\begin{figure}[H]
\centering
\includegraphics[width=0.85\textwidth]{Synthesis-Mechanism.png}
- \caption{Mechanism for the synthesis reaction.\label{fig:syn-mechanism}}
+ \caption{Mechanism for the dehydration reaction.\label{fig:syn-mechanism}}
\end{figure}
%
-The overall equation for this reaction is given by equation~\ref{eqn:overall-reaction}.
+The overall chemical equation for this reaction is given by equation~\ref{eqn:overall-reaction}.
\begin{equation}\label{eqn:overall-reaction}
\ce{C6H11OH(l) ->[H3PO4] C6H10(l) + H2O(l)}
An azeotropic distillation was completed in this experiment with an azeotrope (a mixture of two liquids that boils at constant composition)~\footcite[pp.~53]{oxford-chemistry-dictionary} formed between the water and cyclohexene, hence depressing the boiling temperature of the mixture due to positive excess Gibbs free energy associated with the unfavourable mixing of this liquids relative to an ideal mixture.~\footcite[Azeotropes, pp.~207]{atkins-physical} This distillation removed the products of the reaction from the reaction vessel hence driving the reaction to completion.
%
\section{Experimental Method}
-A mixture of concentrated (\SI{85}{\percent}) phosphoric acid (\SI{10}{\centi\metre\cubed}) and cyclohexanol (\SI{25}{\gram}) was distilled at \SIrange{85}{95}{\celsius} (with collection in a ice-cold flask) until white fumes were evolved from the mixture residue. This produced a colourless, but cloudy distillate containing \SI{3.0}{\centi\metre\cubed} of water (\SI{69}{\percent} of the theoretical maximum volume). The distillate was washed with \SI{15}{\centi\metre\cubed} of concentrated sodium chloride solution and then the organic product was dried using anhydrous calcium chloride. After drying the organic product was very slightly cloudy. A further distillation of the dry organic product was completed (boiling temperature \SI{81.5}{\celsius} compared to the literature value of \SI{82.9(2)}{\celsius})~\footcite[Physical Constants of Organic Compounds, 3-134]{crc-handbook} and the \SI{3.81}{\gram} of perfectly clear cyclohexene (\SI{19.4}{\percent} yield) was collected in a ice-cold flask.
+A mixture of concentrated (\SI{85}{\percent}) phosphoric acid (\SI{10.0}{\centi\metre\cubed}) and cyclohexanol (\SI{24.00}{\gram}) was distilled at \SIrange{85}{95}{\celsius} (with collection in a ice-cold flask) until white fumes were evolved from the mixture residue. This produced a colourless, but cloudy distillate containing \SI{3.0}{\centi\metre\cubed} of water (\SI{69}{\percent} of the theoretical maximum volume). The distillate was washed with \SI{15}{\centi\metre\cubed} of concentrated sodium chloride solution and then the organic product was dried using anhydrous calcium chloride. After drying the organic product was very slightly cloudy. A further distillation of the dry organic product was completed (boiling temperature \SI{81.5}{\celsius} compared to the literature value of \SI{82.9(2)}{\celsius})~\footcite[Physical Constants of Organic Compounds, 3-134]{crc-handbook} and \SI{3.81}{\gram} of perfectly clear cyclohexene (\SI{19.4}{\percent} yield) was collected in a ice-cold flask.
%
\subsection{Calculations}
\subsubsection{Percentage Yield}
\end{align*}
%
\section{Analysis}
-During the experiment both cyclohexanol and water impurities were removed from the product. After the first distillation the product was heavily contaminated with water and hence appeared cloudy due to the emulsion formed. Washing with concentrated sodium chloride solution removed most of this water due to the negative Gibbs free energy change associated with the further solvation of the sodium and chloride ions in this solution. In addition this washing removed any cyclohexanol in the crude product since it is soluble in water,\footcite[Physical Constants of Organic Compounds, 3-134]{crc-handbook} although it is likely that little cyclohexanol was present as an impurity since it has a high boiling temperature of \SI{160.9(2)}{\celsius}.~\footcite[Physical Constants of Organic Compounds, 3-134]{crc-handbook} The subsequent drying with anhydrous calcium chloride was intended to remove any remaining water and the final distillation removed byproducts from the final product.
