SENYAWA HALOGEN ORGANIKAlkyl Halides RX, ArX

Introduction to Alkyl Halides Alkyl halides are organic molecules containing a halogen atom

bonded to an sp3 hybridized carbon atom. Alkyl halides are classified as primary (1), secondary (2), or

tertiary (3), depending on the number of carbons bonded to the carbon with the halogen atom. The halogen atom in halides is often denoted by the symbol X. Uses:

pesticides, refrigerants (freons), solvents, synthetic intermediates, anestesi lokal (kloroetana) dan hisap (1-bromo-1kloro-2,2,2-triflouroetana), plastik yg dipake dlm piringan hitam, tas, pipa dll (PVC)

There are other types of organic halides. These include vinyl halides, aryl halides, allylic halides and benzylic halides. Vinyl halides have a halogen atom (X) bonded to a CC double bond. Aryl halides have a halogen atom bonded to a benzene ring. Allylic halides have X bonded to the carbon atom adjacent to a CC double bond. Benzylic halides have X bonded to the carbon atom adjacent to a benzene ring.

3

Nomenclature:common names: alkyl halide (fluoride, chloride, bromide, iodide)

IUPAC names: use rules for alkanes halogen = halo (fluoro, chloro, bromo, iodo)

ClCH3CH2CH2CH2-Br n-butyl bromide 1-bromobutane 1o CH3CHCH3 isopropyl chloride 2-chloropropane 2o

Nomenclature

5

Physical Properties Alkyl halides are weak polar molecules. They exhibit dipole-dipole interactions because of their polar CX bond, but because the rest of the molecule contains only CC and CH bonds, they are incapable of intermolecular hydrogen bonding.

6

Carbocation Stability The effect of the type of alkyl halide on SN1 reaction rates can be explained by considering carbocation stability. Carbocations are classified as primary (1), secondary (2), or tertiary (3), based on the number of R groups bonded to the charged carbon atom. As the number of R groups increases, carbocation stability increases.

7

8

General Features of Nucleophilic Substitution Three components are necessary in any substitution reaction.

9

11.4 The SN2 Reaction One type of nucleophilic substitution reaction has the

following characteristics:

Reaction occurs with inversion at reacting center Follows second order reaction kinetics rate = k [Nu:-][RX]

The SN2 ReactionH3C HO - + H H3CH 2C (S)-2Bromobutane Br HO H3C H H3CH 2C Br HO H CH 3 Br CH 2CH 3 Transition State HO CH 3H + Br

CH 2CH 3 (R)-2-Butanol

SN2 Transition State The transition state of an SN2 reaction has a planar arrangement of the carbon atom and the remaining three groups.

11.5 Characteristics of the SN2 Reaction Sensitive to steric effects Methyl halides are most reactive Primary are next most reactive Secondary might react Tertiary are unreactive by this path No reaction at C=C (vinyl halides)

Reactant and Transition-state Energy LevelsHigher reactant energy level (red curve) = faster reaction (smaller G).

Higher transitionstate energy level (red curve) = slower reaction (larger G).

Steric Effects on SN2 Reactions

The carbon atom in (a) bromomethane is readily accessible resulting in a fast SN2 reaction. The carbon atoms in (b) bromoethane (primary), (c) 2-bromopropane (secondary), and (d) 2-bromo-2-methylpropane (tertiary) are successively more hindered, resulting in successively slower SN2 reactions.

Steric Hindrance Raises Transition State EnergyVery hindered

Steric effects destabilize transition states

Severe steric effects can also destabilize ground state

Order of Reactivity in SN2 The more alkyl groups connected to the reacting

carbon, the slower the reaction

The Nucleophile

Neutral or negatively charged Lewis base Reaction increases coordination at nucleophile Neutral nucleophile acquires positive charge Anionic nucleophile becomes neutral See Table 11-1 for an illustrative list

Relative Reactivity of Nucleophiles Depends on reaction and conditions More basic nucleophiles react faster (for similar structures. See Table 11-2) Better nucleophiles are lower in a column of the periodic table Anions are usually more reactive than neutrals

The Leaving Group A good leaving group reduces the barrier to a reaction Stable anions that are weak bases are usually excellent leaving groups and can delocalize charge

Poor Leaving Groups If a group is very basic or very small, it is prevents

reaction

The Solvent Solvents that can donate hydrogen bonds (-OH or

NH) slow SN2 reactions by associating with reactants Energy is required to break interactions between reactant and solvent Polar aprotic solvents (no NH, OH, SH) form weaker interactions with substrate and permit faster reaction

11.6 The SN1 ReactionPreviously we learned that tertiary alkyl halides react extremely slowly in SN2 reactions. But tert-butyl bromide reacts with water 1,000,000 times faster than methyl bromide. Tertiary alkyl halides react rapidly in protic solvents by a mechanism that involves departure of the leaving group prior to addition of the nucleophile Called an SN1 reaction occurs in two distinct steps while SN2 occurs with both events in same step If nucleophile is present in reasonable concentration (or it is the solvent), then ionization is the slowest step

