Halogenoalkanes and Reaction Pathways

IB Chemistry Topics 10.5 & 10.6

10.5 Halogenoalkanes 10.5.1 Describe, using equations, the

substitution reactions of halogenoalkanes with sodium hydroxide.

10.5.2 Explain the substitution reactions of halogenoalkanes with sodium hydroxide in terms of SN1 and SN2 mechanisms.

10.5.1

Describe, using equations, the substitution reactions of halogenoalkanes with sodium hydroxide.

Halogenoalkanes Halogenoalkanes consist of a carbon bonded

to an atom of fluorine, chlorine or bromine General formula = CnH2n+1X, where X is a

halogen These are typically oily liquids that do not mix

well with water They are used in many products CFCs have been renowned for their negative

impact on the ozone layer

Substitution Reactions In a substitution reaction, one atom or group of

atoms, takes the place of another in a molecule Examples

CH3CH2Br + KCN CH3CH2CN + KBr (CH3)3CCl + NaOH (CH3)3 COH + NaCl

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Nucleophilic Substitution

A nucleophile is a molecule or ion that has a high electron density… nucleo = nucleus; phile = loving.

It is attracted to atoms in molecules with a lower electron density.

It may replace another group in an organic molecule, such as a halogen.

The hydroxide ion (OH-) from NaOH is an effective nucleophile that will substitute the halogen, turning the product into an alcohol

These reactions are known as substitution nucleophilic, or SN reactions.

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Nucleophilic Substitution One covalent bond is broken as a new covalent

bond is formed The general form for the reaction is Nu:- + R-X R-Nu + X:

Nucleophile Substrate Product Leaving group

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Nucleophilic Substitution Nu:- + R-X R-Nu + X: The bond to the leaving group is broken The leaving group takes both electrons that formed

the bond with it The nucleophile provides the electrons to form the

new bond

Nucleophile Substrate Product Leaving group

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Nucleophilic Substitution Alkyl halides commonly undergo nucleolophilic

substitution reactions. The nucleophile (OH-) displaces the halide leaving group from the alkyl halide.

There are two common ways for nucleophilic substitutions to occur.

They are known as SN1 and SN2.

Nucleophile Substrate Product Leaving group9

Examples of Nucleophilic Substitutions

Nucleophilic substitutions may be SN1 or SN2 10

10.5.2

Explain the substitution reactions of halogenoalkanes with sodium hydroxide in terms of SN1 and SN2 mechanisms.

Nucleophilic Substitution Bimolecular or SN2 A reaction is bimolecular when the rate

depends on both the concentration 2 reactants: the substrate and the nucleophile.

SN2 mechanisms occur most readily with methyl compounds and primary haloalkanes

Takes place in one step

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SN2 Mechanism

The general form for an SN2 mechanism is shown above.Nu:- = nucleophile

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An Example of a SN2 Mechanism

This is a one step process since both the nucleophile and the substrate must be in a rate determining step.

The nucleophilic substitution of ethyl bromide is shownabove. This reaction occurs as a bimolecular reaction.The rate of the reaction depends on both the concentrationof both the hydroxide ion and ethyl bromide

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SN2 Mechanism One-step mechanism with *curly arrows:

*Curly arrows represent the movement of an electron pair.

H3CH2C

C

CH3H

BrOH-

H3CH2C

C

CH3H

BrOH

CH2CH3

C

CH3H

OH

Transition State: As OH- attaches, Br- leaves

+ Br-+

Nucleophilic Substitution Unimolecular or SN1 A unimolecular reaction occurs when the rate of

reaction depends on the concentration of 1 reactant: the substrate but not the nucleophile.

A unimolecular reaction is a two step process since the subtrate and the nucleophile cannot both appear in the rate determining step

SN1 mechanisms occur most readily with tertiary haloalkanes and some secondary haloalkanes.

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SN1 Mechanism

The general form for an SN1 mechanism is shown above.Nu:- = nucleophile

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SN1 Mechanism

The first step is the formation of the carbocation. It is the slow step. The rate of the reaction depends only on theconcentration of the substrate.

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SN1 Mechanism: Heterolytic Fission

SN1 Mechanism

In the second step, the nucleophile attaches to the carbon intermediate (carbocation). This is a very fast step.

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SN1 Mechanism Two-step mechanism with curly arrows:

H3CH2C

C

CH3H

Br

H3CH2C

C+

CH3H

CH2CH3

C

CH3H

OH

Transition State: Formation of Carbocation

Br-OH-

+

Rate determining step: spotaneous dissociation of leaving group

Very fast step: reaction of nucelophile and carbocation

SN1 and SN2 Reactions

SN1 SN2

Rate =k[RX] =k[RX][Nuc:-]

Carbocation intermediate?

Yes No

Number of steps 2 1

Occurs with Tertiary halogenoalkanes

Primary halogenoalkanes

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http://www.youtube.com/watch?v=tAjFraTw0HM

http://www.youtube.com/watch?v=ZtnAR3uOAbo (?)

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10.6 Reaction Pathways 10.6.1 Deduce reaction pathways given the

starting materials and the product.

Reaction Pathways and mechanisms Most organic reactions proceed by a defined sequence

or set of steps. The detailed pathway which an organic reaction follows is called a mechanism.

Knowing a reaction mechanism is very valuable information. It allows the chemist to predict what products will be formed when a chemical reaction occurs.

The organic chemist can use this information to modify compounds and to synthesize new compounds with certain desired characteristics.

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Diagram of common organic reactions

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Reaction Pathway Practice Fill in the reaction pathway chart, showing

the necessary reactants and any other additional conditions necessary for the reaction to take place

You may omit trihalogenoalkanes and poly(alkenes)