[No#1] Best Overview on Hydrogen Iodide (HI)

Hydrogen iodide (HI) is a hetero diatomic molecule. Aqueous solutions of HI are referred to as iodohydroic acid or hydroiodic acid, a robust acid. Gas and solution are interconvertible. HI is employed in organic and inorganic synthesis together of the first sources of iodine and as a reducer in organic synthetic reaction.

Hydrogen iodide cover

Properties of Hydrogen Iodide (HI)

Hydrogen Iodide (HI) is a colorless gas that reacts with oxygen in the atmosphere to offer water and iodine vapor.

With moist air, Hydrogen Iodide (HI) gives a slight brownish mist (or fumes) of acid.

it’s exceptionally soluble in water, giving hydriodic acid. One liter of water will dissolve 425 liters of HI gas, the ultimate solution having only four water molecules surrounding per molecule of HI.

Some Important Physical Properties of Hydrogen Iodide:

  • The molecular formula of hydrogen iodide is HI
  • The molecular weight or mass of hydrogen iodide (HI) is 127.91 g mol−1
  • The exact molecular mass or weight of hydrogen iodide (HI) is 127.912293452 g mol-1
  • The appearance of HI is colorless
  • The density of HI is 2.85 g cm-3 at −47 °C temperature
  • The melting point of HI is -51 °C( 222 K and -60 °F). At the normal atmospheric condition, it stays in a gaseous form.
  • It boils at a very low temperature. The boiling point of HI is -34 °C (239 K and -29 °F)
  • It is very acidic in nature. Its Acidity (pKa) value is –9.5
  • Its Basicity (pKb) value is 23.5
  • Refractive index (nD) value of HI is 1.466
  • Dipole moment of HI is 0.38 D

What is Hydroiodic acid

Hydroiodic acid is a liquid solution of pure HI in water phase. Commercial acid usually contains 57% HI by mass.

The answer forms an azeotrope boiling at 127 °C with 57% HI, 43% water.

Acid is one among the strongest of all common acids thanks to the
high stability of its corresponding conjugate base.

The iodide ion is that the largest of all common halides which ends up within the charge being dispersed over a bigger space.

HI reaction

By contrast, a chloride ion is significantly smaller, meaning its charge is more concentrated, resulting in a stronger interaction between the proton and therefore the chloride ion.

This weaker H+ —I interaction between hydrogen and iodide ions, that facilitated the dissociation of the proton from the anion.

This is that the reason HI is that the strongest acid of the hydrohalides.

The industrial preparation of Hydrogen iodide (HI) involves the reaction of I2 with hydrazine, which also yields nitrogen gas as a by-product.

When reaction performed in water solution, the Hydrogen iodide (HI) must be distilled.

                     2 I2 + N2H4 → 4 HI + N2


HI also can be distilled from an answer of NaI or other alkali iodides in concentrated hypophosphorous acid (note that vitriol won’t work for acidifying iodides because it will oxidize the iodide to elemental iodine).

Std enthalpy of formation ΔH at 298K for hydrogen iodide is 0.0016199 kJ mol-1. Its Specific heat capacity (C) is 0.2283 J/(g·K).


Another way HI could also be prepared is by bubbling sulfide steam through an aqueous solution of iodine, forming acid (which is distilled) and elemental sulfur (this is filtered).

                    H2S +I2 → 2 HI + S

Additionally HI are often prepared by simply combining H2 and I2. This method is typically employed to get high purity samples.

                       H2 + I2 → 2 HI

For many years, this reaction was considered to involve an easy bimolecular reaction between molecules of H2 and I2. However, when a mix of the gases is irradiated with the wavelength of light in the visible region adequate to the dissociation energy of I2, about 578 nm, the rate increases significantly.

Hydrogen Iodide

This evident a mechanism of dissociation of I2, whereby I2 first dissociates into 2 iodine atoms. (I-I)Iodine atoms attach themselves to a side of Hydrogen (H2) molecule and break the H—H bond. As per transition state theory, it can be monitored in ultrafast femtosecond spectroscopy:

H2 + I2 + 578 nm radiation → H2 + 2 I → I – - – H – - – H – - – I → 2 HI

In the chemical laboratory, another method involves hydrolysis of phosphorus triiodide (PI3), the iodine equivalent of PBr3. during this method, I2 reacts with phosphorus to make phosphorus triiodide, which then reacts with water to make HI and hypophosphorous acid .