SN1 Energy Diagram

Step through highest energy point is ratelimiting rate = k[RX]

Rate-Limiting Step The overall rate of a reaction is controlled by the rate

of the slowest step The rate depends on the concentration of the species and the rate constant of the step The highest energy transition state point on the diagram is that for the rate determining step (which is not always the highest barrier) This is the not the greatest difference but the absolute highest point (Figures 11.8 the same step is ratedetermining in both directions)

Stereochemistry of SN1 Reaction The planar

intermediate should lead to loss of chirality A free carbocation is achiral Product should be racemic

SN1 in Reality Carbocation is biased to react on side opposite

leaving group Suggests reaction occurs with carbocation loosely associated with leaving group during nucleophilic addition Alternative that SN2 is also occurring is unlikely

Effects of Ion Pair Formation If leaving group

remains associated, then product has more inversion than retention Product is only partially racemic with more inversion than retention Associated carbocation and leaving group is an ion pair

11.9 Characteristics of the SN1 Reaction Tertiary alkyl halide is most reactive by

this mechanism Controlled by stability of carbocation

Delocalized Carbocations Delocalization of cationic charge enhances

stability Primary allyl is more stable than primary alkyl Primary benzyl is more stable than allyl

Allylic and Benzylic Halides Allylic and benzylic intermediates stabilized by delocalization of charge (See Figure 11-13) Primary allylic and benzylic are also more reactive in the SN2 mechanism

Effect of Leaving Group on SN1 Critically dependent on leaving group Reactivity: the larger halides ions are better leaving groups In acid, OH of an alcohol is protonated and leaving

group is H2O, which is still less reactive than halide p-Toluensulfonate (TosO-) is excellent leaving group

Nucleophiles in SN1

Since nucleophilic addition occurs after formation of

carbocation, reaction rate is not affected normally affected by nature or concentration of nucleophile

Solvent Is Critical in SN1transition state and controls rate

Stabilizing carbocation also stabilizes associated

Solvation of a carbocation by water

Polar Solvents Promote Ionization Polar, protic and unreactive Lewis base solvents facilitate +

formation of R Solvent polarity is measured as dielectric polarization (P) (Table 11-3) Nonpolar solvents have low P Polar SOLVENT have high P values

Effects of stabilizes transitionEnergies Solvent on state and Polar solventintermediate more than reactant and product

11.10 Alkyl Halides: Elimination Elimination is an alternative pathway to substitution

Opposite of addition Generates an alkene Can compete with substitution and decrease yield,

especially for SN1 processes

Zaitsevs Rule for Elimination Reactions (1875) In the elimination of HX from an alkyl halide, the more highly substituted alkene product

predominates

Mechanisms of Elimination Reactions Ingold nomenclature: E elimination E1: X- leaves first to generate a carbocation

a base abstracts a proton from the carbocation E2: Concerted (one step) transfer of a proton to a base and departure of leaving group

11.11 The E2 Reaction Mechanism A proton is transferred to base as leaving group

begins to depart Transition state combines leaving of X and transfer of H Product alkene forms stereospecifically

E2 Reaction Kinetics One step rate law has base and alkyl halide Transition state bears no resemblance to reactant or

product Rate = k[R-X][B] Reaction goes faster with stronger base, better leaving group

GeometryallowsEliminationminimizes of orbital overlap and E2 Antiperiplanarsteric interactions

E2 Stereochemistry

Overlap of the developing orbital in the

transition state requires periplanar geometry, anti arrangementAllows orbital overlap

Predicting Product

E2 is stereospecific Meso-1,2-dibromo-1,2-diphenylethane with base gives

cis 1,2-diphenyl RR or SS 1,2-dibromo-1,2-diphenylethane gives trans 1,2-diphenyl

11.12 Elimination From Cyclohexanes Abstracted proton and leaving group should align

trans-diaxial to be anti periplanar (app) in approaching transition state (see Figures 11-19 and 1120) Equatorial groups are not in proper alignment

11.14 The E1 Reaction Competes with SN1 and E2 at 3 centers

V = k [RX]

Stereochemistry of E1 Reactions E1 is not stereospecific and there is no requirement

for alignment Product has Zaitsev orientation because step that controls product is loss of proton after formation of carbocation

Comparing E1 and E2 Strong base is needed for E2 but not for E1

E2 is stereospecifc, E1 is not E1 gives Zaitsev orientation

11.15 Summary of Reactivity: SN1, SN2, E1, E2 Alkyl halides undergo different reactions in competition, depending on the reacting molecule and the conditions Based on patterns, we can predict likely outcomes (See Table 11.4)

Sintesis Alkil Halida Halogenasi

Alkana dpt bereaksi dg halogen (Cl2 atau Br2) dg adanya energi (h dan kalor)H3C H

+ Cl

Cl

cahaya panas

H3C

Cl

+ HCl

metana

kloro metana

Halogenasi aromatik dipake untuk mendapat aril halida melalui suatu reaksi substitusi elektrofilik aromatikBr

+ Br2

FeBr3 + HBr hangat Bromobenzena