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    CHAPTER 5

    ALKYL HALIDE

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    Objective

    Outcome

    Ability to explain the relationship between the structure, physical andchemical properties of the different bonds and functional group in organiccompounds.(CO2)

    Ability to explain each of functional group activity. (CO3)

    The student should be able to: -

    Name alkyl halides.

    Explain alkyl halides properties.

    Predict, draw and name the products of functional groups reactions.

    Draw the mechanistic pathway.

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    BACKGROUND

    The functional group of alkyl halides is a carbon-halogen bond, the

    common halogens being fluorine, chlorine, bromine and iodine. With the

    exception of iodine, these halogens have electronegativities significantly

    greater than carbon.

    This functional group is polarized so that the carbon is electrophilic and

    the halogen is nucleophilic, as shown in the drawing on the right.

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    In alkyl halides this polarity causes the carbon to become activatedto substitution reactions with neucleophiles.

    Carbon-halogen bonds get less polar, longer and weaker in goingfrom fluorine to iodine.

    Classes of halides :-

    i. Alkyl : Halogen, X, is directly bonded to sp3 carbon

    ii. Vinyl : X is bonded to sp2 carbon of alkene.

    iii. Aryl : X is bonded to sp2 carbon on benzene ring.

    C

    H

    H

    H

    C

    H

    H

    Br

    alkyl halide

    C C

    H

    H

    H

    Cl

    vinyl halide

    I

    aryl halide

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    Solvents very good for a variety of organic compounds.

    Reagents precursors for a variety of syntheses. One bigapplication is the so-called Grignard reagents.

    Freons used as refrigerator coolants, in sprays and as blowingagents. Because of ozone problem, their application is foreseen toterminate in about 10 years.

    Pesticides very powerful, but tend to accumulate in naturebecause of low reactivity, causing lasting contamination. Many of

    them now avoided, DDT banned.

    Anesthetics widely used in the past (chloroform), but because oftoxicity now are generally avoided.

    USESOF ALKYL HALIDES

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    Halogens are more electronegative than C.

    Carbon-halogen bond is polar, so carbon has partial positive charge.

    Carbon can be attacked by a nucleophile.

    Halogen can leave with the electron pair.

    POLARITY AND REACTIVITY

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    REVIEW

    PREPARATIONOF ALKYL HALIDES

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    IUPAC SYSTEM

    An alkyl halide is named as an alkane with a halogen substituent-that is , as a

    halo alkane.

    To name a halogen substituent, change the ine ending of the name of the

    halogen to the suffix o (chlorine -> chloro)

    The halogen is treated as a substituent.

    4

    5

    6 7

    Cl 8r9

    4-bromo-2-chloro-1-methylcyclohexane

    1

    2

    3

    4

    5

    6

    7

    8

    Br9

    2 bromo-5-methylheptane

    1

    2

    3

    4

    5

    6

    7

    8

    Cl9

    1-chloro-5,5-dimethylhexane

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    Example

    How to name an alkyl halide Using the IUPAC System

    1. Give the IUPAC name of the following alkyl halide :

    STEP 1 :- Find the parent carbon chain containing the halogen.

    STEP 2 :- Apply all other rules of nomenclature.

    a. Number the chain

    b. Name and number the substituents

    c. Alphabetize

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    Common Names

    Common names for alkyl halides are used only for simple alkylhalides. To assign a common name:

    - Name all the carbon atoms of the molecule as a single

    alkylgroup.

    - Name the halogen bonded to the alkyl group. To name the

    halogen,change the ine ending of the halogen name to

    the suffix ide; for example, bromine -> bromide.

    - Combine the names of the alkyl group and halide,

    separating the words with a space.

    tert-butyl iodide ethyl chloride

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    Undergoes ionic reactions - Nucleophilic Substitution and Nucleophilic

    Elimination Reactions.

    1. Nucleophilic Substitution Reaction

    2. Nucleophilic Elimination Reaction

    REACTIONSOF ALKYL HALIDES

    Nu:-

    +

    nucleop ile

    :

    :

    alkyl ali e

    R-Nu +:

    :

    :-

    substratepro uct hali e ion

    R-X X: :

    C

    X

    C

    H

    C C(-HX)

    alkyl hali e alkene

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    What does the term "nucleophilic substitution" imply ?

