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Organic Reactions

Organic Reactions. Addition, Oxidation, Reduction, Substitution and Elimination. 4.) (Nucleophilic) Substitution. Like a single displacement reaction (remember grade 10?). Substitution. Substitution. We start with a halogenoalkane (an alkane with some halogens on it  )

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Organic Reactions

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  1. Organic Reactions Addition, Oxidation, Reduction, Substitution and Elimination

  2. 4.) (Nucleophilic) Substitution Like a single displacement reaction (remember grade 10?)

  3. Substitution

  4. Substitution • We start with a halogenoalkane (an alkane with some halogens on it ) R – C – X where X is the halogen • I.e. R—C—Br, R—C—Cl, etc.

  5. Substitution • This is attacked by a nucleophile (“Y”) • “nucleophile” = “nucleus-loving” • The nucleophile (Y) attaches itself to the C, kicking off the halogen (X) Y: + R – C – X R – C – Y + X: • Y Is a Lewis base, it donates both pairs of electrons to its bond with the Carbon • This makes the halogen (X) a leaving group (…because it leaves…) • When the halogen (X) leaves, it takes both the electrons that formed its bond with the Carbon

  6. Substitution • What can act as a nucleophile? • Neutral molecule or anion (- charge) • Has unshared electron pair (lone/free e- pair) • I.e. OH-, H2O, CN-, RNH2, R—OH, R—O, etc. • Substitution reactions tend to occur in water (because involve polar groups)

  7. Substitution • R'O- + RX  X- + R'OR (an ether) • OH- + RX  X- + ROH (an alkanol) • H2O + RX  X- + ROH2+ • R'OH + RX  X- + ROHR'+ • CN- + RX  X- + RCN • R'NH2 + RX  HX + RNHR'

  8. Substitution Examples • CH3-Cl + OH- CH3-OH + Cl- • CH3-CH2-Cl + NaOH (+ Δ)  CH3-CH2-OH + Cl- + Na+ • CH3-Cl + HO-CH2-CH3  CH3-OH-CH2-CH3+ + Cl- • CH3-Cl + -OCH3  CH3-O-CH3 + Cl- • CH3-Cl + NaOCH3  CH3-O-CH3 + Cl- + Na+

  9. 2 types of Substitution • SN2 – New Y-C bond made while the old X-C one is broken • One step • SN1 – First old X-C bond breaks, then new C-Y bond made • 2 steps

  10. SN2 Substitution: “Attack!” • “Substitution, Nucleophilic, Bimolecular” • Everything occurs in one step • No intermediate compounds are made • Often called a “backside attack” • Nucleophile must attack C from opposite side of X (where there’s room to attack)

  11. SN2 Substitution • If you see a weird transition state with Carbon sort-of-bonded to 5 things at once, you are looking at an SN2 substitution. • (A transition state is NOT an intermediate!)

  12. SN2 Substitution Examples

  13. SN1 Substitution: “I’ll just leave now…” • “Substitution, Nucleophilic, Unimolecular” • First X leaves, then Y attaches to Carbon • This makes a carbocationintermediate • (+ charged cation)

  14. SN1 Substitution Examples

  15. Uses • Substitution reactions let us introduce nucleophilic functional groups into molecules we create • I.e. • We can make R—Br into alcohols, ethers, etc. 

  16. When do SN2 vs. SN1 Happen? • If the C being attacked is very branched: SN1 • SN1 occurs on tertiary halogenocarbons • no room for the nucleophile (Y) to get in while the halogen (X) is still attached • So can’t have SN2 • I.e. • SN2 occurs on primary halogenocarbons (lots of room) • Either can occur on secondary halogenocarbons

  17. Rate of Substitution • Depends on what your Halogen (X, leaving group) is • The further down the periodic table, the weaker the C-X bond  the easier it is to kick X off • The faster the reaction

  18. Rate of Substitution • Also depends on whether it’s SN1 or SN2 • SN2 is slower, because that crazy transition state has a relatively high activation energy • Compared to the transition state, the carbocation intermediate in SN1 is surprising stable, actually • So tertiary halogenocarbons (use SN1 – see 2 slides ago) react faster than secondary, which react faster than primary

  19. 5.) (Nucleophilic) Elimination • So called because it eliminates things from the molecule (kicks them off) • I.e. • But this is different from substitution: involves 2 Cs! • Start with a C-C single bond with a leaving group (ie – halogen/“X”) hanging off of one of the Carbons, and a Hydrogen hanging off the other • We kick one atom off of each C (needs to be initiated by a nucleophile) • The 2 carbons end up double-bonded together • The nucleophile ends up bonded to the Hydrogen • The Leaving group (halogen/“X”) ends up alone

  20. Substitution or Elimination? Substitution: • R-X (halogen) + Nu R-Nu + X- • Halogen ends up alone, R’s number of single/double/triple bonds don’t change • If there’s only 1 carbon, it can only be sub Elimination: • C-C-X + Nu  C=C + X- + H-Nu • Add a C-C bond • Nu doesn’t attach to the hydrocarbon, but pulls off an H

  21. Elimination is very different from Substitution • Reason 1: • In substitution, the nucleophile and the leaving group (X) are on the same carbon atom • In elimination, the leaving group(X) is on the carbon adjacent to (next to) the one attacked by the nucleophile

  22. Elimination is very different from Substitution • Reason 2: • In substitution the nucleophile attacks a carbon atom • In elimination, the nucleophile attacks the hydrogen attached to a carbon atom!

  23. Elimination is very different from Substitution • Reason 3: • In substitution the carbon that’s attacked ends up with the same number of C-C bonds and the same number of non C-C bonds • It loses a bond to one non-carbon (X) but gains a bond to another (Y; here called “Nu”)

  24. Elimination is very different from Substitution • Reason 3: • In elimination, the 2 Carbons (the one whose H is attacked by Nu, and the one with X on it) both lose a non-carbon molecule and end up double-bonded to each other!!! • Each carbon loses 1 non-carbon bond (to X and H, respectively) and gains one C-C bond (double bond vs. single bond before)! • Note: this also qualifies as oxidation! (reduce C-H bonds)

  25. 2 Types of Elimination • Like Substitution, there are 2 types of Elimination • They differ in number of steps • E2 – New Nu-H bond and C-C bond forms simultaneously with X leaving • One step with a weird transition state (like SN2) • E1 – First X leaves, then Nu attacks the H and the C-C bond is made • 2 steps with a carbocation intermediate (like SN1)

  26. Mechanisms

  27. E2 Elimination Examples

  28. E1 Elimination Examples

  29. Predicting Substitution vs. Elimination • What’s a strong/weak nucleophile? • A good nucleophile is a good Lewis base • Donates electrons easily • I.e. OH-

  30. Predicting Substitution vs. Elimination • Nucleophilic Substitution (SN2): • 1 RX, non-polar solvent, strong Nu (strong base) RX + Nu-  R-Nu + X- • Nucleophilic Substitution (SN1) • 3 RX, polar solvent, weak Nu (weak base) RX + Nu-  R-Nu + X- • Elimination (E2) • 3 RX, strong Nu RX + Nu-  alkene + NuH + X- • Unimolecular Elimination (E1) • 3 RX, polar solvent, weak Nu RX + Nu-  alkene + NuH + X-

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