1 / 23

Nucleophilic substitution “NS” Lab #4

Nucleophilic substitution “NS” Lab #4. Substitution reaction:. A reaction in which one atom , ion, or group is substituted for another. It is the reaction of an electron pair donor (the nucleophile, Nu) with an electron pair acceptor (the electrophile).

umed
Download Presentation

Nucleophilic substitution “NS” Lab #4

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Nucleophilic substitution“NS”Lab #4

  2. Substitution reaction: A reaction in which one atom, ion, or group is substituted for another. It is the reaction of an electron pair donor (the nucleophile, Nu) with an electron pair acceptor (the electrophile). e.g. HO:¯ + CH3CH2-Br → CH3CH2-OH + Br:¯

  3. Good nucleophile: 1- have free pair of electrons (-ve charge ). e.g. ¯OH 2- the nucleophilicity is decrease while the electronegativity increase. e.g. ¯ OH > ¯ F , I > Br > Cl > F 3-higher molecular weight will be higher nucleophilicity. e.g. I > Br > Cl > F

  4. Leaving group: • Any group that can be displaced from a carbon atom. • Good leaving groups are those capable of forming stable ions or molecules upon displacement from the original molecule.

  5. SN2 mechanism (single step) there is simultaneous formation of the carbon-nucleophile bond and breaking of the carbon-leaving group bond (concerted).

  6. SN2 mechanism (single step) Energy :

  7. SN2 mechanism (single step) • Energy of the molecules (average energy ) • Increase the energy by heating to reach the transition state(kinetic energy). Eact= kinetic energy + average energy

  8. SN2 mechanism (single step) Rate: depend on Temperature Solvent The concentration of the reactants R + Nu

  9. SN2 mechanism (single step) Example :

  10. SN2 mechanism (single step) Example :

  11. SN1 mechanism(a multi-step) In an SN1 there is loss of the leaving group generating an intermediate carbocation which then undergoes a rapid reaction with the nucleophile.

  12. SN1 mechanism(a multi-step) This pathway is multi-step process: step 1:slow loss of the leaving group, LG, to generate a carbocation intermediate (rate determining step). step 2:rapid attack of a nucleophile on the electrophilic carbocation to form a new bond

  13. SN1 mechanism Example: Step1: Step2:

  14. SN1 mechanism Energy:

  15. SN1 mechanism • step1: enough energy must be supplied to break the c-x bond (Eact) & give the carbocation (intermediate). • step2: need lower Eact

  16. SN1 mechanism Rate : Depends on 1- the concentration of the reactant R 2-temperature 3-solvent

  17. SN1 mechanism XX Reactivity order of -R : (CH3)3C-  >  (CH3)2CH-   >  CH3CH2-  >  CH3- 3° 2° 1° Stability: the key step is the loss of the leaving group to form the intermediate carbocation. The more stable the carbocation is, the faster the SN1 reaction will be.

  18. Which substitution mechanism might operate?

  19. Nucleophilic substitution of alkyl halides Reactions : SN1: X= halides (Cl , Br, I ) R= CH3CH2O- = ---> Leaving group ---> electrophile (e acceptor) ---> nucleophile (e donor)

  20. Nucleophilic substitution of alkyl halides Procedure (SN1 reaction): 1- label 5 clean, dry test tubes from 1to5. 2-add 3drops of one of the following halides in test tube and immediately stopper with each addition: (1) 2-bromobutane. (2) 1-chlorobutane. (3) 2-chloro-2-methylpropane. (4) 2-bromo-2-methylpropane. (5) bromobenzene.

  21. Nucleophilic substitution of alkyl halides 3-in each test tube add 1ml of (1% AgNo3 in ethanol solution) then immediately stopper the test tube. 4-mix thoroughly and record the times for each reaction to form a precipitate or cloudiness. 5-after 5 minutes, place any test tubes that do not contain a precipitate in 100°C water bath, after about 1 minute of heating, cool the test tubes to room temperature and note whether a reaction has occurred. NOTE: consider the alkyl halide to be unreactive if no turbidity or precipitate appears after 10 min

  22. Nucleophilic substitution of alkyl halides

  23. THANK YOU

More Related