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CHE 8A: Week 3 Notes

by: Mackenzie Hayes

CHE 8A: Week 3 Notes CHE 8A

Mackenzie Hayes
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These notes cover what was handled in class, with examples, bits from the lecture notes, and other visual aids, happy studying :)
organic chemistry - brief
Sarah Lievens
Class Notes
Organic Chemistry




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This 5 page Class Notes was uploaded by Mackenzie Hayes on Thursday April 14, 2016. The Class Notes belongs to CHE 8A at University of California - Davis taught by Sarah Lievens in Summer 2015. Since its upload, it has received 19 views. For similar materials see organic chemistry - brief in Biological Sciences at University of California - Davis.

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Date Created: 04/14/16
CHE 8A: Week 3 Notes 4/11: ● Acids + Bases: ○ Definitions: ■ Arrhenius - general basics ■ Bronsted-Lowry - working modern definitions ■ Relative acid strengths - pKa + strengths ● Acids + Bases: ○ Arrhenius Acids + Bases: ■ Acids ​= donate H+ in H20; have H+ in formula ● HCl, HBr, H2SO4, H3PO4 ■ Bases ​= donate OH- in H2O; Have OH- in formula ● NaOH, KOH, Ca(OH)2, NH4OH* (*doesn’t actually have OH) ■ Works well in water reactions BUT ● Organics don’t “like” H2O and some things don’t fit ○ Bronsted-Lowry Acids + Bases: ■ Acids ​= donate H+ ● Works for all Arrhenius Acids ● And other stuff outside of water reactions ● CH3COH, H2O, NH3 ■ Bases ​= accept H+ ● Works for all Arrhenius Bases ● And anything that has extra e- (anything with a lone pair) ● Cl-, CN-, NH3, H2O ○ Lewis Acids + Bases: ■ Acids ​= accept electron pair into empty orbital ● H+, CH3+, BH3, Zn2+ ■ Bases ​= donate lone pair into empty orbital ● Essentially same as Bronsted-Lowry Bases ○ Relative Strengths: ■ Relative ability to donate H+ ■ pKa: ● Small # = donates lots of H+ (good acid) ● Large # = keeps H+ (bad acid) ● More about Bronsted-Lowry: ○ Neutralization → conjugate pairs play “pass the proton” ■ HCl + H2O → Cl- + H3O+ ■ Acid + Base → Base + Acid ■ End up with conjugate pairs: ● Acid form has extra H+ ● Base form missing H+ ■ HCl/Cl- H3O+/H2O ■ Stronger Acid forces Weaker Acid to take the H+ (proton) ● Organics will look at strength not dissociation ● Measure Acid Strength by How Well it Donates H+: ○ Ka = [H3O+][A-] / [HA]​ = acid dissociation constant ○ HA + H2O → A- + H3O+ ○ Ka has a huge range so we use pKa ■ pKa = -log(Ka) ■ In Ochem: pKa range = -10 → 50 ■ In H2O: pKa range = 0 → 14 ○ In H2O if pKa is <0 = “strong acid” ; >0 = “weak acid” ○ In Ochem pKa1<pKa2 (pKa1 is stronger; based on relativity) ● What makes a good (stronger) acid? ○ HA → H+ + A- ○ H+ portion is always the same → conjugate base is what varies ■ Conjugate base determines the acid strength ○ Conjugate bases with stable -charge state are easier to form ○ What makes a -charge stable? ■ Atomic size - how big is the orbital with the -charge? ● Larger = greater area to disperse, less concentrated -charge ● -charge = a heavy weight, easier to carry the bigger you are ● Down periodic table = better acids ● *know generally = ​HF < HCl < HBr < HI ​better acid → ) ■ pKa: 3.2 -6 -9 -10 ○ HF: ■ Good overlap ■ Decent bond ○ HI: ■ Poor overlap ■ Weak bond ■ I- is so big that it doesn’t notice a new e- ● (like if you have a billion kids vs. only 2) ● Size doesn’t tend to be a factor in Ochem so: ● Electronegativity: ○ More EN atoms = more willing to carry -charge weight ○ EN are essentially more “buff” ■ CH4 < NH3 < H2O < HF (better acid → ) (size is same) ○ EN: 2.5 3 3.5 4 ○ pKa: ~50 35-40 15.7 3.2 (4/13) ● Acid Strength and Structure ○ How well the conjugate base holds the -charge ■ Stable -charge = stronger acid / weaker base ■ Unstable -charge = weaker acid / stronger base 1. Atomic Size: a. Larger atom with -charge = better acid 2. Electronegativity: a. More EN atom with -charge = better acid 3. Resonance: a. Split -charge over more atoms = better acid Ex:​CH3COO-: (pKA = 3-5) ● Share -charge over 2 atoms (the 2 O’s) ● Resonance​ → multiple valid Lewis Dot structures ○ Vary in placement of electrons (pi e- (pi bonds) / charges / lone pairs) ○ DON’T vary in atom connections (sigma bonds consistent) ○ Electrons = delocalized + spread out over 3+ atoms ■ Ex:​ CH3COO- (above) → the extra electron gets “stolen” back and forth by both of the oxygens so much, that instead of one O having a -1 charge, the both have a -½ charge, which is joined by the C, making the -charge load “lighter” ■ Structures above ^ = “extreme resonance structures” (imaginary) ■ Structure with -½ charge on both O and dotted bond line = “resonance hybrid” (reality) 4.Hybridization: ■ sp has more s orbitals + less p orbitals than sp2 or sp3 ■ More s orbitals = less node @ nucleus ● → e- get closer to the nucleus, +charge lowers the energy ■ sp orbitals = a little bit e- withdrawing, a little bit EN ○ Alkane: pKa ~ 50 ■ sp3 = ¼ s + ¾ p ○ Alkene: pKa ~44 ■ sp2 = ⅓ s + ⅔p ○ Alkyne: pKa ~25 ■ sp = ½ s + ½ p 5. Induction:(minor factor) ■ Electronegativity working at a distance ■ Affects where in normal of that function group the individual molecule falls ■ Starts with EN neighbours (like a cheering section, moral support) ● Don’t actually help at all but still make the load easier ● Pull e- toward themselves + lessen -charge a tiny bit on that atom ■ 1. More EN atom is → more effect ■ 2. More neighbours it has → more effect ● Like if 50 neighbours stole from you, you’d realize faster than if only one was ■ 3. Closer EN atoms are → more effect ● REMEMBER: ○ Many functional groups have specific pKa ranges ■ Predictable due to combos of size, EN, hybridization, resonance ■ Functional group = single central atom w/ specific bonds ○ Functional group: pKa: ○ Carboxylic Acid 3-5 ○ Alcohol 15-18 ○ Amine 35-40 ○ Alkane ~50 ○ Alkene ~44 ○ Alkyne ~25 ● Organic functional groups can be bases (can form conjugate acid) ● Who gets the proton? ○ Stronger acid (smallest pKa) loses H+ first ○ Strongest base (conjugate base of large pKa acid) gain H+ first ○ → conjugate acid gains H+ compared to reference ○ → conjugate base loses H+ compared to reference ○ Can work in H2O if pH < pKa ■ Lots of H+ (will tend to be in acid form) ○ If pH > pKa → little H+ (prefer base form)


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