobutane
Colourless Solution
Tube 2
1-bromobutane
Colourless to misty cloud white solid over 10 minutes
Tube 3
1-iodobutane
Colour to yellow solid almost instant

17. The Reactivity of Halo alkanes
Which halo alkane reacts fastest under these conditions?
Iodoalkanes
Which factor is more important – bond polarity or bond strength in the reaction of halo alkane.
The bond strength seems to be the most important factor – the weaker the carbon halogen bond the faster the reaction.

18. The Reactivity of Halo alkanes
Write balanced equations for the hydrolysis reactions carried out in this practical
The hydrolysis of 1-chlorobutane
CH3CH2CH2CH2Cl H2O CH3CH2CH2CH2OH + H+ + Cl-
The hydrolysis of 1-bromobutane
CH3CH2CH2CH2Br H2O CH3CH2CH2CH2OH + H+ + Br-
The hydrolysis of 1-iodobutane
CH3CH2CH2CH2I H2O CH3CH2CH2CH2OH + H+ + I-
Write the formula of the product formed between silver ions and the halide ions in these reactions. Silver forms ions of formula Ag+
The hydrolysis of 1-chlorobutane AgCl
The hydrolysis of 1-bromobutane AgBr
The hydrolysis of 1-iodobutane AgI

19. The Reactivity of Halo alkanes
What is a hydrolysis reaction?
A reaction that decomposes a molecule by means of adding water
The hydrolysis of 1-bromobutane
The C-F bond is very strong therefore they are very resistant to chemical attack

20. Br- The Reactivity of Halo alkanes
The formation of carbocation’s
Draw the structures of the 3 halo alkanes to be used in the spaces below.
H H H H 1-bromo-butane
H-C- C- C- C-Br primary
H H H H
H H H H 2-bromo-butane
H-C- C - C- C-H secondary
H H Br H
H CH3H 2-bromo-2-methyl propane
H-C- C- C-H tertiary
H Br H
When the C-Br bond is broken in these compounds the Br takes both the electrons from the bond forming a Br-ion
Br-
It would have a full outer shell of eight electrons
Draw the electrons round a Br- ion.
Is a Br- ion stable or not? Explain your answer.
Yes it is - as it has a full outer electron shell

21. The Reactivity of Halo alkanes
The other species formed is called a carbocation.
When the C-Br bond breaks in 1-bromo-butane the carbocation formed looks like this:
H H H H
H-C- C- C- C +
This is called a primary carbocation as the number of carbons directly attached to the carbon with the positive charge is one.
Draw the two carbocation’s formed by the other haloalkanes and classify them:
H H H H
H-C- C - C- C-H
H H + H
 Carbocation from 2-bromo-butane
  H CH3H
H-C- C- C-H
H + H
 Carbocation from 2-bromo,2-methyl propane

22. The Reactivity of Halo alkanes
Practical work
2) The reaction of bromoalkanes with water
For example:
CH3CH2CH2CH2Br H2O CH3CH2CH2CH2OH + H+ + Br-
In this practical we are going to investigate which bromoalkane (primary, secondary or tertiary) reacts the fastest with water. The reaction can be slow so it is going to be performed in a hot water bath.
Procedure:
Place 2cm3 of ethanol in three test tubes (halo alkanes are not very soluble in water therefore ethanol is used to help the halo alkane dissolve)
Add 5 drops of the 1-bromobutane to test tube 1
Add 5 drops of the 2-bromobutane to test tube 2
Add 5 drops of 2-bromo-2-methyl propane the to test tube 3
Then add 1cm3 of silver nitrate solution to each test tube
Place the test tubes in a water bath at 60oC
Note your observations in the table below:
Halo alkane
Type
Observation – speed of reaction
Tube 1
1-bromobutane
 10
 slow
Tube 2
2-bromobutane
 20
fast
Tube 3
2-bromo, 2methylpropane
 30
 very fast

23. The Reactivity of Halo alkanes
Practical work
Which bromoalkane reacts fastest under these conditions?
Tertiary bromoalkane
Chemical fact – hydrocarbon groups such as methyl groups donate electron density.
How does this fact help explain our practical observations?
The more methyl groups that surround the carbo-cation the more stable the carbo-cation will be. Hydrocarbon groups by their nature donate electron density and help stabilise the carbo-cation so it is more likely to be formed.

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