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coursework plan for halogenalkanes

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Introduction

Comparing the rate of hydrolysis from different haloalkanes Sachin Patel Candidate Number : 8169 Form Class : 12TU4 Mrs Greyson AS Chemistry Cousework (planning) Aim To investigate the rate of hydrolysis of different haloalkanes and to see which C-X bond has the fastest rate (X = any haloalkane bonded to a carbon atom) under a precipitation reaction acting as a monitor. Introduction Haloalkanes are compounds which contain at least one halogen atom bonded to a carbon atom and have a general formula of CnH2n+1X. The hydrogen atom in the alkane molecule gets replaces by a halogen atom in a nucleophillic substitution reaction. Nucleophiles are electron rich molecules which have lone pairs of electrons and therefore attack an electron loving carbon atom. The C-X bond is polarised (C ?+-X ?-) due to the halogen atom being more electronegative then the carbon atom. The electrons in the covalent bond are nearer to the halogen atom therefore the halogen atom carries a partial negative charge. This leaves the carbon atom with a partial positive charge causing the whole bond to become polar but not the whole molecule. In a hydrolysis reaction the negative side of the bond causes the attraction of a hydroxyl ion (OH-) which acts as the nucleophile. It attacks the carbon atom of the haloalkane forming a halogen halide and an alcohol. I will monitor the rate of hydrolysis by using silver nitrate (AgN03). The time it takes for the silver halide precipitate to form will indicate the rate of hydrolysis of each haloalkane. AgNO3 C2H5X + OH - C2H5OH + X - Below is a diagram showing a nucleophilic substitution reaction:- Below is a SN1 mechanism:- Below is a SN2 mechanism:- In this reaction the haloalkane 1-Bromobutane undergoes a nucleophilic substitution reaction. The same will happen for 1-Chlorobutane and 1-Iodobutane. A SN1 mechanism is when the bond between the carbon atom and halogen atom breaks due to heterolytic fission forming a carbocation. ...read more.

Middle

This therefore means that it will take longer periods of time to break. My prediction can be backed up by what I mentioned before about the shielding effect and nucleur charge. As you go down group 7, the electronegativity decreases and shielding increases due to the number of energy levels also increasing. The C-Cl bond will need a higher amount of energy because the chlorine atom has less energy levels causing a greater nuclear attraction. However the C-I bond will need a lower amount of energy because the iodine atom has more energy levels and so has a greater shielding effect causing a weaker nucleur attraction. The ability for halogens to depart electrons in a covalent bond decreases down the group. The C-Cl bond holds on to the electrons in the covalent bond so strongly so that it takes a lot of energy to break the bond, and the opposite occurs in a C-I bond. In my experiment, I will predict that 1-iodobutane will take the shortest time to form a silver precipitate and 1-chlorobutane will take the longest time to produce a silver precipitate. Therefore 1-iododbutane will undergo hydrolysis much faster than 1-chlorobutane because it acts as a better leaving group. It is a better leaving group because an iodine atom has more shielding, weaker electronegativity and a lower bond enthalpy. Iodine is less electronegative and therefore has a smaller bond polarity with carbon compared to chlorine. Equipment * LR Ethanol (250cm3 reagent bottle) - The ethanol will act as the nucleophile in the reaction and so will be kept constant * 0.1M Silver Nitrate (250cm3 reagent bottle) - This will help monitor the rate of hydrolysis by forming a silver halide precipitate * Water bath - This will be needed so that all haloalkanes that are being used in the experiment are at a constant high temperature so a fair test could be achieved. ...read more.

Conclusion

1-Bromobutane If this chemical has been inhaled by a pupil then can cause irritation to the upper respiratory tract and mucous membranes. The pupil will start to get a headache, cough, vomit, internal burning and get shortness of breath. Taking in this chemical either by drinking or eating will cause irritation to the gastrointestinal tract. If in contact with the skin can cause irritation such and itching and redness. If in contact with eyes or area around the eyes can also cause irritation and redness but also a little bit of pain. (Medical treatment) - If the chemical has been inhaled and finding it difficult to breath, go out side and get some fresh air. - If the chemical has been taken in by eating or drinking it the drink plenty of water. - If the chemical has been in contact with the skin the remove any spillage clothing and run that area of skin under cold water for about 15 minutes. - If the chemical has been in contact with eyes then add a lot of water to them making sure you lift the top and bottom eye lids occasionally. 1-Iodobutane This chemical may be toxic but the properties have not yet been fully discovered yet. If this chemical has been inhaled then can be harmful and can be irritating inside the body if taken in either by eating or drinking it. If in contact with skin or eye, it can also be irritating causing redness and itching. (Medical treatment) - If the chemical has been inhaled then get some fresh air or if finding it difficult to breath provide oxygen. - If the chemical has been taken in by eating or drinking then drink plenty of water. - If chemical has been in contact with eyes then add a lot of water to them for at least 15 minutes. - If chemical has been in contact with skin the pour a lot of soapy cold water on that area. ...read more.

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