10. alkyl halides · 10.7 preparing alkyl halides from alcohols reaction of tertiary c-oh with hx...

12
10. Alkyl Halides What Is an Alkyl Halide An organic compound containing at least one carbon- halogen bond (C-X) X (F, Cl, Br, I) replaces H Can contain many C-X bonds Properties and some uses Fire-resistant solvents Refrigerants Pharmaceuticals and precursors

Upload: letruc

Post on 06-Sep-2018

214 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: 10. Alkyl Halides · 10.7 Preparing Alkyl Halides from Alcohols Reaction of tertiary C-OH with HX is fast and effective Add HCl or HBr gas into ether solution of tertiary alcohol

10. Alkyl Halides

What Is an Alkyl Halide

An organic compound containing at least one carbon-halogen bond (C-X)

X (F, Cl, Br, I) replaces HCan contain many C-X bondsProperties and some uses

Fire-resistant solventsRefrigerantsPharmaceuticals and precursors

Page 2: 10. Alkyl Halides · 10.7 Preparing Alkyl Halides from Alcohols Reaction of tertiary C-OH with HX is fast and effective Add HCl or HBr gas into ether solution of tertiary alcohol

10.1 Naming Alkyl Halides

Name is based on longest carbon chain(Contains double or triple bond if present)Number from end nearest any substituent (alkyl or halogen)

Naming with Multiple Halides

If more than one of the same kind of halogen is present, use prefix di, tri, tetraIf there are several different halogens, number them and list them in alphabetical order

Page 3: 10. Alkyl Halides · 10.7 Preparing Alkyl Halides from Alcohols Reaction of tertiary C-OH with HX is fast and effective Add HCl or HBr gas into ether solution of tertiary alcohol

Naming if Two Halides or Alkyl Are Equally Distant from Ends of Chain

Begin at the end nearer the substituent whose name comes first in the alphabet

Many Alkyl Halides That Are Widely Used Have Common Names

ChloroformCarbon tetrachlorideMethylene chlorideMethyl iodideTrichloroethylene

Page 4: 10. Alkyl Halides · 10.7 Preparing Alkyl Halides from Alcohols Reaction of tertiary C-OH with HX is fast and effective Add HCl or HBr gas into ether solution of tertiary alcohol

10.2 Structure of Alkyl Halides

C-X bond is longer as you go down periodic tableC-X bond is weaker as you go down periodic tableC-X bond is polarized with slight positive on carbon and slight negative on halogen

10.3 Preparing Alkyl Halides

Alkyl halide is from addition of HCl, HBr, HI to alkenes to give Markovnikov product (see Alkenes chapter)Alkyl dihalide from anti addition of bromine or chlorine

Page 5: 10. Alkyl Halides · 10.7 Preparing Alkyl Halides from Alcohols Reaction of tertiary C-OH with HX is fast and effective Add HCl or HBr gas into ether solution of tertiary alcohol

Reaction of Alkanes with HalogensAlkane + Cl2 or Br2, heat or light replaces C-H with C-X but Gives mixtures

Hard to controlVia free radical mechanismSee mechanism in Figure 1-1

It is usually not a good idea to plan a synthesis that uses this method

10.4 Radical Halogenation of Alkanes

If there is more than one type of hydrogen in an alkane, reactions favor replacing the hydrogen at the most highly substituted carbons (not absolute)

Page 6: 10. Alkyl Halides · 10.7 Preparing Alkyl Halides from Alcohols Reaction of tertiary C-OH with HX is fast and effective Add HCl or HBr gas into ether solution of tertiary alcohol

Relative ReactivityBased on quantitative analysis of reaction products, relative reactivity is estimatedOrder parallels stability of radicalsReaction distinction is more selective with bromine than chlorine (See Figure 10-2)

10.5 Allylic Bromination of Alkenes

N-bromosuccinimide (NBS) selectively brominates allylic positionsRequires light for activationA source of dilute bromine atoms

