III. Relationship of Structure and Energy. A. Overview

Transcription

1 III. Relationship of Structure and Energy A. verview B. Determining Relative Stability from Structure 1. Same type of atom: complete vs. incomplete octet 2. Same type of atom: charged vs. noncharged 3. Two charged atoms (both positive or both negative) a. Electronegativity b. Atom size c. ybridization d. Inductive effects (s network) e. Resonance (p network) A. verview Goal: Determine relative energy of a compound by analyzing it s structure (high/low) Potential energy Reactivity (high/low) Stability (low/high) Analyzing the energy of a stucture allows you to: Determine relative energies of resonance contributors Determine which reaction will proceed faster Determine position of equilibrium 1

2 B. Determining Stability from Structure Incomplete octet? no yes igh potential energy Unstable Reactive! + and B Formal charge? no Relatively low potential energy Relatively stable Relatively unreactive yes onsider: Electronegativity Atom size ybridization Induction Resonance of charged atom Most important parameters in determining : 1. Incomplete octet 2. Formal charge Same Type of Atom 1. Incomplete octet igh potential energy Unstable Reactive! Low potential energy Stable Unreactive 2. harged vs. oncharged Positive egative 4 vs. 3 2 vs. 3 More energy Less stable More reactive 2

3 B.3. Two harged Atoms a. Electronegativity b. Atom size c. ybridization d. Induction e. Resonance Must consider the effect of each parameter for both positive and negative charged atoms For a meaningful comparison, vary only one factor! a. Electronegativity riteria: atoms in same row and have same # of electron pair domains (Atoms in same row are relatively the same size) For egative harges: The more electronegative the atom, the better it can hold a negative charge Electronegativity increases (lowers energy) < < < F relative electronegativity - 3 < - 2 < - < F - of charged compounds For Positive harges: The more electronegative the atom, the worse it can hold a positive charge Electronegativity decreases (raises energy) < < F 4 > 3 > 2 F relative electronegativity of charged compounds 3

4 b. Atom Size riteria: atoms in same column and have same # of electron pair domains (Atoms in same column significantly different size) For egative harges: The larger the atom, the greater the Surplus of electrons, electrons repel each other larger atoms allow this charge to spread out (lowering the charge density) I > Br > l > F relative size I - > Br - > l - > F - For Positive harges: The smaller the atom, the greater the Surplus of protons in the nucleus Smaller atoms allow the valence electrons to be held more tightly to the (+) nucleus S > relative size S < Example: vs vs S When comparing atoms of different size, size matters more than electronegativity 3 S < 3 < S > 3 2 > 3 2 itrogen is most effective at stabilizing a positive charge and least effective at stabilizing a negative charge Sulfur is most effective at stabilizing a negative charge and least effective at stabilizing a positive charge 4

5 c. ybridization sp sp 2 sp > > 3 50% s character 33% s character 25% s character The more s character the closer the electrons are held to the (+) nucleus the more electron withdrawing the orbital For egative harges: For Positive harges: > > < 2 < 3 (-) stabilized by greater s character 1. the electrons are held closer to the (+) nucleus (more stable) 2. more electron withdrawing Energy is decreased! (+) destabilized by greater s character 1. the electrons are held closer to the (+) nucleus - less accessible for bonding to an atom 2. more electron withdrawing Energy is increased! d. Inductive Effects - e - Withdrawing electronegative atoms draw electron density towards them through the sigma bond network stabilizes (-) charge 3 destabilizes (+) charge 3 F 3 l 3 F 3 F F 3 5

6 e. Resonance The more the charge is spread out (delocalized), the more stable the structure