Organic Chemistry Mechanisms for Dummies

1. Organic Chemistry Mechanisms for Dummies By Zachary Denkensohn and Joshua Weiss

2. Free Radical Substitution An alkane plus a halogen yields an alkyl halide and a hydrogen halide

3. Initiation Cl Cl The UV rays break the bond between the two chlorine atoms

4. Products: Cl Cl RADICALS Two chlorine radicals are formed (they are radicals because they need to bond with something)

5. Propagation Cl One of the chlorine radicals steals a hydrogen from the ethane

6. Products: RADICAL Hydrogen and chlorine bond to form HCl and a C2H5 radical remains

7. Termination Cl The other chlorine radical replaces the hydrogen that was stolen before

8. Products: The remaining two radicals have bonded to form an alkyl halide

9. Final:

10. Minor Products: Minor products are formed because this free radical substitution is happening many times and is not completely controlled, so the expected reactions don’t always occur Cl Cl + +

11. Electrophilic Addition An alkene plus a hydrogen halide yields an alkyl halide

12. The approaching positive side of the H-Br dipole causes one of the pairs of electrons in the double bond to go to the left carbon

13. The partially positive hydrogen is attracted to the negative charge on the left carbon and the partially negative bromine is attracted to the positive charge on the right carbon

14. Final: The hydrogen bonded to the left carbon and the bromine bonded to the right carbon to create an alkyl halide

15. Markonikov’s Rule • When a compound HX is added to an unsymmetrical alkene, the hydrogen attaches to the formerly double bonded carbon with the most hydrogens already attached to it • X will attach to the carbon on the other side of the former double bond • If peroxide is present, the opposite will occur: Anti-Markonikov Rule

16. Nucleophilic Substitution (Sn1) A tertiary alkyl halide plus a negative ion yields a tertiary alkane bonded to a negative ion plus a negative halogen ion

17. The already polarized C-Br bond begins to ionize as the electron pair in the bond moves toward the more electronegative bromine

18. The bromine has been ejected as a negative bromine ion and a carbocation (positive carbon ion) remains A negative ion approaches the carbocation

19. Final: The negative ion was attracted to the carbocation and bonded with it

20. Nucleophilic Substitution (Sn2) A primary alkyl halide plus a negative ion yields a primary alkane bonded with a negative ion plus a negative halogen ion

21. The negative ion is attracted to the positive side of the C-Br dipole and begins to bond with the carbon As the bond between the negative ion and the carbon is formed, the bond between the bromine and the carbon is broken This process is much faster than SN1 because, unlike in SN1, ionization in the C-Br bond is forced by the negative ion and happens very quickly

22. Final: The negative ion has replaced the bromine and bonded with the carbon The bromine was ejected as a negative ion

23. Secondary alkyl halides A secondary alkyl halide can undergo SN2 because the carbon is not completely protected, so the negative ion can approach it, but sometimes secondary alkyl halides undergo SN1 because the carbon is partially protected and the negative ion may have trouble bonding with it