SN2 (Williamson Ether Synthesis)
|6 Mar / 7 Mar||A/B|
|sodium chloride||sodium bicarbonate||magnesium sulfate|
Sample 1H-NMR SN1 product 2-methyl-2-butanol (not available for submission for credit)
Stock GC-MS SN1 Product 2-chloro-2-methyl-butane (available for submission for credit, see laboratory manual for details)
Sample 1H-NMR SN2 Reaction of 1- and 2-bromopropanes with 2,6-dimethylphenol (not available for submission for credit)
Stock 1H-NMR FID SN2 Reaction (A) of 1-bromopropane and 2,6-dimethylphenol (available for submission for credit, see laboratory manual for details)
Stock 1H-NMR FID SN2 Reaction (B) of 1- & 2-bromopropanes and 2,6-dimethylphenol (available for submission for credit, see laboratory manual for details)
Stock GC-MS SN2 Reaction (A) of 1-bromopropane and 2,6-dimethylphenol (available for submission for credit, see laboratory manual for details)
Stock GC-MS SN2 Reaction (B) 1- & 2-bromopropanes and 2,6-dimethylphenol (available for submission for credit, see laboratory manual for details)
Summary of above aticle on the SN2 Mechanism: Orr-Ewing, A.J. Reaction Mechanisms: Stripping down SN2. Nature Chem., 2012, 4, 522-523.
Q1) What is a drying agent? Why do we need them? What do they do?
A1) Drying agents dry things; it’s that simple. How they do so depends on the drying agent, but they remove water molecules from an organic solvent. A brief summary of a few common drying agents is provided below.
Calcium chloride (CaCl2), sodium sulfate (Na2SO4) calcium sulfate (CaSO4, also known as Drierite) and magnesium sulfate (MgSO4) are the most common drying agents used in labs, The salts are used in their anhydrous (i.e. water-free) form. Each salt readily forms hydrates as shown below – essentially, these drying agents trap water in their coordination sphere but do not react directly with the water. (M = metal cation, X= anion)
The capacity (n, the maximum number of water molecules that the drying agent can bind) and efficiency (e, the amount of water left in the organic solution after the drying process is completed) are important considerations in selecting a drying agent for an organic compound. However, it is most important to select a drying agent that will not react with the molecule that you are drying!
1. Calcium chloride, CaCl2 (n=6, e=1.5 mg/L) is a good drying agent for a variety of solvents but is generally not compatible with molecules containing hydroxyl (alcohol, phenol), amino (amine, amide) and carbonyl (acid, ketone, ester) functional groups due to basic impurities such as Ca(OH)2 and CaCl(OH).
2. Calcium sulfate, CaSO4 (n=0.5, e=0.004 mg/L) is a neutral drying agent but suffers from low capacity. The commercially available Drierite contains CoCl2 as an indicator (dry: blue, wet: pink). If the compound is wet, the water can be removed by heating to ~200 oC for 1 hr.
3. Magnesium sulfate, MgSO4 (n=7, e=2.8 mg/L) is a slightly acidic drying agent. It works well in solvents like diethyl ether, but not as well for ethyl acetate. Mag sulfate works quickly because it exists as a fine powder with a large surface area.
4. Sodium sulfate NaSO4 (n=10, e=25 mg/L) has a very high capacity and is mainly used for very wet solutions. It is very efficient for drying ether solutions but can absorb other polar compounds (alcohols etc.). In addition, it is fairly slow compared to MgSO4.
5. Potassium hydroxide KOH (n= high, e=0.1 mg/L) and potassium carbonate (K2CO3, n=2, e=moderate) are often used to dry basic solutions containing amines (think about why KOH and K2CO3 should not be used to dry acidic compounds).
Reactive drying agents such as calcium hydride (CaH2), sodium (Na) or potassium (K) metal, lithium aluminum hydride (LiAlH4), and phosphorus pentoxide (P4O10) are used when organic solvents, reagents etc. must be absolutely bone-dry (>99% anhydrous). Such drying agents remove water by reacting with it. We don’t need this level of dryness or purity in CHEM 344 so the non-reactive drying agents are sufficient – they are also safer and easier to handle than the reactive drying agents.