Organic Acids - Carboxylic Acids

Transcription

1 Orgnic Acids - rboxylic Acids Orgnic cids - crboxylic cid functionl group rboxylic cids re redily deprotonted by bses such s NO eg 3 O O - + O - + O 3 O O Acid Bse onjugte Bse onjugte Acid This rection is product fvoured, s the products re more stble thn the rectnts The conjugte bse (cette; 3 O 2- ) is much more stble thn the bse (hydroxide, O - ); therefore, i) O - is stronger bse thn 3 O 2- ii) 3 O 2 (pk = 47) is stronger cid thn 2 O (pk = 14) 1 1

2 Orgnic Acids The strength of n cid is dependent on the stbility of its conjugte bse: onjugte bse of l ( strong cid; pk = -7): l - (stble nion nd thus very wek bse) onjugte bse of 2 O ( wek cid; pk = 14): O - (less stble thn l - thus reltively strong bse) The strength of n cid cn lso be sid to be inversely relted to the strength of its conjugte bse 2 2

3 Orgnic Acids (rboxylic Acids) Is the rection below product- or rectnt-fvoured? 3 O O O + O 3 + O - + O This rection is equivlent to the cid dissocition eqution s long s the solution is sufficiently dilute so tht the X 2O 1, its equilibrium constnt is the K for 3 O 2 : K 3 3O2 O O 3 2 ecll tht we cn relte the K nd pk for n cid vi: pk log[ K ] An cid is stronger if it hs lrge K nd smll pk 3 3

4 lculting p of n Acidic Solution onsider 032 M solution of phenol (pk =995) t O (q) + 2 O(l) 6 5 O - (q) + 3 O + (q) Acid Bse onj Bse onj Acid Initil 032 M 5555 M (pure wter) 0 0 (10-7 M) oncs hnge -x -x x x Eqlm 032 x 1 x x Activities Solvent (X = 1) pk = 995 K = = 1122 * = (onj Bse) (onj Acid)/(Acid) (Bse) = xx/(032 x) ssume x << 032 = x 2 /032 x = (1122*10-10 * 032) 05 = 599*10-6 = [ 3 O + ] ssumption vlid? p = -log[ 3 O + ] = 522 X o = 599*10-6 X 1 = (1122*10-10 *(032 x o )) 05 = 599*

5 Orgnic Acids We cn increse the strength of n cid by dding electron-withdrwing groups, further stbilizing its conjugte bse To increse the cidity of cetic cid, replce one or more hydrogen toms of the methyl group with hlogens: O O O O O F O F F O F F F O pk = 474 pk = 266 pk = 124 pk = 023 This stbiliztion through bonds is clled n inductive effect Inductive effects re strongest when close to the cidic hydrogen NB: NB: F O 2 is not significntly more cidic thn O 2 Previously inductive effects were seen to ffect the strength of the oxocids (eg lo 2 vs lo 3 vs lo 4 ) 5 5

6 Orgnic Acids rboxylic cids re mong the most cidic orgnic molecules; however, number of other functionl groups contin cidic hydrogen toms: i) Alcohols re s cidic s wter Most hve pk s of ~15-18 NB: Inductive effects cn lower these vlues to ~12 ii) Phenols re more cidic thn lcohols Most hve pk s of ~8-10 NB: Their pk s cn go s low s 1 with the right groups ttched to the ring iii) Thiols (-S) re more cidic thn lcohols Most hve pk s of ~10-12 NB: Also subject to inductive effects iv) Amines re not cidic in wter, but cn be deprotonted by strong bses in unrective solvents like lknes or ethers Their pk s re typiclly ~35-40! 6 6

7 lculting p of n Acidic Solution A strong cid, A, (pk < 0) dissocites fully in wter: A stronger cid thn 3 O + (pk = 0) genertes 3 O + nd conj bse, A - This effect is known s solvent leveling: No cid stronger thn the conjugte cid of the solvent cn exist in ny solution of tht solvent No bse stronger thn the conjugte bse of the solvent cn exist in ny solution of tht solvent Wek cids ren t subject to solvent leveling: onsider the percentge of cid molecules tht hve dissocited In concentrted solution of wek cid, the % dissocition is smll The ctul conc of cid is close to the nominl conc As solution becomes more dilute, this ssumption becomes less vlid 7 7

8 lculting p of n Acidic Solution onsider M solution of 3 O 2 (pk = 474) t O 2 (q) + 2 O(l) 3 O - 2 (q) + 3 O + (q) Acid Bse onj Bse onj Acid Initil M 55 M 0 0 (10-7 M) initil onc hnge -x -x x x Eqlm x 10 x x ctivities t eqlm pk = 474 K = = 1820 * 10-5 = (onj Bse) (onj Acid)/(Acid) (Bse) = x 2 /( x) => 1820*10-5 ( x) = x 2 => x *10-5 x - 764*10-10 = 0 x = {-1820* /- [(1820*10-5 ) 2 4(1)(- 764*10-10 )] 05 }/2 = {-1820*10-5 +/- 5821*10-5 }/2 = 2000 * 10-5 = [ 3 O + ] p = -log[ 3 O + ] = -log[200 * 10-5 ] = 470 X o = 2764*10-5 (+38%) X 1 = (182*10-5 (42* *10-5 ) 05 = 1617*10-5 (-19%) X 2 = 2168*10-5 (+84 %) X 3 = 1923*10-5 (-39 %) X 4 = 2036*10-5 (+18 %) X 5 = 1985*10-5 (- 08 %) 8 X 6 = 2008*10-5 (+ 04 %) 8

