Comparing Basicity of Imidazole Phenylamine and Amide

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In this video we want to compare basicity of 3 nitrogen compounds - imidazole, phenylamine and amide. • We should be familiar with basicity of phenylamine and amide, but imidazole is a new compound that we would need some time to understand and explain. • 1. List the 3 compounds in increasing basicity • A stronger base will dissociate to a greater extent to give more OH-, hence its Kb will be bigger, and pKb smaller. • So listing down the 3 compounds from weak basicity (large pKb) to strong basicity (small pKb) is pretty straightforward: • ethanamide, phenylamine, imidazole • 2. Explain why amide is least basic (neutral) • The lone pair on nitrogen is delocalised extensively between 2 electronegative nitrogen and oxygen. • Hence it is not available for donation. • This means amide is neutral. • We can draw the resonance structures to show the delocalisation of lone pair between N to O. • 3. Explain why phenylamine is basic, but less basic than ammonia • The lone pair on nitrogen interacts with the delocalised pi system of benzene. • Therefore it is less available for donation which makes phenylamine a weaker base than NH3. • But phenylamine is still able to function as a base, hence it is more basic and has a lower pKb than amide. • For comparing basicity of common nitrogen compounds such as amine, ammonia, phenylamine and amide, check out my previous video. • 4. Explain why N1 of imidazole is most basic • There are 2 nitrogens in imidazole (let's name them N1 and N4). • Interestingly both nitrogens seem to have lone pairs but N1 is basic while N4 is neutral. • Although not very obviously mentioned in the question, we can deduce that the cyclic structure in imidazole is aromatic. • In the cyclic structure, there are 2 pi bonds (between C2-C3 and N1-C5) and one lone pair on N4 that can interact with each other to form a delocalised pi system. • This delocalisation is interesting because the number of delocalised electrons is 6, which is the same as that of benzene. • This means that like bezene, imidazole has a closed delocalised ring of 6 pi electrons. • Therefore it is considered an aromatic compound with resonance stability, just like benzene. • Since N4 needs to use its lone pair for aromaticity, it is not available for donation and N4 is neutral. • N1 on the other hand already forms a pi bond which contributes to the delocalised system. • N1 is sp2 hybridised and its basic shape is trigonal planar. • Its lone pair is pointing away from the cyclic structure and is unable to interact with the delocalised pi system. • Therefore the lone pair will be available for donation, which makes N1 the strongest base in this question. • Topic: Nitrogen Compounds, Organic Chemistry, A Level Chemistry, Singapore • Found this video useful? • Please LIKE this video and SHARE it with your friends! • Any feedback, comments or questions to clarify? • Suggestions for new video lessons? • Drop them in the COMMENTS Section, I would love to hear from you! • You can also view this video lesson with screenshots and detailed explanation at https://chemistryguru.com.sg/basicity... • Do check out the following for more video lessons: • Organic Chemistry Videos at https://chemistryguru.com.sg/a-level-... • A Level Chemistry Videos at https://chemistryguru.com.sg/a-level-... • If you are looking for JC Chemistry Tuition, do consider taking up my classes at Bishan. More info at https://chemistryguru.com.sg/ • Or you can save both time and cost by learning A Level Chemistry online! Find out more at https://chemistryguru.com.sg/online-t... • ~-~~-~~~-~~-~ • Please watch my latest video: 2019 P1 Q2 - Deflection of Charged Particle in Electric Field • • 2019 P1 Q2 - Deflection of Charged Pa... • ~-~~-~~~-~~-~

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