Alkyl Halides

A workhorse of organic synthesis

Goals

•      After this chapter you should be able to

–   Name alkyl halides

–   Describe a halogen’s effect on carbon

–   Show synthesis of alkyl halides using

•   Addition, Radical, NBS, SOCl₃ and PCl₃

–   Show organometallic synthesis reaction of

•   Grignard reagents

•   Organolithium and Gilman reagents

–   Determine the relative oxidation state of organic molecules 

 

Nomenclature

 

•      Find longest carbon chain

–   Chain must contain multiple bond

•      Number from end nearest first substituent

–   Alkyl group or halogen

•      Assign numbers and appropriate prefix (di, tri, tetra…) for each substituent group

•      If second rule can’t decide numbering give precedence alphabetically

 

Nomenclature

 

 

 

 

 

 

 

 

 

 

 

 

 

Halogen’s Effect on Carbon

•      All C-X bonds are4 polar. X=F, Cl, Br, I

•     
Carbon is positive and halogen is negative

                            Chloromethane

Preparation of Alkyl Halides

•      Anti addition of halogen

–  
Cl₂ Or Br₂

Addition of Halo Acids

•     
HCl, HBr, and HI

 

•      A Markovnikov addition

Radical Halogenation

•      Cl₂, and Br₂

•      Generally a poor synthetic method

•      Not specific

 

Initiation

 

Propagation

 

Termination

 

 

 

Two factors control halogenation:

•      First is relative bond energy of abstracted hydrogen

 

•      Second is stability of the radical intermediate

Problem

•     
Calculate the relative amounts of the mono chlorinated species for the following reaction:

 

 

 

 

Allylic Bromination of Alkenes

•      N-bromosuccinimide

•      Selective bromination

 

 

 

 

    of allylic hydrogen

Allylic Bromination of Alkenes

•      Number of possible products depend on how many allylic hydrogens can be abstracted.

•      Possible products depend on resonance structure of intermediate.

•      Distribution of products depends on relative stability of  intermediates.

 

Nature of the Allylic Radical

Allylic Bromination of Alkenes

Allylic Bromination of Alkenes

•     

What four products do you expect?

Preparing Alkyl Halides from Alcohols

•      General Reaction

•      ROH + HX                RX + H₂O

Preparing Alkyl Halides from Alcohols

•      Best reagent choices are

–   SOCl₂

–   PBr₃

•      These reagents are less acidic than HX

 

Alcohol to Alkyl Halide

 

The Grignard Reagent

 

 

 

 

 

 The Grignard Reaction

Grignard Reaction

•      The ether must be DRY!

 

•      Ether is involved in the Grignard reagent as a complex

 

 

Grignard Reactions

Another Grignard Reaction

•     
Grignard reagents react vigorously with carbonyls

 

Organolithium

•      Chemistry is similar to Grignard reagents

Organometallic Coupling Reactions

 

Organolithium compounds are useful in making Gilman reagents for extending carbon chains

Gilman Reagents

•      Made from reacting organic halides with lithium and CuI

•      Extend carbon chains using other organic halides

•      Can be used on alkyl, vinylic and aryl halides

•      Typically gives excellent yields

 

Redox of Carbon

•      Oxidation

–   Loss of electron density for Carbon

•      Reduction

–   Gain of electron density for Carbon

Recognizing Organic Redox

•      Oxidation

–   Removing hydrogen

–   Adding oxygen

–   Adding element more electronegative than C

–   Adding multiple bond

•      Reduction

–   Adding hydrogen

–   Removing oxygen

–   Adding element less electronegative than C

–   Removing multiple bond

Some Relative Oxidation Levels 

Some Oxidizing and Reducing Agents

 

•      Oxidizing agents

–   KMnO₄

–   K₂CrO₄

–   K₂Cr₂O₇

•      Reducing agents

–   NaBH₄

–   LiAlH₄

–   BH₃