TOPIC 6. NUCLEOPHILIC SUBSTITUTIONS (chapter 6 and parts of chapter 11)

Transcription

1 L TPI 6. NULEPILI SUBSTITUTINS (chapter 6 and parts of chapter 11) BJETIVES 1. Describe two pathways (mechanisms) to account for substitution at sp 3 carbons bearing an electronegative atom (leaving group) 2. Discuss the effect of starting material (substrate), leaving group, reagent (a nucleophile) and reaction conditions on the course of a reaction 3. ecognize functional group transformations and synthesis of new molecules in one step by substitution of an appropriate material 4. Explore the substitution chemistry of alcohols and ethers

2 EATINS T DATE Subst n Subst n substitutions additions 2 /Pt Add n Elim n Add n Elim n Subst n Subst n substitutions Acid-Base hemistry + 3 N + 3 N VEVIEW: NULEPILI SUBSTITUTINS S: l KN N Kl Na Na NaN 3 l S l 2 S 2 l

3 L Nu Nu L We are only considering sp 3 (alkyl) substrates in this section. Molecules with leaving groups on sp 2 (e.g., vinyl, aryl) or sp (ethynyl) carbons do not react in fashion described in this section Nucleophiles Donate a pair of electrons: to an electrophile (lone pair or pi bond) Neutral Anionic Electrophiles eceive a pair of electrons: from a nucleophile ationic Lacks an octet l Polar, electrophilic l Al l X

4 Effect of Leaving Groups on Electrophilicity L Nu Nu L -L bond must be polarizable -L bond must be relatively weak L needs to be able to accommodate a pair of electrons Good leaving groups are weak bases Substitution: What is the Mechanism? substitute / sêb-,stê,t(y)üt/ vb: to put in the place of another What are the rules?

5 TW LASSES F EATIN Substitution reactions can be performed under different conditions which give rise to dramatically different outcomes. Nucleophilic substitution reactions can be classified as one of two types, based on these experimental observations. In order to develop predictive tools, we need to understand reasons why these observations are important. That is, we need to develop proposals for two different mechanisms which are consistent with the two sets of data and which we can use to predict the outcome of other reactions. SUBSTITUTIN AT 1 SUBSTATES: BIMLEULA NULEPILI SUBSTITUTIN (S N 2) Examples S: Prob: , 17aj,25,27, 31,33, Na I + Na S 3 3 S

6 Kinetics NaMe 3 2 Me + Na [ 3 2 ] [NaMe] relative rate 0.01 M 0.01 M M 0.01 M M 0.02 M M 0.02 M 4 onclusion: ate = k[-l][nu] d1 Mechanism of Nucleophilic Substitution of 1 Alkyl alides: The S N 2 eaction

7 Slide 15 d1 d-laptop, 8/6/2007

8 hirality hiral -L forms -Nu with opposite stereochemistry (inversion of stereochemistry, Walden Inversion ) 3 D D 3 Interpretation: Nu L Nucleophilicity Basicity and nucleophilicity are not the same, but they are related phenomena A negatively charged nucleophile is more reactive than its conjugate acid. N > N > 2 > In a group of nucleophiles in which the nucleophilic atom is the same, nucleophilicity parallels trends in basicity. > >> 2 >> > 2 Steric bulk hinders nucleophilicity Across a row in the periodic table, nucleophilicity parallels trends in basicity. 3 > 2 N > > F N 3 > 2

9 Down a column in the periodic table, trends in nucleophilicity of anions depend on the choice of solvent. polar protic Me Et ( 3 ) 2 S DMS dimethyl sulfoxide polar 3 N AN actonitrile aprotic ( 3 ) 2 N DMF N,N-dimethylformamide 3 3 acetone 2 l 2 methylene chloride - TF tetrahydrofuran Et 2 ether diethyl ether non-polar 3 ( 2 ) 4 3 benzene Protic solvents solvate the anions (nucleophiles) by -bonding and cations (counterions). Small electronegative anions particularly are well solvated, lowering their nucleophilicity. Trends in nucleophilicity in polar protic solvents increases going down a column of the periodic table Polar aprotic solvents cannot hydrogen bond to anions (but do solvate cations). Trends in nucleophilicity in polar aprotic solvents parallels trends in basicity. Large neutral nucleophiles ( 2 S, P 3 ) are highly polarizable and can donate more electron density than smaller neutral nucleophiles.

10 Nucleophilicity of Anions nucleophilicity e Li Na K Be Mg a B Al Ga 3 Si Ge base strength N N 2 P S S As Se Se F F l l I I base strength Ne Ar Kr nucleophilicity in APTI solvents nucleophilicity in protic solvents elative Nucleophilicity (in Me) Excellent Nucleophiles N S I yanide Thiolate Iodide elative Nucleophilicity 126, ,000 80,000 Good Nucleophiles ydroxide 16,000 omide 10,000 N 3 Azide 10,000 N 3 Ammonia 8000 N 2 Nitrite 5000 Fair Nucleophiles l hloride Acetate 630 F Fluoride 80 3 Methanol 1 2 Water 1 Nucleophilic relative strength is measured by relative rate in an S N 2 reaction (but depends on substrate, solvent type, etc.)

