SUBSTITUTION REACTION CHARACTERISTICS. Sn1: Substitution Nucleophilic, Unimolecular: Characteristics

Transcription

1 SUBSTITUTION EATION AATEISTIS Sn2: Substitution cleophilic, Bimolecular: haracteristics 1) The 2 means Bimolecular (or 2 nd order) in the rate-determining (slow) step: rate = k [: - ] [-X] or rate = k [:] [-X] if the is neutral. 2) S N 2 reactions are One Step and oncerted (oncerted means bond making and breaking at the same time): X If the nucleophile is neutral; + X hemistry 118A Workshop Jim ollister, Doug Kent,olf Unterleitner Learning Skills enter; U Davis Sn1: Substitution cleophilic, Unimolecular: haracteristics 1) The 1 means Unimolecular (or 1 st order) in the rate-determining (slow) step: rate = k [-X]. The is not in the rate law. 2) Two (or Three) Steps: Step 1. Dissociation of halide from haloalkane. (I 1 = Intermediate 1) Step 1. 1 X I 1 slow 1 + X X + X E I 2 P eaction oordinate E X P eaction oordinate Step 2. Step fast fast 1 equal front & backside attack 1 Step 2. cleophilic Attack forms Intermediate 2 (I 2 ). + Step 3. Deprotonation step forms the Product. 3) There is a Transition State (TS) but no Intermediate) (The symbol indicates a TS) (See above). 3) as Intermediate arbocation (sp 2 carbon) and Transition States (See above).

2 4) Backside Attack- get Inversion (and, if the electrophilic carbon is chiral, usually get a change in absolute configuration, as from to S, but not necessarily, because it depends on the priorities of the attaching nucleophile and the leaving group relative to the other groups attached to the chiral carbon.) 5) Enhanced by aprotic polar solvents which make the nucleophile unhindered, or "naked." 4) Equal Front and Backside Attack; not stereospecific; if start with chiral carbon, get racemic mixture. 5) Enhanced by protic, polar solvents, as 2 O, -O-, and small carboxylic acids, which stabilize the carbocation by bonding. 6) Sensitive to steric hindrance. 6) Solvolysis: Solvents as 2 O, -O-, and small carboxylic acids, can act as. 7) Occurs in methyl > 1 o > 2 o haloalkanes substrates. eacts best in order indicated. 7) Occurs best in 3 o haloalkanes substrates and slowly in 2 o haloalkanes.

3

4 Learning Skills enter S n 2 substitution rxns Factors favoring S n 2 1) Substrate: methyl > 1 >2 (never 3 ) an't have things in the way or the nucleophile won't make it in. 2) Good nucleophile needed. c. has to go after the reactive center or no rxn. 3) The substrate needs to have a good leaving group. Good LG's are very weak bases which can accomodate their negative charge by high electronegativity &/or delocalization. 4) Best with aprotic polar solvents. They leave the nuc. free to react, and help to stabilize the polar species. Mechanism: Backside attack. In the transition state, the nucleophile is coming in as the leaving group is coming off. Inversion changes /S configurations most of the time. E2 Elimination rxns Factors favoring E2 1) cleophile fi strong bases. The nuc. acts like what it is, a strong base. It tears a b- off a b-carbon, which is next to the carbon with the leaving group on the substrate (called the a-carbon). 2) Needs a good leaving group in antiposition to a neighboring b-hydrogen. When the b- is being removed, the leaving group needs to be free to come off. A double bond forms. 3) Solvent is often the conjugate acid of basic nuc., as NaO in 2 O (but could also be in 3 O), 3 2 O - K + in 3 2 O, etc. ( Strong bases are looking for 's. You need the base to remove the 's from the substrate and not the solvent). 4) Substrate should not be methyl or 1 unbranched (which with small, strong bases will favor S n 2 product). Needs a leaving group which is anti to the b- being removed. Mechanism: Anti-elimination. Base removes a b- 180 to leaving group on neighboring carbon. olf Unterleitner, Jim ollister and Doug Kent S n 1 substitution rxns Factors favoring S n 1 1) Substrate: 3 >2 (never 1 and methyl) More alkyl groups help to stabilize the carbocation intermediate. 2) The substrate needs to have a good leaving group (LG). To fall off and leave a carbocation behind, the LG has to be stable on it's own. Good LG's are very weak bases which can accommodate their negative charge by high electronegativity &/or delocalization [as I -, or SO 4 - (sulfates) or SO 3 - (.sulfonates)] 3) Works with poor as 2 O or O. Once the carbocation is formed in the rate determining step, it is very, very reactive; anything with a lone pair will do. 4) Best with protic polar solvents. The carbocation is a charged (+1) species, so polar solvents help stabilize it (-bonds). Mechanism: arbocation intermediate. You get a mixture of both and S. Beware of rearrangements. E1 Elimination rxns Factors favoring E1 1) Substrate 3 >2 (never 1 and methyl) More alkyl groups help to stabilize the carbocation intermediate. 2) The substrate needs to have a good leaving group (LG). To fall off and leave a carbocation behind, the LG has to be stable on its own (needs to accomodate its negative charge by high electronegativity &/or delocalization [as I - or SO 4 - or SO 3 - (sulfonates)]. 3) Best with protic polar solvents. As with S n 1, the carbocation is a charged (+1) species; polar solvents help stabilize it. 4) Poor nucleophile as 2O or O. Mechanism: arbocation intermediate. Look out for rearrangements. The product that comes from the most substituted (most alkyl groups around it) carbocation is usually the major product. Beware of rearrangements.

