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hydrolysis of halogenoalkanes

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Introduction

Hydrolysis of Halogenoalkanes In this piece of coursework I aim to investigate the reactivity of three halogenoalkanes. By doing this it allows me to see which halogenoalkane is the most reactive and which is the least reactive. Halogenoalkanes have an alkane skeleton with one or more halogen atoms in place of hydrogen atoms. The general formula of a halogenoalkane with a single halogen atom is Cn + H2n+1X, where X is the halogen. The three halogenoalkanes that I will be testing are: 1- chlorobutane 1- bromobutane 1- iodobutane As you can see above all the halogens are primary halogens indicated by the number 1. This is where the halogen is at the end of the chain, by keeping all the halogenoalkanes as primary ones it becomes a fairer a test as the halogen is situated in the same place in each. This is because reactions may vary due to the halogen not being in the same place therefore results from the experiments taken out may not necessarily be those which are relevant to the objective of the experiment. ...read more.

Middle

There is also a difference in electronegativities between a Carbon-Halogen bonds where the halogen is more electronegative. This also makes this bond polar meaning the halogen has a slightly negative charge and the carbon has a slightly positive charge. Due to this polarity the electron deficient carbon atom is attacked by the electron-rich species which in this case is the nucleophile hydroxide ion. As the hydroxide ion and carbon atom are attracted to each other, the carbon-halogen bond breaks so halogen leaves as a halide ion. As halogenoalkanes are insoluble in water the reaction will have to carried out in the presence of ethanol, which acts as a solvent. This creates a hydrolysis reaction. There are no colour changes during this reaction as halogenoalkanes are covalently bonded; they give no precipitate of silver halide. Therefore silver nitrate will be used, and gradually the halogen can be detected determined by the colour of the precipitate. If the precipitate is white then this means that chlorine is present, if cream bromine is the halogen present and finally if yellow it is iodine which is present in the halogenoalkane. ...read more.

Conclusion

Halogen Electronegativity Value Chlorine 3.0 Bromine 2.8 Iodine 2.5 This means that the polarity of the C-Cl bond is more polar than C-Br and finally the C-I is the least polar bond. From this the reactivity of the halogens involved is as follows; Cl>Br>I. Bond enthalpy however predicts that the reverse will be true where the iodo-compound will be the most reactive. Below is a table showing the bond enthalpies of the carbon-halogen bond. Bond Bond enthalpy/ kJmol?� Carbon-Chlorine 346 Carbon-Bromine 290 Carbon-Iodine 228 Bond enthalpy which is the energy required to break a bond shows us that iodo-compound will have the weakest bond while the bromo-compounds have stronger bonds and then finally chloro-compounds have the strongest bonds. This is due to the atomic radius of the atoms where iodine is the biggest and chlorine has the smallest atomic radius. The outermost electrons feel less of the nuclear charge as the atoms have a larger atomic radius; this means that it easier for other atoms to react with the electrons of the bigger atom. From this the reactivity of the carbon-halogen bond is as follows; C-I>C-Br>C-Cl. Bond enthalpy governs the rate of reactions of the halogenoalkanes. ...read more.

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