1. Halogenoalkanes and Reaction Pathways
IB Chemistry Topics 10.5 & 10.6

2. 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 SN1 and SN2 mechanisms.

3. 10.5.1
Describe, using equations, the substitution reactions of halogenoalkanes with sodium hydroxide.

4. 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

5. 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 5

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 SN reactions. 6

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 7

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 8

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 SN1 and SN2.
Nucleophile Substrate Product Leaving group 9

10. Examples of Nucleophilic Substitutions
Nucleophilic substitutions may be SN1 or SN2 10

11. 10.5.2
Explain the substitution reactions of halogenoalkanes with sodium hydroxide in terms of SN1 and SN2 mechanisms.

12. 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 12

13. SN2 Mechanism The general form for an SN2 mechanism is shown above.
Nu:- = nucleophile 13

14. An Example of a SN2 Mechanism
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
This is a one step process since both the nucleophile and the substrate must be in a rate determining step. 14

15. SN2 Mechanism One-step mechanism with *curly arrows:
*Curly arrows represent the movement of an electron pair.

16. 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. 16

17. SN1 Mechanism The general form for an SN1 mechanism is shown above.
Nu:- = nucleophile 17

18. SN1 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. 18

19. SN1 Mechanism: Heterolytic Fission

20. SN1 Mechanism
In the second step, the nucleophile attaches to the carbon intermediate (carbocation). This is a very fast step. 20

21. SN1 Mechanism
Two-step mechanism with curly arrows:

22. Tertiary halogenoalkanes Primary halogenoalkanes
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 22

23. http://www.youtube.com/watch?v=ZtnAR3uOAbo (?)
(?) 23

24. 10.6 Reaction Pathways
Deduce reaction pathways given the starting materials and the product.

25. 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. 25

26. Diagram of common organic reactions26

27. 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)