Page 7: 10. Alkyl Halides · 10.7 Preparing Alkyl Halides from Alcohols Reaction of tertiary C-OH with HX is fast and effective Add HCl or HBr gas into ether solution of tertiary alcohol

Allylic Stabilization

Allyl radical is delocalizedMore stable than typical alkyl radical by 40 kJ/mol (9 kcal/molAllylic radical is more stable than tertiary alkyl radical

10.6 Stability of the Allyl Radical: Resonance Revisited

Three electrons are delocalized over three carbonsSpin density surface shows single electron is dispersed

Page 8: 10. Alkyl Halides · 10.7 Preparing Alkyl Halides from Alcohols Reaction of tertiary C-OH with HX is fast and effective Add HCl or HBr gas into ether solution of tertiary alcohol

Use of Allylic BrominationAllylic bromination with NBS creates an allylicbromideReaction of an allylic bromide with base produces a conjugated diene, useful in synthesis of complex molecules

10.7 Preparing Alkyl Halides from Alcohols

Reaction of tertiary C-OH with HX is fast and effective

Add HCl or HBr gas into ether solution of tertiary alcohol

Primary and secondary alcohols react very slowly and often rearrange, so alternative methods are used

Page 9: 10. Alkyl Halides · 10.7 Preparing Alkyl Halides from Alcohols Reaction of tertiary C-OH with HX is fast and effective Add HCl or HBr gas into ether solution of tertiary alcohol

Preparation of Alkyl Halides from Primary and Secondary Alcohols

Specific reagents avoid acid and rearrangements of carbon skeletonThionyl chloride converts alcohols into alkyl chlorides (SOCl2 : ROH RCl)Phosphorus tribromide converts alcohols into alkyl bromides (PBr3: ROH RBr)

10.8 Reactions of Alkyl Halides: Grignard Reagents

Reaction of RX with Mg in ether or THFProduct is RMgX – an organometallic compound (alkyl-metal bond)

R is alkyl 1°, 2°, 3°, aryl, alkenylX = Cl, Br, I

Page 10: 10. Alkyl Halides · 10.7 Preparing Alkyl Halides from Alcohols Reaction of tertiary C-OH with HX is fast and effective Add HCl or HBr gas into ether solution of tertiary alcohol

Reactions of Grignard Reagents

Many useful reactionsRMgX behaves as R- (adds to C=O)RMgX + H3O+ R-H

10.9 Organometallic Coupling Reactions

Alkyllithium (RLi) forms from RBr and Li metalRLi reacts with copper iodide to give lithium dialkylcopper (Gilman reagents)Lithium dialkylcopper reagents react with alkyl halides to give alkanes

Page 11: 10. Alkyl Halides · 10.7 Preparing Alkyl Halides from Alcohols Reaction of tertiary C-OH with HX is fast and effective Add HCl or HBr gas into ether solution of tertiary alcohol

Utility of Organometallic Coupling in Synthesis

Coupling of two organometallic molecules produces larger molecules of defined structure Aryl and vinyl organometallics also effectiveCoupling of lithium dialkylcopper molecules proceeds through trialkylcopper intermediate

10.10 Oxidation and Reduction in Organic Chemistry

In organic chemistry, we say that oxidation occurs when a carbon or hydrogen that is connected to a carbon atom in a structure is replaced by oxygen, nitrogen, or halogen

Not defined as loss of electrons by an atom as in inorganic chemistry

Oxidation is a reaction that results in loss of electron density at carbon (as more electronegative atoms replace hydrogen or carbon)

Oxidation: break C-H (or C-C) and form C-O, C-N, C-X

Page 12: 10. Alkyl Halides · 10.7 Preparing Alkyl Halides from Alcohols Reaction of tertiary C-OH with HX is fast and effective Add HCl or HBr gas into ether solution of tertiary alcohol

Reduction ReactionsOrganic reduction is the opposite of oxidation

Results in gain of electron density at carbon (replacement of electronegative atoms by hydrogen or carbon)

Reduction: form C-H (or C-C) and break C-O, C-N, C-X

Oxidation LevelsFunctional groups are associated with specific levels