5 ELIMINATION EATIONS FOM ALKENES. TEY AE FAVOED BY STEI- ALLY INDEED MOLEULES, EITE ALOALKANES, NULEOPILES, O BOT. E2: Elimination, Bimolecular: haracteristics 1) The 2 means Bimolecular (or 2 nd order) in the rate-determining (slow) step: rate = k [: - ] [-X] or rate = k [:] [-X] if the is neutral. hemistry 118A Workshop Jim ollister, Doug Kent, olf Unterleitner Learning Skills enter; UDavis E1: Elimination, Unimolecular: haracteristics 1) The 1 means Unimolecular (or 1 st order) in the rate-determining (slow) step: rate = k [-X]. The is not in the rate law. 2) E2 reactions are One Step oncerted Acid/Base reaction with elimination of the LG: (Acid/Base reactions happen faster than nucleophilic/electrophilic reactions.) Alkene forms. 1 X strong base Beta 3 B a E2 1 X 1 3 anti-elimination + B + X conj. acid L.G. of nuc. Another view: 3 B E2 1 3) oncerted Mechanism- No Intermediate + B + X 4) Anti-Elimination: The base takes the of the TS which is in anti-conformation (180 o ) to the Leaving Group (LG). (The carbon with the LG on it is called the 3 a carbon and the carbon(s) with the acidic on it is called the b carbon.). 2) E1 reactions are Two Steps: Alkene forms. Step 1. Dissociation of halide X Beta a X Step 2. Acid/Base eaction. A poor nuc. acts as a base to remove the acidic. Alkene forms. B B + X conj. acid L.G. of nuc. 3) arbocation Intermediate 4) The is removed from the arbocation Intermediate in the second step (the LG left when the intermediate was formed by dissociation in the first step. Therefore, the LG and the acidic are not in anti-conformation.)

6 5) Enhanced by sterically hindered, strongly basic as LDA or tert-butoxide. 5) E1 product usually seen as a minor product along with a major S N 1 product; the solvent, usually 2 O or an -O-, acts as a base to deprotonate the carbocation. (E1 can be a minor product with a major E2 also, but do not worry about this.) 6) an have more than one type of alkene product if the haloalkane has more than one different type of b carbon with 's on them. (hapter 11 in Vollhardt and Schore) 7) Occurs with 2 o and 3 o haloalkanes and 1 o haloalkanes branched near the a carbon. 6) Same as E2. 7) Best with 3 o haloalkanes.

7 Type of aloalkane Likely Mechanisms by Which aloalkanes eact with cleophiles of Varying Basicity Very, very poor base; Poor nucleophile (e.g., 2 O, or small alcohols or organic acids) Modified From Schore and Vollhardt, Table 7.4 Type of cleophile and its Basicity Weakly basic, GOOD nucleophile (e.g., I -, Br -, N 3, 3 OO - ) Strongly basic, unhindered nucleophile(e.g., - O, 3 0 -, - N 2, -, - 3 ) Methyl No reaction S N 2 S N 2 S N 2 Primary unhindered No reaction S N 2 S N 2 E2 Primary branched No reaction S N 2 E2 E2 Secondary Slow S N 1, E1 S N 2 E2 E2 Tertiary S N 1, E1 S N 1, E1 E2 E2 Strongly basic, hindered nucleophile(e.g., ( 3 ) 3 O -, LDA)

8 hem. 118A Workshop Notes: * Starred species also happen to be good Leaving Groups (very weak bases) (that is, 2O, I -, Br -, and somewhat l - are good LGs) The pk a s of the conjugate acids of many of the species shown are given in parenthesis. = an alkyl group **ealize that a good nucleophile for S N 2 need not be a good base if it has a larger, more diffuse, polarizable electron density cloud. As one goes down a group, a larger p orbital has a more effective overlap with an electrophile for S N 2. ompare - S and - O. Note that all of the strongly basic are negative (and only some weakly basic, good nucs are negative, and these have lower conjugate pkas.) Examples of Very, very poor bases; Poor 2O (-1.7) water* any -O as: 3O (-2.2) methanol or 32O (-2.4) ethanol maybe organic acids as: O O (3.8) formic acid Examples of Weakly basic, GOOD ** I Br l O (-4.7) iodide* Examples of Strongly basic, unhindered O (15.7) hydroxide (-5.2) bromide* any small O as: (-2.2) chloride* 3 O or 3 2 O (15.5) methoxide Examples of Strongly basic, hindered 3 3 O 3 tert-butoxide 3 3 N 3 3 (18) Li lithium diisopropyl amide (LDA) (40) Jim ollister LS U Davis Notes: Actually, the shaded areas contain GOOD which are very, very, poor bases. They are good because of the larger electron density clouds of their p orbitals, [see double asterisks (**) note far left] not because of 3 O (15.9) (4.7) acetate ethoxide N 3 their being (9.2) ammonia N2 (35) amide N 2 (~11) 1 o ammine N (~11) 2 o ammine N 3 (~10) 3 o ammine N 3 ( N N N) (4.6) azide N ( N) (9.2) cyanide S (7) hydrosulfide S as S 3 (10) (~35) hydride carbanions as: 3 (~50) or 2 3 (~50) good bases.