10.5 Halogenoalkanes

Exam Question Write the reaction between methane and chlorine to form chloromethane. Explain this reaction in terms of the free radical mechanism. 5marks

10.5 Halogenoalkanes Describe, using equations, the substitution reactions of halogenoalkanes with sodium hydroxide Explain the substitution reactions of halogenoalkanes with sodium hydroxide in terms of SN 1 and SN 2 mechanisms.

Assessment Statement 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 = C n H 2n+1 X, 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 CH 3 CH 2 Br + KCN  CH 3 CH 2 CN + KBr (CH 3 ) 3 CCl + NaOH  (CH 3 ) 3 COH + NaCl 6

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 S N reactions. 7

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 8

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 9

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 S N 1 and S N 2. Nucleophile Substrate Product Leaving group 10

Examples of Nucleophilic Substitutions Nucleophilic substitutions may be S N 1 or S N 2 11

Assessment Statement Explain the substitution reactions of halogenoalkanes with sodium hydroxide in terms of SN 1 and SN 2 mechanisms.

Nucleophilic Substitution Bimolecular or S N 2 A reaction is bimolecular when the rate depends on both the concentration 2 reactants: the substrate and the nucleophile. S N 2 mechanisms occur most readily with methyl compounds and primary haloalkanes Takes place in one step 15

S N 2 Mechanism The general form for an S N 2 mechanism is shown above. Nu:- = nucleophile 16

An Example of a S N 2 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 shown above. This reaction occurs as a bimolecular reaction. The rate of the reaction depends on both the concentration of both the hydroxide ion and ethyl bromide

S N 2 Mechanism One-step mechanism with *curly arrows: *Curly arrows represent the movement of an electron pair.

Nucleophilic Substitution Unimolecular or S N 1 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 S N 1 mechanisms occur most readily with tertiary haloalkanes and some secondary haloalkanes. 21

S N 1 Mechanism The general form for an S N 1 mechanism is shown above. Nu:- = nucleophile 22

S N 1 Mechanism The first step is the formation of the carbocation. It is the slow step. The rate of the reaction depends only on the concentration of the substrate. 23

S N 1 Mechanism: Heterolytic Fission Need to mention a term here for IB HETEROLYTIC FISSION: If a covalent bond breaks such that both electrons in the shared pair end up with ONE of the atoms, Hetero = uneven, one (Greek, heteros = other) Lytic = lyse = able to loosen (greek, lutikos) Fission = to break up, the act of splitting into parts. (latin, fissio = cleaving)

S N 1 Mechanism In the second step, the nucleophile attaches to the carbon intermediate (carbocation). This is a very fast step. 27

S N 1 Mechanism Two-step mechanism with curly arrows:

S N 1 and S N 2 Reactions SN1SN1SN2SN2 Rate =k[RX] =k[RX][Nuc: - ] Carbocation intermediate? Yes No Number of steps 2 1 Occurs with Tertiary halogenoalkanes Primary halogenoalkanes 29

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10.6 Reaction Pathways 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. 32

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. 34

Diagram of common organic reactions 35

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)