In other words, weak acids hold on tightly to their protons. (counterintuitive) In other words, weak bases cannot 8. Weak base Weak acid. Section 4.

Transcription

1 Strong/weak acid TA K. Strong acid Strong. Strong acid Weak. Weak acid Weak 4. Weak acid Strong 5. Strong Weak acid 6. Strong Strong acid 7. Weak Strong acid 0% % 58% % % 0% % % Acid: proton donor to X Strong acids donate very easily Weak acids do not donate easily In other words, weak acids hold on tightly to their protons. (counterintuitive) In other words, weak s cannot 8. Weak Weak acid grab protons away from acids. (intuitive) Transformation wheel Why energy profile ow mechanism to X alcohol Starting material What eagent X Section 4.0 X alkyl halide Product Examine simultaneously Substitution rxn Substitution rxn X X ydroxyl and halide switch partners ydroxyl = hydroxy 4 Step l stronger stronger acid pk a ~ -8 pk a ~ - weaker acid l Is this a downhill reaction? weaker There are only a few species that can make an electronegative stay positive the EALLY good negative charge handlers l -, Br -, I - Step l Step continued l 5 eaction oordinate 6

2 l Stronger acid Step continued l weaker We do have an electronegative atom carrying a formal charge Is there anything that can be done to relieve this misery??????? 7 Step Step l eaction oordinate Step Now look at the no longer l sexteted 8 l Step l eaction oordinate Step Step l Step 9 l alkyloxonium ion = protonated alcohol All steps Basic alcohol attacks l creating alkyloxonium ion & l - Alkyloxonium dissociates forming & water l - captures creating alkyl halide intermediates l l 0 Step Step TS l l d charges? d d - l Step bimolecular l l eaction oordinate pk a ~ -8 pk a ~ - d d - l l l eaction oordinate Step TS Step l unimolecular

3 d d - l l l eaction oordinate Step Partial charge Step unimolecular 0% % % 98% d d d - l l l eaction oordinate d Step Step TS Step - unimolecular d - 0 d 4 Step TS l 59 %.. Molecularity 4 %. unimolecular. bimolecular l d d d d - l l l eaction oordinate l l Step 5 l eaction oordinate l l Step 6. unimolecular. bimolecular l l. - - Partial charge l d charges? l eaction oordinate l l Step bimolecular 7 l eaction oordinate l l l 99% 0% % 0%

4 l eaction oordinate l l d charges? d l d - l l 9 d d - l, l bimolecular eaction oordinate, l - All d TS s d unimolecular,, l - Section 4. d l d- bimolecular l, 0 Bronsted acid/ -> X conversion l dissociation l Step start l Name?? sp hybrid planar bonds Empty p-orbital l Step middle Step end d d- l l sp hybrid tetrahedral geometry l 4 4

5 0% % 7% % 89% 0% Lewis acid/ rxn Lewis acid grabbee Lewis acid e - pair acceptor l Section 4.0 grabber Lewis e - pair donor 5 Brønsted vs Lewis Brønsted: Lewis: Lewis vs Bronsted Limited to Acid: proton. donor Base: proton. acceptor Section 4.0 Acid: electron pair. acceptor Base: electron pair. donor All acid/ reactions can be classified as Lewis. 6 All acid/ steps l A) Lewis but not Bronsted B) Both Lewis & Bronsted ) Neither l l 7 Possesses positive charge character & is attracted to species with negative charge character Greek: like Nucleophile & electrophile electrophile Lewis acid Possesses negative charge character nucleophile & is attracted to species with l positive charge character Lewis 8 to X conversion energy diagram steps E, l eaction oordinate d ate determining d step ionic intermediates (polar) ate determining step d d - l, l - E act l - d,, l - DE < 0 Section 4. d - l l, 9 Substitution nucleophilic unimolecular SN S N mechanism d d - l, l l bimolecular eaction oordinate, l - d d ate determining step l - unimolecular d,, l - Section 4. d - l bimolecular l, 0 5

6 Preparation of alkyl halides from alcohols & hydrogen halides: building blocks SN X reactivity X X eactivity of hydrogen halide F << l < Br < I pk a : Preparation of alkyl halides from alcohols & hydrogen halides: building blocks SN reactivity X X eactivity of hydrogen halide F << l < Br < I eactivity of alcohols weakest acid strongest acid Depends upon classification. o alcohol. o alcohol. o alcohol What is the classification? 99% % 0% Preparation of alkyl halides from alcohols & hydrogen halides: building blocks SN reactivity X X eactivity of hydrogen halide F << l < Br < I eactivity of alcohols < < primary least reactive secondary tertiary most reactive o alcohol o alcohol o alcohol 4 eactivity and reaction conditions eactivity of hydrogen halide F << l < Br < I eactivity of alcohols 5 Tertiary o hydrogen chloride < < primary least reactive tert-alkyl chloride secondary l l tert-alcohol water high yield within minutes tertiary most reactive 5 eactivity and reaction conditions Secondary and primary eactivity of hydrogen halide F << l < Br < I eactivity of alcohols hydrogen bromide < < primary least reactive o alkyl bromide secondary o Br Br secondary alcohol hydrogen bromide o alkyl bromide water 0 Br o Br primary alcohol water tertiary 6 most reactive 6

