Radical Reac)ons of Alkanes

Transcription

1 Radical Reac)ons of Alkanes

2 Types of Steps in Reac)on Mechanisms Bond forma)on or breakage can be symmetrical or unsymmetrical Symmetrical- homoly)c Unsymmetrical- heteroly)c

3 Indica)ng Steps in Mechanisms Curved arrows indicate breaking and forming of bonds Arrowheads with a half head ( fish- hook ) indicate homoly)c and homogenic steps (called radical processes ) Arrowheads with a complete head indicate heteroly)c and heterogenic steps (called polar processes )

4 Radical Reac)ons Not as common as polar reac)ons Radicals react to complete electron octet of valence shell A radical can break a bond in another molecule and abstract a partner with an electron, giving subs)tu)on in the original molecule A radical can add to an alkene to give a new radical, causing an addi)on reac)on

5 Steps in Radical Subs)tu)on Three types of steps Ini2a2on homoly)c forma)on of two reac)ve species with unpaired electrons Propaga2on reac)on with molecule to generate radical Example - reac)on of chlorine atom with methane to give HCl and CH 3.

6 Steps in Radical Subs)tu)on Termina2on combina)on of two radicals to form a stable product: CH 3. + CH 3. CH 3 CH 3

7 Alkanes Undergo Chlorina)on or Bromina)on Alkanes are very unreac)ve compounds because they have only strong s bonds and atoms with no par)al charges These halogena)on reac)ons require high temperatures or light.

8 Preparing Alkyl Halides from Alkenes: Radical Halogena)on Alkane + Cl 2 or Br 2, heat or light replaces C- H with C- X but gives mixtures Hard to control Via free radical mechanism It is usually not a good idea to plan a synthesis that uses this method

9 Use Excess Alkane to Get Monochlorina)on Using excess alkane minimizes dichlorina)on, trichloria)on, etc.

10 Rela)ve Stabili)es of Radicals Ter)ary > Secondary > Primary

11 Alkyl Groups Stabilize Carboca)ons Be\er Than They Stabilize Radicals carboca)on both electrons are in the bonding orbital radical one electron is in the an)bonding oribtal

12 Radical Halogena)on of Alkanes If there is more than one type of hydrogen in an alkane, reac)ons favor replacing the hydrogen at the most highly subs)tuted carbons (not absolute)

13 Chlorina)on It is Easier to Make a Secondary Radical Than a Primary Radical

14 Rela)ve Reac)vity Based on quan)ta)ve analysis of reac)on products, rela)ve reac)vity is es)mated Order parallels stability of radicals Reac)on dis)nc)on is more selec)ve with bromine than chlorine

15 Rela)ve Amounts of Products to determine the product distribu)on, consider both probability and reac)vity

16 Rela)ve Rates of Bromina)on a bromine radical is more selec)ve than a chlorine radical

17 The H Values for Removing a Hydrogen Atom from an Alkane

18 Why is a Bromine Radical More Selec)ve than a Chlorine Radical?

19 Because Chlorina)on is Rela)vely Nonselec)ve, It is Useful Only When all the Hydrogens Are the Same

20 Halogena)on Converts an Alkane to a Useful Compound

21 Why Only Chlorina)on and Bromina)on?

22 Forma)on of Explosive Peroxides

23 Addi)on of HBr in the Presence of Peroxide

24 Mechanism for the Addi)on of HBr in the Presence of Peroxide

25 A Carboca)on Intermediate is Not Formed, So a Carboca)on Rearrangement Does Not Occur Peroxide Effect Occurs Only with HBr

26 Why? because both propaga)on steps for HBr addi)on are exothermic

27 The Stereochemistry of Radical Subs)tu)on Reac)ons

28 The Stereochemistry of Radical Addi)on Reac)ons

29 Why are Both Enan)omers Formed? the incoming species has equal access to both sides of the plane

30 Racemiza)on A pair of enan)omers is formed if a hydrogen atom is removed from an asymmetric center.

31 An Allylic Radical Has Two Resonance Contributors A Benzylic Radical Has Five Resonance Contributors

32 Rela)ve Stabili)es of Radicals

33 N- Bromosuccinimide (NBS) is Used to Brominate Allylic Carbons

34 The Mechanism The reac)on takes place in the presence of a low concentra)on of Br 2 and HBr. How is the required Br 2 generated?

35 The Resonance Contributors are Not Iden)cal, So Two Products are Formed

36 Addi)on versus Hydrogen Removal

37 Cyclopropane Undergoes Ring- Opening Reac)ons to Relieve Strain

38 Designing a Synthesis 1

39 Designing a Synthesis 2

40 Designing a Synthesis 3 Retrosynethe)c Analysis: Synthesis: