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ELIMINATION REACTION OF ALKYL HALIDES

ELIMINATION REACTIONS

Involve in loss of elements from the starting material to form a new bond in the product.

ELIMINATION REACTIONS

Removal of the elements of HX is called dehydrohalogenation.

Dehydrohalogenation is an example of β elimination.

Leaving group: Leaving groups are the fragments that retain the

electrons in a heterolytic bond cleavage. Weaker bases are more stable with the extra pair

of electrons and therefore make better leaving groups.

H2O > OHˉ Iˉ > Brˉ > Clˉ > Fˉ

E2 MECHANISMS

The E2 mechanism (Bimolecular Elimination) The most common mechanism for dehydrohalogenation

is the E2 mechanism. E2 eliminations are promoted by a strong base.

The reaction is bimolecular and it involved “second- order kinetics” because two molecules must come together for the reaction to occur.

Second order kinetics is the rate of reaction depends on the concentration of both the base and the substrate.

Since this is a one step mechanism, this is the slow step, and therefore controls the rate of reaction.

Where,

Rate = k [base] [substrate]

rate = k[(CH3)3CBr][¯OH] An energy diagram for an E2 reaction:

In the transition state, the C-H and C-Br bonds are partially broken and the O-H and bonds are partially formed.

Lithium Hydroxide, LiOH

Rubidium Hydroxide, RbOH

Sodium Hydroxide, NaOH

Potassium Hydroxide, KOH

Strontium Hydroxide, Sr(OH)2 Barium Hydroxide, Ba(OH)2

Examples of strong base:

Cesium Hydroxide, CsOH Calcium Hydroxide, Ca(OH)2

The identity of the Alkyl Halide :

The mechanism of an E2 elimination reaction:Base (B:) attacks a neighboring C-H bond and begins to remove the H at the same time as the alkene double bond starts to form and the X group starts to leave.

Neutral alkene is produced when the C=H bond is fully broken and the X group has departed with the C-X bond electron pair.

Notice that the hydrogen that is removed is on the carbon that is adjacent to the one bearing the halogen

Likewise, the “H” and the “X” atoms that are eliminated during the dehydrohalogenation of an alkyl halide must be on the carbon atoms.

Characteristics of the E2 Mechanism

SAYTZEFF’S RULE

The major product of dehalogenation is the most stable alkene.

The most stable alkene is the most substituted C=C due to the electron donating properties of the alkyl group.

MARKOVNIKOV’S RULE

The hydrogen (H) is attached to the C with less alkyl substituents and the halide (X) attached to the C with more alkyl substituents.

Major product has the most stable carbocation intermediate during the addition process.

The most stable carbocation is the more substituted carbocation due to induction and hyperconjugation.

A carbon rich in subtituents will gain more substituents and the carbon with more hydrogens attached will get the hydrogen.

Inductive effect is an experimentally observable effect of the transmission of charge through a chain of atoms in a molecule by electrostatic inductions.

Hyperconjugation is the interaction of the electron in a sigma bond (usually C–H or C–C) with an adjacent empty (or partially filled) non-bonding p-orbital or antibonding π orbital or filled π orbital, to give an extended molecular orbital that increases the stability of the system.

ANTI MARKOVNIKOV’S REACTION

Anti-Markovnikov is exactly opposite of Markovnikov reaction.

The hydrogen (H) is attached to the C with less alkyl substituents and the halide (X) attached to the C with more alkyl substituents.

Anti-Markovnikov behavior extends to other chemical reactions than just additions to alkenes.

Another famous example of anti-markovnikov addition is hydroboration.

STEREOCHEMISTRY OF THE E2 REACTION

In formation of a pi bond, the C-H and C-LG bonds all aligned in a plane to be coplanar.

When C-H bond and C-LG bond at 1800 with respect each other (opposite side), it will called anti-periplanar transition state that has staggered conformation with lower energy.

When C-H bond and C-LG bond at 00 with respect each other (same side), it will called syn periplanar transition state which is in eclipsed conformation with higher energy.

The staggered conformation (anti-periplanar) has more stable than eclipsed conformation (syn-periplanar).

X is a leaving group(LG).

E2 often occur in anti-periplanar.

Anti-periplanar - involves a base reacting with the proton anti-periplanar to the leaving group (that simultaneously leaves) in a single step to give an alkene.

The order in stability of alkene according to Zaitsev’s rule:

<--- More stable alkene --- --- Less stable alkene --->

In cyclohexyl halide such as bromocyclohexane, the more stable conformation has bromine in an equatorial position. As shown at left below, in this conformation, there are no β hydrogens anti to the bromine. In this conformation, an E2 elimination of HX to form cyclohexane is not possible.

In the less stable chair conformation illustrated above right, the bromine is in an axial position.There are hydrogen atoms anti to the bromine on both of the adjacent (β) carbons, so E2 elimination of HX is possible.

For cyclohexyl halides, the requirement for an anti-periplanar transition state geometry means that the halide leaving group must be axial, never equatorial. This has important consequences for how cyclohexane derivatives react.

Explanation: Exists as two conformations (A and B), each of which has one group axial and one group equatorial.

E2 occur from conformation B.

Explanation: Exists as two conformations, C having two equatorial substituents and D having two axial substituents.

E2 occur from conformation D.

STEREOSELECTIVITY

Eliminations often favor the more stable trans-product over the cis-product.

The product of E2 reaction as predicted by the Zaitsev rule (Saytzeff’s rule)

Examples:

This reaction prefers an anti orientation of the halogen and the beta-hydrogen which is attacked by the base. These anti orientations are colored in red in the above equations.

In conclusion, the substituted cyclohexanes, E2 elimination must occur with a trans diaxial arrangement of H and LG to gives the product.

GROUP MEMBERS

Hani Liyana Binti Rahmat UK26352

Nurul Amalina Anati Binti Abdullah UK26346

Amy Madzirah Binti Ramlan UK26347

Norshafiqah Binti Mohamad Saidi UK26360

Wan Azwira Binti Ab Ghani @ W.Ahmad UK26257

Nurul Syazdiana Binti Mohd Zuki UK26258

Sharifah Nurul Aina Binti Sayed Burhanudin UK26335

Nur Hananun Binti Mohd Azimi UK26314

Nurul Huda Binti Alias UK26294

Nadia Farahana Binti Muhammad UK26339

REFERENCES

http://www.personal.psu.edu/the1/e2.htm http://library.tedankara.k12.tr/carey/ch5-7.html http://www.enc.edu/~timothy.t.wooster/courses/CH321/lec

turenotes/chapter6/E2.htm http://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/al

halrx3.htm http://www.mhhe.com/physsci/chemistry/carey/student/ol

c/graphics/carey04oc/ref/ch05eliminationreactions.html

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