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20 - pKa and Reactivity

20 - pKa and Reactivity - Announcements Prof Reisman...

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Unformatted text preview: Announcements Prof. Reisman Office Hours: Wednesday Nights, 8 pm in IOI Schlinger If you have questions about the material we’re covering, this is your opportunity to ask me directly There is a Guide to Molecular Orbitals handout posted on the Website under the “Supplemental Material” heading. Some useful websites: http://www.organicdivision.org/?nd=p_organic_web_links http://www2.chemistry.msu.ed u/faculty/ reusch/VirthtJml/ intro l .htm#contnt Specific to today’s lecture: http://research.cm.utexas.edu/nbauld/teach/acidsbases.html#blacids Useful reading from BPOC: Chapter 8, section I (pages 208-2l l) The pKa System pKa values can be useful for estimating reactivity in organic chemisty K eq 9 (-9 AH (0(1) + H20 (0 = A (04) + H30 (0(7) [H30+][A'] [H30+][A'] K = — K = — K =—| eq [AHHHzO] 3 [AH] P a OgKa Lues w L. “if: (”‘5th ck $3.59 M (MI/L) fig“ ‘33“ W33 H:OMOS/ Wt gases («we :41qu [1&1 “’35 Ham/to; PKa PKa PKa ‘ : : o E S (LC/«J4 _ : __ : : v. "mfi 1—1 Cl : ’+ : ‘00 JL : 20.0 i : H36 CH2 : O : O—fl : I . HO-fi-Ofl E E [05 — E g o : CHSCH2S‘5- : H30 _ fl: 1 .o JOL E w B H3C-Ofl : lg. 3 H H g Hac 0:1 5 5 )—( g ‘13 : HO-fi : i8? H 5 : H : : . ("'3' : q a I H N." I ' ”I I I H’ ‘HH 1 IVN‘HH E 33 ”,6; H H (5 W N6 meiotiq we “KW/W3? V- F001 mud; Periodic Trends and Acidity the stability of the conjugate base determines the PK3 of the acid ° stable conjugate bases lead to low pKa values “ML Mu“ {Avenues ADM“ ‘l'luz 'l'alsle. Mll'k lacrflmlnfi compare the follownng: “WC «mu ( JV“ AW“ GM I; “we W Flag; 01% MVd l" llxoll AtirVerse‘ ovul- l’kn— lanai lb H 55 weakeuufi. TM- colowwslt W365 IL“ “-lio MW PKa pKa fl-NHZ 33 ti-OH l9} (ll-W : : “JIM 5 EN ‘ 0,er H-OH 5 IS 8 1“er H—SH 5 3r o " J fl—F E 5- 9- fl-SeH E 3. ‘1 C It. fivMLB SMer "HQ, O‘l‘l lood> E E N lAcreuzs one/Os: ’l’ke ‘lrlbl’2 - atll’LoWSL EM M00156: go {we W‘I'l’L l-ACXQQJUK3 it“ lAQ/IMllj (Lbilllb (owl m Tlck {S O~ “(ASK/‘2 0‘? 4/“ 4-0 Sl'nLllll—e c-) Oboe/3Q Qor‘). kfifi OclallllXX ’l’b Egg m QuKon. fig RF ll, l3‘l kallMl m Okrll‘DM‘lL (fidli “'AC/ZAJQS égfl“ m W SlSWNS-QJ lao‘flllm l‘l'CQ + a log. flflg‘) Movcwe/lewvns CR“ «012. mll‘) Shgdlk “'8" 8}- (HJPQ/sa lll :H (—3 eta/3e, Mao! H’X 36“ ”alias Mm'lu Electronegative Atoms Stabilize Anions Inductively 'an the stability of the conjugate base determines the pKa of the acid ° stable conjugate bases lead to low PKa values '95 th‘ Jes compare the following: PKa Ll : l o E i , HWJLOH l H 8 Elk °“ 1 8's D H‘H 5 Woods er ' 4 o O 5 76¢“ ZlCC/lrbv‘ KTKKKOH 3 8i i CIYJLOH 2 ‘l Mllywcib \ % ”\‘H i 53th Wgoflsskfi %( 0 ° firm * °° : ‘ Er .