           3 I2 + 2 P + 6 H2O → 2 PI3 + 6 H2O → 6 HI + 2 H3PO3

Main reactions and applications of Hydrogen Iodide (HI)

If you leave hydrogen iodide (HI) out in the open, it will oxidise, according to the reaction pathways below.

4 HI + O2 → 2H2O + 2 I2
HI + I2 → HI3

Since HI3 is dark brown in colour, aged HI solutions also appear dark brown.

HI adds to alkenes in the same way that HBr and HCl do.

HI + H2C=CH2 → H3CCH2I

In organic chemistry, HI is used to convert primary alcohols into alkyl halides. This is an SN2 substitution, in which the “activated” hydroxyl group is replaced by the iodide ion (water). In polar protic solvents, HI is favoured over other hydrogen halides since the iodide ion is a much stronger nucleophile than bromide or chloride, allowing the reaction to proceed at a reasonable rate without much heating.

In polar protic solvents, the broad iodide anion is less solvated and more reactive, causing the reaction to continue faster due to stronger partial bonds in the transition state. Secondary and tertiary alcohols also undergo this reaction, but substitution occurs through the SN1 pathway.

Hi reaction 2

In a reaction close to the replacement of alcohols, HI (or HBr) can be used to cleave ethers into alkyl iodides and alcohols.

This form of cleavage is significant because it allows a chemically stable and inert ether to be converted into a more reactive species.

Diethyl ether is cleaved into ethanol and iodoethane in this case. The reaction is regioselective, as iodide prefers to strike the ether carbon that is less sterically hindered.

HI reaction 3

The same Markovnikov and anti-Markovnikov rules apply to hydroiodic acid as they do to HCl and HBr.

Certain α -substituted ketones and alcohols are reduced by HI by replacing the substituent with a hydrogen atom.

Industrial application of Hydrogen Iodide (HI):

Hydriodic acid is officially classified as a DEA List I chemical by the federal government. Reduction with HI and red phosphorus has been the most common method for producing methamphetamine in the United States due to its utility as a reducing agent.Under fire, clandestine chemists react pseudoephedrine (recovered from decongestant pills) with hydroiodic acid and red phosphorus to form iodoephedrine, an intermediate that is mainly reduced to methamphetamine.

Clandestine chemists now use red phosphorus and iodine to produce hydroiodic acid in situ due to its listed status and closely controlled sales.

Hydrogen Iodide (HI) Application in the salt industry:

To increase the iodine content of salt, hydroiodic acid can be used to make sodium iodide or potassium iodide.

FAQ on Hydrogen Iodide (HI)

What is hydrogen iodide used for?

As a reducing agent and an analytical reagent, hydrogen iodide is used. Pharmaceuticals, disinfectants, and other contaminants are all manufactured from it. It's delivered as a liquefied compressed gas.

What does hydrogen and iodine make?

Hydrogen and iodine bind to form hydrogen iodide in the forward reaction. Hydrogen iodide (HI) decomposes back into hydrogen and iodine in the reverse or backward reaction.

Why is hydrogen iodide a strong acid?

With 57 percent HI and 43 percent water, the solution boils at 127°C and forms an azeotrope. The dispersal of the ionic charge over the anion causes the elevated acidity. The I interaction in HI stimulates proton dissociation from the anion, which is why HI is the best acid of the hydrohalides.

Why is HI covalent?

A polar covalent bond exists between HI. It's because, unlike Iodine, hydrogen is electropositive in nature. So the bonding is covalent, but the charges are partially polarised.

Is hydroiodic acid dangerous?

Hydriodic Acid is a CORROSIVE CHEMICAL that can irritate and burn the skin and eyes, resulting in eye damage. Hydriodic Acid can irritate the nose and throat when inhaled. Hydriodic acid can irritate the lungs, resulting in coughing and/or shortness of breath.

What are the most important intermolecular forces in hydrogen iodide?

Dipole-dipole forces

Which is stronger HF or HI?

Vey strong acids include HCl, HBr, and HI, while weak acids include HF. As the experimental pKa values decrease, the acid intensity increases in the following order: HF (pKa = 3.1) HCl (pKa = -6.0) HBr (pKa = -9.0) HI (pKa = -9.5) HF (pKa = 3.1) HCl (pKa = -6.0) HI (pKa = -9.5) ……. The H-F bond is more difficult to sever since shorter bonds are more secure.

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