PRE-LAB QUESTIONS 1. Rank the following compounds in order of increasing /maxi (

Question

PRE-LAB QUESTIONS 1. Rank the following compounds in order of increasing /maxi (list lowest> highest) 2. Spectroscopic tools are important to the synthetic organic chemist. For each pair of compounds, describe an aspect from one spectroscopic method (UV-vis, proton NMR or IR) that would distinguish one compound in a pair from the other. Be specific (list exact IR stretches, expected NMR chemical shifts, approximate/relative 2max values): a. 1,3-butadiene and butane, by UV-vis b. 1,3-butadiene and 4-bromo-but-1-ene, by H NMR c. 2,4-dibromo-2-butene and 2,3-dibromo-2-butene, by H NMR d. 1-butene and but-3-ene-1-ol, by IR

Explanation / Answer

1) order of compound from lowest to highest lambda max :

3>1>2

3 rd compound has three alkenes in conjugation that causes more delocalisation of pi electrons, due to reduced gap between pi and pi* molecular orbital (owing to resonance).

This reduced gap between HOMO Pi orbital and LUMO PI* molecular orbital requires less energy for electron transition from HOMO TO LUMO, rather longer wavelength or lambda max

Compound one has two double bonds in conjugation , but compound one has none

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