3.2B: Purification of Product Mixtures
- Page ID
- 96652
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)If the crude product of a chemical reaction is a solid, it may be crystallized in order to remove impurities. For example, benzoic acid can be brominated to produce m-bromobenzoic acid (Figure 3.4). The crude solid product could very likely contain unreactive benzoic acid, and this impurity could be removed through crystallization.
![](https://chem.libretexts.org/@api/deki/files/125590/Nichols_Screenshot_3-2-2.png?revision=1)
To demonstrate, a mixture containing roughly \(85 \: \text{mol}\%\) p-bromobenzoic acid\(^1\) (a solid) contaminated with \(15 \: \text{mol}\%\) benzoic acid (another solid) had a yellow tint (Figure 3.5a), and after crystallization the resulting solid was pure white (Figure 3.5c). The crystallization appeared to purify the mixture based on the slight improvement in color.
![](https://chem.libretexts.org/@api/deki/files/125594/Nichols_Screenshot_3-2-3.png?revision=1)
More reliable than appearance, the crystallization was proven to have indeed purified the mixture through melting point and \(\ce{^1H}\) NMR analysis of the crude and crystallized solids. The melting point of the crude solid was \(221\)-\(250^\text{o} \text{C}\) while the melting point of the purified solid was \(248\)-\(259^\text{o} \text{C}\) (literature melting point of p-bromobenzoic acid from Aldrich is \(252\)-\(254^\text{o} \text{C}\)). The melting point of the crystallized solid sharpened significantly, indicating greater purity. Additionally, in the crude solid's \(\ce{^1H}\) NMR spectrum (Figure 3.6a), there are signals from both p-bromobenzoic acid and benzoic acid, and the integrations correlate reasonably well with the original composition of the mixture. In the crystallized solid's \(\ce{^1H}\) NMR spectrum (Figure 3.6b), the benzoic acid signals are entirely gone, demonstrating that the crystallization successfully purified the p-bromobenzoic acid. An analogous purification could be done if a bromination reaction produced a mixture of m-bromobenzoic acid and benzoic acid.
![](https://chem.libretexts.org/@api/deki/files/125595/Nichols_Screenshot_3-2-4.png?revision=1)
\(^1\)p-Bromobenzoic acid was used instead of m-bromobenzoic acid for analysis purposes.