Halogenoalkanes

Halogenoalkanes are compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms (fluorine, chlorine, bromine or iodine

Halogenoalkanes a can be classified as primary, secondary or tertiary depending on the number of alkyl groups attached to the carbon atom holding the halogen atom

Primary

Secondary

Tertiary

Physical properties

Boiling point of the halogenoalkanes increases as the molecular size increases. This is because there are more electrons in larger molecules and more temporary dipoles can be set up, resulting in stronger van der Waals
Primary halogenoalkanes have higher boiling point compared to secondary and tertiary halogenoalkanes of the same molecular weight. This is because the surface area of primary halogenoalkane is higher, More temporary dipoles can be set up, resulting in stronger van der Waals forces.
Halogenoalkanes are insoluble in water but soluble in organic solvents because when dissolved in water, the energy needed to break the hydrogen bonds between water molecules is too high. The energy released when halogenoalkanes-water attraction is set up is not enough to compensate it. This makes the structure to gain energy overall, making it less stable
While when it is dissolved in organic solvents, the halogenoalkanes-solvent attraction is strong enough to compensate the energy needed to break the weak van der Waals forces.

Reactions of Halogenoalkanes

Fluoroalkanes are the least reactive while iodoalkanes are the most reactive. This is because the carbon-halogen bond strength decreases from fluorine to iodine as the size of the halogen atom decreases. Since reactions of halogenoalkanes involve the breaking of the carbon-halogen bond, it follows that carbon-iodine bonds are easiest to break making them easier to react

Nucleophilic substitution

The signature reaction of alkanes is nucleophilic substitution. Nucleophilic substitution is the substitution of an atom by a nucleophile and a nucleophile is a species which is strongly attracted to a region of positive charge of something. It normally carries either a partially or fully negative charge on the molecule

Nucleophilic substitution is possible due to the polarity of the carbon-halogen bond. Halogens are more electronegative than carbon, therefore the electron pair in the carbon-halogen bond will be attracted towards the halogen end, leaving the halogen slightly negative and the carbon slightly positive

Nucleophilic substitution is done via two mechanisms, the Sn1 and Sn2

-Primary halogenoalkanes will use Sn2

-Tertiary halogenoalkanes will use Sn1

-Secondary halogenoalkanes can use both

The Sn2 mechanism

In Sn2 mechanism, the S stands for substitution, the N stands for nucleophilic and the 2 stands for the initial step involving 2 species( halogenoalkane and nucleophile)

For example, bromoethane (CH3CH2Br) as a primary halogenoalkane and Nu- as the nucleophile

The nucleophile Nu- is attracted towards the positive carbon, beginning to form a co-ordinate bond with it. Meanwhile, the negative bromine atom is repelled further due to the approaching nucleophile.
Eventually there is a state called the transition state where the carbon-bromine bond is just at the verge of breaking and the carbon-nucleophile bond is just at the verge of forming. So it has a total of 5 groups attached
The movement goes on until the Nu- is firmly attached to the carbon, and the bromine has been expelled as a Br- ion. The nucleophile has substituted the bromine
ONE HUMP FOR THE GRAPH

The Sn1 mechanism

In Sn1 mechanism, the 1 stands for the initial step. The overall rate reaction is governed by the first rate of the first step

For example, 2-bromo-2-methylpropane as((CH3)3Br) and Nu- as a general nucleophile.

