CHAPTER 5 ALKYL HALIDE

Objective
Ability to explain the relationship between the structure, physical and chemical properties of the different bonds and functional group in organic compounds.(CO2) Ability to explain each of functional group activity. (CO3)

Outcome
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.

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.

 In alkyl halides this polarity causes the carbon to become activated to substitution reactions with neucleophiles. Carbon-halogen bonds get less polar, longer and weaker in going from 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.

H H H C C Br H H alkyl halide

H C C

H Cl vinyl halide

aryl halide

USES OF ALKYL HALIDES

Solvents very good for a variety of organic compounds. Reagents precursors for a variety of syntheses. One big application is the so-called Grignard reagents. Freons used as refrigerator coolants, in sprays and as blowing agents. Because of ozone problem, their application is foreseen to terminate in about 10 years. Pesticides very powerful, but tend to accumulate in nature because of low reactivity, causing lasting contamination. Many of them now avoided, DDT banned. Anesthetics widely used in the past (chloroform), but because of toxicity now are generally avoided.

POLARITY AND REACTIVITY

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.

PREPARATION OF ALKYL HALIDES

REVIEW

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.

8
6 7

5 6 4

3 2

1
1 3

7 5
4

2 bromo-5-methylheptane

1-chloro-5,5-dimethylhexane

4-bromo-2-chloro-1-methylcyclohexane

Br

Cl

Cl

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

Common Names
Common names for alkyl halides are used only for simple alkyl halides. 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

REACTIONS OF ALKYL HALIDES

Undergoes ionic reactions - Nucleophilic Substitution and Nucleophilic Elimination Reactions. 1. Nucleophilic Substitution Reaction
Nu: + R-Nu
pro uct

nucleop ile

alkyl ali e substrate

hali e ion

2.

Nucleophilic Elimination Reaction

C C X H alkyl hali e

(-HX)

C C alkene

R-X

:X

: :

: :

Nucleophilic Substitution Reaction

What does the term "nucleophilic substitution" imply ? A nucleophile is an the electron rich species that will react with an electron poor species.A substitution implies that one group replaces another

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 different mechanisms 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

SN1- Substitution Nucleophilic Unimolecular

SN1 indicates a substitution, nucleophilic, unimolecular reaction, 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, then step 2: rapid attack of a nucleophile on the electrophilic carbocation to form a new bond
Multi-step reactions have intermediates and several transition states (TS). In an SN1 there is loss of the leaving group generating 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.

SN1 Mechanism
Two step reaction with carbocation intermediate. Rate is first order in the alkyl halide, zero order in the nucleophile
STEP 1 :

(CH3)3C Br

(CH3)3C

+ Br

.
STEP 2 : (CH3)3C+

+ H O H

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

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

Rates of SN1 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.

SN2- Substitution Nucleophilic Bimolecular

SN2 indicates a substitution, nucleophilic, bimolecular reaction, 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.
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 the reaction 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.

SN2 Mechanism
Rate is first order in each reactant. (one-step reaction with no intermediate Concerted reaction: new bond forming and old bond breaking at same time.

H H O C Br H H

H HO C Br H H HO C

H + H H Br
-

Rates of SN2 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

Uses for SN2 Reactions

Synthesis of other classes of compounds. Halogen exchange reaction.

N cleo - + I RRRRRRRRR+ + +
-

ile

p p p p p p p p p
-

Prod ct R-I RR- R' RR- R' R-N R- N

+

Class of Prod ct k l li lc et er t i l t i et er l

R'

+ R' + N + N + +
-

i e s lt zi e alk ester e itrile

| -R' |N

R- | -R' R- |N -R' R-

+ R-

SN2
SN2
Primary or methyl Strong nucleophile ( Strong Lewis base) Polar aprotic solvent (DMF, DMSO) Rate = k[halide][Nuc] Inversion at chiral carbon No rearrangements

vs

SN1
SN1
Tertiary

Weak nucleophile (Weak Lewis base) Polar protic solvent, (alcohol and water) Rate = k[halide] Racemization of optically active compound Rearranged products

Leaving Group :- for both SN1 and SN2 ( the weaker the base the group departs, the better the leaving group)

Nucleophilic Elimination Reaction

An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism

The two most important methods are: Dehydration (-H2O) of alcohols, and Dehydrohalogenation (-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 are possible: 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)

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. E1 Mechanism

H Br H C C CH3 H CH3 H H O H H

H H C C CH3 H CH3
Halide ion leaves, forming carbocation

CH3 C C H CH3 + H3O

H C C CH3 H CH3

Base removes H from adjacent carbon and pi bond forms

E2 Reaction
Bimolecular elimination Requires a strong base Halide leaving and proton abstraction happens simultaneously - no intermediate
E2 Mechanism

H Br H H O H C C CH3 H CH3 H C C

CH3 CH3

+ H2O + Br

How do we determine whether a reaction will go via an elimination or a substitution and whether it will be first or second order?
Primary (1) carbons Normally react by an SN2 pathway. With good nucleophiles such as Br-, I-, CN-, RS-, or NH3 get only SN2 reactions. However, with strong base (hydroxide or alkoxide) get some competition by E2 reaction, though SN2 still predominates. Secondary (2) carbons Go by either SN2 or E2: - With good nucleophiles get mostly SN2. - With strong base E2 predominates. Tertiary (3) carbons Go 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.

Substitution and Elimination

Secondary alkyl halides, often react with simple basic nucleophiles to give a mixture of products arising from both substitution and elimination.

Substitution or Elimination
Control of the reaction pathway between substitution and elimination is generally accomplished by careful choice of the reactants; strong, sterically hindered bases tend to favor elimination, while weak, unhindered nucleophiles tend to favor substitution. The choice for a "strong, hindered base" is generally tert-butoxide

TRY THIS..

Formation of Alcohol
Example 1
60oC

CH3

Br

OH

H2O

CH3 O

Br

H H O C Br H H

H HO C Br H H HO C

H + H H Br
-

Example 2

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

(CH3)3C

+ Br

+ H O H

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

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

Formation of Ether ( Williamson synthesis)

General reaction
RO Na R'L R O R' Na L
This is a good route for synthesis of unsymetrical ethers. 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, and dialkyl sulfates.

Example

CH3CH2CH2OH
Propyl alcohol

NaH

CH3CH2CH2O Na
Sodium propoxide

H H

CH3CH2I CH3CH2OCH2CH2CH3
Ethyl propyl ether

Na I

The alkyl halide ( alkyl sulfonate) should be primary to avoid E2 reaction. Substitution favored over elimination at low tempertaure.

Formation of Amino compound

Using ammonia as a nucleophile in a reaction with an appropriate (methyl, primary, or secondary) alkyl halide in an SN2 reaction to prepare primary amines does work, but it requires a huge excess of ammonia, because the product primary amine is also reactive toward the 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.

Formation of Alkane ( Wurtz synthesis)

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.

2 Na

dry (CH2H5)2O

2 NaX

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.

RX

R2'CuLi

R'

R'Cu

LiX

Formation of Alkene compound

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