    A nucleophile is an the electron rich species that will react with anelectron poor species.A substitution implies that one group replacesanother

    There are two fundamental events in these substitution reactions:

    i. formation of the new bond to the nucleophile

    ii. breaking of the bond to the leaving group

    Depending on the relative timing of these events, two differentmechanisms are possible:

    1. Bond breaking to form a carbocation preceeds the formation of

    the new bond : SN1 reaction

    2. Simultaneous bond formation and bond breaking : SN2 reaction

    NucleophilicSubstitution Reaction

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    SN1 indicates a substitution,nucleophilic,unimolecularreaction,

    described by the expression rate = k [R-LG]

    This pathway is a multi-step process with the following characteristics:

    step 1: rate determining (slow) loss of the leaving group, LG, to

    generate a carbocation intermediate, thenstep 2: rapid attack of a nucleophile on the electrophilic carbocation to

    form a new bond

    SN1- SubstitutionNucleophilicUnimolecular

    Multi-step reactions have intermediates and

    several transition states (TS).

    In an SN1 there is loss of the leaving groupgenerating an intermediate carbocation which then

    undergoes a rapid reaction with the nucleophile.

    The reaction profiles shown here are simplified

    and do not include the equilibria for protonation of

    the -OH.

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    Two step reaction with carbocation intermediate.

    Rate is first order in the alkyl halide, zero order in the nucleophile

    .

    SN1 Mechanism

    (CH3)3C Br (CH3)3C+

    + Br-

    STEP 1 :

    (CH3)3C+

    + H O H (CH3)3C O H

    H

    STEP 2 :

    (CH3)3C O H

    H

    H O H+ (CH3)3C O H + H3O+

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    RatesofSN1 Reactions

    3 > 2 > 1 >> CH3X

    Order follows stability of carbocations (opposite to SN2)

    More stable ion requires less energy to form

    Better leaving group, faster reaction (like SN2)

    Carbocations can rearrange to form a more stable

    carbocation.

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    SN2 indicates a substitution,nucleophilic,bimolecularreaction,

    described by the expression rate = k [Nu][R-LG]

    This pathway is a concerted process (single step) as shown by the

    following reaction coordinate diagrams, where there is simultaneous

    attack of the nucleophile and displacement of the leaving group.

    SN2- SubstitutionNucleophilicBimolecular

    Single step reactions have no intermediates

    and single transition state (TS).

    In an SN2 there is simultaneous formation of

    the carbon-nucleophile bond and breaking of

    the carbon-leaving group bond, hence thereaction proceeds via a TS in which the central

    C is partially bonded to five groups.

    The reaction profiles shown here are simplified

    and do not include the equilibria for protonation

    of the -OH.

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    Rate is first order in each reactant. (one-step reaction with no

    intermediate

    Concerted reaction: new bond forming and old bond breaking atsame time.

    .

    SN2 Mechanism

    CH

    Br

    HH

    H O CHO Br

    H

    HH

    CHOH

    HH

    + Br-

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    RatesofSN2 Reactions

    Relative rates for SN2: CH3X > 1 > 2 >> 3

    Tertiary halides do not react via the SN2 mechanism, due to steric

    hindrance.

    Must have a good leaving group

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    SN2 vs SN1

    SN2 SN1

    Primary or methyl Tertiary

    Strong nucleophile ( Strong Lewis

    base)

    Weak nucleophile (Weak Lewis

    base)

    Polar aprotic solvent

    (DMF, DMSO)

    Polar protic solvent, (alcohol and

    water)

    Rate = k[halide][Nuc] Rate = k[halide]

    Inversion at chiral carbon Racemization of optically activecompound

    No rearrangements Rearranged products

    Leaving Group :- for both SN1 and SN2 ( the weaker the base the group

    departs, the better the leaving group)

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    An elimination reaction is a type oforganic reaction in which two substituentsare removed from a molecule in either a one or two-step mechanism

    The two most important methods are: Dehydration (-H2O) of alcohols, andDehydrohalogenation (-HX) of alkyl halides.

    There are three fundamental events in these elimination reactions:

    i. removal of a proton

    ii. formation of the CC p bond

    iii. breaking of the bond to the leaving group

    Depending on the relative timing of these events, different mechanisms arepossible:i. Loss of the LG to form a carbocation, removal of H and formation of

    C=C bond : E1 reaction

    ii Simultaneous H removal, C=C bond formation and loss of the LG :

    E2 reaction

    iii. Removal of H to form a carbanion, loss of the LG and formation of C=C

    bond (E1reaction)

    Nucleophilic Elimination Reaction

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    E1 Reaction

    Unimolecular elimination

    Two groups lost (usually X- and H )

    Nucleophile acts as base)

    Also have SN1 products (mixture SN1 and E1 have common first step.