11 L Leaving Group Ability ate: -I > - > -l >> -F -L bond must be broken weaker bonds are more polarizable, easier to break Bond strengths (kcal/mole): -F 116 -l I 52 verall: A good substrate for bimolecular nucleophilic substitution should have: 1. Weak -L bond 2. Polarizable -L bond (ease with which the electron distribution in the bond is distorted) 3. Leaving group that can accommodate a pair of electrons Leaving group pk a of conjugate acid Good Leaving Groups I Iodide -10 omide -9 l hloride -7 S 3 Sulfonate Water -1.7 Very Poor Leaving Groups F S N Fluoride Thiolate yanide ydroxide Alkoxide ydride 35 N 2 Amino 38 3 Methyl 48

12 Alkyl Substrates ate: methyl > 1 > 2 ( 3 unreactive) Me Nu L Et ipr Nu Me Me Me L tbu Adjacent alkyl groups also slow the reaction L L L Energetics of ne-step (i.e., concerted) eaction S N 2 ΔG eaction coordinate

13 Practical Applications of S N 2 eactions: Functional Group Transformations at 1 o and 2 o arbons al S N S N source of nucleophile Na Na Na S Na Na S Na N N 3 N 3 Na N 3 N' 3 N' 3 N' 3 NaN 2 Na Intramolecular S N 2 eactions - yclizations Na Na

14 ~ MAKE FLASADS ~ NaN N methyl, 1 o, and 2 o front inversion of stereochemistry S N 2 back Substrate + eagent (and conditions) Substrate + eagent (and conditions) Substrate and Product Substrate and Product eagent (and conditions) and Product eagent (and conditions) and Product Problem: ow would you prepare 2-phenylethanethiol from 1-iodo-2- phenylethane? I? S Problem: ow would you make the following compound from 1- bromopropane and any other starting materials? N

15 Problem: Which of the following would undergo the fastest reaction with 1-bromopropane? (a) Ph 3 N or Ph 3 P (b) 1.0 M 3 Na or 2.0 M 3 Na Synthesis of Alkynes and Alkanes: Alkylation of Acetylide Anions NaN 2 ' ' Na 2 /cat 2 2 ' ' must be 1

16 L SUBSTITUTIN AT 3 SUBSTATES: UNIMLEULA NULEPILI SUBSTITUTIN (S N 1) S: Prob: 6.16,30,35 Example 2 Experimental observations of kinetics, chirality, substrate structure and effect of nucleophiles for this reaction are inconsistent with the S N 2 mechanism Kinetics [t-bu] [ 2 ] relative rate 0.01 M 0.01 M M 0.01 M M 0.02 M M 0.02 M 2 esult: ate = k[-l] independent of concentration of Nu

17 Mechanism of Nucleophilic Substitution of 3 Alkyl alides: S N 1 Practical Applications of S N 1 eactions: Solvolysis The only practical S N 1 reactions are solvolyses, reactions in which the solvent also acts as the nucleophile. These reactions arise because solvents which are polar enough to facilitate dissociation of the substrate are also nucleophilic S- (protic solvent) S- =, 2,

18 Substrates ate: 3 > 2 (1, methyl not reactive) Interpretation: Me Me L Me Stability of arbocations arbocation elative Energy (kcal/mol) Methyl 0 Ethyl -37 i-propyl -65 t-butyl More substituted carbocation stabilized by hyperconjugation Effect of Leaving Group ate: -I > - > -l (F: unreactive) Me Me L Me -L bond must be broken weaker, polarizable bonds are easier to break Bond strength: -F > -l > - > -I verall: A good substrate for unimolecular nucleophilic substitution should have: 1. Weak -L bond 2. Polarizable -L bond (ease with which the electron distribution in the bond is distorted) 3. Leaving group which can accommodate a pair of electrons

19 Effect of Nucleophile/Solvent ate independent of concentration of nucleophile (solvent) ate depends on polarity of solvent hirality A single enantiomer of substrate with the leaving group on the stereogenic center reacts to give product that consists of a mixture of both enantiomers. Me Pr Et 2 Me Pr Et Me Et Pr Energetics of a Two-Step eaction S N 1 ΔG eaction coordinate