+During the experiment both cyclohexanol and water impurities were removed from the product. After the first distillation the product was heavily contaminated with water and hence appeared cloudy due to the emulsion formed. Washing with concentrated sodium chloride solution removed most of this water due to the negative Gibbs free energy change associated with the further solvation of the sodium and chloride ions in the saturated solution. In addition this washing removed any cyclohexanol from the crude product since it is soluble in water,\footcite[Physical Constants of Organic Compounds, 3-134]{crc-handbook} although it is likely that little cyclohexanol was present as an impurity since it has a high boiling temperature of \SI{160.9(2)}{\celsius}.~\footcite[Physical Constants of Organic Compounds, 3-134]{crc-handbook} The subsequent drying with anhydrous calcium chloride was intended to remove any remaining water and the final distillation removed byproducts formed from side reactions (e.g. nucleophilic substitution).
%
\subsection{Tests Performed}
Three drops of a solution of bromine in dichloromethane (since cyclohexene is insoluble in water,\footcite[Physical Constants of Organic Compounds, 3-134]{crc-handbook} also a different product of (1S,2S)-2-bromocyclohexan-1-ol would result from using an aqueous bromine solution although this is inconsequential since this is also colourless) were added to approximately \SI{1}{\centi\metre\cubed} of the collected cyclohexene. This caused the bromine solution to change colour from red-orange to colourless as the reaction shown in figure~\ref{fig:electro-add-mechanism} occurred. This demonstrated the presence of the nucleophilic double bond in the cyclohexene product, hence suggesting the intended product was successfully synthesised.
\end{figure}
%
\subsection{Boiling Temperature}
-The boiling temperature of the final product collected was appears to be very close to the published value (\SI{1.4}{\celsius} lower) which suggests that the final product produced was fairly pure. However the true difference in boiling temperature is slightly greater than this since the air pressure in the laboratory was greater (by approximately \SI{0.575}{\kilo\pascal}) than the air pressure for which the literature value was taken (\SI{101.325}{\kilo\pascal}). This had the effect of slightly elevating the boiling temperature of the product (since the vapour pressure of the liquid would have to be greater for boiling to occur) thus the true difference in boiling temperatures between pure cyclohexene and the product was instead approximately \SI{1.6}{\celsius}.
+The boiling temperature of the distillate collected appears to be close to the published value (\SI{1.4}{\celsius} lower) and had no noticeable range of boiling temperatures which suggests that the cyclohexene produced was fairly pure. However the true difference in boiling temperature is slightly greater than this since the air pressure in the laboratory was greater (by approximately \SI{0.575}{\kilo\pascal}) than the air pressure for which the literature value is reported at (\SI{101.325}{\kilo\pascal}). This would have slightly elevated the boiling temperature of the product (since the vapour pressure of the liquid would have to be greater for boiling to occur) thus the true difference in boiling temperatures between pure cyclohexene and the product was instead approximately \SI{1.6}{\celsius}. The calculations used to determine these numbers are included in section \ref{sec:boiling-calculations}.
This lower boiling temperature could be due to a combination of factors such as a slight cooling of the vapour as it travelled up the still head or the presence of impurities in the vapour. The product could have been analysed using NMR spectroscopy to definitively identify any impurities in the product, and the thermometer could have been temporarily moved to a position just above the boiling liquid during the distillation in order to obtain a more accurate value for its boiling temperature.
It is likely that there was still a little water impurity in the product after the drying since it was very slightly cloudy when compared to the distilled product. This may have been since the solution was not left to dry for 10 minutes as recommended in the laboratory manual due to time constraints, however a possible improvement to the method could be be grind the drying agent into a fine powder to increase its surface area and thus reduce the time required to dry the product.
%
-\subsubsection{Calculation}
-The approximate pressure difference from the published value was calculated using an adjusted sea-level air pressure of approximately \SI{102.6}{\kilo\pascal}~\footcite[Pressure at 4:20 PM]{weather-underground-egnt} and laboratory altitude of \SI{55.30}{\metre} above mean sea level~\footcite[Latitude: \SI{54.7683}{\degree}, Longitude: \SI{-1.5711}{\degree}]{os-maps-lab} with equation~\ref{eqn:pressure-diff}.
