Hydrolysis is the breaking of covalent bonds by the reaction with water. Halogenoalkanes do not mix with water this is because they attract the water molecules by dipole – dipole forces which are weaker so they must be mixed with ethanol before being treated with dilute aqueous sodium hydroxide. Warming the mixture then causes a Nucleophilic substitution. Which produces an alcohol.
Aim: The aim of this experiment is to investigate which Halogenoalkane hydrolyses first and why?
Electronegativity Values
As you descend down the group the Electronegativity decreases (Cl → Br → I). This is due to increased electron shielding higher up the group, greater positive charge from the nucleus so therefore more attraction to the negative electron and distance from the nuclei.
Table of Bond Enthalpies
Prediction
Initially looking at the Electronegativity of the halogenoalkanes, I would have predicted that Cl would hydrolyse the fastest; this is because it is the most electronegative of all the halogenoalkanes that we are investigating. The initial thought is that C – I bond would be broken the slowest as the carbon is the least positively charged.
However after examining the bond enthalpies of the halogenoalkanes I have come to a different conclusion, the table above show a significant decrease in bond enthalpy from C – Cl to C – I. This corroborates that the ease of which the bond is broken is more important than the than the polarity of the halogenoalkane. A nucleophile may be more attracted to the carbon atom than the halogen but unless a stronger bond can be formed between them it will not displace the halogen.
So I predict that the 1-iodobutane will hydrolyse first, then the 1-bromobutane and finally the 1-chlorobutane.
The Experiment
Apparatus
The apparatus needed for this experiment are: -
- 1-Chlorobutane
- 1-Bromobutane
- 1-Iodobutane
- Ethanol
- Silver Nitrate
- Nitric Acid
- Sodium Hydroxide
- 3 Boiling Tubes
- Water Bath
- Test tube rack
- Thermometer
- Stop Clock
Method
- Add 1cm3 of ethanol to each test tube.
- Then add the 2-3 drops of each halogenoalkane to each test tube.
- 5cm3 of silver nitrate, AgNO3 must then be added.
-
Remove from water bath, place in test tube rack, and then add 1cm3 of Nitric Acid, HNO3.
- Then add 1cm3 of Sodium hydroxide to each test tube, and then heat in water bath to around 50°C. Check with thermometer.
- Then record the time taken for each precipitate to form for each Halogenoalkane. Using the stop clock
Table of reactions
Fair Test
To ensure the experiment is kept fair and allows for the most accurate results that would be able to be obtained. Certain aspects of the experiment would have to be kept the same such as:
- Temperature. This could be kept constant using a thermostatically controlled water bath.
- Same concentration of the chemicals used.
- Same quantities of the chemicals used.
Risk Assessment:
Chlorobutane / Bromobutane / Iodobutane
(Hazcard No. 44)
- Highly Flammable
- Harmful by inhalation, ingestion and skin contact
- Narcotic in high concentrations
- Irritating to eyes and skin
- Especially dangerous with sodium, as explosive reaction may occur
Silver Nitrate
(Hazcard No. 87)
- Causes burns
- Dangerous to eyes and can blacken skin.
- If ingested can cause internal damage due to adsorption of blood
Sodium Hydroxide
(Hazcard No. 91)
- Causes severe burns
- Dangerous to eyes and skin
- Dangerous when reacted with small amount of water, can reach high temperature, due to exothermic reaction
Actions Taken
With the above chemicals, if contact with skin or eyes occurs, fully irrigate the area with water. If the solution is ingested, do not induce vomiting; drink glasses of water to irrigate the system.
Safety Precautions: that must be taken when using the above chemicals are:
- Where goggles at all time
- Take care when pouring chemicals
- Wear lab coats to avoid getting chemicals on clothes
- Do not sit down when carrying out experiment
- All unnecessary equipment i.e. bags and coats, put away safely.
Bibliography
Textbooks
Advanced Sciences Chemistry 1
Internet
