Bond Polarity
Halogens are electronegative atoms. Attaching a halogen atom to a carbon atom creates a polar covalent bond. Carbon and halogens have different electronegativities (shown in table below) and halogenoalkanes have polar molecules with a polar C-X bond.
The polarity produces an electron deficient carbon atom, C δ+ which is important in the reactions of halogenoalkanes as it promotes nucleophilic attack, resulting in the displacement of the halide ion. The polarity decreases from fluorine to iodine reflecting the decrease in electronegativity down the halogen group.
Nucleophilic Substitution reactions of halogenoalkanes
The reaction mechanism undergone by the carbon-halogen bond in halogenoalkanes when they are substituted is nucleophilic substitution.
Nucleophiles are electron pair donors and are attracted to the δ+ carbon atom of the carbon- halogen bond. Furthermore, the approaching nucleophile repels electron density towards the polarizable halogen atoms, thus inducing a larger dipole in the bond. The carbon- halogen bond eventually breaks and the incoming nucleophile takes its place.
The rate of nucleophilic substitution depends on which halogen is present. Carbon- halogen bonds have different strengths. The C-Cl bond is stronger than the C-Br bond which in turn is stronger than the C-I bond. It is the carbon-halogen bond that breaks during a nucleophilic substitution reaction. As a result iodo-compounds substitute faster than bromo-compounds which substitute faster than chloro-compounds.
Risk Assessment and safety
- Only perform experiment specifically assigned by your teacher, familiarize yourself with the investigation and all safety precautions before entering the lab.
- During this procedure you must make sure to wear safety glasses at all times to protect your eyes from any dangers there may be in the experiment.
- Make sure if your hair is long to keep it tied back and out of your face as it could prove irritating during the experiment as well as avoiding any loose clothing that could knock things over, catch on fire or absorb chemical solutions.
- Make sure the area around you is kept tidy and uncluttered so spillage does not occur, or that you trip or fall during the experiment.
- Ethanol is highly flammable. Do not have any naked flames near the tubes or ethanol bottle.
- The bromobutane and iodobutane is harmful therefore you must take care when handling these substances.
- Clean your work area at the conclusion of each lab as directed by your teacher.
- Know the location of all safety and emergency equipment used in the laboratory, immediately report any accident or hazard to your teacher
Apparatus:
- Safety glasses
- Test tube rack
- 4 test tubes
- labels
- 250 cm³ beaker
- 0ºC - 100º C thermometer
- 10 cm³ measuring cylinder
- digital timer
- 3 dropping pipettes
-
1-cholobutane*
-
1-bromobutane*
-
1-iodobutane*
- ethanol
- silver nitrate solution (approximately 0.1 mol dm- ³)
* use separate pipettes for each bottle of those indicated above with asterisks
Method
- Collect all apparatus on the list above.
- Set up three test tubes in a test tube rack. Label each tube with 1 cm³ ethanol. Add to the label of the first test tube 2 drops of 1-chlorobutane, to the second add 2drops of 1-bromobutane and to the third add 2 drops of 1-iodobutane.
- Position each test tube into a large beaker of water heated to about 50º C.
- Place a tube containing 5 cm³ of 0.1 mol dm-³ silver nitrate solution into the same beaker of water as before
- Let the tubes sit for about 10 minutes; this will allow them to adjust to the temperature of the water bath.
-
Add 1 cm³ of the silver nitrate solution to each of the three test tubes, * you must do so quickly and take noting of the time.
- Shake each test tube to mix the contents
- Observe the test tubes over a period of time take note of your observations and any differences or changes you discover. \also note the time at which the first precipitate appears.
I have been asked to design an experiment to examine how the rates of reaction of halogenoalkanes vary with respect to the Cδ+- Xδ bond. I will be varying the Cδ+- Xδ bond using the halogens Chlorine, Bromine and Iodine (C-Cl, C-Br or C-I) to see which of these bonds give the fastest rate of hydrolysis.
Factors affecting the rate of hydrolysis
- Temperature
- Concentration of reagents
- Pressure
- Surface area
- Use of a catalyst
I will make sure these factors remain constant throughout the entire experiment, this will ensure that only the chosen variable (the halogenoalkanes) is causing any difference to the rate of hydrolysis.
Prediction
I predict that during warming, reactions will occur in all the test tubes except the control. I predict that the liquids in these test tubes will change from transparent to cloudy causing the “cross” on each test tube to no longer be visible. I predict that this will be due to an insoluble precipitate being formed in each test tube. Furthermore, I predict that this precipitate will be either silver chloride, silver bromide or silver iodide depending on the halogenoalkanes used and as these precipitates will be formed at different rates the “crosses” on each test tube will disappear at different times.
I predict that as the halogen is changed down the group, i.e. from chlorine to bromine to iodine, the rate of reaction will increase. This is because of the bond enthalpy of a carbon- iodine bond being lower in energy than that of a carbon-bromine bond, which is subsequently lower than that of a carbon-chlorine bond. This shown by the following extract from the website-
The enthalpies of the C-X bonds decrease from C-Cl to C-I due to the difference in size of the halogen atoms. As group 7 is descended, the size of the halogen atom increases, i.e. iodine is a much larger atom than chlorine. The larger the halogen atom is the less the bond overlap with the carbon atom, resulting in a longer C-X bond. The longer the bond the weaker the bond is and this means less energy is required to break the long carbon-iodine bond than the short carbon-bromine or even shorter carbon-chlorine bonds. Therefore I predict that iodine will be displaced in the least amount of time, followed by bromine and then chlorine.
Bibliography
- Beavon, Rod: AS/A-Level Chemistry Essential word Dictionary
- Clugston and Flemming: Advanced Chemistry
- Eccles and Wooster: Revise for AS chemistry for OCR A
- King, Rob: AS/A-Level chemistry exam revision notes
- Letts: Revise AS Chemistry
- Lister and Renshaw: New Understanding Chemistry for advanced level
- Ratcliff, Brian: chemistry 1 endorsed by OCR