———- - CI>FJL0H E l 3 Q2 0 :c< CI H E 06’ \ 1Q : Ce C.Q_ helm/5H1 ell-eta f o—fc M K’e/KQ-QJ‘ O ' «\Vkrokslm 5M3. ”Tl—c ”Ck/W 414, EN 0(le Enc< 21%0H i OH, is elem.“ «Hoe claws-2‘ “l’kfi mm if) _ l Cl 5 \lecfim gléilliaillm. ii: on EN od' Elma/live, QHT/Ul‘: are wh‘u 1' u M Moira/HZ W ‘2‘" “l CLEAN“, we \qu ’l’ke indmc’hwe ShLliM‘o-a 01C ’l‘ke, Wlb‘hfislkl‘t M‘W‘ 04cm Delocalization Stabilizes Anions The stability of the conjugate base determines the pKa of the acid ° stable conjugate bases lead to low pKa values compare the following: pKa PKa ‘ # i' ‘ i H,C;:H '18 CH3O-fl - is 3 H >=o p O O >=o \ II .L w H3c EH2 ' H3C o Cwsicluw 44a ooh‘yxjodre loaSCI 09 gas .— 0 n0 an M1 6 E ; W30. Cl-lz/lk C/l'l (3*sz H e l H O 06 * » CH *flé H oil k1} he Kp§l<l§n~£ W132 ls de lo CRlll‘d 21> O 3 Ole ( l0“ QM)“ (Hal \Wl 039», M “Mom C/Lwfi" ‘3 olkslvlbvt’md Mnl3 our \5 (095le Zlc Cl‘fbh‘z’yothuz. *W 0 015305) ‘5 ole lo 1 elm—1,! ”Ore ‘ ' ' “AER gay}; : to dolomlx %’l\ux oulnA docxb'lf lllOQ 0w ukjlhlolll~fd C”) alto/SQ The Same Rationale Applies to Bascity Weaker acids have stronger conjugate bases. ° high pKa values indicated strong conjugate bases cm #—> ems NI-l3 ~—> 9M2 ilc“ : :33 1+2? flco‘33 5:05? C‘l’MVBl MUM m QOH3> The more accessible the unpaired electrons, the stronger the base ° negatively charged species are more basic relative to neutral species M16 ENI Touts—(4&2. ale/mm; Mole ’hachj' lOuJ-Q/ W33, H‘OMO CZM Pay) 6’5 @ H9. 0 H30 Nl'ls pl—l‘l ‘pbc‘ ~l'3 Flag (13* Liam &l%€xgwce in louci k3) Lone pair basicity increases from sp to sp2 to sp3 M2 />,[email protected] Ma,“ C ENG) Amides are Weak Bases that Protonate on Oxygen remember \CSS wk 0 amide nitrogens are sp2 hybridized to allow delocalization ( ° delocalization lowers the energy of the HOMO, makes lone pair less available lama Valr l.“ m V—orlolllkll Q Q dip/localibes ‘ml'o Tr/Sj£l‘€« .41 H/lKN/M‘ E @17: Si’2 Laynmul lcmfiesl- Coegqul’ lg 0" 0x339“ (more clad/w WW2; on More. EM Slfiloshlwea‘l‘B ?mvl-Mal{m 0n mafia» mlSo mllOvéj X1» ShLluhlbvk cl: (+5 Wicks; k3 raw Aelockllhlx‘m oh 0'3 aka-15¢ Cs El‘kbllififi HE C 1+ \Ga 0 (9 H® O i “i Oljn we C9 M / 5—» H “3/ é—>> RAN/M 99 RAKT<U 1+ l’ 1 l «L9 «L0 M9 H’ H. ouL CILMLDl' Shiite Pl' 6 ‘02 la a to NC: ”Agi/NL ‘l’kix G") M 3) Hr P/rxe ‘Q YeSmov'LQ’Qll l M ...
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