Grignard Lab Why are the ether layers washed using BRINE (a saturated solution o
ID: 502369 • Letter: G
Question
Grignard Lab
Why are the ether layers washed using BRINE (a saturated solution of salt in water) instead of just plain water? Sodium sulfate is used to dry the ether solutions. What are the intermolecular forces that help the sodium sulfate to dry the ether? Why do you place rubber septa on the reaction vessels for this synthesis? Why do you keep the syringe stored with the needle inserted into a septum when it is not in use always? Why do you add the 6M hydrochloric acid to the reaction mixture from last week at the start of this weeks' lab? Why do you hold the separator funnel with the stopcock upwards when shaking? Why do you hold the separator funnel with 2 hands when shaking, particularly with the stopper resting in one of the hands (this is not a trick question, the answer is very basic and obvious)? Petroleum ether is added before crystallizing the product. Characterize petroleum ether as polar or nonpolar and as protic or aprotic?Explanation / Answer
1) Brine solution is water saturated with sodium chloride. Brine solution is used in extractions primarily due to two reasons:
(a) The solubility of organic compounds is lower in brine solution than in water. Hence, brine is used to prevent the extraction of organic compounds into the aqueous phase.
(b) Brine helps to absorb moisture from organic solvents. Sodium chloride has a strong affinity for water and hence can suck up moisture from organic solvents.
2) Sodium sulfate is used to dry ether layers. Sodium sulfate absorbs moisture from ether layer. Sodium sulfate is an ionic compound and dissociates into sodium and sulfate ions in water. The sodium ion is positively charged while sulfate is negatively charged. These ions are attracted to water because water is dipolar in nature (hydrogen is positively polarized while oxygen is negatively polarized). Therefore, the predominant interaction between water and sodium sulfate is ion-dipole interaction.
3) Rubber septa is placed on the reaction vessel during synthesis because Grignard reagents are highly sensitive to moisture and carbon dioxide. Grignard reagents react with moisture forming alkanes and with CO2 forming carboxylic acids. Therefore, the reactivity of Grignard reagent decreases (in the sense that the complete amount of Grignard reagent taken for the reaction is not available for reaction with a carbonyl compound). Hence, the reaction vessels are flushed with nitrogen to remove moisture and CO2 and closed with rubber septa. The rubber septa prevent the CO2 and moisture from coming in contact with the Grignard reagent.
4) The syringe is kept with the needle inserted into the septum so that the open end of the needle doesn’t come in contact with moisture or CO2 of air. Moisture and CO2 deactivates Grignard reagent and hence exposure of the needle to these two must be prevented.
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