![]() ![]() Better orbital overlap allows for stronger pi-bonding between the nitrogen lone pair and the carbonyl p-orbital, which results in an overall lowering of energy. The drawing below tries to show how a change in hybridization from sp 3 to sp 2 brings the p-orbital closer to the adjoining p-orbitals of the pi bond, allowing for better orbital overlap. Why is trigonal planar geometry favoured here? Better orbital overlap of the p orbital with the pi bond vs. The difference in energy varies widely, but a typical value is about 10 kcal/mol favouring the trigonal planar geometry. The nitrogen in amides is planar (sp 2), not trigonal pyramidal (sp 3), as proven by x-ray crystallography. This turns out to be the case in many situations where the lone pair is adjacent to a pi bond! The most common and important example is that of amides, which constitute the linkages between amino acids. What if there was some compensating effect whereby a lone pair unhybridized p-orbital was actually more stable than if it was in a hybridized orbital? ![]() That might not sound like a lot, but for two species in equilibrium, a difference of 5 kcal/mol in energy represents a ratio of about 4400:1]. Let’s think back to why atoms hybridize in the first place: minimization of electron-pair repulsion.įor a primary amine like methylamine, adoption of a tetrahedral (sp 3) geometry by nitrogen versus a trigonal planar (sp 2) geometry is worth about 5 kcal/mol. Lone Pairs In P-Orbitals (Versus Hybrid Orbitals) Have Better Orbital Overlap With Adjacent Pi Systems Why? The quick answer is that lowering of energy from conjugation of the p-orbital with the adjacent pi-bond more than compensates for the rise in energy due to greater electron-pair repulsion for sp 2 versus sp 3 In the cases below, a nitrogen or oxygen that we might expect to be sp 3 hybridized is actually sp 2 hybridized (trigonal planar). This is most common for nitrogen and oxygen. Quick shortcut: Lone pairs adjacent to pi-bonds (and pi-systems) tend to be in unhybridized p orbitals, rather than in hybridized sp n orbitals. The main exception is for atoms bearing lone pairs that are adjacent to pi bonds. Exception #1: Lone Pairs Adjacent To Pi-bonds It should be noted that by the time your course explains why these examples are exceptions, it will likely have moved far beyond hybridization.īottom line: these probably won’t be found on your first midterm. Helpfully, these exceptions fall into two main categories. Although as with many things, explaining the shortcut takes about 2 minutes, while explaining the exceptions takes about 10 times longer. More practice quizzes for hybridization can be found here(MOC Membership unlocks them all) 3. Try picking out the hybridization of the atoms in this highly poisonous molecule made by the frog in funky looking pyjamas, below right. How To Determine Hybridization Of An Atom: Two Exercises
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