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To investigate the effect of the halogen atom on the rate of hydrolysis.

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Introduction

Background knowledge The halogenoalkanes are a vital homologous series, this is due to their importance in organic synthesis, since the halide ion, X-(aq) is an excellent leaving group and may be readily substituted. Halogenoalkanes exist as simple molecules. Between these molecules, van der waals forces as well as dipole- dipole attractions (resulting from the polar C-X bond) operate. The existence of the dipole-dipole attractions explains why halogenoalkanes have slightly higher melting and boiling points when compared with the corresponding alkane. The general formula for halogenoalkanes is: CnH2n+1X. * Where X= fluorine, chlorine, bromine or iodine. Types of halogenoalkanes * Primary (1�) a halogen is present at the end of the chain: * Secondary(2�) a halogen is present in the body of the chain: * Tertiary(3�) a halogen is present at a branch in of chain: Strength of bonds I have included a table of the carbon - halogen bond energies, in which you can see trends between them. The main trend that you see is that the bond energies decrease from Fluorine to Iodine. With that in mind one can predict that due to the shared electrons being closer to the nucleus in fluorine than iodine, iodo-compounds will have the weakest bonds and greatest reactivity while fluorine compounds will have the strongest bonds and least reactivity. ...read more.

Middle

* 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 cm� beaker * 0�C - 100� C thermometer * 10 cm� measuring cylinder * digital timer * 3 dropping pipettes * 1-cholobutane* * 1-bromobutane* * 1-iodobutane* * ethanol * silver nitrate solution (approximately 0.1 mol dm- �) * 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 cm� 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. ...read more.

Conclusion

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- http://www.webelements.com/webelement/text/c/enth.html C-X Bond Bond Enthalpy kj mol-� C-Cl 397� 29 C-Br 280�21 C-I 209�21 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. ...read more.

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