13.2 Halogenoalkanes

• Chemistry
• Secondary Education
• Age 12,13

HALOGENOALKANES

Organic Chemistry

05 March 2024

• apply the SN1 and SN2 mechanisms in halogenoalkane reactions to show how nucleophilic addition occurs
• create a quick slide presentation to describe the chosen reaction that halogenoalkanes undergo
• orally present the created slides to describe how halogenoalkanes react with different reagents
• accurately respond to the given question in the 3-question sheet

Learning expectations

HALOGENOALKANES

Organic Chemistry

05 March 2024

 Illustrate the following:

• (a) a halogenoalkane
• (b) a nucleophile
• (c) nucleophilic addition

Picture me this!

• substitution of an atom by a nucleophile
• possible due to the polarity of the carbon-halogen bond
• two mechanisms, the SN1 or SN2 mechanism

Nucleophilic Substitution

HALOGENOALKANES

Organic Chemistry

05 March 2024

Polarisation of carbon-halide bond

What's happened to the molecule?

• Primary halogenoalkanes will use SN2.
• Tertiary halogenoalkans will use SN1.
• Secondary halogenoalkanes will use both.

Two Mechanisms of Nucleophilic Substitution

• The 'S' stands for substitution, the 'N' stands for nucleophilic and the '2' stands for the initial step (rate-determining step) 
• involves two species(the halogenoalkane and the nucleophile)

SN2 MECHANISM

How to write the mechanism in exams

What do you notice about the orientation of the atoms?

It is crucial to get the orientation of the atoms right, the molecule has been

inverted at the end of the reaction. This is called the inversion of configuration(product has a configuration opposite to the reactant).

Is it possible for a tertiary halogenoalkane to react by SN2 mechanism?

THINK ABOUT:

• It is unlikely for tertiary halogenoalkanes to react by the SN2 mechanism. This is because SN2 mechanism involves the nucleophile approaching and forming bond from the opposite side of the halogen. In the case of tertiary halogenoalkane, it is unlikely because the opposite of the halogen is cluttered with CH₃ group(s).

EXPLANATION

• The '1' stands for the initial step(rate-determining step).
• involves one species only(the halogenoalkane)

SN1 MECHANISM

i. The reaction happens in two stages. In the first stage, a small proportion of

the halogenoalkane ionises to give a carbocation and a bromide ion.

ii. Once the carbocation is formed, it will react immediately when it comes into contact with a nucleophile, Nu−. The lone pair of the nucleophile is strongly attracted towards the positive carbon, and moves towards it to create a new bond.

Why do secondary halogenoalkanes react using both SN1 and SN2 mechanisms?

THINK ABOUT:

1. The opposite of the halogen is uncluttered by CH3 groups.
2. There is no other way than both SN1 and SN2.
3. The second-degree carbocation formed is more stable.
4. Both A and C.

HALOGENOALKANES

Organic Chemistry

05 March 2024

  Goal and Expected Output: Make and share a quick diagram presentation about:

• Team 1 (Vathana and Romnea) - Substitution reaction with NaOH(aq) and heat to produce an alcohol
• Team 2 (Kosoma and Vinisa) - Substitution reaction with KCN in ethanol and heat to produce a nitrile
• Team 3 (Kanika and Paulika) - Substitution reaction with NH3 in ethanol heated under pressure to produce an amine
• Team 4 (Malika) - Reaction with aqueous silver nitrate in ethanol as a method of identifying the halogen present as exemplified by bromoethane

Classwork 4

USES OF HALOGENOALKANES

Organic Chemistry

29 February 2024

Uses of CFCs

• chlorofluorocarbons, compounds containing carbon with chlorine and fluorine atoms attached. Two common CFCs are CFC-11(CCl3F) and CFC-12(CCl2F2).

Effects of CFCs to the environment

• destruction of the ozone layer (O3)

REACTIONS OF HALOGENOALKANES

Organic Chemistry

29 February 2024

• Substitution with hydroxide ions, OH⁻.
• Substitution with cyanide ions, CN⁻.
• Substitution with ammonia, NH₃.
• Elimination

Halogenoalkanes reactions

Substitution with hydroxide ions, OH⁻ (Hydrolysis)

• Reagent: Aqueous sodium hydroxide, NaOH or water
  
• Condition: Heat under reflux
• Product: Alcohols

it can also happen with water molecule as a nucleophile

Substitution with cyanide ions, CN⁻

• Reagent: Potassium cyanide, KCN in ethanol (ethanolic KCN)
  
• Condition: Heat under reflux
• Product: Nitriles

• The nitrile produced can be converted to carboxylic acid by heating under reflux with acid or alkali (hydrolysis reaction).
• (CH3)3CN + 2H2O + H+ → (CH3)3COOH + NH4+ 
• (CH3)3CN + H2O + OH− → (CH3)3COO− + NH3

Substitution with ammonia, NH3

• Reagent: Excess ammonia, NH3 in ethanol
  
• Condition: Heat in a sealed tube
• Product: Amines

it can also happen with water molecule as a nucleophile

Elimination

• Reagent: Sodium hydroxide, NaOH in ethanol
  
• Condition: Heat under reflux
• Product: Alkenes

it can also happen with water molecule as a nucleophile