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Preparation of a Haloalkane

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Introduction

Experiment 2 Preparation of a Haloalkane Date: 11-9-2006 Objective To produce 2-chloro-2-methylpropane from 2-methylpropan-2-ol. To find the yield of 2-chloro-2-methylpropane. Introduction 2-chloro-2-methylpropane is formed when 2-methylpropan-2-ol and concentrated hydrochloric acid are added together. Because alcohols undergo substitution rapidly, the reaction takes place at room temperature. Afterwards, the haloalkane would be separated with a separating funnel, dried with anhydrous sodium sulphate and extracted by distillation. Procedure 1. About 9 ml of 2-methylpropan-2-ol was poured into a measuring cylinder and the measuring cylinder was weighed. 2. The 2-methylpropan-2-ol was poured into a 50ml separating funnel. The mass of the empty measuring cylinder was weighed again. 3. About 20ml of concentrated hydrochloric acid was added into the separating funnel, 3ml at a time. This procedure was carried out by the window side. 4. After each addition, the funnel was sealed and was shaken. The tap was opened at intervals to allow the gas produced to be released. ...read more.

Middle

Data and Calculation 1. The range of boiling point of 2-chloro-2-methylpropane was 46~53oC. 2. Mass of measuring cylinder + 2-methylpropan-2-ol 26.6233 g Mass of measuring cylinder after emptying 19.869 g Mass of 2-methylpropan-2-ol 6.574 g Mass of collecting flask 21.328 g Mass of collecting flask + 2-chloro-2-methylpropane 22.993 g Mass of 2-chloro-2-methylpropane 1.665 g Molar mass of 2-methylpropan-2-ol: 12x4 + 1x10 + 16 = 74g/mol Molar mass of 2-chloro-2-methylpropane: 12x4 + 1x9 + 35.5 = 92.5g/mol Number of moles of 2-methylpropan-2-ol used: 6.574/ 74 = 0.0888 moles For 100% yield, the number of moles of 2-methylpropan-2-ol should also be 0.0888 moles. So, the maximum mass of 2-chloro-2-methylpropane produced should be 0.0888 x 92.5 = 8.2175g Actual mass of 2-chloro-2-methylpropane produced is 1.665g So, the percentage yield is 1.665/8.2175 x 100% = 20.3 % Conclusion 1. The mass of 2-chloro-2-methylpropane produced is 1.665g. 2. The percentage yield is 20.3%. ...read more.

Conclusion

4. The drying agent itself also absorbs the haloalkane. This reduces the percentage yield. Another reason for a yield lower than expected may be due to too much organic layer in the separating funnel being treated as the junction layer and disposed of during separation. 5. The preparation of haloalkane from tertiary alcohol takes place at room temperature. This is due to 3 alkyl group being attached to the carbon atom. As a result, the carbocation intermediate is stabilized by 3 electron release alkyl groups, thus lowering the charge on the carbocation. Consequently, the activation energy required is reduced. If a primary or secondary alcohol was used, heating would be required in addition to zinc chloride catalyst. 6. When we do not know which layer of the separating funnel is aqueous, we simply add a drop of water into the separating funnel. The drop of water would merge with the aqueous layer, thus the layer which is aqueous can be determined. 7. The boiling point range for the product during distillation was 46oC to 53oC. The product is not very pure. ...read more.

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A well written, concise investigation of average quality. All the basics are present but further explanation of procedure and experimental calculations would make this piece more detailed and clear, important for showing mastery of the subject and understanding. Being scientific ...

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Response to the question

A well written, concise investigation of average quality. All the basics are present but further explanation of procedure and experimental calculations would make this piece more detailed and clear, important for showing mastery of the subject and understanding. Being scientific and concise is important but it’s always worth making sure your work is understandable even by a non-scientist to ensure the experiment could be easily replicated.

Level of analysis

Giving some background to the experiment and why it is carried out in industry etc. would help to show relevant but additional knowledge that may not be on the syllabus, evidence of reading around the subject is particularly vital at A level. I feel that at the moment the candidate has listed too much and not evaluated their method enough, there needs to be a section dedicated to evaluating and more could be written on experimental error, possible causes etc. as this is what the top candidates are including to achieve the best marks. Similarly expanding the conclusion to include more than just the bare findings would help but the particular aims of this investigation make this difficult. The lack of health and safety analysis is also a problem as including this would both help show your knowledge of chemical properties and ensure safety when the experiment is repeated.

Quality of writing

Spelling, punctuation and grammar are all accurate allowing for a fluent piece.


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