Haloform Reaction Mechanism

The Haloform reaction mechanism is the one that starts with the halogen disproportionation with the presence of the hydroxide ion. This gives either the hypohalite or the halide. Then, the hydroxide abstracts the proton by producing enolate. Hypohalite can also react with any of the present methyl ketones, forming a haloform, eventually. This reaction is the type of nucleophilic substitution. An example of the haloform reaction is given below.

(Image to be added soon)

In the above-given reaction, it can be observed that when the methyl ketone is treated with the bromine halogen in the aqueous sodium hydroxide solution, there occurs Polyhalogenation, followed by the methyl group's cleavage. The resulting reaction products are the tribromomethane and carboxylate, which is the essential haloform. Formerly, this reaction was used to produce bromoform, iodoform, and chloroform industrially. Tracing its roots to 1822, it is the oldest known organic reaction, when the Georges-Simon Serullas added potassium to the Iodine solution in water and ethanol, resulting in the formation of iodoform and potassium formate.

Haloform Reaction Mechanism

Step 1

The base (which is the hydroxide ion) takes out the alpha hydrogen, producing enolate. After that, the halogen and enolate reaction takes place, leading to the halogenated ketone formation, including the halogen corresponding anion.

(Image to be added soon)

Step 2

Now, Step 1 is repeated twice to produce a tri-halogenated ketone. Then, the net reaction until the tri-halogenated ketone formation can be written as follows.

(Image to be added soon)

Step 3

The hydroxide ion will act as a nucleophile, and it attacks the electrophilic carbon, doubly bonded to the oxygen ion. This double bond carbon-oxygen changes to a single bond by making the oxygen atom anionic. Moreover, this makes the reformation of the carbon-oxygen double bond favorable. Also, the carbon attached to the 3 halogens is displaced by leaving us with the carboxylic acid. When an acid-base reaction happens, the carboxylic acid donates a proton to the tri-halomethyl anion by offering the requisite haloform product.

(Image to be added soon)

Hence, the three-step haloform reaction mechanism yields the halogenation of a methyl ketone with excess halogen, and the essential haloform precipitate results in the formation of haloform and carboxylate ion. The substrates used in this reaction include secondary alcohols that are oxidizable to methyl ketones, methyl ketones, acetaldehyde, and ethanol. However, fluoroform cannot be prepared through the haloform reaction because the hypofluorite ion is highly unstable.

Iodoform Test and the Compound Types Producing a Positive Iodoform Test

Any compounds that contain either the CH3CH(OH) group or the CH3C=O group produces a positive result with the iodoform test or iodoform reaction mechanism. When NaOH and I2 are added to a compound, that contains one of these groups, a pale yellow colored precipitate of iodoform (otherwise triiodomethane) is formed.

Therefore, the iodoform test can be used to identify ketones and aldehydes; if the compound is an aldehyde, then it should be ethanal (where it is the only aldehyde, including the CH3C=O group). Also, this occurs only due to the 3 I atoms replace the H atoms of CH3C=OR, and the C-C bond breaks due to the electron-withdrawing effect of the 3 I atoms (since I is more electronegative than that of C), by forming CHI₃ and the carboxylic acid's salt anion (based on the R group of the original compound, that influences the carbon chain's anion RCOO- length, that is formed).

We can also use this test to identify the alcohols.  If the alcohol is tertiary, then it gives no result because it cannot be oxidized. Whereas, if the alcohol is primary, then it must be ethanol (because this is oxidized to ethanal, which can be given as the only aldehyde that presents a positive result with the iodoform test). All the secondary alcohols produce a positive result because they are oxidized to ketones.

Exothermic Reaction

An exothermic reaction can be described either as a chemical or physical reaction that releases heat. It also gives net energy to its surroundings. It means the energy required to initiate the reaction is less than the released energy.

When the medium where the reaction is taking place gains heat, the reaction is called exothermic. When we use a calorimeter, the total amount of heat that flows into or through the calorimeter is given as the negative of the system's net change in energy.

The absolute amount of energy present in a chemical system is difficult either to calculate or measure. The enthalpy change (ΔH) of a chemical reaction is easier to work with. This change equals the change in the system's internal energy, including the work needed to change the system's volume against the constant ambient pressure. A bomb calorimeter is more suitable to measure the energy change (ΔH) of a combustion reaction. The ΔH values measured and calculated are related to bond energies by the equation given below.

ΔH = energy used in forming the bonds of the product - energy released in breaking the reactant's bonds.

Did You Know?

• The only primary alcohol, which provides the triiodomethane (otherwise called iodoform) reaction is ethanol. If 'R' belongs to the hydrocarbons category, we have secondary alcohol. Furthermore, this group does not hold tertiary alcohols. So, none of the tertiary alcohols with the -OH group can contain a hydrogen atom attached to the carbon.