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

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

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? ___________________________________________________________________

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? __________________________________________________________

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

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. _________________________________________________________

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 _______________________________

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

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.   ________________________________________________________

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:

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

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?

The Reactivity of Halo alkanes Answers

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

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

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

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.

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

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

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

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  

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

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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