7 eactivity and reaction conditions LQ #) Both l and Br are added to pentan--ol & the solution is heated to 80 o. Draw MP. o alcohol All comparison MP = major organic product l 5 o l o alcohol Br Br o Br Br o alcohol Br 0 o Br 7 eactivity and reaction conditions Why can o alcohols react with X at cooler temperatures than can o or o? All comparison o alcohol LQ #) Both l and Br are added l 5 o l o alcohol Br Br o Br Br o alcohol Br 0 o Br 8 reactive Which species is more reactive? I 99% 0% % 0% II I. A. A D. B 4. B D II S N mechanism o SN E diagram` d d o d Section 4. d - l B d d - l Why????, l - A D, l, l - eaction oordinate eaction oordinate 9 eaction oordinate l 40 Structure, bonding & stability of s o arbocation classifications propyl cation -methylpropyl cation - is the charged Section 4.0 -methylcycloheptyl cation o o tert-butyl cation,-dimethylethyl cation 4 A B. o 0. o. o 4. o 0 5. o 6. o 7. o 0 8. o 9. o lassification A) lassification of B) # s bonded to arbocation for next clicker? 96% 0% 0% 0% % % 0% 0% 0%

8 A B. o 0. o. o 4. o 0 5. o 6. o 7. o 0 8. o 9. o lassification A) lassification of B) # s bonded to 0% 0% 98% 0% 0% % % 0% 0% Structure & bonding of s arbocation electronic description 0 o electronic description sp planar Section 4.0 hybridization of? empty pure p orbital - s bonds sexteted 4 44 Stability of s methyl arbocation stabilities o o Alkyl groups donate e - density to. electrophilic Alkyl group is an electron pump Section 4.0 o 45 Dr. Kay Sandberg Stability of s Inductive effect Alkyl group is an electron pump d d Section 4.0 Inductive effect: polarization of s bonding e - s (- s bonding e - s more polarizable than - s bonding e - s) 46 Dr. Kay Sandberg Stability of s Alkyl groups donate e - density to. a a a yperconjugation b b b yperconjugation delocalization of s bonding e - s in a p -like interaction d d Section 4.0 b 47 Dr. Kay Sandberg Alkyl groups are electron donating Electronic (electron pumps) effects Inductive effect yperconjugation d d via s - bond d d via p - fashion 48 Dr. Kay Sandberg 8

9 Stability of s < methyl (least stable) arbocation stabilities < Most stable? < Section 4.0 tertiary (most stable) The slowest rxn has the highest hill E hill analogy Alkyl groups donate e - density to. inductive effect hyperconjugation The more alkyl groups the more stable the The slowest rxn has the highest hill E hill analogy continued DS of S N Structure and reaction rate: S N mechanism: eactivity of alcohols ate = k[alkyloxonium ion] E Step analysis k k Section 4. < < < k methyl d d d d d d,,, E act E act E act E act, d d k tertiary A B 5 eaction oordinate 5 Smaller activation E = The species with the LWE energy hill to cross over will be the ME reactive species Step comparison S N mechanism Substitution nucleophilic bimolecular d- d SN E diagram Br Section 4. o o eaction oordinate 5 E d d- Br Br Br Br methyl & o Br 54 9

10 ther methods to convert alcohols to alkyl chlorides Thionyl chloride Section 4. DG = D - TDS g Sl l l S thionyl chloride rx w/ ther methods to convert alcohols to alkyl chlorides o & o -> l reagents Sl pyridine or K N l Section 4. DANGE! SIVE. AUSES BUNS T ANY AEA F NTAT. MAY BE FATAL IF INALED. AMFUL IF SWALLWED. VAPS AUSE SEVEE IITATIN T SKIN, EYES AND ESPIATY TAT. WATE EATIVE. 55 o l typcial s l 56 ther methods to convert alcohols to alkyl bromides o & o Phosphorus tribromide water soluble PBr Br P require higher temperatures o, o & o Br Br Section Alcohol Transformation Wheel S Alcohol N ( transformation o, o ) wheel P Br Br o, o, o Br o & o Br S N ( o ) Br o, o, o o & o PBr o S N l o & o Sl LQ #: MP formed when 4-methylheptan--ol is treated with thionyl chloride in o l l Section 4.0 o & o l l l S 58 0