Investigate The Relationship Between Structure And Properties Of Alkyl Halides - Session 1

Alkyl halides are a class of organic compounds that consist of a halogen atom bonded to a carbon atom of an alkyl group. In alkyl halides, the halogen atom is directly bonded to a carbon atom. The general formula for alkyl halides is R-X. Can you tell what does the R and X represent? An example of alkyl halide is chloroethane. The boiling points of alkyl halides are higher than similar alkanes. They are also more reactive than simple alkanes. But why are they more reactive and have higher boiling points than alkanes?.

The high boiling point and reactivity of alkyl halides is due to polar nature of alkyl halides. Alkyl halides exhibit a polar nature due to the difference in electronegativity between the carbon and halogen atoms in the carbon halogen bond. Electronegativity is the ability of an atom to attract shared pair of electrons towards itself in a chemical bond. For example, in a molecule of hydrogen chloride, chlorine atom is more electronegative than hydrogen. So the electron density is more concentrated towards chlorine atom.

The halogen atoms are more electronegative than carbon. The electronegative halogen atom attracts the shared electrons in the carbon halogen bond closer to itself. This creates a partial negative charge on the halogen atom. Simultaneously, the carbon atom becomes partially positive. This is because the electron density is pulled away from carbon towards the halogen. This polarity in the bond gives rise to the overall polarity of the alkyl halide molecule.

Now we know that alkyl halides have polar nature. Higher boiling point of alkyl halides is due to the presence of attractive forces between the molecules of alkyl halides. These attractive forces are called Dipole-Dipole interactions. Dipole-Dipole interactions occur between the partially positive end of one molecule and the partially negative end of another molecule. More energy is required to break these Dipole-Dipole interactions. As a result the alkyl halides have higher boiling points.

Alkyl halides are very reactive due to their polar nature. We know that the halogen atom of alkyl halide withdraws electron density from the carbon atom. As a result the halogen atom detaches itself from the alkyl halide. The carbon atom of the alkyl halide becomes positively charged. This alkyl group with positively charged carbon atom is called carbocation. We already know that carbocations are reactive. They can react with nucleophile.

Alkyl halides readily undergo Nucleophilic substitution reaction. During this reaction, the halogen atom of the alkyl halide is replaced by the nucleophile. For example, chloromethane reacts with Sodium Hydroxide to form methanol. The hydroxide ion of the sodium hydroxide acts as nucleophile. It replaces the chlorine atom of chloromethane.

There are two types of nucleophilic substitution reactions. These are bimolecular nucleophilic substitution and unimolecular nucleophilic substitution. The halogen atom of alkyl halide is called leaving group. During bimolecular nucleophilic substitution reaction, nucleophile displaces a leaving group from an organic molecule in a single step. In alkyl halides, the leaving group is halogen atom. Bimolecular nucleophilic substitution reaction is called so because it involves the two molecules in a single step. One is nucleophile, other is alkyl halide.

Lets discuss the mechanism of this reaction. First of all, the nucleophile attacks the carbon atom of alkyl halide from the opposite side of the halogen atom. The bond between halogen atom and carbon atom of alkyl halide starts breaking. At the same time, the bond formation between nucleophile and carbon atom takes place. In other words we can say that a transition state is formed. In this transition state, bond formation and bond breakage takes place at the same time. Finally bond between halogen and carbon atom is broken and product is formed. Bimolecular nucleophilic substitution reactions are also called as SN2 reactions.

Primary Alkyl halides prefer bimolecular nucleophilic substitution. However, tertiary alkyl halides do not prefer bimolecular nucleophilic substitution. But why? Tertiary alkyl halides have alkyl groups attached to carbon atom from all sides. In other words we can say carbon atom is crowded. Due to this, nucleophile cannot attack carbon atom from behind opposite to the halogen atom. On the other hand primary alkyl halides are not crowded. They can easily undergo bimolecular nucleophilic substitution.

Now lets understand unimolecular nucleophilic substitution reaction of alkyl halides. Unimolecular means only one molecule is involved in the first step of the reaction. This reaction has two steps. In the first step, the halogen atom of alkyl halide detaches. As a result carbocation is formed. In second step, nucleophile attacks the carbocation and forms the product. Tertiary alkyl halides undergo unimolecular nucleophilic substitution. Unimolecular nucleophilic substitution reactions are also called as SN1 reactions.

Have you thought why tertiary alkyl halides would prefer to undergo unimolecular nucleophilic substitution? This is because in unimolecular nucleophilic substitution, carbocation is formed in first step. More stable carbocation means a stable product will be formed. So tertiary carbocation is very most stable. Meanwhileprimary alkyl halides would form primary carbocation. Primary carbocation is not stable. That is why tertiary alkyl halides will undergo unimolecular nucleophilic substitution.

An elimination reaction is a type of organic reaction in which atoms or groups of atoms are removed from a molecule. This results in the formation of a double bond between two carbon atoms in molecule. During elimination reaction of alkyl halide, the halogen atom attached to the carbon atom detaches. Hydrogen atom attached to next carbon atom is removed by a base. A base is a specie that can accept hydrogen ions. As a result double bond is formed between two carbon atoms. Alkene is formed as a product in this reaction.

There are two types of elimination reactions. One is bimolecular elimination reaction and other is unimolecular elimination reaction. During bimolecular elimination reaction of alkyl halides two things happen at the same time. They are the process of leaving of a halogen atom and attack of base to remove hydrogen atom. Hydroxide ion acts as a base to remove hydrogen from alkyl halide. As a result alkene is formed. Bimolecular elimination reactions are also called as E2 reactions.

In unimolecular elimination reaction of alkyl halides, in first step the halogen atom detaches from the alkyl halide. As a result, the carbocation is formed. In second step base removes the hydrogen atom from alkyl halide. The double bond forms between two carbon atoms. Alkene is formed as a product. Tertiary Alkyl halides undergo unimolecular elimination reaction. Unimolecular elimination reactions are also called as E1 reactions.

As we have studied in previous reactions, carbon atom of alkyl halides is electron deficient. It acts as electrophile. But the same carbon atom can become an electron rich specie. It can also act as a nucleophile. But how? When alkyl halides react with magnesium metal in the presence of dry ether, they form Grignard reagent. Grignard reagent has general formula RMgX . R represents alkyl group. X represents halogen atom.

In Grignard reagent, magnesium metal is attached directly with carbon atom. We know that carbon is more electronegative than magnesium. In this case, carbon become electron rich. It acts as nucleophile. Grignard reagent acts as a very strong base and nucleophile.