7 Elimination Reactions

Formation of Alkenes

The elimination of HX from adjacent atoms of a haloalkane is widely used for synthesizing alkenes

The elements of a hydrogen halide are eliminated from a haloalkane in this way, the reaction is called dehydrohalogenation

Dehydrohalogenation of most haloalkanes yields more than one product

• The major product will be the more stable alkene

• The more stable alkene has the more highly substituted double bond

• Elimination follows the Saytzeff’s rule when the elimination occurs to give the more highly substituted alkene as the major product

• The stabilities of alkenes:

Elimination Versus Substitution

• Nucleophiles are potential bases

• Bases are potential nucleophiles

• In S_N2 pathway, elimination and nucleophilic substitution compete each other

• Substitution is favoured when the substrate is primary alcohol and the base is hydroxide ion

• Elimination is favoured when the substrate is secondary alcohol

• With tertiary haloalkanes, S_N2 reactions cannot take place Þ Elimination is highly favoured especially at high temperatures Þ Substitution occurs through S_N1 mechanism only

Eliminations will be favoured when using:

1. higher temperatures

2. strong sterically hindered bases (e.g. (CH₃)₃CO^–)

Summary of the reaction pathways for the substitution and elimination reactions of simple haloalkanes

CH3X Methyl	RCH2X 1°	R2CHX 2°	R3CX 3°
Gives SN2 reactions only	Gives mainly SN2 and gives mainly E with a strong sterically hindered base (e.g. (CH3)3CO–)	Gives mainly SN2 with a weak base (e.g. I–, CN–, RCO2–) and gives mainly E with a strong base (e.g. RO–)	No SN2 reaction. In hydrolysis, gives SN1 or E. At low temperatures, SN1 is favoured. When a strong base (e.g. RO–) is used or at high temperatures, E predominates.

Formation of Alkynes

• Alkynes can be produced by dehydrohalogenation of dihaloalkanes

• Two molecules of hydrogen halides are eliminated

e.g.

8 Uses of Halogeno-compounds

As Solvents in Dry-cleaning

• Chlorinated hydrocarbons are good solvents for oil and greases Þ widely used in the dry-cleaning industry

e.g. trichloroethene, CCl₂ = CHCl tetrachloroethene, CCl₂ = CCl₂

• Properties that favour the use: 1. Relatively non-flammable

2. Volatile

3. Little or no structural effect on fabrics

As Raw Materials for Making Addition Polymers

Poly(chloroethene) (also known as PVC):

• Produced by means of the addition polymerization of the chloroethene monomers in the presence of a peroxide catalyst

• Polar C – Cl bond results in dipole-dipole interactions between polymer chains, making PVC hard and brittle and used to make pipes and bottles

• PVC becomes flexible when plasticizer is added

• Used to make shower curtains, raincoats, artificial leather, insulating coating of electrical wires

Poly(tetrafluoroethene) (PTFE, ‘Teflon’):

• Produced through addition polymerization of the tetrafluoroethene monomers under high pressure and in the presence of catalyst

• Teflon has a high melting point and is chemically inert

• Used to make non-stick frying pans

Questions and problems

1. Give the name to the following compound:

2. Write the equations of butanol formation from 1-bromobutane.

3. What type of isomers are created while monochlorination of isobutane? What quantity (in percent) of each isomer is there in products of monochlorination if it is known that relative rate of replacement of one hohydrogen atom is changed in order: 1⁰ >2⁰ >3⁰ such as 5,0 : 3,8 : 1.

4. Find out the structure of compound using the empirical formula and the products of chemical reactions: С₄Н₉Br, while hydrolysis 1⁰ alcohol is created, while dehydrohalogenation with next hydrohalogenation 3⁰ bromoalkane is created. Write the equations.

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