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Alkyl Halides: Nucleophilic Substitution and Elimination

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Background for Alkyl Halides
Alkyl halides have dipole-dipole attractions, but do not have intermolecular hydrogen bonding.  As the size of the halogen increases, the boiling point of the alkyl halides increases.  Alkyl fluorides and alkyl chlorides are less dense than water.  Alkyl chlorides with more than two chlorides, alkyl bromides, and alkyl iodides are more dense than water.

Uses of Alkyl Halides
Alkyl halides can be used as solvents, dry cleaning solvents, anesthetics, freons, and pesticides.

IUPAC Nomenclature for Alkyl Halides
For the IUPAC nomenclature, the longest continuous carbon chain is the parent compound.  Number from whichever end that gives the substituents the lowest possible combination of numbers.  Give the location of each substituent with a number.  Alphabetize the groups, ignoring all numerical prefixes.

Common Nomenclature for Alkyl Halides
For common nomenclature, name as alkyl halide.   Use n, iso, sec, tert, and neo prefixes.

Synthesis of Alkyl Halides
Halogenation of alkanes occurs with halogen in the presence of heat or light.  Alkenes react with hydrogen halide to produce alkyl halides. Halogenation of alkenes produce dihaloalkanes.  Alkynes react with two equivalent of hydrogen halide to produce dihaloalkanes.  Dihalogenation of alkynes result in tetrahaloalkanes.  Alcohols undergo reaction with hydrogen halide, phosphorous trihalide, phosphorus pentahalide, or thionyl chloride to yield alkyl halides.

SN1 Mechanism
The slow step is the bromide takes the electron pair and departs as leaving group with the formation of a carbocation.  The faster step is rearrangement, if present.  The fast step is the weak nucleophile attacks the carbocation from both sides.  The nucleophile abstracts the extra proton from the product and the electron pairs move from O-H bond to O.  The rate is equal to k[RX].  The mechanism is first order kinetics.  A pair of enantiomers are formed.  The products are retention and inversion.  The mechanism is not concerted.  The reactivity of RX is 3o > 2o > 1o > 0o.  A protic solvent, with intermolecular hydrogen bonding, is best.  Rearrangement is possible.

SN2 Mechanism
The nucleophile attacks on the carbon, but on the opposite side of the halogen.  Then the bromide takes its electron pair and leaves.  In the intermediate, the nucleophile and leaving group are drawn 180o apart.  The product is inversion.  The mechanism is concerted.  The rate is equal to k[RX][Nu].  The mechanism is second order kinetics.   The  reactivity of RX is 0o > 1o > 2o > 3o.  An aprotic solvent, without intermolecular hydrogen bonding, is best.  Rearrangement is not possible.

E1 Mechanism
The bromide takes the electron pair and departs as the leaving group with the formation of a carbocation.  The slow step is the formation of the carbocation.  The rate is equal to k[RX].  The reaction is first order kinetics.  The weak base abstracts hydrogen on carbon next to carbocation, then electron pair goes from C-H bond to C-C bond to form double bond.  The faster step is rearrangement, if present.  The fast step is the weak base abstracts proton from carbon attached to carbocation from either side.  Both E and Z will be formed.  The reactivity of RX is 3o > 2o > 1o > 0o.  The more substituted product is preferred, according to Sayteff=s rule.  A protic solvent best.  Rearrangement is possible.

E2 Mechanism
A strong nucleophile abstracts proton from carbon adjacent to carbon with X, but on opposite side from X.  This is anti elimination.  The C-H electrons go to C-C bond to form double bond.  The C-Br bond electrons go to Br.  Br leaves with electron pair.  Both E and Z products may be formed.  Rotation occurs to get hydrogen into correct position.  The reactivity of RX is 3o > 2o > 1o > 0o.  The more substituted product is preferred, based upon Sayteff=s rule.  An aprotic solvent is best.  Rearrangement is not possible.

Reaction of Alkyl Halides with Lithium and Magnesium
A lithium replaces the halogen to give an alkyllithium.  Lithium halide is formed as a by-product. A magnesium inserts between the alkyl group and the halogen to form a Grignard reagent.

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"Title" Tutorial Summary :
Properties and uses of alkyl halides are described.  Alkyl halides are named by IUPAC and common nomenclature.  Alkanes undergo free radical halogenation to yield alkyl halides.  Hydrohalogenation or halogenation of alkenes and alkynes yield alkyl halides.  Alcohols are converted to alkyl halides.  Alkyl halides undergo substitution reactions via SN1 and SN2 mechanisms.  Alkenes are synthesized from alkyl halides through E1 and E2 mechanisms.  The SN1, SN2, E1, and E2 mechanisms are described in detail.  Alkyl halides react with lithium to give organolithiums.  Grignard reagents are produced from the reaction of alkyl halides with magnesium.

Tutorial Features:

This tutorial provides the comprehensive coverage of the chapter with easy introduction and simple illustration. It features:

  • Concept map showing interconnections of new concepts in this tutorial and those previously introduced.
  • Definition slides introduce terms as they are needed.
  • Visual representation of concepts.
  • Use of colors to emphasize points.
  • Outline of chemical properties and uses.
  • Easy-to-follow animations of stated rules of nomenclature.
  • Detailed stepwise explanations and animations of reactions and mechanisms.
  • Examples worked out step-by-step throughout the tutorial.
  • A concise summary is given at the conclusion of the tutorial.

"Title" Topic List:

Properties and Uses of Alkyl Halides

Nomenclature of Alkyl Halides

  • Common

Synthesis of Alkyl Halides

  • From Free Radical Halogenation of Alkanes
  • From Alkenes
  • From Alkynes
  • From Alcohols

Reactions of Alkyl Halides

    • Substitution (SN1 and SN2)
    • Elimination (E1 and E2)
    • Organometallics

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