    H C

    H

    H

    C

    CH3

    CH3

    Br

    C

    H

    H

    H

    C CH3

    CH3

    OH

    H

    C

    H

    H

    H

    C CH3

    CH3

    C C

    H

    CH3

    CH3

    H

    + H3O+

    Halide ion leaves, forming carbocation

    Base removes H from adjacent carbon

    and pi bond forms

    E1 Mechanism

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    E2 Reaction

    Bimolecular elimination

    Requires a strong base

    Halide leaving and proton abstraction happens simultaneously - no

    intermediate

    H C

    H

    H

    C

    CH3

    CH3

    Br

    C C

    H

    CH3

    CH3

    H

    O

    H

    + H2O Br-

    +

    E2 Mechanism

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    Primary (1) carbonsNormally react by an SN2 pathway. With good nucleophiles such asBr-, I-, CN-, RS-, orNH3 get only SN2 reactions. However, withstrong base (hydroxide or alkoxide) get some competition by E2reaction, though SN2 still predominates.

    Secondary (2) carbonsGo by either SN2 orE2:

    - With good nucleophiles get mostly SN2.

    - With strong base E2 predominates.

    Tertiary (3) carbonsGo by SN1, E1, or E2:

    - SN1 and E1 are both favored by acid conditions, but acidic

    nucleophiles such as HCl, HBr, and HI favor SN1, while sulfuric

    acid favors E1

    - E2 is favored by strong base again.

    How dowe determinewhethera reactionwill goviaan

    elimination orasubstitutionand whetheritwill befirst

    orsecond order?

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    Secondary alkyl halides, often react with simple basic nucleophiles

    to give a mixture of products arising from both substitutionand

    elimination.

    Substitutionand Elimination

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    Control of the reaction pathway between substitution and elimination isgenerally accomplished by careful choice of the reactants; strong,stericallyhindered basestend tofavorelimination,whileweak,unhinderednucleophilestend tofavorsubstitution. The choice for a "strong,hindered base" is generally tert-butoxide

    Substitutionor Elimination

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    TRY THIS..

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    FormationofAlcohol

    (CH3)3C Br (CH3)3C+

    + Br-

    (CH3)3C+

    + H O H (CH3)3C O H

    H

    (CH3)3C O H

    H

    H O H+ (CH3)3C O H + H3O+

    CH3 Br OH60oC

    H2OCH3 O Br

    C

    H

    Br

    HH

    H O CHO Br

    H

    HH

    CHO

    H

    HH

    + Br-

    Example1

    Example2

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    CH3CH2CH2OH NaH CH3CH2CH2ONa H H

    Propyl alcohol Sodium propoxide

    CH3CH2I

    CH3CH2OCH2CH2CH3Ethyl propyl ether

    Na I

    FormationofEther ( Williamsonsynthesis)

    RO Na R'L R O R' Na LGeneral reaction

    Thisisa good routeforsynthesisofunsymetrical ethers.

    Example

    The alkoxide ion reacts with the substrate in an SN2 reaction, with resulting

    formation of ether.The substrate must be unhindered and bear a good leaving

    group.Typical substrates are 1o

    and 2o

    alkyl halide halides, alkyl sulfonate, anddialkyl sulfates.

    The alkyl halide ( alkyl sulfonate) should be primary to avoid E2 reaction.

    Substitution favored over elimination at low tempertaure.

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    FormationofAminocompound

    Using ammonia as a nucleophile in a reaction with an appropriate (methyl, primary, or secondary) alkylhalide in an SN2 reaction to prepare primary amines does work, but it requires a huge excess of

    ammonia, because the productprimary amine is also reactive towardthe alkyl halide. This would produce

    a secondary amine, and then even further reaction with alkyl halide would give a tertiary amine. Thus, a

    mixture of primary, secondary, and tertiary amines would be generated unless ammonia is used in large

    excess.

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    Wurtz synthesis- Coupling of alkyl halide with organometallic compounds.

    The alkane is prepared by the synthesis of metallic sodium and the haloalkane in a dry

    etheral ( ethoxyethane) solution.

    R-Na ( an intermediate compound) is so reactive that is attacks RX itself, thus the method

    can only be used to prepare symmetrical alkane.

    The reaction is limited in its preparation, giving only low yields with haloalkanes of low

    relative molecular mass, although much better yields are obtained with those of higher

    relative molecular mass. A more versatile coupling reaction of this type is the Corey-House

    reaction involving the haloalkane and a lithium dialkylcopper.

    2 X 2 Nadry

    (CH2H5)2OR R 2 NaX

    FormationofAlkane ( Wurtzsynthesis)

    RX R2'CuLi R R'R'Cu LiX

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    Suggest.

    FormationofAlkenecompound