+\subsubsection{Boiling Temperature Calculations}\label{sec:boiling-calculations}
+The approximate laboratory air pressure was calculated using an adjusted sea-level air pressure of approximately \SI{102.6}{\kilo\pascal}~\footcite[Pressure at 4:20 PM]{weather-underground-egnt} and laboratory altitude of \SI{55.30}{\metre} above mean sea level~\footcite[Latitude: \SI{54.7683}{\degree}, Longitude: \SI{-1.5711}{\degree}]{os-maps-lab} with equation~\ref{eqn:pressure-diff}.
\begin{equation}\label{eqn:pressure-diff}
\Delta P = - \rho g \Delta h
\begin{align*}
\text{Laboratory Air Pressure} &\approx \SI{102.6e3}{\pascal} - \SI{1.29}{\kilo\gram\per\metre\cubed} \times \SI{9.81}{\metre\per\second\squared} \times \SI{55.30}{\metre} = \SI{102}{\kilo\pascal} \\
- \therefore \text{Pressure Difference} &\approx \SI{101.325}{\kilo\pascal} - \SI{102}{\kilo\pascal} = \SI{0.575}{\kilo\pascal}
+ \therefore \text{Difference from Published Pressure} &\approx \SI{101.325}{\kilo\pascal} - \SI{102}{\kilo\pascal} = \SI{0.575}{\kilo\pascal}
\end{align*}
-The Clausius-Clapeyron equation (equation~\ref{eqn:clausius-clapeyron}) and can be rearranged to make $T_2$ the subject (equation~\ref{eqn:clausius-clapeyron-rearranged}) and hence can be used to determine the boiling temperature elevation caused by the pressure difference.
+The Clausius-Clapeyron equation (equation~\ref{eqn:clausius-clapeyron}) and can be rearranged to make $T_2$ the subject (equation~\ref{eqn:clausius-clapeyron-rearranged}) and hence can be used to determine the boiling temperature elevation caused by this pressure difference.
\begin{equation}\label{eqn:clausius-clapeyron}
\log_e\left(\frac{P_1}{P_2}\right) = \frac{\Delta H_{\text{vap}}}{R} \left(\frac{1}{T_2} - \frac{1}{T_1}\right)
\subsection{Yield}
The overall percentage yield obtained was very low (\SI{19.4}{\percent}), despite the fairly high percentage of water obtained compared to the theoretical maxima (\SI{69}{\percent}) after the initial distillation. However this water percentage is likely to be a fairly inaccurate measure of yield since the water which formed an emulsion in the organic layer is not represented in this figure and it doesn't account for any water which was originally contained within the reagents (such as the water in the phosphoric acid solution) which was collected.
-Some of the cyclohexene was lost during transfers both due to some liquid remaining the in previous vessel and due to evaporative losses. To reduce these evaporative losses the flask containing the product could be cooled in an ice bath at all intermediate stages. In addition after the distillations some of the cyclohexene remained in the distillation apparatus. To reduce this a 'chaser' solvent with a higher boiling temperature than the cyclohexene -- such as toluene -- could have been added to the reaction vessel at the end of the first distillation to force any remaining cyclohexene into the collection flask.~\footcite{unmass-cyclohexene-synthesis} This solvent would then be removed from the cyclohexene in the second distillation step.
+Some of the cyclohexene was lost during transfers both due to some liquid remaining the in previous vessel and due to evaporative losses. To reduce these evaporative losses the flask containing the product could be cooled in an ice bath at all intermediate stages in the experiment, not just during distillation. In addition after the distillations some of the cyclohexene remained in the distillation apparatus. To reduce this a 'chaser' solvent with a higher boiling temperature than the cyclohexene -- such as toluene -- could have been added to the reaction vessel at the end of the first distillation to force any remaining cyclohexene into the collection flask.~\footcite{unmass-cyclohexene-synthesis} This solvent would then be removed from the cyclohexene in the second distillation step.
The yield calculated is likely to also have been low since not all of the product was distilled for a second time (due to the small size of the round-bottomed flask used) and the second distillation itself was stopped prematurely due to time constraints.
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