halide

Examples of halide in the following topics:

• Alkyl Halide Reactions

  • The functional group of alkyl halides is a carbon-halogen bond, the common halogens being fluorine, chlorine, bromine and iodine.
  • and do not share any of the reactivity patterns shown by the other alkyl halides.
  • The second factor to be considered is the relative stability of the corresponding halide anions, which is likely the form in which these electronegative atoms will be replaced.
  • This stability may be estimated from the relative acidities of the H-X acids, assuming that the strongest acid releases the most stable conjugate base (halide anion).

• Halogen Compounds

  • Halogens are highly reactive and can form hydrogen halides, metal halides, organic halides, interhalogens, and polyhalogenated compounds.
  • The halogens all form binary compounds with hydrogen, and these compounds are known as the hydrogen halides: hydrogen fluoride (HF), hydrogen chloride (HCl), hydrogen bromide (HBr), hydrogen iodide (HI), and hydrogen astatide (HAt).
  • When in aqueous solution, the hydrogen halides are known as hydrohalic acids.
  • Metal halides are generally obtained through direct combination or, more commonly, through neutralization of a basic metal salt with a hydrohalic acid.
  • Many synthetic organic compounds, such as plastic polymers, as well as a few natural organic compounds, contain halogen atoms; these are known as halogenated compounds, or organic halides.

• Variation of Physical Properties Across a Period

  • Many salts are halides; the hal- syllable in halide and halite reflects this correlation.
  • All Group 1 metals form halides that are white solids at room temperature.
  • All of the alkali halides and alkaline earth halides are solids at room temperature and have melting points in the hundreds of degrees centigrade.
  • In contrast, when an alkali halide or alkaline earth halide melts, the resulting liquid is an excellent electrical conductor.
  • This tells us that these molten compounds consist of ions, whereas the non-metal halides do not.

• Main Group Organometallic Compounds

  • The first reported organometallic compounds were prepared by the reductive substitution of alkyl halides, as shown in the following three equations.
  • Halide reactivity increases in the order: Cl < Br < I.
  • This can also be a problem when allyl or benzyl halides are converted to Grignard or lithium reagents.
  • Since magnesium halides are moderate Lewis acids, their presence in solution may influence the outcome of certain chemical reactions.
  • Pure dialkylmagnesium reagents may be prepared by alternative routes (vida supra), or by removing the magnesium halide by precipitation (dioxane is added).

• Alkyl Halide Occurrence

• Substitution

  • This apparent nucleophilic substitution reaction is surprising, since aryl halides are generally incapable of reacting by either an SN1 or SN2 pathway.
  • This two-step mechanism is characterized by initial addition of the nucleophile (hydroxide ion or water) to the aromatic ring, followed by loss of a halide anion from the negatively charged intermediate.
  • Three additional examples of aryl halide nucleophilic substitution are presented on the right.
  • When applied to aromatic halides, as in the present discussion, this mechanism is called SNAr.

• Reactions of Alcohols

  • Under stronger conditions, carboxylic acids, esters, acid anhydrides, and acyl halides can also be reduced to primary acids.
  • The Williamson ether synthesis involves an alkoxide nucleophile reacting with an electrophilic alkyl halide to produce an ether.
  • Alcohols can be converted to alkyl halides by SN1 and SN2 modes of nucleophilic substitution.
  • Alcohols can react with carboxylic acids, acid anhydrides, and acyl halides to form esters.
  • Both afford alkyl halides.

• Substitution and Elimination

  • The characteristics noted above lead us to anticipate certain types of reactions that are likely to occur with alkyl halides.
  • Finally, there are some combinations of alkyl halides and nucleophiles that fail to show any reaction over a 24 hour period, such as the example in equation 4.
  • In order to understand why some combinations of alkyl halides and nucleophiles give a substitution reaction, whereas other combinations give elimination, and still others give no observable reaction, we must investigate systematically the way in which changes in reaction variables perturb the course of the reaction.
  • The first four halides shown on the left below do not give elimination reactions on treatment with base, because they have no β-hydrogens.
  • The two halides on the right do not normally undergo such reactions because the potential elimination products have highly strained double or triple bonds.

• Irreversible Substitution Reactions

  • If an alkyl halide undergoes an SN2 reaction at the carbon atom of an enolate anion the product is an alkylated aldehyde or ketone.
  • The reaction of alkyl halides with enolate anions presents the same problem of competing SN2 and E2 reaction paths that was encountered earlier in the alkyl halide chapter.
  • Since enolate anions are very strong bases, they will usually cause elimination when reacted with 2º and 3º-halides.
  • Halides that are incapable of elimination and/or have enhanced SN2 reactivity are the best electrophilic reactants for this purpose.
  • The dichloro alkylating agent used in reaction #1 nicely illustrates the high reactivity of allylic halides and the unreactive nature of vinylic halides in SN2 reactions.

• Trihalides: Boron-Halogen Compounds

  • When in a compound with halides, boron has the formal oxidation state +3.
  • These include oxides, sulfides, nitrides, and halides.
  • NMR studies of mixtures of boron trihalides shows the presence of mixed halides, which may indicate a four center intermediate (that is, a dimer).