coursework plan for halogenalkanes

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Comparing the rate of hydrolysis from different haloalkanes

Sachin Patel

Candidate Number : 8169

Form Class : 12TU4

Mrs Greyson

AS Chemistry Cousework (planning)


To investigate the rate of hydrolysis of different haloalkanes and to see which C-X bond has the fastest rate

(X = any haloalkane bonded to a carbon atom) under a precipitation reaction acting as a monitor.


Haloalkanes are compounds which contain at least one halogen atom bonded to a carbon atom and have a general formula of CnH2n+1X.

The hydrogen atom in the alkane molecule gets replaces by a halogen atom in a nucleophillic substitution reaction. Nucleophiles are electron rich molecules which have lone pairs of electrons and therefore attack an electron loving carbon atom.

The C-X bond is polarised (C δ+-X δ-) due to the halogen atom being more electronegative then the carbon atom. The electrons in the covalent bond are nearer to the halogen atom therefore the halogen atom carries a partial negative charge. This leaves the carbon atom with a partial positive charge causing the whole bond to become polar but not the whole molecule. In a hydrolysis reaction the negative side of the bond causes the attraction of a hydroxyl ion (OH-) which acts as the nucleophile. It attacks the carbon atom of the haloalkane forming a halogen halide and an alcohol.  

I will monitor the rate of hydrolysis by using silver nitrate (AgN03). The time it takes for the silver halide precipitate to form will indicate the rate of hydrolysis of each haloalkane.


C2H5X   +   OH -                            C2H5OH   +   X -

Below is a diagram showing a nucleophilic substitution reaction:-

Below is a SN1 mechanism:-    





Below is a SN2 mechanism:-


In this reaction the haloalkane 1-Bromobutane undergoes a nucleophilic substitution reaction. The same will happen for 1-Chlorobutane and 1-Iodobutane.

A SN1 mechanism is when the bond between the carbon atom and halogen atom breaks due to heterolytic fission forming a carbocation. The carbocation is highly positively charged and so is attacked by the negatively charged nucleophile.

A SN2 mechanism occurs in primary haloalkanes where the 2 stands for there being 2 species (molecules or ions) involved in the initial stage of the reaction.

Below shows three symbol equations showing nucleophilic substitution taking place for each hydrolysis reaction that I will carry out:-

                                                            AgNO3 & CH3CH2OH

1. CH3CH2CH2CH2Cl (l) + :OH - (aq)                                              CH3CH2CH2CH2OH (aq)  + :Cl - (aq)

                                                                        AgNO3 & CH3CH2OH

2. CH3CH2CH2CH2Br (l) + :OH - (aq)                                              CH3CH2CH2CH2OH (aq)  + :Br - (aq)

                                                      AgNO3 & CH3CH2OH

3. CH3CH2CH2CH2I (l) + :OH - (aq)                                                 CH3CH2CH2CH2OH (aq)  + :I - (aq)

Below shows three ionic equations for each hydrolysis reaction that I will carry out:-

1. Ag +   +   Cl -                                            AgCl   (white precipitate)

2. Ag +   +   Br -                                            AgBr   (pale yellow precipitate)

3. Ag +   +    I -                                             AgI      (darker yellow precipitate)

Factors affecting the rate of hydrolysis

1] Polarisation of the C-X bond

Bond polarity occurs due to the difference in electronegativity in the C-X bond. The carbon atom is less electronegative then the halogen atoms so the electrons in the bond are more nearer to the halogen. The carbon atom will now have a partial positive charge known as δ+ (delta plus) and the halogen atom will have a partial negative charge known as δ- (delta minus). As you go down group 7 on the periodic table, polarisation of the C-X bond decreases. This is because the attraction between the two gets weaker due to a greater shielding effect because the number of electrons increase and a weaker nuclear attraction between the central atom of the carbon atom and the outer shell electrons of the halogen atom.  

C-I > C-Br > C-Cl 

       Electronegativity decreases as you

       go down group 7 in the periodic table

When a hydrolysis reaction takes place, it’s always the carbon atom which gets attacked by a nucleophile. The halogens are more electronegative than the carbon therefore pull electrons in the bond leaving the carbon atom electron deficient, in other words seeking for electrons. Now a negatively charged nucleophile with lone pairs attacks the carbon atom (or carbcation). I think the C-I bond will have the fastest rate of hydrolysis compared to the C-Br and C-Cl bonds. Not only that I can say the C-I bond has more shielding and a weak nuclear attraction, but because the electrons in the covalent bond are quite far away from the Iodine atom, less  energy will be required to break the bond and therefore it will take less time for hydrolysis to be completed.

2] Bond Enthalpy

Bond enthalpy is basically the energy needed to break covalent bonds. As you go down group 7 in the periodic table, bond enthalpy decreases due to polarisation also decreasing. The nuclear attraction gets weaker between the carbon atom and halogen atom due to greater shielding and a larger atomic radius. The halogens which are at the top of the group will need much more energy to break their bond with carbon; however the ones which are at the bottom will need less energy. In conclude the rate of hydrolysis will increase down the group because the bond will break quicker and therefore form new bonds faster.

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Shielding will increase because of more electron shells being present and so weaken the attraction between the outer shell bonding electrons and the central nucleus in the halogen atom. The bigger the bond enthalpy, the greater the amount of energy needed to break the bond and the lower the bond enthalpy, the lower the amount of energy needed to break the bond. This means that if more energy is needed to break the bond then the rate of hydrolysis will decrease and if less energy is needed to break the bond the rate of hydrolysis will increase.


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