20 SUMMAY: FATS EFFETING S N 1 AND S N 2 EATINS S:6.15; Prob: 6.18,26 S N 1 S N 2 L Nu Nu Nu Nu L Nu Dissociation-Nucleohipic addition-deprotonation Substrate: 3 >2 >>1 or methyl - Stability of carbocation intermediate ate = k [substrate] - independent of [nucleophile] - rate depends on solvent polarity hirality - racemization Generally only useful for solvolyses (reactions with 2,, 2 ) L ne-step (concerted) mechanism Substrate: Methyl>1 >2 >>3 - Steric bulk hinders attack of Nu ate = k [substrate] [nucleophile] hirality - Inversion ften performed in polar aprotic solvents, e.g., DMF (Me 2 N), DMS (Me 2 S) to dissolve substrate and ionic reagent, and increase reaction rate A Limitation to eactions of Alkyl alides with Nucleophiles Elimination competes with substitution if the nucleophile is too basic or if the electrophile is too crowded (we will explore this further in Topic 7) X L X L

21 L SUBSTITUTIN EATINS F ALLS S: Alcohols as Acids and Bases B A Alcohols are weak acids and weak bases. Acid-base chemistry is important in activating the electrophilic and nucleophilic character of alcohols, respectively. onversion f Alcohols To Alkyl alides Alcohols do not react with sodium halides to give alkyl halides! 3 Na Alcohol + -al 3 o Alcohols: S N 1

22 1 o Alcohols: S N 2 ther reagents (1 o or 2 o ) P 3 (1 o or 2 o ) Sl 2 l

23 MESYLATES AND TSYLATES IN S N 2 EATINS l S ' 1 o Nu S ' Nu Nu = Me: mesylate (Ms) 4-methylphenyl: tosylate (Ts) = al S N etc. S:11.10 Problem: ow would you prepare 2-phenylethanethiol from 2-phenyl-1- ethanol?? S Problem: ow would you make the following product from 1-propanol and any other starting materials?? N + l

24 SYNTESIS F ETES Synthesis of Ethers: Williamson Ether Synthesis S: Prob: verall eaction and Mechanism Na 1 or 2 o (l or I) Designing Williamson Ether Syntheses You could suggest making either - bond of the ether

25 Synthesis of Symmetrical Ethers: Dehydration of Alcohols 2 S o At higher temperature a competing reaction predominates (elimination of water to form an alkene, see Topic 7) 2 S o eaction of Ethers: Acid-catalyzed ydrolysis ' + '

26 Δ 3 Δ 3 L SUBSTITUTINS IN SYNTESIS Prob: 6.17,20, 32a-h,33 You should prepare a chart of all of the types of reactions that have been covered so far.we ll add more later. 2, hν substitutions 2 /Pt Elimn Addn Elimn Subst substitutions

27 NE-STEP SYNTESES S: It is important to recognize transformations which can be performed in a single step. Use the thought process: - What can the product can be made from? PDUT STATING MATEIAL - The synthesis itself is STATING MATEIAL PDUT Problem: ow would you prepare the following from appropriate alkyl chlorides? (a) 3 2 S (b) ( 3 ) 2 N (c) 2 3 from starting materials with 7 or fewer carbon atoms

28 Problem: Why does the following reaction not take place? Problem: Explain why reaction of 1-bromopropane with potassium cyanide gives a mixture of N (major product) and N (minor)? Draw Lewis structures of the products and nucleophiles.

29 Problem: ow can you prepare the following two compounds from the appropriate alkyl bromide? (a) Methyl phenyl ether, Me--Ph (b) (S)-2-Pentanol, 3 () Problem: ow could you perform the following synthesis? [An introduction to designing multi-step syntheses] N 3 from

30 Problem: Predict the structure of the product of the following reaction l Na Et 6 10 S S An important limitation on acid catalyzed S N reactions tert-butanol must be protonated to make the hydroxyl group a better leaving group. Ammonia is nucleophilic So why doesn t this reaction work? N 3 2 S 4 N 2 Why can t you do this sequentially? i.e., 2 S 4 N 3 2 N 2 Acid promoted S N 2 reactions are limited to cases where the nucleophile is very weakly basic (i.e., reaction with -al). For other nucleophiles, convert the alcohol to the tosylate or mesylate first.

31 The role of nucleophilicity, leaving group ability, and substrate on S N processes. Which of the following proceed efficiently? Na Na 2 Na 2 2 Na 2 2 What about secondary alkyl halides? 1 -L: 3 -L: 2 -L: might display features of both S N 1 and S N 2 reactions (!) Under conditions strongly favoring S N 1 reactions S N 1 Under conditions strongly favoring S N 2 reactions S N 2...but strongly basic nucleophiles promote elimination! (see Topic 7) What about vinyl halides and aryl halides?

32 TPI 6 N EXAM 3 Types of Questions - ecognize factors which influence the mechanism of nucleophilic substitutions - Predict outcomes of substitution reactions - The problems in the book are good examples of the types of problems on the exam. Preparing for Exam 3 - Work as many problems as possible. - Work in groups. - Do the Learning Group Problem at the end of the chapter. - Work through the practice exam