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Chapter 3

CHEM 212 Chapter Notes - Chapter 3: Ethylene, Substituent, Ion


Department
Chemistry
Course Code
CHEM 212
Professor
Laura Pavelka
Chapter
3

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3-Acids & Bases
!
Reaction!mechanism:!
description!of!how!events!that!take!place!on!a!molecular!level!
as!reactants!become!products!
Can!have!
intermediates
!that!intervene!between!each!step!
!!
3.1 ACID-BASE REACTIONS
Bronsted-Lowry!acid-base!reactions
:!involve!the!transfer!of!protons!
o
B-L!acid
:!substance!that!can!donate/lose!a!proton!
o
B-L!base:
!substance!that!can!accept/gain!a!proton!
o
Conjugate!base
:!molecule/ion!that!forms!when!acid!loses!its!proton!
o
Conjugate!acid:
!molecule/ion!that!forms!when!base!accepts!a!proton!
!!
i.e.!reaction!of!HCl!with!water!-->!H3O+$and$Cl-!
o In$this$case,$the$H$from$HCl$was$given$to$water$to$form$hydronium$ion!
o HCl$is$therefore$the$proton$donor$(acid)$and$water$the$proton$acceptor$(base)!
o Cl-$ion$is$conjugate$base,$and$hydronium$ion$is$conjugate$acid!
$$
Acid$strength$is$a$measure$of$the$percentage$of$ionization$and$not$concentration!
Strong'acid:$transfer$of$proton$in$water$proceeds$essentially$to$completion!
o Dissociate$completely$into$ions$in$water!
o Hydrohalic$acids:$HI,$HBr,$HCl,$!
o Oxoacids$in$which$#$O$atoms$exceeds$number$H$by$2$or$more:$H2SO4$HSO4-$and$
HClO4!
Sulfuric$acid$=$diprotic$acid$bc$transfers$two$protons$(first$one$completely$and$second$
one$only$to$extent$(10%))!
$$
Acids!&!Bases!in!Water!
Hydronium$ion$(H3O+)$strongest$acid$that$can$exist$in$water,$hydroxide$ion$strongest$
base$(OH-)!
When$ionic$compound$dissolved$in$water,$ions$solvated$!
o +$ions$stabilized$by$interaction$with$unshared$electron$pairs$of$water$molecules!
o -$ions$stabilized$by$H$bonding$of$unshared$electron$pairs$with$partially$+$H$of$
water!
o Ions$surrounded$by$water$molecules!
Spectator'ions:'play$no$part$in$acid-base$reaction$(i.e.$Na+$and$Cl-$ions)!
!!
3.2 HOW TO USE CURVED ARROWS
Curved'arrows:$show$direction$of$electron$flow$in$reaction$mechanism!
o From$source$of$pair$to$atom$receiving!
o From$site$of$higher$to$lower$electron$density!
o Never$use$curved$arrows$to$show$the$movement$of$atoms!
o Must$not$violate$octet$rule!
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Begin$with$covalent$bond$or$unshared$electron$pair$(high$e$density)$and$point$
towards$site$of$electron$deficiency!
Also$indicates$bond$cleavage$--indicates$that$bond$breaks$and$electron$pair$leaves$
with$ion!
!!
3.3 LEWIS ACIDS & BASES
Acids:
!electron!pair!acceptors!
o i.e.!HCl,!BF3,$AlCl3,$SO2,$CO2,$zinc/iron$(III)$halides$(alkali$metal$+$Zn/Fe)!
o Electron$deficient$atom$can$act$as$Lewis$acid$!
Bases
:!electron!pair!donors!
o NH3,$CH3Cl,$H2O!
Requires!that!base!have!electron!pair!to!donate,!doesn’t!expand!classes!of!bases!
However,!greatly!expands!classes!of!acids!–many!species!don’t!contain!H!in!formula!
so!cannot!be!B-L!acids!but!can!be!Lewis!acids!bc!accept!electron!pair!in!reactions!(i.e.!
CO2!and!Cu2+)!
o Proton!itself!is!Lewis!acid!(H+)!
!!
Opposite!Charges!Attract!
In!Lewis!acid-base!theory,!attraction!of!opposite!charges!fundamental!to!reactivity!
Negative!charge!localized!on!nonbonding!electron!pairs!
Electrostatic!properties!of!a!Lewis!base!and!acid!molecules!perfectly!suited!for!
reaction!
o Leads!to!separation!of!charge!
!!
3.4 HETEROLYSIS OF BONDS TO CARBON
Heterolysis:!
cleavage/breaking!of!a!covalent!bond!so!that!one!fragment!departs!with!
both!electrons!of!the!covalent!bond!that!joined!them!(one!electron!takes!all)!
o Heterolysis!of!bond!to!a!carbon!atom!produces!either!of!two!ions:!
!!
Carbocation:
!ion!with!positive!charge!on!C!atom!
o Electron!deficient,!only!6!electrons!in!valence!shell!
o Are!Lewis!acids!
o Short-lived,!highly!reactive,!occur!as!intermediates!in!some!organic!reactions!
Carbanion:
!ion!with!negative!charge!on!C!atom!
o Electron!rich,!have!unshared!electron!pair!
o Lewis!bases!
!!
Electrophiles!&!Nucleophiles!
Electrophiles
:!reagents!that!seek!electrons!to!achieve!a!stable!shell!like!noble!gas!
o Electron-loving!
o All!Lewis!acids!(carbocations!included)!are!electrophiles!
Accept!electron!pair!from!Lewis!base!
o Carbon!atoms!that!are!electron!poor!bc!of!bond!polarity,!but!are!not!
carbocations,!can!also!be!electrophiles!
React!with!electron-rich!centers!of!Lewis!bases!
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