CHEM 257-chapter 10: organohalides

organohalides

an organic compound containing at least one carbon-halogen bond (C-X), X (F, Cl, Br, I) replaces H

naming alkyl halides step 1

find the longest chain, and name it as the parent

if a double or triple bond is present, the parent chain must contain it

naming alkyl halides step 2

number the carbons of the parent chain beginning at the end nearer the first substituent, whether alkyl or halo

(if different halogens are present, number each one and list them in alphabetical order when writing the name)

naming alkyl halides step 3

if the parent chain can be properly numbered from either end by step 2, begin at the end nearer the substituent that has alphabetical precedence

structure of alkyl halides

C-X bond is longer as you go down periodic table, C-X bond is weaker as you go down periodic table (shorter bond=stronger bond and longer bond=weaker bond, C-X bond is polar because halogen is more electronegative which results in a dipole moment with the carbon atom behaving as a nucleophile)

simple alkyl halides

can sometimes be prepared by the radical reaction of an alkane with Cl2 or Br2 in the presence of ultraviolet light (initiation, propagation, and termination), alkane halogenation is a poor synthetic method for preparing alkyl halides because mixtures of products invariably result and hard to control

alkanes can have >1 type of H

reaction favors replacing the H at the most highly substituted carbons

least reactive to most reactive = 1° < 2° < 3°

stability of radicals is the same as reactivity, 3° is most stable (1° is least)

strength of C-H bonds parallels radical stability

N-bromosuccinimide (abbreviated NBS)

another laboratory method for preparing alkyl halides from alkenes by reacting NBS in the presence of ultraviolet light, to give products resulting from substitution of hydrogen by bromine at the allylic position

allylic position

carbon atom next to a carbon-carbon double bond, is easily formed and contains least bond-energy association which results in it being the most stable position

position stability

vinylic < methyl < primary < secondary < tertiary < allylic

stability of allylic position

the radical carbon atom with an unpaired electron can adopt sp2 hybridization, placing the unpaired electron in a p orbital and giving a structure that is electronically symmetrical

the stability of the allyl radical is due to the fact that the unpaired electron is delocalized, or spread out, over an extended π-orbital network rather than localized at only one site,

electronically symmetrical

because the allyl radical is electronically symmetrical, it has two resonance forms—one with the unpaired electron on the left and the double bond on the right and another with the unpaired electron on the right and the double bond on the left, resonance results in more stability

consequence of allylic bromination

allylic bromination of an unsymmetrical alkene often leads to a mixture of products, the two products are not formed in equal amounts, however, because the intermediate allylic radical is not symmetrical and reaction at the two ends is not equally likely, reaction at the less hindered, primary end is favored

preparing alkyl halides from alcohols

most generally useful method for preparing alkyl halides is to make them from alcohols, treat the alcohol with HCl, HBr, or HI, works best with tertiary alcohols because reaction is super rapid

methyl < primary < secondary < tertiary (reactivity)

alkyl halides from secondary and primary alcohols

primary and secondary alcohols are best converted into alkyl halides by treatment with either thionyl chloride (SOCl2) or phosphorus tribromide (PBr3)

making alkyl chloride

use thionyl chloride (SOCl2) with a 1° and 2° alcohols, byproducts are gases (generates HCl) and mild reaction conditions (less acidic conditions)

making alkyl bromide

use phosphorus tribromide (PBr3), however PBr3 is reactive with water and releases HBr on contact with air

reactions of alkyl halides: grignard reagents

alkyl halides, RX, react with magnesium metal in ether or tetrahydrofuran (THF) solvent to yield alkylmagnesium halides, RMgX, the products are called Grignard reagents (RMgX) which is an organometallic compound (alkyl-bond)

formation of grignard reagent

RX + Mg with Et2O = RMgX

R is alkyl, X is Cl, Br, I

halide reactivity: I > Br > Cl

involves radicals, surface reaction (takes place on surface of Mg)

properties of grignard reagent

carbon is basic (strongly) and nucleophillic, grignard reagents act as nucleophiles, abstracts a proton from weak acids

organometallic coupling reactions

many other kinds of organometallic compounds can be prepared in a manner similar to that of Grignard reagents (RLi and LiR2Cu)

alkyllithium reagents (RLi)

can be prepared by the reaction of an alkyl halide with lithium metal, RLi is both nucleophillic and a strong base

Gilman reagents (LiR2Cu)

2 RLi + CuI → lithium dialkylcopper (LiR2Cu, Gilman reagents), gilman reagents add to alkyl halides to form C-C bonds (comes from one of the R's in R2), aryl and vinyl organohalides also effective

suzuki-miyaura reaction

another coupling reaction of an aromatic or vinyl substituted boronic acid with an aromatic or vinyl substituted organohalide in the presence of a base and a palladium catalyst

reduction on carbon

increases e- density on C (RIG), reduction is gaining electrons, makes carbon more nucleophillic

forming C-H

breaking: C-O, C-N, C-X

oxidation on carbon

decreases e- density on C (OIL), oxidation is losing, makes carbon more electrophillic

forming: C-O, C-N, C-X

breaking: C-H