Chapter 5Organohalides

Compounds that contain one or more halogen atoms

Naming alkyl halides

• Commonly called alkyl halides- identifying first alkyl group and then the halogen

• Named systematically as haloalkanes (halogen as substituent)

• find longest chain and name parent• Number carbon of parent chain beginning at end

nearer the first substituent whether alkyl or halo.• If numbering can be made from either end begin

at end nearer first substituent.

Exercise• Name the following alkyl halides

• Draw structures corresponding to the following IUPAC names– 2,3-dichloro-4-methylhexane– 4-bromo-4-ethyl-2-methylhexane– 3-iodo-2,2,4,4-tetramethylpentane– Cis-1-bromo-2-ethylcyclopentane

Structure of alkyl halides

• Increase in size down periodic table

• Length of carbon-halogen bonds increase accordingly and C-X bond strength decrease

• C-X bond polar

• Chemistry of alkyl halides dominated by their electrophilic behavior

Preparing alkyl halides

• Preparing alkyl halides from alkanes– simple alkyl halides can be prepared by

radical reaction of alkane with Cl2 or Br2 in presence of UV light

– initiation, propagation, termination– reaction continues in a self sustaining cycle– mixture of products result– Br more selective because it depends on

stability of radical

• Preparing alkyl halides from alkenes– Reaction of alkenes with HX or X2

– Reaction of N-bromosuccinimide (NBS) in presence of light to give products resulting from substitution of hydrogen by bromine at allylic position (analogous to alkane halogenation)

– Allylic radical- greater stability because greater number of resonance, electrons attracted to more nuclei.

– Allylic bromination of unsymmetrical alkenes gives mixture of products

• Preparing alkyl halides from alcohols– Treat alcohols with HCl, HBr, HI.

Reactions of alkyl halides

• Grignard reagent– React with Mg metal in ether/THF solvent to

yield alkylmagnesium halides, RMgX (Grignard reagent)

– Halogens: Cl, Br, I– C-Mg polarized making C nucleophilic and

basic

Oxidation and reduction in organic chemistry

• Oxidation: decreases electron density on carbon by:– Forming one of these; C-O, C-N, C-X– Or breaking this; C-H

• Reduction: increases electron density on carbon by:– Forming this; C-H– Or breaking one of these; C-O, C-N, C-X

Exercise• Reaction below in not favorable to be

apply in industry. Why?

• A large amount of 1-bromopent-2-ene is needed for a synthesis reaction. The following reaction was carried out. What is wrong with the following reaction.explain

• Predict the product(s) of the following reactions

SN1

• SN1 reaction (substitution, nucleophlic, unimolecular)

– Loss of leaving group before nucleophile approaches

– Characteristic of reaction• Substrate: yield most stable carbocation• Nucleophile: nonbasic prevent competition with

elimination. Does not affect rxn rate. Neutral nucleophiles work well

• Leaving group: good leaving group • Solvent: polar solvent

SN2

• SN2 reaction (substitution, nucleophlic, bimolecular)

– Single step without intermediate– Incoming nucleophile reacts with the substrate

(alkyl halide or tosylate) from opposite direction of leaving group.

– Invert stereochemical configuration– Characteristic of reaction

• Substrate: steric hindrance decreases rxn rate. Best methyl, 10 substrates, 2o react slowly, 3o do not react.

• Nucleophile: basic,-ve charge • Leaving group: good leaving group more stable• Solvent: polar aprotic solvent

Elimination reaction

• E2 reaction (elimination, bimolecular)– Anti periplanar geometry for p orbital overlap– Non zaitsev’s rule

• E1 reaction (elimination, unimolecular)– E1 and SN1 reactions occur together– No geometric requirement– zaitsev’s rule

• E1cB reaction– Carboanion intermediate– Common in substrates with poor leaving group

Summary

• Primary alkyl halides: – SN2 reaction occur if good nucleophile is used– E2 reaction occur if strong base is used– E1cB reaction occur if the leaving group is 2 C away from

carbonyl group

• Secondary alkyl halides: » SN2 reaction occur if weakly basic nucleophile is used in a

polar aprotic solvent» E2 reaction occur if strong base is used» E1cB reaction occur if the leaving group is 2 C away from

carbonyl group» 2o allylic and benzylic alkyl halides can undergo SN1 and

E1 reactions if weakly basic nucleophile is used in a protic solvent

• Tertiary alkyl halides:

• E2 reaction occur if base is used

• SN1 and E1 reactions occur together under neutral conditions

• E1cB reaction occur if the leaving group is 2 C away from carbonyl group