9 Distribution urves Working bckwrds, using the K eqution, the percent dissocition of n cid t given p cn be determined: The p tells us the ctivity of 3 O + (q), hence s long s the K vlue is known, the clcultion proceeds s: K O 3 A A so A O 3 K A D A A % A 0 A A A A A O 3 K 1 1 O 3 K K K O 3 lculte the percent dissocition of cetic cid (pk =474) t p 400 K = = 1820*10-5 M 3O+ = 10-4 M 5 K 182*10 % D K *10 100*10 O = 154 % 9 9

10 Distribution urves If we repet this clcultion t lrge number of different p vlues, we generte wht is known s distribution curve: cette ( 3 O 2- ) [cette]=[cetic cid] pk cetic cid ( 3 O 2 ) NB: the two curves cross t the pk of the cid! The pk of n cid is the p t which it is exctly 50% dissocited 10 10

11 Distribution urves It is esy to see why n cid will be 50% dissocited t its pk onsider the definition of pk: 3O A log K log A pk log log A O 3 A p A pk log A enderson-sselbch eqution A p log A note tht if: A- / A = 1, then: A log 0 A hence : p = pk 11 11

12 Distribution urves for Polyprotic Acids If n cid hs multiple cidic protons whose pk vlues re different by severl units, the distribution curve will look like severl monoprotic distribution curves superimposed: If the pk vlues re too close, the distribution curve will become more complex 12 12

13 Orgnic Bses - Amines Orgnic bse: generlly n mine Amines re redily protonted by cids: eg N N + + O + + O bse cid conjugte cid conjugte bse The bsicity of mines is due to the lone pir on the nitrogen tom which mkes ll mines ( =, lkyl or combintion) Lewis bses 13 13

14 Orgnic Bses There re other functionl group contining nitrogen Why is n mine considered good bse, but n mide is not? The nitrogen tom of n mide is so wekly bsic tht the oxygen of the crbonyl group will be protonted over it! 14 14

15 A bse s strength is ssessed by : 1) K b ( or pkb) Quntifying Bsicity or 2) The K (or pk ) of the conjugte cid Since the strength of bse is inversely relted to the strength of its conjugte cid, N + pk = 93 3 N + pk = 106 Smller pk = Stronger onjugte Acid = Weker Bse 15 15

16 Quntifying Bsicity: K b This is the equilibrium constnt for rection of the bse with wter: N Bse Acid onj Acid onj Bse K b is inversely proportionl to K for the conjugte cid: K K K w b 3 N pk b = 34 N + - O + + K b N pk b = 47 K K w K log( K K ) log( K ) b w b O log( K ) log( K ) log( K ) b w B B O where K w = under stndrd conditions pk pkb pkw 14 Lrger K b = Smller pk b = Stronger Bse 16 16

17 Quntifying Bsicity: K b Ex) ompute the K b of niline ( 6 5 N 2 ) given tht the pk of 6 5 N 3 + is 47 K b = K w /K = /10-47 = /200*10-5 = 500*10-10 pk b =-log(500*10-10 )= 930 Alterntively pk b = 14 pk = =

18 lculting p of Bsic Solution B (q) + 2 O (l) B + (q) + O - (q) Using the K b expression nd nominl solution concentrtion to clculte Othen Using the K w expression clculte + from O- K b O B B O K 3 w O ecll: t 25, K w = Finlly use + to clculte p 18 18

19 lculting p of Bsic Solution lculte the p of 071 M queous solution of niline t N 2 (q) (l) 6 5 N 3 + (q) + O - (q) Initil 071 M 55 M 0 0 (10-7 M) onc hnge -x -x x x Eqlm 071-x 1 x x Activities pkb = 10 => K b = K b = (B + )(O - )/ (B) ( 2 O) = xx/(071 x) Assume x<< = x 2 /071 x = (10-10 *071) 05 = 842*10-6 (O - ) = 842*10-6 ( 3 O + ) = Kw/ (O - ) = / 842*10-6 = 1188*10-9 p = -log(1188*10-9 ) = 893 Alterntively : po = -log(842*10-6 ) = 507 p = 14 po =

20 Amino Acids: Acid nd Bse Some textbooks present the structure of n mino cid s follows in order to show tht the nme comes from hving n mine group nd crboxylic cid group This is not how n mino cid ctully exists in biologicl systems (or other p ~7 solution) Insted, it exists s the zwitterion: N O O N + O O

21 Why is this? Amino Acids: Acid nd Bse The pk for the O 2 proton is typiclly ~2 (slightly lower thn the stndrd 3-5 rnge for crboxylic cids) The pk for the conjugte cid of the mine group (ie for the N 3 + proton) is typiclly ~9-10 Use this dt to sketch distribution curve for the mino cid, lbeling with the mjor species within ech p rnge 21 21

22 Beyond Wter: Implictions of Acid-Bse hemistry It s esy to forget tht not ll chemistry is done in queous solution, but the pk scle doesn t just go from 0 to 14! Often times, the solvent for rection is chosen bsed (t lest in prt) on cid-bse considertions onsider the following rection: O - + I O + - I Wht would hppen if we tried to perform it in queous solution? 22 22

23 Beyond Wter: Implictions of Acid-Bse hemistry Solvent leveling cn be useful, though Sodium methoxide (NO 3 ) cn be prepred vi couple of different methods, both of which use methnol s the solvent: We sw in EM 1000 tht dding N (s) to 2 O gives NO nd 2(g) Similrly, dding N (s) to 3 O gives NO 3 nd 2(g) : Adding very strong bse (eg source of - such s N) to 3 O will lso produce NO 3 long with the conjugte cid of the bse: 23 23