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
Once the carbocation is formed, It will react instantly when it comes into contact with the nucleophile, Nu-. The lone pair of the nucleophile is strongly attracted towards the positive carbon, and moves towards it to create a new bond.
Tertiary halogenoalkanes react via Sn1 because the tertiary carbocation intermediate formed is relatively stable. If primary halogenoalkanes were to react in the same manner, the primary carbocation formed will be relatively unstable, resulting in high activation energy of the reaction.
Secondary halogenoalkanes can react with both Sn1 and Sn2 mechanisms because:

-The opposite of the halogen is not cluttered by CH3 groups

-The secondary carbocation formed is more stable than the primary carbocation

TWO HUMPS FOR THE GRAPH

Substitution with hydroxide ions, hydrolysis

Reagent: Aqueous sodium hydroxide, NaOH on water

condition: Heat under reflux

Product: Alcohols

When aqueous sodium hydroxide is heated with bromoethane under reflux, the bromine is substituted by hydroxide ion, OH and ethanol is produced

CH3CH2Br + NaOH = CH3CH2OH + NaBr

Heating under reflux means heating with a condenser placed vertically in the flask to prevent loss of volatile substances from the mixture
This reaction uses Sn1 mechanism, the nucleophile is the hydroxide ion, OH
Water can also be used as the nucleophile in this reaction. However, hydrolysis using water occurs much slower. This is because the negatively-charged OH- is a moe effective nucleophile than water
For a particular alkyl group R, the rate of hydrolysis decreases in order

R-I> R-Br> R-Cl> R-F

As the strength of carbon-halogen increases exponentially from iodine to fluorine. In fact, carbon-fluorine bond is so strong that fluoroalkanes undergo hydrolysis

Substitution with cyanide ions CN-

Reagent: Potassium cyanide, NaCN or KCN in ethanol

Condition: Heat under reflux

Product: nitriles

When ethanoic potassium cyanide, KCN is heated with 2-bromo-2- methylpentane under reflux, the bromine is substituted by cyanide ion, CN-. 2-methyl-2propanentrile is produced

(CH3)3Br + KCN = (CH3)3CN + KBr

This reaction uses the Sn2 reaction, the nucleophile ion is the cyanide ion, CN-
This is a very useful reaction in organic synthesis as it serves as a mean of increasing the length of carbon chain.

The nitrile produced can be converted to carboxylic acid by heating under reflux with acid or alkali

-(CH3)3CN + 2H2O + HCL = (CH3)3COOH + NH4CL, acidic hydrolysis with dilute HCL

-(CH3)3CN + H2O + OH- = (CH3)3COO- + NH3, alkaline hydrolysis ONLY FOR ESTER AND NITRILES

Substitution with ammonia, NH3

Reagent: Excess ammonia, NH3 in ethanol

Condition: Heat under reflux

Product: amines

When ethanolic ammonia, NH3 is heated with bromoethane in a sealed tube, the bromine is substituted by amine group, NH2-. The reaction occurs in two stages. In the first stage, a salt is formed

CH3CH2Br + NH3 = CH3CH2NH3+Br-

In the second stage, a reversible reaction occurs between this salt and the excess ammonia. Ethylamine, a primary amine is formed

CH3CH2NH3Br- + NH3 = CH3CH2NH2 + HBr

If ammonia not used in excess, a complicated mixture containing secondary and tertiary anime is obtained. This is because ethylamine is a good nucleophile and it can attack the unreacted bromoethane

Elimination

Reagent: Sodium hydroxide, NaOH in ethanol

Condition: Heat under refluxed

Product: alkenes

In this alcoholic condition, the hydroxide ion, OH- acts as a base rather than a nucleophile. Hence it will accept a proton from the carbon atom next door to the one holding the bromine. The resulting re-arrangement of the electrons expels the bromine as a bromide ion and produces ethene

CH3CH2Br + OH- = CH2CH2 + Br- +H20

Uses of halogenoalkanes

CFCs are chlorofluorocarbon compounds containing carbon with chlorine and fluorine atoms attached. Two common CFCs are CFC-11 and CFC-12

Properties

Chemically unreactive or inert
Non-flammable
Low toxicity
odourless
volatile

Uses

As refrigerants
As aerosol propellants
As foam-blowing agents
As cleaning agents in electric industry
As flame retardant in fire extinguisher

CFCs and the environment

CFCs have many uses in our daily lives. However, due to their stability and inertness, they are also largely responsible for the destruction of the ozone layer. Ozone layer prevents harmful UV radiation from reaching us.

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