Answers to Lab Questions

Lab Questions 1.​ What are organometallic halides, and why are they essential in chemistry? Organometallic halides are compounds with a carbon-metal bond and a halogen atom. Organometallic halides comprise a metal atom bonded to an organic group (alkyl or aryl group) and a halogen (chlorine, bromine, or iodine). The metal atom can be alkali (lithium) or transition metal (zinc or magnesium). The significance of organometallic halides lies in their very high reactivity, making them powerful organic synthesis reagents. They are responsible for forming carbon-carbon bonds, thus enabling complex organic molecules to be constructed from more straightforward starting materials. This is vital in synthesizing pharmaceuticals, polymers, and other fine chemicals. Nucleophilic carbon reactivity is made possible by the metal-carbon bond polarity, which is required to add carbon chains to electrophilic positions like carbonyl groups. Furthermore, these complexes are handy reagents for introducing particular functional groups into organic compounds, expanding the applicability of synthesis. 2.​ How do organomagnesium bromide and organozinc bromide reactions differ? Discuss the advantages of using an aqueous-based organozinc reaction from a green chemistry perspective compared to traditional Grignard or organolithium reactions. Organomagnesium bromide (Grignard reagents) and organozinc bromide reagents both facilitate carbon-carbon bond formation but are highly different in reactivity and demands for manipulation. Grignard reagents, like organolithium reagents, are highly reactive and extremely moisture- and air-sensitive, demanding rigorously anhydrous conditions and ethereal solvents. This sensitivity poses safety hazards and environmental concerns due to the requirement for voluminous amounts of volatile and highly flammable solvents. Organozinc bromide reagents are, however, less reactive and less water sensitive and, therefore, may be operated in mixed aqueous-organic solvents. That is a valuable consideration from the perspective of green chemistry. Water-based organozinc reactions make hazardous organic solvents unnecessary, giving less waste and less environmental tension. The ability to perform reactions in water or mixed solvents simplifies workup processes. It reduces the need for large-scale purifications, favoring a more sustainable and ecologically sound approach to synthesis. Reduced sensitivity toward moisture also improves safety by reducing the hazards of handling extremely reactive organometallic reagents. 3.​ Write the reaction of an organozinc compound with a ketone (such as 3-pentanone) and an alkyl bromide (such as 2-bromopropene). The reaction is the addition of 2-bromopropene and zinc to give the organozinc reagent, followed by its addition to 3-pentanone. The net reaction may be represented as follows: This reaction produces a tertiary alcohol, where the alkyl group of the organozinc reagent adds to the carbonyl carbon of the ketone. 4.​ Write the reaction of an organozinc compound with a carbonyl compound. For example, use (a) ethylzinc bromide (C2H5ZnBr) with acetaldehyde (CH3CHO), and (b) phenylzinc bromide (C6H5ZnBr) with formaldehyde (CH2O). What would be the expected products in each case? 5.​ What is the theoretical yield in moles and mass? Show the complete steps to calculate the moles of reactants, and explain how to determine the limiting reagent. Use the lab notebook values of reactants to calculate the theoretical yield. 6.​ What is the principle behind a separating funnel? Where do you think your final product will be found, in the upper or lower layer, and why? A separating funnel is used to divide immiscible liquids by their densities. The process relies on the separation of liquid of different densities in layers. The heavier liquid will fall to the bottom and can be drained off, whereas the lighter liquid remains in the funnel. The organic phase containing the product 3-methyl-3-pentanol in ether solution will likely be the upper layer in this reaction. Diethyl ether is lighter than water and will hence float on top. The product, thus, in ether solution, will be found in the upper layer. 7.​ What precautions should you take to ensure an accurate % yield for this reaction? Some precautions are necessary to guarantee a correct percentage yield. First, accurate reactant and solvent measurements must be used. Secondly, the reaction should be controlled to prevent side reactions. The organic layer has to be dried carefully with anhydrous sodium sulfate to eliminate all the water, which will influence the mass of the product. Losses during transfer, filtration, and evaporation should be prevented using good technique and apparatus. Finally, the product should be separated and carefully weighed to measure yield accurately. 8.​ What characterization techniques can be used to identify the product of an organozinc reaction, and how do they help confirm its structure and purity? Characterization techniques are employed in determining and detecting an organozinc reaction product's product structure and purity. Nuclear Magnetic Resonance (NMR) Spectroscopy elucidates structural detail by analyzing chemical shifts and splitting of protons and carbons. This confirms the presence of specific functional groups and the product's structure. Fourier Transform Infrared (FTIR) spectroscopy identifies functional groups by scanning the absorption of infrared light. This confirms the presence of hydroxyl groups (OH) and other tell-tale functional groups in the product. Mass spectrometry (MS): It determines the product's molecular weight and provides information on its fragmentation pattern, verifying its purity and identity. Gas chromatography-mass spectrometry (GCMS): It identifies and separates the components of a mixture and informs about the purity of the product.