H H main carbon atom)
This nucleophile forms an intermediate and so the carbon has 5 bonds (10 electrons) in its outermost shell.
H H OH
ﺍ ﺍ
H-C-C
ﺍ ﺍ Br
H H
The spare pair of electrons must be removed from the intermediate and these will leave via an electronegative atom (in this case Br) which is known as the leaving group.
H H
ﺍ ﺍ
H-C-C-OH + H-Br
ﺍ ﺍ
H H
The reaction takes place in ethanol as halogenoalkanes do not mix readily with water. This is also a hydrolysis reaction.
Prediction
The most reactive halogen will be iodine up to Cl being the least reactive.
I → Br → Cl
Most least
Reactive Reactive
The outer electrons of iodine are further from the positive nucleus as iodine has more electrons than Br and Cl. Therefore it has more shells and the outer electrons are further from the positive nucleus and there is also shielding from inner electron shells. This makes the C-I bond easier to break and so less energy is needed to break the bond (Ea). Chlorine has fewer electrons than iodine so the outer electrons of chlorine are closer to the nucleus and so more energy is required to break these bonds. However if the polarity of the bonds is considered, the halogens are more electronegative than carbon so the polarity will be C - X‾ (where X is a halogen).
These bonds are attacked by nucleophiles (as in introduction) at the carbon atom. As you go down the group of halogens, their electro negativity decreases (essential as chemistry for OCR). This makes the polarity of the bonds decrease as you go down the group because the difference in charge between the C and X decreases.
However, this would predict that Chlorine is the most reactive as it is the most polar, but I predict that iodine is the most reactive because of bond enthalpy, that iodo-compounds have the weakest bonds (essential AS chem. OCR pg 115)
Apparatus
- Three test tubes and test tube rack
- Ethanol (as a solvent)
- Water (to provide OH ion)
- Measuring cylinder
- Safety goggles
- Chloroethane, bromoethane and iodoethane( these are primary substrates and good for substitution reactions, if I was to do an elimination I would use tertiary structures)
- Silver nitrate (to test halogenoalkanes)
- Pipette
Method
To find out the reaction rates of the halogens I will react halogenoalkanes with water using ethanol as a solvent. Ethanol is used as a solvent to dissolve the reagents so that they may come into contact with one another. It must not react with or decompose any of the reagents. It is a polar solvent and so is good at stabilizing ions (organic chemistry by Joseph M Hornback).
- Firstly, the reaction takes place at room temperature so no control of this is needed.
- The three halogenoalkanes used will be iodoethane, chloroethane and bromoethane.
- In one test tube place 1cm³ of ethanol with 2 drops of 1-chloroethane, in the next two test tubes place 1cm³ ethanol and 2 drops of 1-bromoethane and in the other, 1cm³ 1-iodoethane
- When the silver nitrate reacts with a halide ion a silver halide precipitate forms which will be white if there are chlorine ions present in solution, cream if there are bromine ions and pale yellow if there are iodide ions. (essential as chemistry for OCR section 6.2.4)
- 1cm³ of 0.05 mol dm³ silver nitrate solution is added to each of the test tubes, a bung put on each and the tubes are shaken and the time taken for the precipitate to form can be recorded using a stop watch.
- When the precipitate forms, stop timing.
- The precipitate that forms first will have the fastest reaction because, firstly the halogenoalkane reacts with the nucleophile to leave an alcohol (in this case, ethanol) and the halide ions, which will then form precipitates with the aqueous silver nitrate.
Precautions
Safety glasses must be worn to prevent any mixtures from entering the eye. Care must be taken using ethanol as it is flammable so no naked flames. (Although, the experiment doesn’t require heat).
Stools and bags must be under desks to prevent tripping up whilst carrying chemicals and hair tied back as to prevent it obstructing sight and getting in chemicals. A lab coat is also essential to prevent spillages onto clothing.
